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Page 2 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

The n_TOF Collaboration (2009-2011)

E. Chiaveri1,2), S. Andriamonje1), J. Andrzejewski3), L. Audouin4), V. Avrigeanu5), M. Barbagallo6), V. Bécares7), F. %HþYiĜ8), F. Belloni2), E. Berthoumieux1,2), J. Billowes9), D. Bosnar10), M. Brugger1), M. Calviani1), F. Calviño11), D. Cano-Ott7), C. Carrapiço12), F. Cerutti1), M. Chin1), N. Colonna6), G. Cortés11), M.A. Cortés-Giraldo13), M. Diakaki14), I. Dillmann15), C. Domingo-Pardo16), I. Duran17), N. Dzysiuk18), C. Eleftheriadis19), M. Fernández-Ordóñez7), A. Ferrari1), K. Fraval2), S. Ganesan20), G. Giubrone21), M.B. Gómez-Hornillos11), I.F. Gonçalves12), E. González-Romero7), F. Gramegna18), E. Griesmayer22), C. Guerrero1), F. Gunsing2), M. Heil16), D.G. Jenkins23), E. Jericha22), Y. Kadi1), F. Käppeler24), D. Karadimos25), M. Kokkoris14), M. .UWLþND8), J. Kroll8), C. Lederer26), H. Leeb22), L.S. Leong4), R. Losito1), M. Lozano13), A. Manousos19), J. Marganiec3), T. Martinez7), C. Massimi27), P.F. Mastinu18), M. Mastromarco6), M. Meaze6), E. Mendoza7), A. Mengoni28), P.M. Milazzo29), M. Mirea5), W. Mondalaers30), C. Paradela17), A. Pavlik26), J. Perkowski3), A. Plompen30), J. Praena13), J.M. Quesada13), T. Rauscher31), R. Reifarth16), A. Riego11), F. Roman1,5), C. Rubbia1,32), R. Sarmento12), P. Schillebeeckx30), G. Tagliente6), J.L. Tain21), D. Tarrìo17), L. Tassan-Got4), A. Tsinganis1), S. Valenta8), G. Vannini27), V. Variale6), P. Vaz12), A. Ventura28), M.J. Vermeulen23), V. Vlachoudis1), R. Vlastou14), A. Wallner26), T. Ware9), C. Weiß22), T.J. Wright9)

1) European Organization for Nuclear Research (CERN), Geneva, Switzerland 2) &RPPLVVDULDWjO¶eQHUJLH$WRPLTXH &($ 6DFOD\- Irfu, Gif-sur-Yvette, France 3) 8QLZHUV\WHWàyG]NLLodz, Poland 4) Centre National de la Recherche Scientifique/IN2P3 - IPN, Orsay, France 5) Horia Hulubei National Institute of Physics and Nuclear Engineering - IFIN HH, Bucharest - Magurele, Romania 6) Istituto Nazionale di Fisica Nucleare, Bari, Italy 7) Centro de Investigaciones Energeticas Medioambientales y Technologicas (CIEMAT), Madrid, Spain 8) Charles University, Prague, Czech Republic 9) University of Manchester, Oxford Road, Manchester, UK 10) Department of Physiscs, Faculty of Science, Zagreb, Croatia 11) Universitat Politecnica de Catalunya, Barcelona, Spain 12) Instituto Tecnológico e Nuclear (ITN), Lisbon, Portugal 13) Universidad de Sevilla, Spain 14) National Technical University of Athens (NTUA), Greece 15) Physik Department E12 and Excellence Cluster Universe, Technische Universität München, Munich, Germany 16) GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 17) Universidade de Santiago de Compostela, Spain 18) Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Italy 19) Aristotle University of Thessaloniki, Thessaloniki, Greece 20) Bhabha Atomic Research Centre (BARC), Mumbai, India 21) Instituto de Fìsica Corpuscular, CSIC-Universidad de Valencia, Spain 22) Atominstitut, Technische Universität Wien, Austria 23) University of York, Heslington, York, UK 24) Karlsruhe Institute of Technology, Campus Nord, Institut für Kernphysik, Karlsruhe, Germany 25) University of Ioannina, Greece 26) University of Vienna, Faculty of Physics, Austria 27) Dipartimento di Fisica, Università di Bologna, and Sezione INFN di Bologna, Italy 28) $JHQ]LDQD]LRQDOHSHUOHQXRYHWHFQRORJLHO¶HQHUJLDHORVYLOXSSRHFRQRPLFRVRVWHQLELOH (1($ %RORJQD,WDO\ 29) Istituto Nazionale di Fisica Nucleare, Trieste, Italy 30) European Commission JRC, Institute for Reference Materials and Measurements, Geel, Belgium 31) Department of Physics and Astronomy - University of Basel, Basel, Switzerland 32) /DERUDWRUL1D]LRQDOLGHO*UDQ6DVVRGHOO¶,1)1$VVHUJL $4  Italy

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TABLE OF CONTENTS 1. Executive Summary ...... 5 2. Introduction ...... 6 3. Scientific Case for EAR-2 ...... 7 3.1 Astrophysical Research ...... 7 3.1.1 MACS measurement on stable isotopes to 1% accuracy ...... 8 3.1.2 MACS measurement on unstable isotopes ...... 9 3.1.3 Neutron induced charged particle reactions ...... 11 3.2 Nuclear technology applications ...... 11 3.3 Basic nuclear research ...... 14 4. Dosimetry and Radiation Damage Studies ...... 16 5. Facility Performance ...... 18 5.1 Neutron Fluence ...... 19 5.2 Neutron Energy Resolution ...... 20 5.3 Charged Particle Fluence ...... 21 5.4 Photon Fluence ...... 22 5.5 Higher Signal to background ratio and Equivalent Half-Life ...... 23 6. Radiation Protection Analysis ...... 26 7. Engineering Study ...... 29 7.1 Civil Engineering ...... 29 7.2 Beam line study ...... 31 8. Conclusion ...... 32 APPENDIX I: BUDGET (EAR-2) ...... 36 APPENDIX II: STAFF (EAR-2) ...... 38 APPENDIX III: TENTATIVE PLANNING (EAR-2) ...... 40 Appendix IV. EXPRESSIONS OF INTEREST FOR EAR-2 ...... 41

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1. EXECUTIVE SUMMARY

The outstanding features of the existing CERN n_TOF neutron beam (with a flight path of 185 m) are the very high instantaneous neutron flux, excellent TOF resolution, low intrinsic backgrounds and coverage of a wide range of neutron energies, from thermal to a few GeV. These characteristics provide a unique possibility to perform neutron-induced cross-section and angular distribution measurements for applications in nuclear astrophysics, nuclear reactor technology and basic nuclear physics. A wide variety of measurements have already been performed since the facility became operational in 2001, most of them already published [1-24] and made available to the nuclear data and nuclear physics community.

A study has been performed investigating the feasibility of new Experimental Area called EAR-2 which, having a flight path of only 20 m from the existing spallation target (90 degrees with respect to the incoming proton beam), would fulfil the demands of the neutron science community for a neutron time-of-flight facility with a higher neutron flux [25]. The construction of the EAR-2 with a short flight path would offer the possibility of improving the quality of the data essential for nuclear energy applications, nuclear astrophysics, basic nuclear physics, dosimetry and radiation damage.

The main advantages of the planned EAR-2 with respect to existing facilities, in particular the existing n_TOF beam line, are:

The number of neutrons at the sample position is on average increased by a factor 25.

Neutron-induced reaction measurements can be performed on very small samples (< 1 mg). This feature is of key importance for reducing the activity of unstable samples and in cases where the available sample material is particularly rare. Limitations in sample mass are crucial in astrophysics as well as for the field of nuclear technologies. Depending on the particular isotope, this may enable some measurements for the first time at all.

Measurements can be performed on isotopes with very small cross-sections, which act as neutron poisons or as bottlenecks in the reaction flow of the s-process. An optimized signal-to-background (S/B) ratio is an essential prerequisite for such experiments.

Measurements can be performed on much shorter time scales for significantly improved accuracy. Repeated runs with modified conditions are essential to verify corrections and reduce systematic uncertainties.

Measurements of neutron-induced cross-sections at high energies (En >10±100 MeV), which are very difficult in the existing EAR-1, become possible thanks to the strongly UHGXFHGȖ-flash. This concerns the important measurements of inelastic scattering cross- sections or for reactions with charged particles in the exit channel, where the preferred Si DQG*HGHWHFWRUVDUHVWURQJO\DIIHFWHGE\WKHȖ-flash.

EAR-2 will contribute to a substantial improvement in experimental sensitivity and open a new window to astrophysics, technological issues (such as transmutation or design of safety of future nuclear energy systems) and basic nuclear physics by allowing measuring neutron- induced reactions which are not accessible so far in existing facilities worldwide. EAR-2 should be considered complementary to EAR-1; since they would run in parallel.

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2. INTRODUCTION

The overall efficiency of the experimental programme and the range of possible measurements could be significantly improved by constructing EAR-2, located vertically 20 m above the n_TOF spallation target (see Fig. 1). This possibility, already evoked in the past during the first phase of n_TOF, was recently analysed in detail within the framework of an interdepartmental working group. During the last year, this group considered in detail the feasibility of the project and in particular the clarification and better definition of the scientific case, expected characteristics of the neutron beam, civil engineering challenges and radiation protection issues. The present document is a summary of the work and conclusions reached to date.

Fig.1: Experimental Area 2 (EAR-2) schematic view

The configuration of the presently designed n_TOF EAR-2, presented in detail in Section 5, allows neutron-induced reactions to be measured with the following advantages compared to the existing EAR-1:

Reduction RIȖ-flash: Since most of the relativistic particles produced in the spallation process and which generate the so-called µȖ-flash¶ are emitted in the forward direction, placing an experimental area at an angle of 90° with respect to the primary beam axis strongly reduces the related background effects. The large reduction of these signals, which for some detectors mask the signal from neutron reactions for the first few ȝs, opens the possibility to extend, in these cases, the measurement of neutron-induced reactions up to higher energies compared to those presently achievable in EAR-1.

Higher neutron flux: Being closer to the spallation target (flight path of 20 m) the configuration provides a higher instantaneous neutron flux with respect to the present neutron fluence in EAR-1 (flight path of 185 m). This is a clear advantage for measuring reactions on samples with very small masses and/or very small cross-sections.

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Higher neutron rate: Another effect of the ten times shorter flight path is that the interaction time of the neutron pulse with the sample is also ten times shorter. In this way, the ratio between neutron-induced signals and the constant background from the activity of radioactive samples is increased by an order of magnitude. This feature is extremely important in studies on high activity radioactive isotopes, where the intrinsic activity represents the dominant background component.

Small samples:. The set-up with C6D6 detectors at the present flight path appears to be limited to samples of the order of 6 × 1017 atoms or 150 g. With the 25 times higher flux at EAR-2 this limit can be reduced to 6 µg. Another factor of five can be gained by replacing the C6D6 set-up with the Total Absorption Calorimeter (TAC), thus decreasing the limit to about 1 µg or 5 × 1015 atoms. These numbers refer to samples in the important mass range A >150, which includes major s-process branchings, but also the actinide isotopes. Assuming ISOLDE intensities of 1011 s±1, this means that attractive n_TOF samples for measurements at EAR-2 could be made in a single shift at ISOLDE. This possibility is fascinating but needs of course further investigation.

3. SCIENTIFIC CASE FOR EAR-2

The realization of the 2nd Experimental Area with its short flight path will contribute to a substantial improvement in experimental sensitivity and will open a new window to stellar nucleosynthesis, nuclear technology issues (design and safety investigations for future nuclear energy systems), and basic nuclear physics by allowing neutron-induced reactions which are not accessible in the existing facilities to be measured. Together with measurements of interest for nuclear technology and nuclear astrophysics, the EAR-2 will permit studies of radiation damage on electronics devices and detectors to be performed, and to study the response and resistance of new materials, such as moderators, in future high-flux facilities, in particular the European Spallation Source (ESS), as well as in fusion systems, like ITER.

3.1 Astrophysical Research

In the field of Nuclear Astrophysics, neutron capture reactions are responsible for the origin of the heavy elements between the Fe peak and the actinides. Roughly equal abundances of the heavy elements were produced by the slow and rapid neutron capture processes (s- and r-process) [26,27]. Whilst the s-process is associated with comparably quiescent stellar conditions during He and C burning, the r-process is associated with an explosive environment. More specifically, the s-process takes place either during He shell burning in thermally pulsing low-mass asymptotic giant branch (AGB) stars, or in the core He and shell C burning phases of massive stars. The prevailing temperatures and neutron densities at the s-process sites are rather moderate, corresponding to thermal energies kT = 10±100 keV and typical neutron capture times of about a year. Owing to the slow time scale, the s-process reaction path follows the valley of stability, starting at the pronounced abundance maximum at Fe and Ni all the way up to the terminal point at Pb and Bi, where further neutron captures produce short-lived Į-unstable nuclei, which eventually decay back into the Pb isotopes.

As a consequence of the slow time scale, the final s-abundances are determined by the respective neutron capture reactions. Therefore, accurate (n,) cross-sections for all isotopes involved in the reaction chain represent the key ingredients for any quantitative picture of

Page 7 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration element synthesis during the advanced He and C burning phases of stellar evolution. These data determine the reliability of abundance predictions and are, accordingly, the basis for testing stellar s-process models.

In a stellar environment, data are needed in the form of Maxwellian-averaged cross- sections (MACS), (1)

ିா೙Τ௞் ௡ܧ݀ ݁ ௡ ήܧ ௡ሻ ήܧ׬ ߪሺ ʹ  ൌ ିா೙Τ௞் ௡ܧ݀ ݁ ௡ ήܧ ߨ ׬ where En denotes the neutron energy,ξ ı(En) the energy-dependent cross-section, and kT the respective thermal energy. In mass regions between magic neutron numbers, where MACS values are large enough for flow equilibrium to be established, there is a direct correlation between s abundances and the respective cross-sections. Under this condition one finds that the product MACS times the resulting s abundance, also known as ıN value, is constant to rather good approximation.

3.1.1 MACS measurement on stable isotopes to 1% statistical accuracy

The (n,) cross-sections have to be known for neutron energies between 0.3 and 300 keV for a complete coverage of the thermal energy range 8 < kT < 90 keV determined by the temperatures of the stellar s-process zones. For meaningful abundance predictions, cross- sections should be available with an accuracy of 5% or better, but uncertainties as low as 1% statistical are desired for stable isotopes and in particular for a number of key isotopes, e.g. for the 33 s-only nuclei on the s-path and for the approximately 70 isotopes needed for the interpretation of s-process signatures discovered in presolar grains. While the MACS collection in the KADoNiS data base [24] contains experimental values for the 279 stable isotopes on the s-process path, the criteria of completeness and accuracy are met only in a minority of cases. For the accuracy aspect this is illustrated in Fig. 2, where the respective uncertainties are plotted versus mass number. Further improvements are clearly required, especially in the mass regions below A = 120 and above A = 180, where a large number of cross-sections with uncertainties in excess of 10% await improvement. 7KH($5FDQUHGXFHWKH³VWDWLVWLFDO´DFFXUDF\RIWKHPHDVXUHPHQWVGRZQWRKRZHYHU WKH³V\VWHPDWLF´RQHZLOO VWLOOEHVXEMHFW WR WKHNQRZOHGJHRIWKHUHIHUHQFHFURVVVHFWLRQV Which can give a good chance to redefine the reference cross section at EAR2 with a future proposal.

Fig. 2: Presently quoted uncertainties for the stellar (nȖ) cross-sections required in s-process studies.

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The cross-sections of the stable s-RQO\LVRWRSHVDUHUHTXLUHGZLWKXQFHUWDLQWLHVRI§7KLV goal has been reached for only half of the cases between 70Ge and 204Pb. At present, high- quality measurements on s-only isotopes are still needed for 64Zn, 76Se, 80,82Kr, 86,87Sr, 96Mo, 104Pd, 164Er, 180Tam, 192Pt, 198Hg, and 204Pb. State of the art experiments were performed at 186,187 n_TOF on Os, using C6D6 liquid scintillators combined with modern pulse height weighting techniques [25]. The more efficient TAC [26]DODUJHʌdetector array of 40 BaF2 FU\VWDOV ZKLFK RIIHUV ILYH WLPHV EHWWHU HIILFLHQF\ VXIIHUHG IURP WKH Ȗ-flash in the existing experimental area, and was therefore limited to neutron energies below about 10 keV. However, the higher sensitivity of the TAC is crucial in cases, where the sample material is only available in small quantities or with low enrichment, e.g. for 180Tam [31].

3.1.2 MACS measurement on unstable isotopes

In contrast to the situation for stable isotopes, experimental data are almost completely missing for the unstable branch point isotopes. Branchings occur when neutron capture and ȕ-decay compete whenever an isotope with a half-life comparable to the neutron capture time is reached by the s-process flow. The relative strength of the two branches is given by the branching ratio

ȕ Ȝȕ Ȝȕ Ȝ (2)

ˆ ൌ Τሺ ൅ ୬ሻ which formally depends on the ȕ-decay rate Ȝȕ = ln2/t1/2 and on the neutron capture rate Ȝn= nn vT ı, where nn denotes the neutron density, vT the mean thermal velocity, and ıthe MACS for the radioactive branch point nucleus. Therefore, reliable branching analyses depend critically on accurate MACS values not only for the s-only isotopes involved, but also for the unstable branch point nuclei. An illustrative example of this type of situation is given in Fig. 3.

