Physics of Atomic Nuclei, Vol. 66, No. 8, 2003, pp. 1399–1406. From Yadernaya Fizika, Vol. 66, No. 8, 2003, pp. 1445–1452. Original English Text Copyright c 2003 by Ibrahim. The ALTO Project at IPN Orsay* F. I b r a h i m ** Institut de Physique Nucleaire,´ Orsay, France Received December 15, 2002 Abstract—In order to probe neutron-rich radioactive noble gases produced by photofission, a PARRNe 1 experiment (Production d’Atomes Radioactifs Riches en Neutrons) has been carried out at CERN. The incident electron beam of 50 MeV was delivered by the LIL machine (LEP Injector Linac). The experiment allowed one to compare under the same conditions two production methods of radioactive noble gases: fission induced by fast neutrons and photofission. The obtained results show that the use of the electrons is a promising mode to obtain intense neutron-rich ion beams. Thereafter, with the success of this photofission experiment, a conceptual design for the installation at IPN Orsay of a 50-MeV electron accelerator close to the PARRNe 2 device has been worked out: the ALTO project. This work has started within a collaboration between IPNO, LAL, and CERN groups. c 2003 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION of such beams. The neutron-rich radioactive nuclides are to be produced by fissioning a heavy nuclide, There is currently in the nuclear physics commu- such as 238U. The technique originally proposed [3] nity a strong interest in the use of beams of acceler- is the use of energetic neutrons to induce fission of ated radioactive ions. Although a fast glance at the depleted uranium. The neutrons are generated by the nuclide chart immediately shows the vast unknown breakup of deuterons in a thick target, a so-called territories on the neutron-rich side of the valley of beta converter, of sufficient thickness to prevent charged stability (see Fig. 1), only few projects are concerned particles from escaping. The energetic forward-going with the neutron-rich nuclides. neutrons impinge on a thick production target of The availability of intense neutron-rich ion beams fissionable material. The resulting fission products will open new perspectives in the study of nuclei very accumulate in the target, diffuse to the surface from far away from the valley of stability. It would allow one which they evaporate, are ionized, mass-selected, and to apprehend the behavior of the nuclear matter under finally postaccelerated. An ISOL device has been de- extreme conditions [1, 2]. Several laboratories have veloped to carry out various R&D projects [4] (see concentrated their efforts in studies aiming to pro- Fig. 2). duce beams intense enough for the next generation of experiments (SPIRAL II and EURISOL projects). This method has several advantages. The highly To get such beams, a large R&D effort is required. activated converter can be kept at low temperature Uranium fission is a very powerful mechanism to pro- without affecting the neutron flux. The target is bom- duce such radioactive beams. A substantial part of the barded by neutral projectiles losing energy only by PARRNe program (Production d’Atomes Radioactifs useful nuclear interactions and having a high pene- Riches en Neutrons) at the IPN Orsay is dedicated to trating power allowing very thick targets. the development of neutron-rich isotope beams by the One of the main objectives of the R&D program ISOL (Isotopes Separator On-Line) method. wastodeterminetheenergyoftheprimarydeuteron beam giving the best yields of radioactive nuclides of interest for radioactive beams while taking into 2. INVESTIGATION OF FAST NEUTRON account beam power evacuation and safe operation PRODUCTION MODE of the facility. The approach has consisted in carry- The aim of the PARRNe program is to establish ing out simulations with various codes available or the feasibility of producing neutron-rich radioactive developed by different task groups of the SPIRAL II beams for the SPIRAL II project at GANIL and to project and performing a number of key experiments determine the optimum conditions for the production to validate the simulations. In this way, confidence is gained about the predictive power of the codes ∗This article was submitted by the author in English. for situations where experiments could not be set up **e-mail: [email protected] within the allocated time for the study. 1063-7788/03/6608-1399$24.00 c 2003 MAIK “Nauka/Interperiodica” 1400 IBRAHIM 114 184 162 Fission 82 Z 126 Sn 50 132Sn N/Z = 1.6 4 Ni 28 82 20 78Ni / ≅ 1.79 8 50 N Z 28 2 20 N 8 Fig. 1. The production of fission fragments (upper light gray part) in comparison with the production for SPIRAL (dark gray part). The concept of using neutrons generated by neutrons since evaporation is not implemented in the deuteron breakup implies a study of the production code. LAHET reproduces the low-energy neutron yields, energy spectrum, and angular distributions spectrum, while it tends to slightly underestimate of neutrons in converters made of various materials (less than a factor of 2) the neutron distributions at and as a function of deuteron energy. Experiments very forward angles. were performed at IPN Orsay, KVI Groningen, and A strong increase in neutron production is ob- SATURNE at Saclay. They explored a deuteron served for deuteron energy between 14 and 100 MeV. energy range between 14 and 200 MeV. The main It is much less pronounced between 100 and features of neutron spectra are listed below. 200 MeV (see Fig. 3). Among converters tested, Be At forward angles, the energy distribution has a is slightly more productive than C. It has, however, broad peak centered at about 0.4 times the deuteron disadvantages related to its physical and chemical energy. The angle of emission becomes narrower with properties. increasing energy. For 100-MeV deuterons, the en- Productions of radioactive noble gases on a cryo- ergy width (FWHM) of the neutron spectrum is about genic finger have been measured for different deuteron 30 MeV and the FWHM opening angle of the cone of ◦ energy in the same experimental conditions with the emission is about 10 [5, 6]. so-called PARRNe 1 setup [8] in the framework of the There is a rather isotropic distribution of neutrons European RTD program SPIRAL II. The PARRNe 1 of a few MeV due to evaporation in the fusion reac- setup has been designed to be compact and portable tion. to enable its installation at various accelerators. The angular distributions and energy spectra are The search of the optimal energy of the deuterons in fair agreement with calculations with an extended was done by installing this setup successively at version of the Serber model [7] and with the LAHET IPN Orsay (20 MeV) [9], at CRC Louvain La code. The Serber model reproduces the distributions Neuve (50 MeV), and at KVI Groningen (80 and of high-energy neutrons but not of the low-energy 130 MeV) [10]. PHYSICS OF ATOMIC NUCLEI Vol. 66 No. 8 2003 THE ALTO PROJECT AT IPN ORSAY