Fig. 3: The nucleosynthesis mechanisms beyond Fe illustrated in the example of the Kr-Rb-Sr- region. The neutron capture path of the s-process proceeds along the stability valley because ß-decay is usually much faster than neutron capture. Exceptions are certain unstable isotopes ± in this case 85Kr with t1/2 = 10.8 years ± where competition between neutron capture and ß -decay leads to branchings in the reaction flow. The explosive r- and p-processes occur in the region of unstable isotopes and FRQWULEXWHWRWKHREVHUYHGDEXQGDQFHVDVLQGLFDWHGE\GDVKHGDUURZV,VRWRSHVVKLHOGHGDJDLQVWĮ- decays from the r-process region (full green boxes) are considered to be of s-process origin (apart from minor p-process contributions). The small cross-sections of neutron magic nuclei act as a bottleneck for the reaction flow and give rise to sharp s-process maxima in the observed abundance distribution.

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Whilst the demand for accurate MACS values of stable isotopes can be satisfied with present techniques, measurements on unstable branch point isotopes are still at the edge of feasibility. The main difficulties are (i) to obtain suitable samples and (ii) to handle the background induced by the radioactivity of the samples. Consequently, MACS data on unstable isotopes are scarce and mostly limited to cases that could be measured at kT = 25 keV by activation in a quasi-stellar spectrum. So far, TOF experiments on unstable samples were limited to very long-lived nuclei and suffered from samples of low isotopic purity. Future efforts will have to face the production of purer samples in sufficient quantities and, most importantly, find ways of reducing the required amount of sample material. In this way, the future EAR-2 at n_TOF would provide the most promising options for such studies. First priority should be given to the important branch points 63Ni, 79Se, 147Pm, 151Sm, 152,154,155Eu, 153Gd, 163Ho, 170Tm, 171Tm, 179Ta, 204Tl, and 205Pb.

This list needs to be extended further in view of recent results in stellar evolution, which suggest a superpulse at the beginning of the He burning phase in low metallicity stars. The very high neutron densities in excess of 1015 cm±3 that are predicted for such superpulses imply that the s-process path would be shifted by a few mass units into the region of ß-unstable nuclei, thus enormously increasing the number of unstable isotopes in the s-path. For example, the neutron capture chains in Xe and Cs would easily reach the magic nuclei 136Xe and 137Cs, thus bypassing the stable Ba isotopes 134 to 137, which are commonly considered to be an integral part of the s-process scenarios.

Numerous other requests are related to (i) the interpretation of the s-process signatures of single stars which are preserved in pre-solar grains and ȝm sized dust particles formed in the s-process rich outflows from red giant stars, (ii) the neutron poison effects of the very abundant light elements which strongly affect the s-process neutron balance despite the extremely small cross-sections involved, and (iii) the accurate characterization of the bottleneck regions at magic neutron numbers N = 50, 82, and 126, which are responsible for the structure of the ı1 $ curve that reflects the overall s-process efficiency. These requests are described in more detail in Refs. [26,27] and the corresponding deficits in the present status of recommended MACS values can be identified in the KADoNiS data base [28] where experimental MACS values and the respective uncertainties are listed for the 279 stable isotopes on the s-process path. This collection also shows the situation for the 77 radioactive nuclei on the common s-path, which mostly rely on theoretical results with typical uncertainties of 30±50%. The reasons for missing or uncertain experimental data in KADoNiS are clearly related to difficulties in the underlying measurements. The highly improved sensitivity and the much lower backgrounds reached in EAR-2 would obviously also boost these important aspects of the s-process.

In some particular cases both the mass and the cross section may be so small that it would be impossible to perform a differential measurement in the EAR-1 neither in EAR-2 or any other facility in the world. )RUVXFKFDVHVWKHUHDUHQ¶WDQ\H[SHULPHQWDOGata available and thus any experimental information would be a breakthrough. It is in those situations that the availability of an irradiation point at only 1.5 m accessible only from the new EAR-2 opens the door to preforming integral cross section experiment. The sample (A,Z) would be irradiated at a short distance form the target where the neutron fluence is very intense, and the number of (n,g), (n,a) or (n,p) reactions would be calculated from the number of isotopes (A+1,Z), (A-3,Z-2) and (A, Z-1) found in the sample, respectively. If these reaction products were radioactive we would be talking about an activation measurement. In any case, the result from such experiments would be the convolution of the cross section with the distribution of neutrons as function of neutron energy. Although without any information on

Page 10 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration neutron energy, such integral cross section value would be enough to gain some information on those cross sections which are the most difficult to measure and, if level densities were available, could lead to semi-empirical differential cross sections. This becomes even more interesting due to the proximity of the ISOLDE facility to n_TOF because it open the door to measure cross section from using very small masses, even below the microgram limit. 3.1.3 Neutron induced charged particle reactions Apart from neutron capture, (n,p) and (n,Į) cross-sections are also important in astrophysics for two reasons. (i) In the mass range below Fe, these reactions can dominate the QȖ) channel and determine the reaction flow and the local abundance patterns. Prominent examples are the reactions 14N(n,p), 172 QĮ), 26Al(n,p), 250J QĮ), and 33S(n,Į). (ii) The nuclear physics of the explosive p-process, which occupies the proton-rich side of the chart of nuclides, is strongly determined by Į-induced reactions. Since the reaction network of the p- process includes about 20000 reactions predominantly among unstable nuclei, the corresponding reaction rates have to rely on theoretical predictions. Theory, however, was found to suffer from our poor knowledge of the Į-nucleus potential at the relevant energies below 1±2 MeV. In this situation, experimental input is extremely valuable, even for stable isotopes between Mo and Hg. It has been found that (n,Į) reactions are especially suited to characterize the Į-nucleus potential, but measurements of these very small cross-sections are difficult because only very thin samples can be used due to the limited range of the reaction ejectiles. Therefore, very few data exist so far, in spite of the large variety of possible reactions that can be studied. As successful measurements are crucially dependent on the available neutron flux, the EAR-2 would be per se the ideal place for studying this yet unexplored territory of the p-process.

Another measurement that could be performed at EAR-2 is the 7Be(n,p) and 7Be(n,) cross-section, of interest for the Big Bang nucleosynthesis. A sample of 7Be, which has a 53 day half-life, could be produced in microgram quantity, through the 7Li(p,n) reaction at existing cyclotron laboratories in Europe, and by chemical processing of cooling water from spallation sources. The accurate knowledge of these cross-sections is important to gain information about the so-called 7Li anomaly in Big Bang nucleosynthesis.

3.2 Nuclear technology applications

The construction of EAR-2 would be a great advantage also for measurements related to projects of nuclear waste transmutation, as well as for feasibility studies of future generation nuclear energy systems. In fact, the much higher flux that would be available in EAR-2, relative to the current experimental area at n_TOF and other existing facilities, could allow to measure capture and fission cross-section of various U, Pu and Minor Actinides (MA) isotopes with half-lives as short as a few years. Among them, some of the most important ones are 238Pu (87.7 yr), 241Pu (14.1) and 244Cm (18.1 yr). To reduce the short-term radiation hazard, it is desirable to burn those actinides in future systems (either ADS or Gen IV fast reactors), together with their neighbouring long-lived isotopes. 238Pu is produced in nuclear systems mainly by (n,2n) from 239Pu, n capture in 237Np and alpha decay of 242Cm. 241Pu is produced by n capture in 240Pu and 244Cm is produced by neutron Capture in 243Cm, following a neutron capture in 242Pu. So 241Pu and 238Pu will be present in significant fractions in any reactor fuel loaded with Pu or Np, and both plus the 244Cm will be abundantly produced in transmutation fuels loaded with Pu and minor actinides. Their short lifetime imply that they (and their decay chains) have important contributions to the activity, heat load and neutron emission of the spent fuels from present and future reactors during the first years after discharge from the reactor. This period of time is very important for the

Page 11 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration intermediate storage and, in the case of direct disposal of spent fuel, for the geological repository. These parameters can limit the conditions, minimum acceptance time and the capacity of the facilities foreseen for the storage and final disposal of those spent fuels and associated high level wastes. Furthermore, the 244Cm is the main door for the production of heavier Cm, Bk and Cf isotopes by neutron capture. Therefore, the knowledge of their cross- sections (capture and fission) is important for the estimation of the fuel composition of several nuclear systems, and in this way they are key parameters for the optimization and design of such systems, as confirmed by several sensitivity analyses. Therefore, the knowledge of their cross-section is important for the design of such systems, as indicated by several sensitivity analyses.

In particular, as reported in the µNEA Nuclear Data High Priority Request List¶ (NEA- HPRL) [35], new measurements of the fission cross-section of 238Pu are required in the range between 9 keV and 6 MeV. Improving nuclear cross-section data for 238Pu(n,f) is important for reliable understanding and simulation of the behaviour of the Sodium-cooled Fast Reactors (SFR), Lead-cooled Fast Reactors (LFR), accelerator-driven minor actinide burners (ADMAB), and Gas-cooled Faster Reactors (GFR), in order of significance. Furthermore, improved data for the 238Pu(n,f) reactions are fundamental for estimating the peak power of ADMAB and the void coefficient of SFR. According to EXFOR, due to the short half-life of this isotope, only a few recent measurements have been performed in the energy range of interest, and discrepancies and inconsistencies between various results need to be resolved.

The high flux that would be available in the second experimental area at n_TOF may also allow an even more difficult measurement of interest for transmutation projects and new generation reactors to be undertaken: the 241Pu(n,f) reaction. The very short half-life of 14.1 years makes direct measurements on this isotope almost impossible to perform with reasonable accuracy at current neutron facilities, even with low-resolution lead-slowing down spectrometers (LSDS). Some results have also been obtained recently by means of surrogate reactions, but the accuracy of such measurements is still subject to debate. The very short half-life imposes the use of a very small amount of material, to obtain a reasonable count-rate and extremely high instantaneous neutron flux, such as predicted at n_TOF-EAR-2.

The measurement of the 241Am(n,f) cross-section is relatively easier and has already been attempted in the past at n_TOF. Good accuracy data have been obtained from thermal to a few keV of neutron energy, in particular for the most important resonances. However, the presence of a large ܤ-decay background has resulted in low-accuracy results in the range between 180 keV and 20 MeV, a range of interest for Fast Neutron Reactors (FNR) and Accelerator-Driven Minor Actinides Burners (ADMAB), as indicated in the NEA-HPRL. A recent experiment using the surrogate method was performed on this isotope but the accuracy of the results is affected by model assumptions. A direct measurement, taking advantage of the much higher flux of EAR-2, compared to EAR-1, would improve the situation significantly in the fast neutron energy range.

The Nuclear Data Request List also calls for new measurements on the 242mAm(n,f) cross- section in the range between 500 keV and 6 MeV, for its interest especially for Fast Reactors. The short half-life of this isotope, 141 years, has prevented measurements with the required accuracy of a few per cent the fission cross-section. Some measurements were performed with lead-slowing-down spectrometers (LSDS), but with scarce energy resolution and covering mostly the low-energy region. The EAR-2 would allow new data in a wide energy range to be collected, with reasonable resolution and with improved accuracy.

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The same argument applies to another short-lived minor actinide, 244Cm (18 yr), which has only been studied so far by means of LSDS and surrogate reactions. The fission cross-section of this isotope is required in the range between 65 keV and 6 MeV for the feasibility study of Accelerator-Driven Minor Actinides Burner (ADMAB). As for 241Pu, the very large ܤ-decay background, combined with a relatively high probability of spontaneous fission, makes it extremely difficult to measure the fission cross-section of 244Cm at existing neutron facilities, particularly in the fast neutron energy range where the cross-sections are typically low. In contrast, the much enhanced flux of EAR-2 may allow data to be collected with a few per cent accuracy. Apart from the fission cross-section, the neutron spectrum between thermal energies and 10 MeV is also required for the fission reaction of 244Cm. At present, it is very difficult to study such a spectrum in a direct measurement. Such a measurement, again, is a good candidate for EAR-2.

Another isotope, for which fission cross-section could be studied in n_TOF EAR-2 is 243Cm (29.1 yr). Since its half-life is just slightly higher than that of 244Cm, it is also very difficult to measure, as demonstrated by the very few results currently available. In the case of the 232Th/233U cycle the 232U isotope plays a particular role, although its amount remains limited, because its short half-life of 69 years and the emission of a very hard photon of 2.6 MeV at the end of its radioactive chain impose severe constraints on the shielding for the preparation and handling of the fuel. It is produced by (n,2n) reactions on 233U and by neutron capture on 231Pa,but it is also consumed by capture and fission reactions. The final amount in the fuel results from this balance. Fission is an important process because 232U is fissile although it has an even number of neutrons. Due to the short half-life very few measurements of the fission cross section have been achieved and in narrow energy ranges. The high flux available at EAR-2 would allow a fission cross section measurement covering the full energy range, from to thermal to hundred MeV, with a very limited amount of material.

For capture measurements, the required mass is typically larger than for fission, and this limits the number of measurements that can be performed. For transmutation projects, the capture cross-section measurement of 245Cm (~8500 yr half-life) would become feasible, as would that for 231Pa (32400 yr half-life), an important isotope involved in the Th/U fuel cycle.

However, in the case of capture reactions, another advantage of the high flux in the EAR-2 is that measurements of capture reactions of fissile isotopes will become feasible. When isotopes are fissile, capture and fission reactions are competing and thus one can only measure (n,) cross-section when both fission and capture reactions are measured simultaneously, so that anti-coincidence techniques can be applied [38]. This implies the use of very thin samples (~300 g/cm2) so that the fission fragments can escape the samples and be detected. Thus the total mass that can be reached with a few such thin samples mounted in parallel is only a few mg. With such mass, the measurements in the existing EAR-1 would take several months and even years, and that is why the new EAR-2 is very appealing for these kinds of measurement. Of course, the use of lower masses for these fissile isotopes would also reduce the background from their intrinsic activity with respect to EAR-1.

Finally, the higher neutron flux also offers the possibility of using the C6D6 for this type of measurement, since their lower efficiency with respect to the TAC would no longer be important. In this case, the high neutron energy limit of the measurements would increase, since C6D6 detectors are less affected by the Ȗ-flash.

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3.3 Basic nuclear research

Neutron-induced reactions measured with the time-of-flight technique form a unique tool to investigate nuclear structure at high excitation energies by observing resolved nuclear levels that are revealed by resonances in the reaction yields. This information can be completed by the analysis of measured gamma-ray spectra corresponding to the decay of the compound nucleus, which also provide information on transition probabilities below the neutron separation energy.

Neutron resonance spectroscopy is used to obtain crucial information on level densities in the vicinity of the neutron binding energy, i.e. at several MeV above the ground state. Level densities are an important part in the calculation of nuclear reaction rates, having applications in astrophysical processes and in nuclear reactor devices based on fission or fusion reactions. A large number of level density models exist which are all calibrated by the level density observed with neutron resonances.

As an example of the observation of nuclear levels in neutron-induced experiments, Fig. 4 shows the total cross-section as a function of neutron energy for several nuclei with increasing mass ranging from 6Li to 241Am. The resonance structure present in the cross- sections corresponds to nuclear levels in the compound nucleus. Note that the zero of the neutron energy scale corresponds to the excitation energy of the neutron binding energy, i.e. several MeV above the ground state. On the logarithmic energy scale one can observe the decrease of the spacing between two levels, or the increase of the level density, when the mass of the nucleus increases. The significant shell effects are illustrated by the case of 208Pb, a nucleus with closed neutron and proton shells, where a decrease in the level density can be observed.

Experiments already performed at n_TOF have provided valuable information on this topic and a specific proposal for the particular case of 88Sr has been approved by the INTC [35] and will be performed in 2012 in the present EAR-1.

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4 10

6 Li

0 10

2 10 27 Al 0 10 3 55 10 Mn

0 10

107 3 10 Ag

0 ) 10 b

( 4 10 197 Au on i t 0 10 sec

1

ss 10

o 208 r Pb c 0 l l 10

a 4

t 10 o t 235 U 0 104 10

241 Am 0 10 -2 -1 0 1 2 3 4 5 6 7 10 10 10 10 10 10 10 10 10 10 neutron energy (eV) Fig. 4: The total neutron cross-section as a function of the incident neutron energy. The resonances in the cross-section correspond to nuclear levels in the compound nucleus at an excitation energy of several MeV above the ground state.

Another very interesting field of research would be the study of -ray transition probabilities, so-called Photon Strength Functions (PSF), which is based on the study of the detector response to -rays from neutron capture reactions and its comparison with predictions from different models for PSF. The advantage of this technique with respect to others is that it provides information below the neutron separation energy Sn, which is not the case in photo-absorption experiments. Indeed, the study of the region below Sn is key to understanding hot topics such as the existence and systematics of scissor modes and low- energy pigmy structures in the PSF. Previous experiments at n_TOF using the TAC have already provided for the first time very valuable information on PSF of actinides beyond 238U [36].

The availability of a higher neutron flux at EAR-2 would imply that measurements could be carried out in a much shorter time scale and this would open the door to dedicated proposals devoted to the investigation of nuclear levels and PSF. This is not the case in

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EAR-1 because the number of measurements is very limited (5±8 each year), while in the EAR-2 this number could be increased by a factor of 5 or 10, depending on the masses available for the different samples.

Last, the study of the fission mechanism near the fission threshold, where large asymmetries in angular distribution are found, would benefit from increase flux. In EAR-1, these measurements are limited due to the very low fission cross section below the threshold (~MeV).

4. DOSIMETRY AND RADIATION DAMAGE STUDIES FOR ELECTRONICS

The new experimental area at n_TOF may also be useful for the characterization and calibration of passive and active dosimeters as well as detectors. The radiation resistance of electronic and structural components and cables can be tested in its stray radiation field. In order to assess quantitatively the potential of the new facility for these applications, a series of dedicated FLUKA simulations was performed and various quantities of interest were scored in two different positions:

Position 1: at 1.4±1.8 m above the target. This position offers very high neutron fluences and dose rates. Samples and/or detectors of up to 16.5 cm in diameter could be placed in this position if no neutron cross-section measurement is taking place in EAR-2. There is also the possibility of exposing small passive dosimeters, detectors or components in parasitic mode during normal EAR-2 cross-section measurements. A schematic layout is shown in Fig. 1. Position 2: the standard EAR-2 experimental area at about 19 m from the target, where active and passive detectors and components with diameters of up to 31 cm can be exposed when not performing neutron cross-section measurements

The main results are summarized in Table 1, where the following quantities of interest are reported, normalized to a standard pulse of 7 × 1012 protons on target.

1 MeV (Si) equivalent neutron fluence rate. This quantity is a measure for displacement damage in (electronic) silicon-based devices. It can be correlated to its long-term damage. Ambient dose equivalent (H*(10)) rate. This quantity is of relevance for passive and active dosimeters and radiation monitors. Divided by a µtypical¶ quality factor it can also give an indication of the expected absorbed dose rate. Exact values for the absorbed dose rate are not reported since they will depend greatly on the irradiated material composition, as is customary for an almost pure neutron spectrum with a substantial amount of sub-MeV neutrons. The absorbed dose rate is also of interest for radiation damage of materials such as organic insulators. The fluence rate of µhigh energy¶ hadrons. The fluence rate of thermal (En < 0.4 eV) neutrons. This quantity is important as the charged hadrons above ~20 MeV (high-energy hadrons, HEH) or neutrons above a few MeV are assumed to cause single event upsets (SEUs) in electronic equipment. At the same time another part of SEUs are induced by thermal neutron capture process on light element (such as 10B), which might generate light recoils and -particles, which contribute to the release of energy in the semiconductor. Levels above 107 HEH/cm2/pulse are considered to be critical for standard electronics equipment; these levels could be reached after ~25 pulses or 1 minute for a 2.4 s PS cycle.

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High energy (> 20 MeV) Thermal 1MeV (Si) eq.n H*(10) Position hadron fluence neutrons n/cm2/pulse Sv / pulse h / cm2 / pulse n / cm2 / pulse Position 1 3.03 × 109 1.47 1.44 × 108 1.22 × 1010 Position 2 3.4 × 106 1.3 × 10-3 4 × 105 1.5 × 106 Table 1: Rates (per pulse) of various quantities of interest for dosimetry and radiation damage studies. Position 1 has been assumed to be at 1.5 m from the n_TOF target for the purpose of this table

The contribution of particles other than neutrons is minimal. At Position 1, their contribution to the 1 MeV (Si) equivalent neutron fluence, to the high-energy hadron fluence, and to the ambient dose equivalent is respectively 0.3%, 2.6%, and 3.1%. For Position 2, the corresponding figures are 0.6%, 1.5%, and 1.1% with no sweeping magnet: with the sweeping magnet there remains only 0.3% of contribution to H*(10) caused by photons.

The results presented in Table 1 demonstrate that the proposed EAR-2 will offer a unique opportunity for the calibration of active and passive radiation detectors in an almost pure neutron field at moderate (Position 2) and high (Position 1) dose rates. The available fluence and dose range is very large. A short irradiation in Position 2 (10 pulses) would integrate a few 107/cm2 1 MeV n Si equivalent and 10 mSv, while a long irradiation (105 pulses) in Position 1 will integrate 3 × 1014/cm2 1 MeV (Si) equivalent neutrons and 1.5 × 105 Sv.

The neutron spectrum at Position 1 is shown in Fig. 5. Both spectra are similar to those expected in the LHC caverns, as well as to those in the tunnel, even though in the latter case there is also a significant component of charged hadrons. Therefore, n_TOF EAR-2 could be successfully exploited to test and calibrate components and instrumentations used in the LHC under very similar neutron irradiation conditions.

Fig. 5: Neutron spectrum at Position 1

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5. FACILITY PERFORMANCE

In order to quantitatively assess the performance of the facility and the improvement with respect to the fluence achievable in the existing n_TOF Experimental Area (EAR-1), a FLUKA Monte Carlo simulation was carried out. The geometry of the facility was set up according to the preliminary plans discussed with the Civil Engineering (GS Department).

The implemented geometry of the proposed installation can be seen in Fig. 6.

The neutron tube extends from the spallation target up to the beam dump. The dump is implemented with a Fe core that slows down fast neutrons, surrounded by borated polyethylene and a thin Cd layer at the entrance that captures slow neutrons and reduces backscattering towards the experimental area.

The bunker housing the experimental area is implemented as a square room of 40.8 m2 in surface and 5.5 m high. $FFRUGLQJWRWKH*6'HSDUWPHQW¶VSODQVWKHposition of the beam tube is off-centre.

A realistic neutron collimation system is implemented. The implemented collimator has been positioned right before the EAR-2 (to allow it to be removed if needed), starting at a height of 15.4 m from the target¶V centre and finishing at 18.4 m. It has a 2 m long Fe section of conical shape, followed by a 1 m long straight section of borated polyethylene, with an inner diameter of 8 cm up to the entrance to the experimental area, similar to the collimation used before EAR-1 during fission cross-section measurements.

Fig. 7: Cut through the lid of the target housing (top view) and target area with the beam tube. The position of a possible additional moderator is drawn in blue at the bottom of the tube.

Fig. 6: Implemented geometry. Page 18 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

The neutron tube, having an inner diameter of 32 cm from 1.2 m up to the beam dump, has a polygonal bottom section for the part entering in the target¶V structural container as shown in Fig. 7. Its horizontal cross-section at this level follows the geometry of the opening present in the target-housing lid. At the level of the target, the tube follows the curvature of the Al target housing and is positioned at 1 mm distance to the housing. For additional moderation of the neutron spectrum, the bottom part of the tube could be filled with a layer of polyethylene or similar moderating material, if the radiation hardness could be proven in the long term.

5.1 Neutron Fluence

The results of the simulations for the neutron fluence per cm2 per PS pulse of 7 × 1012 protons for the new proposed EAR-2 are shown in Fig. 8, in comparison with the existing EAR-1.

The strong dips in the neutron fluence spectrum of EAR-2 are due to the additional handling and reinforcement structural material on top of the n_TOF spallation target, which is not optimized for a vertical flight path.

It should be noted that the neutron spectrum at EAR-2 ranges from thermal to about 300 MeV, while it extends up to several GeV at the current experimental area EAR-1, due to its forward location with respect to the incident beam on the spallation target. The reduction of the maximum energy of neutrons (200-300MeV) is not something negative, but rather a desired feature by design. The very fast neutrons >250 MeV opens numerous UHDFWLRQFKDQQHOVDVZHOOVSDOODWLRQUHDFWLRQWKDWLQPRVWRIWKHFDVHV³EOLQGV´WKHGHWHFWLRQ system, together with the gamma flash. Therefore we wanted to have a line at 90 degrees to strongly suppress this contribution.

Comparison of the Neutron Fluence in EAR1 and EAR2 107 EAR2 EAR1 106 ppp

5 12

e 10 7

/

2

m 4

c 10

/

) E (

n 3 l 10 d

/

dn 102

101 10-12 10-10 10-8 10-6 10-4 10-2 100 Neutron Energy [GeV] Fig. 8: Simulated neutron fluence per cm2 in the existing n_TOF experimental area (EAR-1, blue line) and in the proposed facility above the n_TOF target (EAR-2, black line). It is worth noting that, while the neutron spectrum extends up to several GeV for the EAR-1, there is a sharp cut at ~300 MeV in EAR-2.

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Statistical Statistical EAR-2 EAR-1 Energy Interval uncertainty uncertainty Gain n / cm2 / pulse n / cm2 / pulse [%] [%] 0.02 ± 10 eV 1.64e6 2.0 1.07e5 0.2 15.4 10 eV ± 1 keV 1.07e6 1.4 3.98e4 0.3 26.8 1 keV ± 100 keV 1.36e6 1.3 5.02e4 0.2 27.0 0.1 ± 10 MeV 3.00e6 0.9 1.76e5 0.1 17.1 10 ± 200 MeV 4.78e5 2.0 4.15e4 0.3 11.5

Total range 7.54e6 0.6 4.14e5 0.08 18.2 (0.02 eV - 200 MeV)

Table 2: Integrated neutron fluence for EAR-1 and EAR-2 for different energy intervals.

The integrated neutron fluence per cm2 in EAR-1 and EAR-2 for different energy intervals is listed in Table 2, with the gain defined as the ratio between the neutron density in EAR-2 and in EAR-1 for the same collimator diameter. On average, the gain in neutron fluence of EAR-2 with respect to EAR-1 is a factor of ~20, being 27 in the keV region of interest in astrophysics.

The expected high dose and neutron fluence suggest the possibility of using the upgraded facility also for material irradiation tests in the neutron beam, positioned at 1.5 m above the target¶V centre (see Section 4). According to the simulations, a neutron fluence of 6.22e15 neutrons/cm2/week would be obtained at this position, assuming a repetition rate of one pulse of 7 × 1012 protons every 2.4 s.

5.2 Neutron Energy Resolution

In order to investigate the quality of the possible measurements in the EAR-2, the energy resolution in the proposed experimental area was studied. In Fig. 9 the results are given as apparent time-of-flight, t, as a function of neutron energy. Equivalently, one can express the UHVROXWLRQ LQ HIIHFWLYH QHXWURQ IOLJKW SDWK Ȝ/0) as a function of neutron energy, with L0 being the geometrical flight path. The full width at KDOI PD[LPXP ):+0 ǻȜ RI WKLV distribution at a given energy is considered to be representative of the neutron energy resolution. The values for three different neutron energies and the resulting neutron energy UHVROXWLRQǻ(( ǻȜ Ȝ/0) are listed in Table 3 for EAR-1 and EAR-2.

Overall, the energy resolution is degraded by an order of magnitude. This does not concern the study of heavy isotopes resonances (actinides for instance) because the corresponding resonances are found at low neutron energies where the Doppler broadening is still dominant. Neither will the study of (n, charged particles) reactions near and above threshold be affected by this loss of resolution. Only the measurements of resonances in the keV region, of interest in astrophysics, will be affected by this lower resolution. However, in these cases the quantity of interest is the resonance integral (the MACS at 30 keV and 90 keV) and the loss of resolution, therefore, does not have a great influence on the quality of the results.

The effect of the proton beam pulse width on the resolution has not been taken into account in this evaluation, but it is comparable to the moderation length around 1 MeV.

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Neutron EAR-2: EAR-2: EAR-1: EAR-1: Energy L0 = 18.9 m L0 = 187.5 m

ǻȜ>FP@ ǻ(( ǻȜ>FP@ ǻ(( 1 eV 4 4.3e-3 3 3.0e-4 1 keV 8 8.5e-3 5.1 5.4e-4 1 MeV 38 4.1e-2 34.1 3.6e-3 Table 3: Neutron energy resolution in EAR-2 compared to the resolution in EAR-1

Fig. 9: Resolution in the EAR-2, showing the relation between time of arrival and neutron energy.

5.3 Charged Particle Fluence

An estimate of the expected charged particle fluence in EAR-2 was made and is reported in Fig. 10 for different contributions, as it could disturb the measurements of neutron cross- sections. A magnet of 0.2 Tm positioned in the service gallery located 10 m above the target centre and 8.4 m below the EAR-2 would be sufficient to deflect all charged particles from the neutron flight path before the collimator entrance. The construction of such a magnet is currently under study with an already existing preliminary design that fulfils all the above requirements.

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Charged Particle Fluence at 18.9 m 107 Electrons Protons 106 Pions Kaons ppp

Muons 5 12 10 e 7

/

2 4 m

c 10

/

) p (

n 3 l 10 d

/

dn 102

101 10-2 10-1 100 101 Momentum [GeV / c] Fig. 10: Expected charged particle fluence over particle momentum at EAR-2

5.4 Photon Fluence

The photon fluence in EAR-2, being a background component for measurements with liquid or solid scintillators, has also been investigated in the context of this study.

In Fig. 11 the arrival time of photons (red line) and neutrons (black line) in EAR-2 (left) and EAR-1 (right) are compared. In both facilities the prompt photons from the spallation SURFHVVFDQEHVHHQDVDQLQLWLDOȖ-flash arriving together with the first high-energy neutrons.

Fig. 11: Arrival time of photons (red) and neutrons (black) at EAR-2 (left) and EAR-1 (right).

The green lines in Fig. 11 indicate the time which has been selected for each of the IDFLOLWLHVWR VHSDUDWHWKH SKRWRQVFODVVLILHGDV FRQWULEXWLQJWR WKHȖ-flash, from the photons classified as delayed photons. The energy spectrum of the prompt and the delayed photons in both facilities are plotted in Fig. 12.

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Fig. 12: Photon energy spectrum of prompt (left) and delayed (right) photons in both facilities.

The peak at 2.2 MeV, resulting from the neutron capture reaction of 1H in the water surrounding the target can be seen in both spectra. The smaller absolute value of the prompt photon fluence and the lack of very high energy photons to EAR-2 (Emax~300 MeV instead of ~10 GeV in EAR-1) are expected due to the orthogonal location of the area with respect to the impinging proton beam on the spallation target. 7KLVZLOOKHOSUHGXFHWKHȖ-flash effects in the different detection system and shall therefore allow higher energies to be measured in EAR-2 than in EAR-1. The higher absolute fluence of the delayed photons is understandable considering the significant amount of structural material at the spallation target in the direction of the vertical flight path and the closer position of the experimental area to the collimator and to the target.

The effect of an additional collimator closer to the spallation target (for example in the service gallery, located 10 m away from the spallation target) in reducing this photon distribution will be investigated.

5.5 Higher Signal to background ratio and Equivalent Half-Life

The neutron fluence, energy resolution and background are the basic parameters that describe the performances of each facility. Typically each facility makes a great effort to reduce the background by introducing appropriate shielding, collimation system, etc. However, when short-lived radioactive targets are to be measured (as are the majority of most of the recent n_TOF proposals) the background, due to the natural radioactivity, can be decreased by decreasing the sample mass, with, as a direct consequence, a proportional decrease on the reaction rate dNreaction/dt. The reaction rate is directly proportional to the neutron fluence which, in the case of EAR-2, is ~25 times greater than what is available in EAR-1 (see section 5.1).

In the case of time-of-flight measurements on radioactive samples, the background induced by the radioactive decay of the sample is directly proportional to the time needed for the measurement. A range of neutron energy ¨( corresponds to a window in time-of-flight ¨7, and the signal-to-noise ratio is therefore proportional to the ratio ¨(¨7. From the classical relation between time-of-flight and neutron energy, it follows that ¨(¨7 is inversely proportional to the flight length L. Therefore the new short flight path will result in

Page 23 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration a more favourable signal to radioactive background ratio by a factor 185/19 which equals nearly a factor 10.

The advantage of the last two points can be combined in comparisons when defining the µequivalent half-life ¶. We would like to compare the rate of neutron-induced reactions

(3) ĭ ݀ܰ୰ୣୟୡ୲୧୭୬ ݀݊ሺܧሻ ʹܧ ൌ ߪሺܧሻ ൈ ൈ ܰ ൌ ߪሺܧሻ ൈ ൈ ሺܧሻ ൈ ܰ ሻܧwith the background݀ݐ from radioactive݀ݐ decays. ݐሺ (4)

݀ܰୢୣୡୟ୷ Ž‘‰ሺʹሻ ൌ ߣ ൈ ܰ ൌ ൈ ܰ ݐ ݐଵȀଶ݀ where dNreaction/dt is the reaction rate, dNdecay/dt the natural decay rate, t1/2 the isotope half-life and N the number of atoms, ı ( the reaction cross-section under investigation, ĭ ( the neutron fluence as a function of the energy, t(E) the time-of-flight for each neutron.

From Eq. (3) and Eq. (4) we can calculate the equivalent half-life t1/2 (Eq. (5) when the reaction rate is equivalent to the decay rate.

(5) ୪୭୥ሺଶሻ ĭ ଵȀଶ మಶ ݐ ൌ ఙሺாሻൈ೟ሺಶሻൈ ሺாሻ The equivalent half-life t1/2 value only depends on the performances of the facility and the cross-section to be measured. This representation does not include the discrimination efficiency of alphas versus fission fragments of the detection system to be used. Therefore, this value gives us an indication of the measuring capabilities of the shortest lived isotopes in each experimental area. For an accurate determination of the shortest lived isotopes, the efficiency of the detection system should be also taken into account. It is evident that in the EAR-2, due to the shorter flight path and the low repetition rate of the PS machine, the measuring capabilities are increased further by a factor of 10 on top of the increase of the neutron fluence (factor 25) resulting in a total gain of a factor of about 250 times (with respect to what is presently achievable by performing measurements in EAR-1) on measuring radioactive samples with half-lives as little as a few tenths of a year (Fig. 13). Figure 14 shows the neutron reaction rate assuming a cross section of 1 b (red) and 1 kb (blue) cross section expressed in isolethargic time units, per unit of mass expressed in moles. The reaction rate is compared with the background rate, mainly alpha background from the radioactive decays for one mole of the isotopes. Particular attention should be paid that in this plot the efficiency of the detector in discriminating the alpha events from the fission ones is missing. Even in the extreme case of Cm244 the directly competing reaction (spontaneous fission) will be below the flux induced reaction for most of the energy range. Still one has to develop techniques able to handle very important backgrounds, because the main decays (alpha or beta decays) of some of the samples will have a rate comparable or higher to the induced reaction rates except in the stronger resonances of the cross sections. Figure 15 shows the equivalent half-life in years as a function of the incoming neutron energy, for both measuring stations, the present EAR-1 at 185 m distance and the future EAR-2 at 19 m distance, and for two representative values of cross-section ı ( of 1 b and 1 kb. The 1 b was chosen as a typical value of cross-section for higher energies (above 1 MeV) and the 1 kb as a typical value for the resonance region (1 eV to a few keV). Also, the half-life of certain isotopes of the future proposals and past measurements are shown. Page 24 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

Fig. 13: Neutron rate (dn/dt) for the two experimental areas EAR-1 and EAR-2.

Fig.14. Isolethargic neutron rate (dn/dlnt/mole) per unit of mass, plotted together with the expected radioactive decay rate of various isotopes.

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Fig. 15: Equivalent half-life in years as a function of the incoming neutron energy, for both measuring stations, EAR-1 @ 185m and EAR-2 @ 19m distance, and both for two representative values of cross- section ı ( of 1 b and 1 kb. Area of the half-lives of some proposed isotopes to be measured in EAR-2 (238,241Pu) as well as the one corresponding to 245Cm, already measured in EAR-1.

6. RADIATION PROTECTION ANALYSIS

There are two radiation protection aspects associated to the operation of the EAR-2 facility. The first one concerns the radiation levels in accessible areas induced by the neutron flux between the target and the experimental area. The adopted mitigation measure in that case is to implement sufficient shielding to absorb the neutrons and reduce the radiation levels to acceptable values. The shielding analysis was performed using the Monte-Carlo code FLUKA. A detailed description of the experimental area was implemented as a FLUKA geometry in order to determine the thickness of the concrete walls and the dimensions of the beam dump absorbing the neutrons which do not interact in the sample. A three dimensional view of the new experimental area enclosure as it is implemented in FLUKA for the radiation protection analysis is shown in Fig 16.

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Fig 16. Three dimension view of the experimental area as it is implemented in FLUKA for shielding calculations. The lateral wall shielding could be reinforced at a later stage if the 30 cm collimator is implemented.

The source term used for the calculations takes into account the intensity and the energetic distribution of the neutrons exiting the collimation system and entering the experimental area. For the neutron flux intensity, it was first considered that a collimator with an 8 cm radius would be implemented. To determine the wall thickness, it was conservatively assumed that a bulky target scattering the neutrons toward the side walls was present in the beam path. Based on this scenario the wall thickness was determined using a shielding design criterion of 2.5 µSv/h, which corresponds to the limit for non-designated areas with no permanent workplace [39]. In addition, the same exercise was repeated considering a collimator with a 30 cm radius in order to determine the thickness of concrete which would need to be added if the facility was upgraded to benefit from a higher number of neutrons. With those two specifications, it was decided to build the wall between the experimental area and the rest of the building using the requirement for the 30 cm-radius collimator and the remaining three lateral walls using the requirement for the 8 cm collimator. Those three walls could be made thicker at a later stage by adding an extra layer of concrete blocks if the collimator is upgraded from 8 cm to 30 cm. Fig. 17 shows the dose rate profile perpendicular to the beam direction for the 8 cm and 30 cm collimator cases. The dose rate values show that sufficient safety margin is available to ensure that the dose rate is below the design value of 2.5 µSv/h (shown in blue in Figure 17).

Page 27 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

Fig. 17. Dose rate profile perpendicular to the beam direction considering the neutrons intensity which correspond to the 30 cm collimator (red curve) and 8 cm collimator (black curve) impinging on a scattering target. The thick wall on the right side of the figure is in place for the two configurations while an extra wall (in purple) is only present for the 30 cm collimator on the left.

For the beam dump design a similar approach considering the same shielding criteria was used. However in that case, a different scenario corresponding to the use of a very thin sample absorbing a negligible fraction of the incoming neutrons was considered. The neutron intensity taken into account for the beam dump design corresponds to the 30 cm radius collimator and the VDPHGHVLJQOLPLWRIȝ6YKLQWKHVXUURXQGLQJDFFHVVLEOHDUHD URRIRIWKHH[SHULPHQWDODUHD  was used. Figure 18 shows a dose rate profile perpendicular to the beam direction and going WKURXJKWKHEHDPGXPS7KHGRVHUDWHYDOXHVFDQEHFRPSDUHGWRWKHGHVLJQOLPLWRIȝ6YK (plotted in blue) in accessible areas on the side of the beam dump shielding.

Fig.18. Dose rate profile perpendicular to the beam direction at the beam dump level considering the neutrons intensity which correspond to the 30 cm collimator.

Page 28 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

In addition to the shielding for the experimental area and for the beam dump, it is also foreseen that additional shielding will be needed in the technical gallery hosting the crane used to lower the target into the pit. Due to the implementation of the new collimation system inside the shielding plug filling the pit, the additional shielding will be needed in order to limit the radiation levels inside the ISR8 hall on the other side of the technical gallery. The technical gallery itself will then become part of the primary area and will not be accessible anymore during beam operation. The second aspect to be considered from a radiation protection point of view is the use of radioactive samples in the form of unsealed sources. For this reason, some technical features are implemented in the design of the facility. Such features for example concern the performances of the ventilatLRQ V\VWHP SUHVVXUH GLIIHUHQFH EHWZHHQ DUHDV XVH RI VSHFLDO ILOWHUV«  WKH ILUH resistance of the walls, the possibility to easily decontaminate the area or the presence of a buffer area in order to comply with the procedural requirements to access the experimental area. The specifications and the implementation of those special requirements will benefit from the experience gained from the design and operation of the existing experimental area where the same rules apply. Finally, as significant residual dose rates are expected close to the target, a detailed work planning and optimization analysis will be performed following the ALARA principle for the installation of the new collimation system.

7. ENGINEERING STUDY

7.1 Civil Engineering

A complete study of the Civil Engineering work associated to the construction of the EAR-2 has been carried out by the GS department at CERN. The challenges were identified and tackled without finding any glitches. In the following there is a brief description of the characteristics of the construction and the solutions adopted.

The function of the so-called bunker (see Fig. 19 and Fig. 20) is to house the Experimental Area 2 (EAR-2). The bunker is envisaged to be approximately 7.9 m long and 7.8 m wide. The EAR-2 will partly be located on top of the TT2 tunnel and partly on top of the ISR building (see Fig. 19). The bunker will be connected with the n_TOF underground facilities, in the TT2A tunnel, via a duct of 60cm in diameter. Due to the foreseen weight of the bunker of the Experimental Area, support pillars of roughly 12 m will have to be built with the feet located on the concrete structure of the TT2 target foundations. This will require the opening of a trench of roughly 8 m from the ground (at the maximum point), while the rest of the trench would be sitting on top of the ISR building. A venting chimney on top of the TT2 tunnel will have to be partly dismantled.

Page 29 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

Fig. 19: Overview of n_TOF spallation target area.

The walls and roof of the bunker will be made of concrete with a minimum thickness of 50 cm. This design is optimized for n_TOF measurements, for which a collimator will also be put in place, and meets the requirements for radioprotection of the surrounding area during physics measurements. A shield made of prefabricated blocks will be placed on the roof surrounding the beam dump.

Additional shielding for an upgrade of the facility for testing electronics components or material tests, which will be done without collimator, can be implemented in a second building stage by adding a base plate around the building and by positioning concrete shielding blocks as a wall around the facility as shown in Fig. 20.

Fig. 20: n_TOF EAR-2 surface and underground installations.

Page 30 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

7.2 Beam line study

An extensive feasibility engineering study has been done concerning the possible beam line construction (Fig.21,22). As already mentioned in section 5, a possible collimator system having an iron section of conical shape with a straight section of borated polyethylene has been designed (Fig.21). To complete the beam line, two other elements have been considered; a permanent magnet (0.2Tm) and a shutter acting when with the beam off an access in the area will be requested. The detailed design of the beam line is still on-going because the full and optimized collimation system is still under study.

Alignment ±30mm

borated polyethylene 2x0.5m

Steel 4x0.5m

Fig. 21: Sketch of the proposed collimator design.

Shutter

Permanent magnet

Fig. 22: Sketch of the proposed implementation of the vertical beam line.

Page 31 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

8. CONCLUSION

An additional measuring station, called Experimental Area 2 (EAR-2), has been proposed for construction at the n_TOF (neutron time-of-flight) installation at CERN. The project consists in realizing a vertical flight path of roughly 20 m above the neutron spallation target as well as the EAR-2 bunker itself, where neutron-nucleus interaction experiments would be performed. EAR-2 should be considered complementary to EAR-1; since they would run in parallel.

An interdepartmental working group was created one year ago and the preliminary outcome has been presented in this document: The studies on the construction have been carried out by the Civil Engineering Group (GS/SE). A feasibility study was performed and discussed along with a possible construction plan. The conceptual design considered a realistic schedule, with the minimum requirement of about 3FTEs/year over a period of less than 3 years. This flexible planning offers the great opportunity to be synchronized with the period of the /+&¶V ORQJ VKXWGRZQ DQG WKH FRUUHsponding injector chain refurbishing work (see Appendix I, II, III). Results regarding the required shielding and beam dump design have already been discussed and detailed radiation protection studies are ongoing. No problems have been identified so far in the current implementation.

Detailed Monte Carlo simulations of the proposed EAR-2 beam line have been realized as input for the radioprotection studies and to support the physics case. The presented study shows that experimental campaigns will benefit from an increase of more than one order of PDJQLWXGH LQ QHXWURQ IOXHQFH FRPELQHG ZLWK D VWURQJ UHGXFWLRQ RI WKH VR FDOOHG ³Ȗ-IODVK´ with respect to the EAR-1. The n_TOF facility with its EAR-1 is already unique in the world in terms of the instantaneous neutron flux and low background, but the addition of the EAR-2 with its enhanced capabilities will be of utmost importance; due to the beam characteristics the installation will open new opportunities for measurements of neutron-induced reactions with unprecedented accuracy for various important fields of physics, among which we could cite nuclear technology, nuclear astrophysics and stellar evolution, basic research, medical applications, dosimetry and radiation damage. In more details, the characteristics of the EAR- 2 will enhance the capabilities of the n_TOF facility and of the related experiments allowing ± for example ± to: a) Measure samples with very low mass or very high activity; b) Measure neutron-induced reactions up to high neutron energies (roughly 300 MeV); c) Use very thin samples suited for (n, charged particle) reactions.; d) Perform irradiation of various material and electronic devices for dosimetric studies, detector development, radiation damage and other applications which could further increase the range of possible applications of the n_TOF neutron beam.

The n_TOF Collaboration showed a strong interest (including the access of new institutes in the Collaboration) in supporting the project, based on the new opportunities that the EAR- 2 will offeU 7KH PHPEHUV RI WKH &ROODERUDWLRQ KDYH SUHSDUHG VHYHUDO ³([SUHVVLRQV RI ,QWHUHVW´ (2,  UHODWHG WR SRVVLEOH H[SHULPHQWV ZKLFK FRXOG EH SHUIRUPHG LQ WKH QHZ proposed EAR-2 and which are impossible to be carried out in the existing EAR-1. These EOIs are included in Appendix 2. The n_TOF Collaboration is willing to contribute, despite their limited resources, in the construction of the n_TOF facility extension, assuming that the civil engineering and beam line construction activities could be covered by extra FRQWULEXWLRQVIURP&(51¶VEXGJHt (see Appendix I).

Page 32 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

Last, but not least, a vast campaign of collaboration has been launched with other Institutions like IRMM (Institute for Reference Materials and Measurements, Geel, Belgium), University of München (Germany), Oak Ridge National Laboratory (USA), and PSI (Switzerland) for producing high purity samples, the essential ingredient for performing measurements in both EAR-1 and in EAR-2.

Page 33 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

References

[1] U. Abbondanno et al. (n_TOF Collaboration), Neutron Capture Cross-section Measurement of 151Sm at the CERN Neutron Time of Flight Facility (n_TOF), Phys. Rev. Lett. 93 (2004) 161103. [2] C. Domingo-Pardo et al. (n_TOF Collaboration), New measurement of neutron capture resonances in 209Bi, Phys. Rev. C74 (2006) 025807. [3] C. Domingo-Pardo et al. (n_TOF Collaboration), Resonance capture cross-section of 207Pb, Phys. Rev. C74 (2006) 055802. [4] C. Domingo-Pardo et al. (n_TOF Collaboration), Measurement of the neutron capture cross-section of the s-only isotope 204Pb from 1 eV to 440 keV, Phys. Rev. C75 (2007) 015806. [5] R. Terlizzi et al. (n_TOF Collaboration), 7KH /D QȖ  cross-section: Key for the onset of the s-process, Phys. Rev. C75 (2007) 035807. [6] C. Domingo-Pardo et al. (n_TOF Collaboration), Measurement of the radiative neutron capture cross-section of 206Pb and its astrophysical implications, Phys. Rev. C76 (2007) 045805. [7] G. Tagliente et al. (n_TOF Collaboration), Neutron capture cross-section of 90Zr: Bottleneck in the s-process reaction flow, Phys. Rev. C77 (2008) 035802. [8] G. Tagliente et al. (n_TOF Collaboration), ([SHULPHQWDOVWXG\RIWKH=U QȖ UHDFWLRQ up to 26 keV, Phys. Rev. C78 (2008) 045804. [9] M. Calviani et al. (n_TOF Collaboration), High-accuracy 233U(n,f) cross-section measurement at the white-neutron source n_TOF from near-thermal to 1 MeV neutron energy, Phys. Rev. C80 (2009) 044604. [10] C. Massimi et al. (n_TOF Collaboration), $X QȖ  cross-section in the resonance region, Phys. Rev. C81 (2010) 044616. [11] G. Tagliente et al. (n_TOF Collaboration), 7KH=U QȖ) reaction and its implications for stellar nucleosynthesis, Phys. Rev. C81 (2010) 055801. [12] K. Fujii et al. (The n_TOF Collaboration), Neutron physics of the Re/Os clock. III. Resonance analyses aQGVWHOODU QȖ cross-sections of 186,187,188Os, Phys. Rev. C82 (2010) 015804. [13] M. Mosconi et al. (The n_TOF Collaboration), Neutron physics of the Re/Os clock. I. 0HDVXUHPHQWRIWKH QȖ cross-sections of 186,187,188Os at the CERN n_TOF facility, Phys. Rev. C82 (2010) 015802. [14] C. Paradela et al. (n_TOF Collaboration), Neutron-induced fission cross-section of 234U and 237Np measured at the CERN Neutron Time-of-Flight (n_TOF) facility, Phys. Rev. C82 (2010) 034601. [15] C. Lederer et al. (n_TOF Collaboration), 197Au(n,Ȗ) cross-section in the unresolved resonance region, Phys. Rev. C83 (2011) 034608. [16] D. Tarrío et al. (n_TOF Collaboration), Neutron-induced fission cross-section of natPb and 209Bi from threshold to 1 GeV: An improved parametrization, Phys. Rev. C83 (2011) 044620. [17] G. Tagliente et al., Neutron capture on 94Zr: Resonance parameters and Maxwellian- averaged cross-sections, Phys. Rev. C84 (2011) 015801. [18] R. Sarmento et al. (The n_TOF Collaboration), Measurement of the 236U(n,f) cross- section from 170 meV to 2 MeV at the CERN n_TOF facility, Phys. Rev. C84 (2011) 044618. Page 34 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

[19] G. Tagliente et al. (n_TOF Collaboration), 96Zr(n,Ȗ PHDVXUHPHQWDWWKHQB72)IDFLOLW\ at CERN, Phys. Rev. C84 (2011) 055802. [20] C. Guerrero et al., (The n_TOf Collaboration), Measurement and resonance analysis of the 237Np neutron capture cross-section, submitted to Phys. Rev. C (November 2011). [21] S. Marrone et al., Measurement of the 151Sm (n,) 152 Sm cross section at n_TOF, Nucl. Phys. A 758 (2005) 533 [22] F. Belloni et al., Neutron induced fission cross section of 233U in the energy range 0.5 En 20 MeV, Eur. Phys. J.A, 47 (2011) 2 [23] D. Tarrio et al., Neutron induced fission cross section of nat Pb and 209 Bi from threshold to 1GeV: An improved parametrization, Phys. Rev. C83 (2011) 044620 [24] F. Belloni et al., Measurement of the neutron induced fission cross section of 243 Am relative to 235 U from 0.5 to 20 MeV, Euro. Phys. J.A, accepted December 2011. [25] NuPECC Long Range Plan 2010: Perspectives of Nuclear Physics in Europe, pp. 142, 146, 179 (December 2010). [26] F. Käppeler et al., The origin of the heavy elements: The s process, Progress in Particle and Nuclear Physics 43 (1999) pp. 419±483. [27] F. Käppeler et al., Reaction cross sections for the s, r, and p process, Progress in Particle and Nuclear Physics 66.2 (April 2011) pp. 390±399. [28] I. Dillmann, R. Plag, F. Käppeler, T. Rauscher, KADoNiS v0.3 ± The third update of WKHµ.DUOVUXKH$VWURSK\VLFDO'DWDEDVHRI1XFOHRV\QWKHVLVLQ6WDUV¶Proc. EFNUDAT Fast Neutrons: Scientific Workshop on Neutron Measurements, Theory and Applications, Geel, Belgium (28-30 April 2009). [29] U. Abbondanno et al., New experimental validation of the pulse height weighting technique for capture cross-section measurements, Nucl. Instrum. Methods Phys. Res. A521 (2004) pp. 454±467. [30] C. Guerrero et al., The n_TOf Total Absorption Calorimeter for neutron capture experiments at CERN, Nucl. Instrum. Methods Phys. Res. A608 (2009) pp. 424±433. [31] K. Wisshak et al., Stellar neutron capture on 180Tam. I. Cross-section measurement between 10 keV and 100 keV, Phys. Rev. C69 (2004) 055801. [32] R. Reifarth et al., Stellar Neutron Capture on Promethium: Implications for the s- Process Neutron Density, Astrophys. J582 (2003) pp. 1251±1262. [33] S. Marrone et al., Measurement of the 1516P QȖ cross-section from 0.6 eV to 1 MeV via the neutron time-of-flight technique at the CERN n_TOF facility, Phys. Rev. C73 (2006) 034604. [34] G. Aerts et al., Neutron capture cross-section of 232Th measured at the n_TOF facility at CERN in the unresolved resonance region up to 1 MeV, Phys. Rev. C73 (2006) 054610. [35] NEA Nuclear Data High Priority Request List HPRL http://www.oecd-ea.org/dbdata/hprl/ [36] C. Guerrero et al., Simultaneous measurement of neutron-induced capture and fission reactions at CERN, submitted to n_TOF Ed. Board (to be submitted to Eur. J. Phys.). [37] F. Gunsing and F. Becvar et al., Spin assignments of nuclear levels above the neutron binding energy in 88Sr, ISOLDE and neutron Time-of-Flight Experiments Committee, CERN-INTC-2011-030, INTC-P-304. [38] C. Guerrero et al., Study of photon strength functions of actinides: the case of 235U, 238Np and 241Pu, Journal of the Korean Physical Society 59.2 (2011) pp. 1510±1513. [39] D. Forkel-Wirth and T. Otto, CERN General Safety Instruction: Consignes Generales G¶([SORLWDWLRQ5*(6HFWLRQ6-GSI1, Document EDMS 810149, 2006. Page 35 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

APPENDIX I: BUDGET (EAR-2) n_TOF

INDICO Estimated Budget Group [KCHF]

Dismantling barrack 559 GS-SE 100

CE new building n_TOF GS-SE 1,300

Ventilation EN-CV 540

Electric services EN-EL 120 Elec. general services 80 UPS 40 Access, alarms & fire detection GS-ASE 200 Access, interlock system 120 Fire detection, alarms 80 Handling equipment EN-HE 100 Crane 70 Monorail modification 30 Radioprotection, monitoring DGS-RP 100

Beam line EN-MEF 340 New target concrete tap 70 New shaft collimation, shielding 50 Dump 50 Vacuum chambers, pump, control 70 Shielded door entrance 50 Gas supply facility 20 Detector support facility (vertical) 30 Permanent Magnet TE-MSC 110

Total [KCHF] 2,910 Contingency 10%

Total [KCHF] 3,201

Page 36 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

The most important items for the budget, as shown in the pie chart above, are the civil engineering, ventilation for Class A condition in the Area, and the beam line. The construction of the bunker represents 45% of the total budget due to the foreseen weight of the bunker of the Experimental Area, supporting pillars of roughly 12 m will have to be built with the feet located on the concrete structure of the TT2 tunnel foundations.

Page 37 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

APPENDIX II: STAFF (EAR-2)

n_TOF Estimated Staff INDICO

Group FTE YEAR

Dismantling barrack 559 GS-SE 0.25 2012

CE new building n_TOF GS-SE 0.75 2012 Study&Purchaising Procedure 1 2013/2014 Civil Engineering work Ventilation EN-CV 0.25 2012 Call for tender 0.50 2014 Installation Electric services EN-EL 0.2 2012 Study

0.5 2014 Installation GS- Access, alarms & fire detection 0.5 2014 Installation ASE Handling equipment EN-HE 1.0 2013/2014 Installation+actual pit modification DGS- Radioprotection, monitoring 1.0 2012/2013 Study RP 0.5 2014 Installation+monitoring EN- Beam line 1.0 2012 Study MEF TE- Permanent Magnet 1 2013 Study&Construction MSC

8.45 Total [FTE] Contingency 10%

Total [FTE] 9.3

Page 38 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

The calculations of the FTEs needed for the construction and installation of the Area has been done taking into account the availability of the staff avoiding any interference with the activities for the refurbishment of the LHC. Most of the preparation for the different actions will be done during 2012.

Page 39 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

APPENDIX III: TENTATIVE PLANNING (EAR-2)

From the point of view of the n_TOF Collaboration, the years 2013/2014 are the ideal period for the construction of the new EAR-2. The table shows the tentative SURMHFW¶V JOREDO VFKHGXOH$PRUHGHWDLOHGDQDO\VLVZLOOEHGRQHIRUHDFKLWHPWDNLQJLQWRDFFRXQWWKH/+&¶V activity priorities.

Page 40 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

APPENDIX IV. EXPRESSIONS OF INTEREST FOR EAR-2

1. Cross sections and prompt -ray emission of fissile Pu isotopes 2. Measurement of the 25Mg(n,Į)22Ne cross section 3. The role of 238Pu and 244Cm in the management of nuclear waste: simultaneous measurements of their capture and fission cross sections 4. Measurements of (n,xn) reaction cross sections for heavy target nuclei 5. Fission cross section of the 230Th(n,f) reaction 6. First measurement of the capture (and fission) cross sections of the fissile 245Cm 7. Neutron capture measurement of the s-process branching point 79Se 8. Cross section and angular distribution of fragments from neutron-induced fission of 232U 9. 'HVWUXFWLRQRIWKHFRVPLFȖ-ray emitter 26Al by neutron induced reactions 10. Measurement of 7Be(n,p)7Li and 7Be(n,)4He cross sections, for the cosmological Li problem.

Page 41 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

TITLE: 1. Cross sections and prompt g-ray emission of fissile Pu isotopes

AUTHORS: C. Guerrero (CERN), D. Cano-Ott (CIEMAT), E. Berthoumieux (CEA)

SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 239,241Pu (n,) (n,f) Thermal to 1-20 MeV MASS: 2x10mg DETECTION SYSTEM (name and short description): C6D6(TAC)+MGAS: Combination of C6D6 (TAC) capture detectors with MicroMegas fission detectors

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: Higher neutron fluence delivered in shorter time, which allows measuring mg samples: - The combination 25 times higher fluence per pulse delivered in 10 times less time per pulse than in EAR-1 results in a reduction of factor 250 reaction to background from sample activity with respect to EAR-1. - The higher fluence allow using samples of only few mg: i) the material in this quantities is available (not the case of hundreds of mg), ii) samples can then be SURGXFHG E\ ³WKLQ VDPSOH´ WHFKQLTXHV OLNH HOHFWURGHSRVLWLQJ HYDSRUDWLRQ HWF QRW the case for thick samples: pressed powders, matrices, etc.) - The higher fluence allows to use a single 1 or 2 thin layers (instead of many like in EAR-1), thus making it possible to use C6D6 detectors (more sensitive to the VDPSOH¶VSRVLWLRQ LQVWHDGRID calorimeter (as the TAC in EAR-1).

MAIN ADVANTAGE(S) OF EAR-2 wrt. to other facilities for this particular measurement: The n_TOF EAR-2 neutron fluence covers in a single shot the full energy range of interest. The availability of a type-A experimental area makes it possible to measure Pu isotopes, which is not the case of instance in other TOF facility. The high instantaneous fluence makes is possible to measure low mass samples. These sample would have to be of small mass because 239Pu is a strategic material and because 241Pu (14y) has a very high activity. MOTIVATION (Nuclear technology, astrophysics, basic physics): Nuclear Technology: The capture cross section of fissile isotopes (particularly difficult to measure due to the fission background) is of utmost importance in the operation of current Nuclear Systems and the design of future devices. The simultaneous measurement of capture and fission improves the accuracy, reducing systematic errors associated to absolute cross- section normalization. The measurement would provide as well information on prompt g-ray emission following capture and fission reaction, which are the dominant sources of heat in a nuclear reactor and need to be measured with high accuracy according to the NEA.

BEAM TIME (assume neutron fluence of EAR-1 x25, and En<300 MeV): 3x1018 protons per isotope (equivalent measuring time in EAR-1 = 5 years) SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES (3 pages maximum): The goal of these measurements would be to measure the capture and fission cross sections,

Page 42 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration as well as alpha ratios, of the fissile isotopes of plutonium 239Pu and 241Pu. The main objective would be in the thermal and unresolved resonance regions. Both isotopes play a key role in the operation of existing reactors, but their (n,) and (n,f) cross sections lack the required accuracy for burn-up calculations of present PWR and calculations for all types of fast reactors. The four cross sections are indeed included among those of in the NEA High Priority Request List [1]. The measurements are very difficult because of the very high activity of the samples to be measured, and also because independent measurement of the two reaction channels may not help to solve the existing problems of the evaluations. For that reason, a simultaneous (n,) and (n,f) measurement is highly desirable. The particularities of the 2nd experimental area at n_TOF (the highest intense neutron flux covering the full energy range of interest: thermal to 20 MeV) [2] make such a measurement feasible. The setup would consist in a MGAS fission detector loaded with a 2 thin samples of 2 350µg/cm of PuO2 each placed in between 2 C6D6 -ray detectors. The samples would be placed back-to-back in 2 MGAS detectors in order to cancel out the effects of the non- uniform angular distribution of fission fragments. Both have been individually tested at n_TOF in the EAR-1 and their combination would be straightforward. The only open question would be their response to the -flash in EAR-2 and the associated high energy limit that could be reached. The expected counting rates for 2x10 mg samples and a total of 3x1018 protons per measurements are shown in the Figure, where it is observed that a minimum of 3% statistical uncertainty would be reached in the complete energy range.

Fig. 1. Expected counting rates for a total of 3x1018 protons allocated to each isotope. The simultaneous measurement of the two reactions channels with such accuracy and covering in a single shot the thermal to MeV regions would be the first of a kind, improve significantly the actual evaluations and opening the door to a new range of measurements of fissile isotopes such as 233,235U, 244Am or 245Cm.

References [1] NEA High Priority Request List http://www.oecd-nea.org/dbdata/hprl/ [2] E. Chiaveri et al., n_TOF Experimental Area 2 (EAR-2) preliminary feasibility study, CERN-INTC-2011-032; INTC-O-013 (2011)

Page 43 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

TITLE: 2. Measurement of the 25Mg(n,ܤ)22Ne cross section

AUTHORS: C. Massimi (University of Bologna and INFN), M. Barbagallo (INFN), Jozef Jan Andrzejewski (Uniwersytet Lódzki, Lodz, Poland), N. Colonna (INFN), F. Käppeler (KIT), Paul Koehler (ORELA), C. Lederer (University of Vienna), P.F. Mastinu (INFN), G. Tagliente (INFN), P. Milazzo (INFN), G. Vannini (University of Bologna and INFN), Christina Weiss (Atominstitut, Technische Universität Wien, Austria)

SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 25Mg (n,ܤ) Thermal to 1 MeV MASS: few mg

DETECTION SYSTEM (name and short description): SiMon and/or MGAS. Combination of an ionization chamber and a modified silicon detector (SiMon). Other solutions are available, but must be reconsidered according to the final EAR- 2 setup.

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: Higher neutron fluence delivered in shorter time, which allows measuring very small reaction cross section: - The 25 times higher fluence per pulse, relative to EAR-1 results in a higher count- rate, allowing one to measure the cross section with reasonable uncertainty. - This measurement has never been performed because the fluence at existing neutron time-of-flight facilities is much too low.

MOTIVATION (Nuclear technology, astrophysics, basic physics): Nuclear Astrophysics: The 22Ne(ܤ,n)25Mg reaction is the main neutron source of the s- process, which is responsible for the production of elements heavier than iron. Despite its importance no data is available in the energy range of interest: 1

BEAM TIME (assume neutron fluence of EAR-1 x25, and En<300 MeV): The measurement can be performed in parallel with other measurements, since there is practically no effect on the neutron beam. The protons available in a whole measurement campaign would be the ideal case: 2x1019 protons (impossible to measure in EAR-1)

SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES: The measurement is very difficult because the Q-value of the reaction is 480 keV and the (n,ܤ) reaction cross section is expected to be small. Ultra thin layers of material are required to let the low-energy ܤ particles escape from the sample. The range of 480-keV ܤ particles in magnesium is few micrometers. The corresponding areal density of a 1-µm thick 22Mg sample is 4.2×10-6 atoms/barn. In literature there are only two measurements [1,2] of the 25Mg(n,ܤ)22Ne cross section, using monoenergetic neutrons with energies of 5, 7, 13 and14 MeV produced at a Van de Graaff accelerator. The cross section in this energy region ranges is comparably high, from 1.4 to 95 Page 44 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration mb. The expected counting rate CR (counts per second) for a thin sample is proportional to the reaction cross section ı (barn), the areal density of the Mg sample n, the neutron fluence particles ܤ and the escape probablilty of the ,ڙ ࢥ (neutrons/s), the overall detection efficiency from the sample Pъ:

CR n P . (1) Assuming the following values: n = 4.2×10-6 atoms/barn, ı = 1 mb which correspond to the ,particle detector), Pъ=80%ܤ using a thin magnesium layer and a 2) %50 = ڙ ,worst case the resulting counting rate, as a function of the neutron fluence is: 5 3 9 CR 4.2 10 10 0.4 2 10 . (2) Equation 2 shows that a higher neutron fluence is crucial for obtaining a reasonable counting rate. 4 For instance in EAR-1, ࢥ EAR-1=7.1×10 neutrons are delivered per proton burst in the 10-100 keV energy ฀range. Therefore, the count rate in EAR- LV OLPLWHG WR § FRXQWV SHU SXOVH LQ WKH HQHUJ\ region of greatest interest. On the other hand, in EAR-2 the fluence is 25 times larger (ࢥ EAR-2=25 ࢥ EAR-1), providing a rate of about 1 counts per 300 pulses.

Unless other interfereing reaction channels are open, e.g. the (n,p) channel, particle- identification is not required. As a consequence the setup can be very simple. Suited detectors for the reaction products are, for instance, Si detectors, MicroMegas, compensated ionization chambers, and diamond detectors. At the moment we propose two use two versions, which are already in use in EAR-1: A) a modified version of the silicon detector SiMon. It would consist of two magnesium samples viewed by 8 Si detectors outside of the beam; B) a micromegas detector loaded with two magnesium samples. In both cases, very thin samples of highly enriched 25Mg are placed back-to-back in order to compensate angular distribution effects of reaction products. Option A) has the advantage of a very low background, since the only material in the beam are the Mg samples on ultrathin substrate layers. Drawbacks are the low geometric efficiency of about 10% and A remaining sensitivity to angular distribution effects. The advantage of option B) is the higher efficiency close to 90%, but the background induced by neutron reactions in the detector windows and backing materials can be important.

Fig.2. Experimental cross section of the 25Mg(n,ъ)22Ne reaction. It is worth nothing that this measurement could be considered as a pioneering step for a series of (n,ܤ) measurements of great importance for Nuclear Astrophysics, which could not be performed in the past.

Page 45 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

References [1] B. Lavielle, H. Sauvageon, and P. Bertin, Phys. Rev. C 42, 305±308 (1990) [2] M. Brendle, R. Enge and G. Steidle, EPJ A 285, 293-304 (1978) [3] www.cern.ch/ntof

Page 46 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

TITLE: 3. The role of 238Pu and 244Cm in the management of nuclear waste: simultaneous measurements of their capture and fission cross sections AUTHORS: E. Mendoza (CIEMAT), D. Cano-Ott (CIEMAT), E. González (CIEMAT), C. Guerrero (CERN)

SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 238Pu, 244Cm (n,) (n,f) Thermal to 1-20 MeV MASS: ~0.1mg each

DETECTION SYSTEM (name and short description): C6D6 or a new Total Absorption Calorimeter+MGAS: Combination of C6D6 (or a new Total Absorption Calorimeter) capture detectors with MicroMegas fission detectors

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: Higher neutron fluence delivered in shorter time, which allows measuring mg samples: - The combination 25 times higher fluence per pulse delivered in 10 times less time per pulse than in EAR-1 results in a reduction of factor 250 reaction to background from sample activity with respect to EAR-1. - The higher fluence allows using samples of less than 1 mg: i) the material in these quantities is available (not the case of hundreds of mg), ii) samples can then be SURGXFHG E\ ³WKLQ VDPSOH´ WHFKQLTXHV OLNH HOHFWURGHSRVLWLQJ HYDSRUDWLRQ HWF QRW the case for thick samples: pressed powders, matrices, etc.) - The higher neutron fluence allows using a 1 or 2 thin samples (instead of the several necessary at EAR-1). The samples can be placed close to the C6D6 detectors (more VHQVLWLYHWRWKHVDPSOH¶VSRVLWLRQ RUDWWKHFHQWUH of a 4 calorimeter (as the TAC used in EAR-1).

MAIN ADVANTAGE(S) OF EAR-2 for this particular measurement: The n_TOF EAR-2 neutron fluence covers in a single shot the full energy range of interest. The availability of a type-A experimental area offers the required safety conditions for measuring short lived Pu and Cm isotopes. The high instantaneous neutron fluence makes it possible to measure high activity samples like the 238Pu and 244Cm isotopes. MOTIVATION (Nuclear technology, astrophysics, basic physics): Nuclear Technology: The capture cross section of 238Pu and 244Cm are highly relevant for the nuclear waste managment: both of them constitute an important source of heat in the irradiated fuel due to their high specific activities (i.e. short half lives). Furthermore, the neutron capture in 244Cm opens the path to the build up of higher mass and longer lived Cm isotopes. The simultaneous measurement of their capture and fission cross sections improves the accuracy, reducing systematic uncertainties associated to the normalisation to absolute cross-sections. BEAM TIME (assume neutron fluence of EAR-1 x25, and En<300 MeV): 3x1018 protons per isotope (equivalent measuring time in EAR-1 = 5 years) SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES (3 pages maximum): The goal of these measurements would be to measure the capture and fission cross sections, as well as alpha ratios, of the isotopes 238Pu (88 y) and 244Cm (18.1 y). The energy range of range of interest comprehends both the resolved and unresolved resonance regions. Both

Page 47 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration isotopes have high specific activities due to their short half lives and thus play a key role in the management of irradiated fuels, the separation of minor actinides and the preparation of fresh nuclear fuels for the transmutation of minor actinides: 238Pu is an intense alpha emitter and 244Cm undergoes spontaneous fission and is one of the main neutron emitters in the irradiated nuclear fuel. Furthermore, the neutron capture in 244Cm builds up heavier and longer lived Cm isotopes. Both fission cross sections are indeed included among those of in the NEA High Priority Request List [1]. The capture cross sections of both isotopes have been poorly measured. For the case of 238Pu, there is only one capture measurement [3], excluding the thermal point, between 17.8 eV and 200 keV, and one transmission measurement [4], between 0.0082 eV and 6.5 keV. For the case of 244Cm, there are two capture measurements available [5,6], between 20eV and 10keV and between 1 and 950keV, respectively. No capture measurements below 20 keV, excluding the thermal point, have been performed. Two transmission measurements have been performed for the 244Cm isotopes [7,8], between 4.27 and 780 eV and between 1keV and 3.5 MeV. Differences in the evaluated cross sections are presented in Figure 1.

Fig.1. Existing differences in the libraries in the evaluated capture cross sections of 238Pu and 244Cm.

The measurements are very difficult because of the very high activity of the samples to be measured, and also because independent measurement of the two reaction channels may not help to solve the existing problems of the evaluations. For that reason, a simultaneous (n,) and (n,f) measurement is highly desirable. The particularities of the 2nd experimental area at n_TOF (the highest intense neutron flux covering the full energy range of interest: thermal to 20 MeV) [2] make such a measurement feasible. The setup would consist in a MGAS fission detector loaded with a 2 thin samples of 2 238 244 ~1.7µg/cm of PuO2 or CmO2 each placed in between 2 C6D6 -ray detectors (or a new Total Absorption Calorimeter if available). The samples would be placed back-to-back in 2 MGAS detectors in order to cancel out the effects of the non-uniform angular distribution of fission fragments. All detector types have been individually tested at n_TOF in the EAR-1 and a test experiment with the combined use of the TAC + MGAS for measuring the 2358 QȖ FURVVVHFWLRQKDVEHHQSHUIRUPHGDVZHOO7KHRQO\RSHQTXHVWLRQ would be their response to the -flash in EAR-2 and the associated high energy limit that could be reached. The expected counting rates for 2x0.05 mg samples and a total of 3x1018 protons per measurements are shown in Figure 2.

Page 48 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

Fig. 2. Expected counting rates for a total of 3x1018 protons allocated to each isotope. 30bins/decade has been used in both figures.

References [1] NEA High Priority Request List http://www.oecd-nea.org/dbdata/hprl/ [2] E. Chiaveri et al., n_TOF Experimental Area 2 (EAR-2) preliminary feasibility study, CERN- INTC-2011-032; INTC-O-013 (2011) [3] M.G.Silbert, J.R.Berreth, Neutron Capture Cross Section of Plutonium-238; Determination of Resonance Parameters, Nucl.Sci.Eng. 52, 187 (1973). [4] T.E.Young et al., Neutron Total and Absorption Cross Sections of Pu238, .Sci.Eng. 30, 355 (1967). [5] M.S. Moore, G.A. Keyworth, Analysis of the Fission and Capture Cross Sections of the Curium Isotopes, Phys. Rev. C, v3 (1971) 1656. [6] H.Fernandez Gianotti, Fast neutron cross-sections for curium-244, EXFOR # V0006.007. [7] R.E.Cote et al., Total Neutron Cross Sections of Cm244, Physical Review Vol.134, p.B1281 (1964). [8] H.Fernandez Gianotti, Fast neutron cross-sections for curium-244, EXFOR # V0006.002.

Page 49 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

TITLE: 4. Measurements of (n,xn) reaction cross sections for heavy target nuclei

AUTHORS: R.Vlastou (NTUA), M.Kokkoris (NTUA), V. Avrigeanu (IFIN-HH), E. Berthoumieux (CEA), Strasbourg group

SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 197Au, 181Ta, (n,2n), (n,3n), (n,4n), (n,5n) 10 to ~60 MeV etc MASS: 2-3g

DETECTION SYSTEM (name and short description): 2 Ge detectors specially modified by the Strasbourg group +MGAS: Combination of 2 HPGe detectors at 120o and 90o with respect to the beam direction and MicroMegas fission detectors for the determination of the neutron flux via the 235-238U(n,f) cross section. The cross section for the exit channels of each reaction will be determined by the prominent gamma rays emitted from the residual nuclei [1].

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: The higher neutron fluence of EAR-2 delivered in shorter time, allows measuring low (n,xn) cross sections (~100-200 mb at high energies). The lower gamma-flash in EAR-2 would allow HPGe detectors to be operative.

MOTIVATION (Nuclear technology, astrophysics, basic physics): Nuclear Technology: The generation, maintenance and validation of nuclear data libraries relevant to ITER, IFMIF and DEMO nuclear engineering design demand, among others, improvement of nuclear models and update of databases. Moreover, the neutron energy range concerned in this respect is extended to 50 MeV. On the other hand, the cross sections for nuclear reactions induced by fast neutrons below 20 MeV are generally considered to be reasonably well known in spite of many fast neutron reactions for which the several data are either conflicting or incomplete even around 14 MeV. It is the reason why having recent sets of accurate measured cross sections still below 20 MeV and especially between 20 and ~60 MeV is highly desirable. Basic physics: Accurate and consistent data of (n,xn) reactions over a wide energy range (10- 50MeV) are very important to test and improve nuclear models and investigate reaction mechanisms. BEAM TIME (assume neutron fluence of EAR-1 x25, and En<100 MeV): 3x1018 protons per isotope (equivalent measuring time in EAR-1 = 5 years) SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES (3 pages maximum): The objective of the present proposal is two-fold. Since only several data with large uncertainties are known above 20-30 MeV (e.g., the case of the target nucleus 197Au in Figure 1), it is important to estimate these cross sections theoretically. Therefore the predictive power of the nuclear model calculations is a major challenge. On the other hand, the accuracy of these predictions should be checked on the basis of some reference experimental reaction cross sections. Data with error bars larger than even 50% or clearly distinct in spite of their

Page 50 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration already 15-20% errors, as shown in Figure 1, can not be used in this respect. Further n_TOF EAR-2 measurements may bring the data status at larger energies close to the enhanced accuracy presently achieved around 10 MeV ([2] and Refs. therein).

2.5 197 196 0.25 197 196m Au(n,2n) Au Au(n,2n) Au - Tewes+ (1960) [12 , 9.6h ] Prestwood+ (1961) 2.0 Tewes+ (1960) Bayhurst+ (1975) 0.20 Paulsen+ (1975) Prestwood+ (1961) Veeser+ (1977) Frehaut+ (1980) Goutiere+ (1981) Ghorai+ (1984) 1.5 Goutiere+ (1981) Csikai (1982) 0.15 Ikeda+ (1988) Daroczy+ (1985) Ikeda+ (1988) Soewarsono+(1992) Lu Hanlin+ (1989) Filatenkov+ (1999) Soewarsono+ (1992) Filatenkov+ (2003) 1.0 Uwamino+ (1992) 0.10 Filatenkov+ (1999) Tsinganis+ (2011) Filatenkov+ (2003) Philis+ (1977): eval. No PE Tsinganis+ (2011) No PE ENDF/B-VII.1 0.5 g (): FGM 0.05 g (): FGM p p g ()=a+b g ()=a+b p p BSFG BSFG

) 0.0 0.00 10 15 20 25 30 35 40 10 15 20 25 30 35 40 b (

2.0 197Au(n,3n)195Au 197Au(n,4n)195Au 2.0 No PE No PE g (): FGM p 1.5 g (): FGM g ()=a+b p p g ()=a+b 1.5 BSFG p BSFG 1.0 1.0

Bayhurst+ (1975) Bayhurst+ (1975) Veeser+ (1977) 0.5 Philis+ (1977): eval 0.5 Lu Hanlin+ (1989) Uwamino+ (1992) Iwasaki+ (1993) Soewarsono+(1992) Philis+ (1977): eval. Svoboda+ (2011) ENDF/B-VII.1 ENDF/B-VII.1 15 20 25 30 35 40 25 30 35 40 E (MeV) Fig. 1: Comparison of experimental [2,3], ENDF/B-VII.1 evaluated [4], and calculated (n,2n), (n,3n), and (n,4n) reaction cross sections by using various model assumptions, for incident energies up to 40 MeV. [5]. Beyond the nuclear technology motivation of these measurements there is also a basic interest for the related studies. Actually the model calculations of the fast-neutron reactions cross sections data below 20 MeV are most sensitive to the parameters related to nuclei in the early stages of the reaction, i.e. within the pre-equilibrium emission (PE) processes which then become dominating at higher energies. Thus, there is a good opportunity through these studies to look for the understanding of the model constraints which are responsible for the calculated cross section variations, concerning particularly (a) the incident energies below 20 MeV, where the statistical model (SM) calculations are most sensitive to the parameters related to residual nuclei and emitted particles which are populating them, and (b) the energies above 20-30 MeV, where the PE processes become dominating so that the measured data analysis may better validate the corresponding model assumptions of the PE model (several being shown in Figure 1 and discussed in Ref. [5]). Certain conclusions on the suitability of the corresponding models could be obtained obviously only on the basis of an increased accuracy of the measured data.

Page 51 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

181 Thomas+ (1967): (p,n) Ta(p,xn) Thomas+ (1967): (p,2n) 1.5 Birattari+ (1971): (p,3n) ? Birattari+ (1971): (p,4n) Zaytseva+ (1994) Michel+ (2002) 2n No PE 3n Shubin+ (1994, MEDNL-2) TALYS-0.64 [(p,3n)179mW] 1.0 4n STAPRE-H.6 [(p,3n)179mW] (b)

0.5

1n

0.0 10 20 30 40 50 60 70 E (MeV) n Fig. 2. Comparison of measured [2], evaluated (MENDL-2) and calculated cross sections for the 181Ta(p,xn) reactions [6].

The experimental procedure for these measurements will be tried for the first time at the n_TOF facility. The cross section for the formation of the residual nucleus for each exit channel of the reaction under study, will be determined by the prominent gamma rays emitted from the residual nucleus, after applying the appropriate corrections with the help of statistical model calculations [1]. The gamma rays will be measured by using a HPGe o detector placed at 125 where the second order Legendre polynomial P2 vanishes and the spectra can be assumed to represent angle integrated yields. A second HPGe detector at 90o will help for the identification of certain Doppler-shifted delayed gamma-rays. The neutron flux will be determined via the 235-238U(n,f) cross section measurements with the fission Micromegas detector already used in n_TOF, while the neutron time-of-flight will be determined by the gamma-flash as START signal and the gamma-ray as STOP signal. Overall, the final goal of this proposal would be to bring the neutron induced reactions studies above 20 MeV, at the experimental accuracy and theoretical understanding of charged-particle induced reactions (e.g., the (p,xn) reactions on 181Ta shown in Figure 2 [6], while there are no similar neutron-induced reaction data). They are altogether quite useful for nuclear technology achievements and the nuclear model validation as well, due to their large cross sections and decreased number of questionable parameters which may affect the calculated cross sections. The present proposal, if adopted for EAR-2, opens a vast field of research on a variety of medium and heavy isotopes, not just 197Au and 181Ta.

References [1] C.T. Papadopoulos, R. Vlastou, E.N. Gazis, P.A. Assimakopoulos, C.A. Kalfas, S. Kossionides and A.C. Xenoulis, Phys. Rev. C 34, 196 (1986). [2] A. Tsinganis, M. Diakaki, M. Kokkoris, A. Lagoyannis, E. Mara, C. T. Papadopoulos, and R. Vlastou, Phys. Rev. C 83, 024609 (2011). [3] Experimental Nuclear Reaction Data (EXFOR), http://www-nds.iaea.or.at/exfor. [4] M. B. Chadwick et al., Nucl. Data Sheets 107, 2931 (2006). [5] V. Avrigeanu, M. Avrigeanu, and F.L. Roman, in: Third International Workshop on Compound Nuclear Reactions and Related Topics, Sept. 19-23, 2011, Prague, EPJ Web of Conferences (in press); http://www-ucjf.troja.mff.cuni.cz/cnr11/presentations_dir/avrigeanu_v.pdf. [6] M. Avrigeanu, R.A. Forrest, F.L. Roman, and V. Avrigeanu, in: Advances in Nuclear Analysis and Simulation (PHYSOR 2006), Vancouver, Sept. 10-14, 2006, ANS, La Grange Park, 2007, p. D102.

Page 52 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

TITLE: 5. Fission cross section of the 230Th(n,f) reaction AUTHORS: R.Vlastou (NTUA), M.Kokkoris (NTUA), N.Colonna (INFN-Bari), M. Calviani (CERN)

SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 230Th (n,f) 0.01-50 MeV MASS: 10mg

DETECTION SYSTEM (name and short description): MGAS: MicroMegas fission detectors currently used for Pu fission measurements in EAR-1.

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: Higher neutron fluence delivered in shorter time, which allows measuring mg samples: - The combination 25 times higher fluence per pulse delivered in 10 times less time per pulse than in EAR-1 results in an increase by a factor of 250 of the reaction to background signal from sample activity with respect to EAR-1. - The higher fluence allows using samples of only few mg: i) the material in these quantities is available (not in the case of hundreds of mg), ii) samples can then be SURGXFHG E\ ³WKLQ VDPSOH´ WHFKQLTXHV OLNH HOHFWURGHSRVLWLQJ HYDSRUDWLRQ HWF QRW the case for thick samples: pressed powders, matrices, etc.) - The higher fluence allows to measure low reaction cross sections at a high accuracy impossible to be achieved in EAR-1.

MAIN ADVANTAGE(S) OF EAR-2 wrt. to other facilities for this particular measurement: The n_TOF EAR-2 neutron fluence covers in a single shot the full energy range of interest. The availability of a type-A experimental area makes it possible to measure Pu isotopes, which is not the case of instance in other TOF facility. The high instantaneous fluence makes is possible to measure low mass samples. These sample would have to be of small mass because 239Pu is a strategic material and because 241Pu (14y) has a very high activity. MOTIVATION (Nuclear technology, astrophysics, basic physics): Nuclear technology: The 230Th(n,f) cross section is important for the Th-U fuel cycle which has several advantages with respect to radioactive waste management and nonproliferation, as compared with the conventional U-Pu fuel cycle. There is lack of data in the low energy region and the existing data of the 230Th(n,f) reaction cross section above the fission threshold, cover only the energy range up to 25 MeV and with many discrepancies and low accuracy. Basic physics: The peak in the excitation function of the cross section measured by James et.al. [1] in the vicinity of 720 keV, has been interpreted in terms of a vibrational mode resonance state in the secondary minimum of a double-humped fission barrier, with K=1/2 [2]. Better experimental data in the energy region close to the fission barrier may enable us to reveal finer structures of the fission mode and extract all possible spectroscopic information on the states associated with the second well of the fission potential.

BEAM TIME (assume neutron fluence of EAR-1 x25, and En<50 MeV): 3x1018 protons (equivalent measuring time in EAR-1 = 5 years)

Page 53 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES (3 pages maximum): The goal of these measurements would be the accurate and consistent fission cross sections of the natural but very rare isotope 230Th, which is very interesting for the investigation of the fission process and very important for the Th-U fuel cycle. To achieve improved design calculations for thorium-based reactors, the determination of neutron-induced reaction cross sections on isotopes of thorium is required. Little data exist on the 230Th(n,f ) cross section in the energy range relevant for fast reactor systems with poor agreement between the measurements, as can be seen in Fig. 1. The most recent measurements are based on an indirect determination of the 230Th(n,f) cross section via the surrogate 232Th(3HeĮ) reaction, over an equivalent neutron energy range of 220 keV to 25 MeV [3]. Due to the lack of data both in the low and high energy region and to the discrepancies among the existing data, especially in the energy region around and below the fission threshold, this reaction is included among those of High Priority Request List in NEA [4]. The measurements are difficult due to the fact that 230Th is a very rare isotope and can be produced in very small quantities, while the fission cross section is relatively low (less than 1b), thus requiring a very high neutron flux in order to achieve accurate measurements. The particularities of the 2nd experimental area at n_TOF, with the high-intensity neutron flux covering the full energy range of interest from 0.01 to 50 MeV)[5], make such a measurement feasible. The setup would consist of a MGAS fission detector loaded with a thin sample of 10mg of Thorium oxide of high isotopic composition (>99%), spread over a circular area of 8 cm diameter on an Al backing. The cross section will be deduced with respect to the 255U(n,f) cross section. The Thorium and Uranium samples would be placed in 2 MGAS detectors, which have already been successfully used at n_TOF in the EAR-1.

Fig. 1. Fission cross section data for 230Th available in literature. References [1] G.D.James et. al. Nucl.Phys. A189(1972)225. [2] G.Yuen et al. Nucl.Phys. A171(1971)614. [3] B. L. Goldblum et al. Phys.Rev.C80(2009) 044610 [4] NEA High Priority Request Listhttp://www.oecd-nea.org/dbdata/hprl/ [5] E. Chiaveri et al., n_TOF Experimental Area 2 (EAR-2) preliminary feasibility study, CERN-INTC-2011-032; INTC-O-013 (2011)

Page 54 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

TITLE: 6. First measurement of the capture (and fission) cross sections of the fissile 245Cm

AUTHORS: E. Mendoza (CIEMAT), D. Cano-Ott (CIEMAT), E. González (CIEMAT), C. Guerrero (CERN)

SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 245Cm (n,) (n,f) Thermal to 1-20 MeV MASS: 0.1mg

DETECTION SYSTEM (name and short description): C6D6 (or a new Total Absorption Calorimeter)+MGAS: Combination of C6D6 (or a new Total Absorption Calorimeter) capture detectors with MicroMegas fission detectors.

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: Higher neutron fluence delivered in shorter time, which allows measuring mg samples: - The combination 25 times higher fluence per pulse delivered in 10 times less time per pulse than in EAR-1 results in a reduction of factor 250 reaction to background from sample activity with respect to EAR-1. - The higher fluence allows using samples of less than 1 mg: i) the material in these quantities is available (not the case of hundreds of mg), ii) samples can then be SURGXFHG E\ ³WKLQ VDPSOH´ WHFKQLTXHV OLNH HOHFWURGHSRVLWLQJ HYDSRUDWLRQ HWF QRW the case for thick samples: pressed powders, matrices, etc.) - The higher neutron fluence allows using a 1 or 2 thin samples (instead of the several necessary at EAR-1). The samples can be placed close to the C6D6 detectors (more VHQVLWLYHWRWKHVDPSOH¶VSRsition) or at the centre of a 4 calorimeter (as the TAC used in EAR-1).

MAIN ADVANTAGE(S) OF EAR-2 for this particular measurement: The n_TOF EAR-2 neutron fluence covers in a single shot the full energy range of interest. The availability of a type-A experimental area offers the required safety conditions for measuring radioactive samples such as the Cm isotopes. The high instantaneous fluence makes it possible to measure enriched samples only available in small amounts, as it is the case for the Cm isotopes.

MOTIVATION (Nuclear technology, astrophysics, basic physics): Nuclear Technology: The capture cross section of 245Cm is particularly difficult to measure due to the five times larger fission background. Accurate cross section data for 245Cm are of utmost importance for the transmutation of the long lived nuclear waste. The simultaneous measurement of capture and fission will improve the accuracy, reducing systematic uncertainties associated to the normalisation to absolute cross-sections.

BEAM TIME (assume neutron fluence of EAR-1 x25, and En<300 MeV): 3x1018 protons per isotope (equivalent measuring time in EAR-1 = 5 years) SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES (3 pages maximum):

The goal of this measurement is to measure the capture and fission cross sections, as well as alpha ratios, of the fissile isotope 245Cm (and heavier Cm isotopes when available). The

Page 55 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration energy range of range of interest comprehends both the resolved and unresolved resonance regions. The (n,) and (n,f) cross sections of 245Cm lack the required accuracy for the calculations for evaluating the performance of different transmutation strategies. The 245Cm fission cross section is indeed included among those of in the NEA High Priority Request List [1], and it has been measured several times, including a measurement at n_TOF in the past [3]. However, there is no capture or transmission data available in EXFOR. Some capture measurements have been performed in the past [4], but without the required accuracy: the measurement was performed using a nuclear explosion as a neutron source, with several Cm isotopes together, and the capture cross section of 245Cm was only ³REWDLQHG´EHWZHHQ DQGH92WKHUPHDVXUHPHQWVHVWLPDWHVRPHUHVRQDQFHLQWHJUDOV [5]. The very poor information of neutron capture in 245Cm leads to big differences between evaluations, as it is presented in the left panel of Figure 1. 7KH QȖ PHDVXUHPHQWLVYHU\GLIILFXOWEHFDXVHRIWKHODUJHILVVLRQȖ-ray background due to the unfavourable fission to capture cross section ratio, and also because independent measurement of the two reaction channels may not help to solve the existing problems of the evaluations. For that reason, a simultaneous (n,) and (n,f) measurement is highly desirable. The particularities of the 2nd experimental area at n_TOF (the highest intense neutron flux covering the full energy range of interest: thermal to 20 MeV) [2] make such a measurement feasible. The setup would consist in a MGAS fission detector loaded with a 2 thin samples of 2 245 ~1.7µg/cm of CmO2 each placed in between 2 C6D6 -ray detectors or inside the TAC. The samples would be placed back-to-back in 2 MGAS detectors in order to cancel out the effects of the non-uniform angular distribution of fission fragments. All detector types have been individually tested at n_TOF in the EAR-1 and a test experiment with the combined use of the TAC + MGAS for measuring the 2358 QȖ FURVVVHFWLRQKDVEHHQSHUIRUPHGDVZHOO The only open question would be their response to the -flash in EAR-2 and the associated high energy limit that could be reached. The expected counting rates for 2x0.05 mg samples and a total of 3x1018 protons per measurements are shown in Figure 1.

Fig.1. Different evaluated capture cross sections on the left and expected counting rates for a total of 3x1018 protons allocated on the right.

References [1] NEA High Priority Request List http://www.oecd-nea.org/dbdata/hprl/ [2] E. Chiaveri et al., n_TOF Experimental Area 2 (EAR-2) preliminary feasibility study, CERN- INTC-2011-032; INTC-O-013 (2011) [3] M.Calviani HWDO³7KHQHXWURQ-induced fission cross-section of 245Cm: new results from QB72)´WREHVXEPLWWHGWR3\V5HY&

Page 56 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

[4] M.S. Moore, G.A. Keyworth, Analysis of the Fission and Capture Cross Sections of the Curium Isotopes, Phys. Rev. C, v3 (1971) 1656. [5] V.D.Gavrilov,V.A.Goncharov, Thermal Cross Section and Resonance Integrals of Radiation Capture of Neutrons for Cm-244-248 Isotopes and for Cf-250, Journ.: Atomnaya Energiya Vol.44, Issue.3, p.246.

Page 57 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration

TITLE: 7. Neutron capture measurement of the s-process branching point 79Se

AUTHORS: C. Domingo-Pardo (IFIC), C. Guerrero (CERN), R.Reifarth (Univ.Frankfurt), J.L. Tain (IFIC).

SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 79Se (n,) Thermal to 1 MeV, of particular MASS:2.7g, approx. interest for astrophysics 1keV to 1/10000 content in 79Se, 100 keV. the rest mostly 78Se

DETECTION SYSTEM (name and short description): BaF2-TAC

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: Higher neutron fluence delivered in shorter time, which allows measuring (n,g) CS with the sub-mg content of 79Se in the sample (mostly 78Se), in a reasonable beam-time of aobut 30-40 days. Furthermore, the goal of the present measurement is to extract later the Maxwellian averaged neutron capture cross section in the relevant neutron energy range for astrophysics, i.e. between 1keV and 100 keV. In this respect, the high TOF resolution of EAR-1 is less relevant than the high neutron fluence that will be available at EAR-2. Finally, the one order of magnitude lower neutron flux at EAR-1 would imply a prohibitively low capture-to- background ratio and too long beam-time request.

MAIN ADVANTAGE(S) OF EAR-2 wrt. to other facilities for this particular measurement: The n_TOF EAR-2 neutron fluence covers in a single shot the full energy range of interest. With respect to other high-resolution TOF facilities, the advantage of n_TOF-EAR-2 is the same, but of even higher magnitude, than when comparing to n_TOF-EAR-1 (see section above). Both the high instantaneous flux, as well as the availability of type-A experimental area are of relevance for measuring a sample, which has an intrinsic activity of more than 10 kBq.

MOTIVATION (Nuclear technology, astrophysics, basic physics): Astrophysics: Selenium-79 is a branching point in the slow neutron capture process (s- process) with relevant implications in nucleosynthesis and in AGB stellar models. Indeed, the products of the s-process nucleosynthesis after 79Se are the s-only 80,82Kr, whose solar system abundance are accurately known from chemical analysis of pre-solar grains. This information, in conjunction with the experimental CS of 79Se, will allow one to extract reliable conditions for the neutron density, as well as the role of the main and weak s-process contributions to the nucleosynthesis in the A=80 mass region.

BEAM TIME (assume neutron fluence of EAR-1 x25, and En<300 MeV): One month. SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES (3 pages maximum): Introduction:

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Neutron capture reaction cross sections are a crucial element for stellar models, which describe the production of elements heavier than iron. The slow neutron capture process (s- process) is responsible for about half of the solar system isotopic abundances of elements heavier than iron. This mechanism operates in AGB stars, where two alternate burning shells, one of H and an inner of He, surround an inert degenerate CO-core. S-process elements are produced by neutron captures on seed nuclei in the He-rich intershell, between the two burning shells. The He intershell is periodically swept by convective instabilities induced by He-burning runaway, where 12C is synthesized. The main neutron source for AGB stars of 8 13 16 mass M<4MĴ and T=10 K is the C(ܤ,n) O reaction (main s-process component) with a 7 -3 neutron density of Nn<10 cm [1]. In the case of more massive stars, the s-process takes place during pre-supernova evolution, i.e. during convective core He-burning and convective C-shell burning. In this case also the neutron source 22Ne(ܤ,n)25Mg is activated at temperatures of 3.5·108 K and neutron densities larger than 1010 cm-3 can be reached. Such high neutron density strongly favours the production of neutron rich nuclides for s-process branching. Therefore, a different s-process pattern is expected depending on whether the first or the second neutron source reaction is more active. The knowledge of the neutron capture cross section sin such branching nuclei together with observed element abundances allow to estimate the neutron density at the s-process site. Since such s-process branching nuclei are unstable, direct neutron capture measurements are generally very difficult at conventional neutron facilities. The reaction to be considered in this expression of interest for n_TOF EAR-2 is 79Se(n,). The knowledge of the neutron capture cross section of 79Se provides a crucial test for the understanding of s-process nucleosynthesis in massive stars[2]. The unstable isotope 79Se is an s-process branching point, and it is located in a region where two scenarios may contribute, the one from massive stars (weak s-process component) and AGB stars main s- process component). Furthermore, the 79Se(n,) reaction is particularly relevant, because it leads to the production of the s-only 80Kr and 82Kr isotopes, which are shielded from the rapid 80 82 20 neutron capture process by their stable (or almost stable) isobars Se and Se (t1/2 = 10 a). The solar abundances of 80Kr and 82Kr isotopes are well characterized from chemical analysis of pre-solar grains [3], thus allowing to extract reliable conclusions about the stellar conditions by comparing predicted abundances (based on the stellar models and the 79Se(n,) versus the measured abundances (in the pre-solar grains). Furthermore, since 80Se is stable, the activation method is not applicable in this case. Thus, the only possibility to measure the 79Se cross section is to use a neutron source with a very intense flux and detect prompt gamma-rays emitted in the capture events.

Experimental approach: The n_TOF EAR-2 facility at CERN represents, at present, the unique place to accomplish the challenging measurement proposed here. A sample of 79Se can be produced with sufficient mass by several methods. At present, the most promising technique seems to be the irradiation of an enriched 78Se sample, of about 2.6 g (2 cm diameter, 2 mm thickness) at the ILL facility of Grenoble or at the HIRF reactor of Oak Ridge. After one week of thermal neutron irradiation in these facilities, one would obtain about 1018 and 1019 atoms of 79Se, respectively. A high enrichment of 78Se in the primary sample is convenient in order to isolate reliably the CS contributions from the several Se isotopes, which have similar Q-values. Another possibilities for the production of a 79Se sample rely on the chemical separation and purification of either fission residues, or from decommissioned spallation targets (e.g. from ISOLDE or from PSI). The measurement of the capture events will be carried out with the Total Absorption Calorimeter (TAC). This way, a high detection sensitivity for the channel of interest will be achieved. Other techniques, like the PHWT with C6D6 are excluded owing to the low

Page 59 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration efficiency of the gamma-ray detectors, which would lead to an excessively long measuring time and too low signal-to-background ratio.

References

[1] F. Käppeler, et al., Rep. Prog. Phys. 52, 945 (1989). [2] G. Walter, H. Beer, F. Käppeler, et al., Astron. Astrophys. 167, 186 (1986). [3] R.S. Lewis, S. Amari, and E. Anders, Geochim. Cosmochim. Acta 58, 471 (1994). [4] Z.Y. Bao, H. Beer, F. Käppeler, et al., Atomic Data Nucl. Data Tables 76, 70 (2000).

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TITLE: 8. Cross section and angular distribution of fragments from neutron-induced fission of 232U

AUTHORS: L. Tassan-Got (IPNO), L. Audouin (IPNO), L. S. Leong (IPNO), C. Paradela (USC), I. Duran (USC)

SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 232U (n,f) Thermal to 100 MeV MASS: 4x60mug

DETECTION SYSTEM (name and short description): PPAC assembly : 10 PPAC detectors enclosed in a single reaction chamber

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: Higher neutron fluence delivered in shorter time, which allows measuring fission of mug samples: - The combination 25 times higher fluence per pulse delivered in 10 times less time per pulse than in EAR-1 results in a reduction of factor 250 reaction to background from sample activity with respect to EAR-1. - The higher fluence allow using samples of only few tens of mug : the material in this quantities is available and possible to handle (not the case of mg). Samples should be produced by the usual electro-deposition method

MAIN ADVANTAGE(S) OF EAR-2 wrt. to other facilities for this particular measurement: The n_TOF EAR-2 neutron fluence covers in a single shot the full energy range of interest The availability of a type-A experimental area makes it possible to measure very active U isotopes. The high instantaneous fluence makes is possible to measure high activity samples (like the mentioned 232U).

MOTIVATION (Nuclear technology, astrophysics, basic physics): Nuclear Technology: 232U is an especially dangerous byproduct of the Th/U3 fuel cycle because its decay chain includes the emission of a 2,6 MeV gamma ray. 232U is mainly formed by (n,2n) reaction on 233U. Its concentration in the spent fuel will be the main factor in the definition of gamma shielding required to handle this fuel. Hence, the measurement of its fission cross section is a key data in order to quantify its presence in spent fuel. Unfortunately, the very high activity of this nuclei has hindered neutron-induced reactions measurements: only 6 measurements have ever been performed, the most recent in 1986, and several energy ranges (< 6 eV, 5-50 keV, > 10 MeV) have never been measured at all.

BEAM TIME (assume neutron fluence of EAR-1 x25): 4x1018 protons

SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES (3 pages maximum): The goal of this measurement would be to measure the fission cross section, as well as the angular distribution of fission fragments, of 232U. We will take advantage of the full energy range of the n_TOF EAR-2 facility, from thermal region to several MeV: 232U, although being an even- even nuclei, behaves as a fissile (thresholdless) isotope. As all even-even nuclei, it is expected to

Page 61 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration exhibit strong fission anisotropies. Existing data are extremely scarce: only 6 data sets exist. They cover only parts of the energy range of interest and, where overlap exists they are significantly discrepant with one another.

The preparation of the experiment, its decommissioning and the measurement itself are all exceptionally challenging because of the very high activity of the samples of interest : the 69 yr half life corresponds to an activity of 840 MBq/mg. Samples of 60 mg would have an activity of 50 MBq each, which appears as a manageable value for PPACs detectors. Obviously such ultra low mass samples may only be measured at the 2nd experimental area at n_TOF.

A total of 1,5x1018 protons are necessary for a clean measurement of fission cross sections for a 15 mg target: hence a factor of 60 in the sample mass and a factor 25 of gain in the fluence results in a requirement of 4x1018 protons. This number of protons should be obtained in the shortest possible time, as the harmfulness of the targets will increase with time due to the build- up of the 232U decay chain.

The setup would consist in a 10 PPAC fission detector, with 4 232 reference target of 235U, each placed in between 2 PPACs. In empty target slots, inert material would be in order to limit the stress of each detector to the reactions taking place in one single target. Targets and detectors would be inclined by 45 degrees with respect to the beam direction in order to measure all angles with respect to the neutron beam, a configuration which has been successfully used at n_TOF in the EAR-1.

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TITLE: 9. DHVWUXFWLRQRIWKHFRVPLFȖ-ray emitter 26Al by neutron induced reactions

AUTHORS: P.J. Woods (University of Edinburgh), C. Lederer (University of Frankfurt) F. Käppeler (Karlsruhe)

SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 26Al (n,p)(n,ܤ) Thermal to 1 MeV MASS: aȝJ DETECTION SYSTEM (name and short description): Large solid angle Double-6LGHG6LOLFRQ6WULS'HWHFWRUǻ(-E telescope system

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: ~25 higher neutron flux ± low statistics experiment to due small sample of radioactive target material.

MOTIVATION (Nuclear technology, astrophysics, basic physics): Nuclear Astrophysics: 26 7KH ILUVW REVHUYDWLRQ RI Ȗ-rays from the radioisotope Al (t1/2 ~ 1 Myear) showed nucleosynthesis is ongoing in our galaxy. Detailed satellite telescope observations indicate that galactic 26Al is predominantly produced in different burning stages of massive Wolf- Rayet stars [1]. Recent reaction network calculations have shown the rates of the neutron induced destruction reactions 26Al(n,p) and 26$O QĮ DUHWKHPDMRUVRXUFHRIXQFHUWDLQW\LQ predicting the amount of 26Al material ejected into the interstellar medium by Wolf-Rayet stars. The limited experimental data sets on the 26Al(n,p) and 26$O QĮ) reactions exhibit major discrepancies, and do not cover all the relevant stellar burning energy range. Consequently stellar modelers have identified new measurements on these reactions as the highest priority for predicting 26Al abundances. We propose to measure the 26Al(n,p) and 26$O QĮ FURVV-sections at the n_TOF facility from thermal energies up to 1 MeV covering the entire energy range of astrophysical interest.

BEAM TIME (assume neutron fluence of EAR-1 x25, and En<300 MeV): Reduction of statistical (and systematic errors) are critical for this experiment, a good measurement could be made with ~3 months running, possibly in parasitic mode although this would compromise Silicon detector geometry/solid angle for experimental set-up.

SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES: 26 26 Trautvetter and Kaeppeler reported the first measurement of the Al(n,p1) Mg reaction [2] which was followed up by a more comprehensive study [3], in which measurements of both 26 26 26 26 the Al(n,p0) Mg and Al(n,p1) Mg reactions were reported. A later study of the 26 26 26 23 Al(n,p1) Mg reaction, and a first study of the $O QĮ0) Na reaction, both performed at the Los Alamos Neutron Science Center (LANSCE), were reported by Koehler et al. [4]. The cross-section data from ref. [4] are plotted in Figures 1 and 2 for these reactions as a function 26 23 of neutron energy. The Al(n,p1) Na reaction rate was found to be in disagreement with that of [3] in the one region of burning temperature overlap between the two sets of measurements (see Figure 16 from ref. [5]).

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26 26 Fig. 1. Cross-section data for the Al(p,n1) Mg reaction [4].

Fig.2. Cross-section data for the 26$O QĮ 23Na reaction [4].

Illiadis et al. [5] highlighted this as the key issue that needs to be resolved for calculating abundances of 26Al produced in the convective carbon shell and explosive neon carbon shell burning phases of Wolf-Rayet stars. Koehler et al. suggested discrepancies between the measurements could be due to angular distribution effects, since the experiments covered different angular ranges, or more likely differences in normalization procedure [4]. Ref [3] used the 197$X QȖ 198Au reaction for normalization, which Koehler et al. comment is sensitive to any small excess in thermal neutrons in the beam [4], whereas Koehler et al. used 26 26 10 7 the Al(n,p1) Mg cross-section at thermal neutron energies, and the % QĮ Li reaction, for normalization purposes [4]. Most recently, De Smet et al. reported measurements of the

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26 23 $O QĮ0+1) Na reactions using the GELINA neutron time-of-flight facility at the Institute for Reference Materials (IRMM), Geel [6], with the 10% QĮ 7Li reaction used for the neutron flux determination. The GELINA measurements overlapped the lower neutron regime covered by the LANSCE experimental study (maximum energy 10 keV [4]) and also obtained data at higher energies, identifying a number of new resonances for the 26$O QĮ 23Na reaction. A resonance at 6 keV was identified in both studies (the only one identified in [4]) but there was disaJUHHPHQWLQWKHWRWDOZLGWKȽof the resonance. De Smet suggested this may be due to the relatively small solid angle covered by the silicon detection system used in the LANCSE measurements, rendering the measurements sensitive to anisotropy effects - the 6 keV resonance was assigned to a p-wave [6]. The GELINA measurements had a much larger statistical accuracy mainly due to the use of much larger sample of 26Al atoms [7]. Illiadis et al. noted the experimental discrepancies between these low energy measurements of the 26$O QĮ 23Na reaction rate (see Figure 15 from [5]), and the absence of measurements of the 26$O QĮ 23Na reaction rate in the higher energy astrophysical burning regime above T~0.3-3 GK , and called for new measurements [5]. This regime covers the burning temperatures for convective shell C/Ne burning, and explosive Ne/C burning in Wolf-Rayet stars, and requires measurements up to 1MeV in neutron energy. In particular, there may be hitherto unobserved strong resonances at high energies that can significantly influence the Maxwellian Averaged Cross-Section (MACS) in the high temperature burning regime.

References [1] R. Diehl et al., Nature (London), 298, 445 (2006). [2] H.P. Trautvetter and F Kaeppeler, Z. Phys. A318, 121 (1984). [3] H.P. Trautvetter, H.W. Becker, U. Heinemann, L. Buchmann, C. Rolfs, F. Kaeppeler, M. Baumann, H. Freiseleben, H.-J. Lutke-Stezkamp, P. Geltenbort and F. Gonnenwein, Z. Phys. A323, 1 (1996). [4] P.E. Koehler, R.W. Kavanagh, R.B. Vogelaar, Yu. M. Gledenov and Yu. P. Popov,Phys. Rev. C56, 1138 (1997). [5] C. Iliadis, A. Champagne, A. Chieffi and M. Limongi, Ast. Phys. J. Supp. 193, 16 (2011). [6] L. de Smet, C. Wagemanns, J. Wagemanns, J.Heyse, J. van Gils, Phys. Rev. C76, 045804 (2007). [7] C. Ingelbrecht et al., Nucl. Inst. Meth. A480, 114 (2002).

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TITLE: 10. Measurement of 7Be(n,p)7Li and 7Be(n,)4He cross sections, for the cosmological Li problem AUTHORS: N. Colonna (INFN), M. Calviani (CERN), F. Kaeppeler (KIT), C. Massimi (INFN), D. Schumann (PSI), C. Guerrero (CERN), E. Berthoumieux (CEA), M. Barbagallo (INFN), A. Ferrari (CERN), E. Chiaveri (CEA) SAMPLE(S) OF REACTION(S) OF ENERGY RANGE OF INTEREST: INTEREST: INTEREST: ISOTOPE: 7Be (n,p) (n,) Thermal to 1-20 MeV MASS: < 10 µg

DETECTION SYSTEM (name and short description): To be defined, possibly MicroMegas, Si-based E-E telescope, diamond detector

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular measurement: Higher neutron fluence delivered in shorter time, which allows measuring sub-mg samples: - The combination 25 times higher fluence per pulse delivered in 10 times less time per pulse than in EAR-1 results in a reduction of factor 250 reaction to background from sample activity with respect to EAR-1. - The higher fluence allow using samples of less than 1 mg: i) the material in this TXDQWLWLHVLVDYDLODEOHIURP36,LL VDPSOHVFDQWKHQEHSURGXFHGE\³WKLQVDPSOH´ techniques like electrodepositing, evaporation, etc. (not the case for thick samples: pressed powders, matrices, etc.) - The higher fluence minimizes the background related to the natural radioactivity of the sample, an aspect particularly important in this measurement, considering the very short half-life of 7Be (53 d), which results in an extremely high specific activity.

MAIN ADVANTAGE(S) OF EAR-2 wrt. to other facilities for this particular measurement: The n_TOF EAR-2 neutron fluence covers in a single shot the full energy range of interest. 7KH DYDLODELOLW\ RI D ³W\SH-$´ H[SHULPHQWDO DUHD PDNHV LW SRVVLEOH WR PHDVXUH WKH FURVV section for short-lived 7Be isotope, which is not the case of several other facilities. The high instantaneous fluence makes it possible to measure high activity samples (like the mentioned 7Be isotopes), and low cross sections as the one expected for the (n,ъ) reaction.

MOTIVATION (Nuclear technology, astrophysics, basic physics): Nuclear Astrophysics: The (n,p) and (n,) reaction on 7Be are of interest for Big Bang Nucleosynthesis. In particular, they are responsible for the destruction of 7Be, from which 7Li is produced. An accurate measurement of their cross section could provide important information on the so-FDOOHG³&RVPRORJLF/LSUREOHP´9HU\IHZGDWDH[LVWRQWKLVUHDFWLRQ in particular on the (n,) one, which reflects in completely different evaluated cross sections. According to some recent calculations of BBN, a factor of 100 in the (n,) cross section may ease the problem on the primordial Li abundance. BEAM TIME (assume neutron fluence of EAR-1 x25, and En<300 MeV): 5x1018 protons per isotope (equivalent measuring time in EAR-1 = several years) SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES (3 pages maximum):

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One of the important unresolved problems of Astrophysics is the so-FDOOHG ³&RVPRORJLFDO /LWKLXPSUREOHP´,WUHIHUVWRWKHODUJHGLVFUHSDQF\RIPRUHWKDQDIDFWRURIEHWZHHQWKH abundance of primordial 7Li predicted by the standard theory of Big Bang Nucleosynthesis (BBN), and the value inferred from observations of galactic halo dwarf stars, the so-called ³6SLWHSODWHDXKDORVWDUV´ IRUDUHYLHZRQWKHVXEMHFWVHH5HI>@ 6HYHUDOPHFKDQLVPV have been put forward to explain this difference: new physics beyond the Standard Model, errors in the inferred primordial 7Li abundance from the Spite plateau stars and, finally, systematic uncertainties in the nuclear physics inputs of the BBN calculations. Recently, several measurements of charged-particle induced reactions have mostly ruled out a possible solution of the cosmological Li problem based on conventional nuclear physics [3,4]. However, new and accurate measurements of neutron-induced reactions are necessary in order to completely clarify this issue. In this letter of intent, we propose to perform a new measurement of (n,p) and (n,) reactions on 7Be, of relevance for the cosmological 7Li problem, as well as for basic nuclear physics. Given the intrinsic difficulty of the measurements, related to the relatively short half-life of 7Be (53.29 d), the second experimental area (EAR-2) at n_TOF could be one of the few facilities in the near future where accurate results on these reactions could be obtained in the energy range of interest for Big Bang Nucleosynthesis. In standard BBN, primordial 7Li is mostly produced by electron capture of 7Be relatively late after the Big Bang, when the Universe has cooled down sufficiently for nuclei and electrons to combine into atoms. Therefore, the abundance of 7Li is essentially determined by the production and destruction of 7Be in the early stages of BBN. The main reaction leading to the production of 7Be is the 3He(ĮȖ)7Be, while several reactions are responsible for its destruction. Among them, the 7Be(n,p)7Li followed by 7Li(SĮ)4He is one of the most important. In 1988 Koehler et al. [5], measured the cross-section of the 7Be(n,p) reaction from thermal to 13.5 keV at the 7 m station of the LANSCE neutron facility, Los Alamos. The results were used to estimate the astrophysical reaction rate as a function of the temperature. The authors found reaction rates 60 to 80% lower than previously thought, thus excluding a significant impact of this reaction on the 7Li problem. It should be considered, however, that for the reaction rate at high temperatures, the authors had to rely on some assumptions, given the limited energy range covered by their measurement. Although it is highly unlikely that a solution to the 7Li problem could be related to a large error in the 7Be(n,p) cross section, a more precise estimate of the reaction rates at temperatures between 0.3 and 1 GK (i.e. 25-80 keV) would be desirable to improve BBN calculations. Together with the (n,p) reaction, the (n,) reaction could in principle contribute to the destruction of 7Be during Big Bang Nucleosynthesis. However, the contribution of this channel is neglected in BBN calculations at present, due to its much lower cross-section compared to the (n,p) reaction. The figure below shows the comparison between the evaluated cross sections, in ENDF/B-VII and RUSFOND-2010, for the two reactions. In the energy range of interest, the (n,) cross section is between two and four orders of magnitude lower than the (n,p) channel. It should be considered, however, that there are essentially no direct measurements of this reaction in the energy range of interest for BBN. In EXFOR, a single (n,) measurement at thermal energy performed at the ISPRA reactor is reported. For this reason, an uncertainty of a factor of 10 is typically assigned to this reaction in BBN calculations. In view of the lack of data on the 7Be(n,)4He cross section, it can not be excluded that the astrophysical reaction rate for this reaction might be significantly underestimated (note the opposite trend in the cross section between ENDF/B-VII.0 and RUSFOND-2010 evaluations). Recent BBN calculations [6] have estimated that a cross section a factor of 100 higher than currently thought may possibly account for a large reduction in the primordial 7Li

Page 67 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration abundance, thus at least partially solving the 7Li problem. Although an error of two orders of magnitude in the (n,) cross section is not very likely, it cannot be completely excluded. It is therefore mandatory to measure the 7Be(n,) cross section in the keV energy range.

Fig.1: Comparison of the evaluated 7Be(n,p)7Li and 7Be(n,)4He cross sections in ENDF/B-VII.0 (left panel) and RUSFOND-2010 (right panel) from 1 eV to 20 MeV neutron energy.

The second experimental area at n_TOF would offer the unique opportunity to perform the very difficult measurements of the important 7Be(n,p) and 7Be(n,) reactions, thanks to the very high instantaneous neutron flux, and the possibility to cover a wide neutron energy range, from thermal to over 1 MeV. Such a measurement would be the first to be performed in this region, with the results of interest for other fields as well. One of the main difficulties in the measurement is related to the small mass typically available for a 7Be sample. In the measurement of Koehler et al., a sample of 90 ng was used, which had to be produced not long before the measurement, due to the relatively short half- life of the isotope. This problem could be easily solved at n_TOF, thanks to the recent agreement between the n_TOF Collaboration and the RadWasteAnalytics group at PSI. The long-time experience of this group, and the availability of a large amount of 7Be from the cooling water of the SINQ spallation neutron source at PSI, guarantees for the production of a sample of mass as high as 8 µg (100 GBq), just when needed for the measurement. The main remaining difficulty for this measurement is related to the detector, which should be able to clearly identify protons and alpha particles, with high background rejection capability. For this aspect, the n_TOF Collaboration is currently investigating various possibilities, in particular the use of high performance gas detectors (MicroMegas), Si-based E-E telescopes, or devices based on diamond detectors. It should be considered that the energy of the reaction products is very different, being 1.64 MeV for protons and several MeV for alpha particles (the Q-value of the 7Be(n,) reaction is 19 MeV). We are therefore confident that this very difficult measurement can be performed at n_TOF EAR-2, thanks of the combination between high-flux, availability of a sufficient quantity of 7Be, and state of the art

Page 68 of 69 09.02.2012 The n_TOF Experimental Area2 n_TOF Collaboration detection and acquisition systems.

References [1] D. Tytler et al., Physica Scripta 85, 12 (2000) [2] R.H. Cyburt et al., Phys. Rev. D 69, 123519 (2004) [3] C. Angulo et al., Astrophys. J. 630, L105 (2005) [4] O.S. Kirsebom and B. Davids, Phys. Rev. C 84, 058801 (2011) [5] P. Koehler et al., Phys. Rev. C 37, 917 (1988) [6] F.L. Villante, Private communication

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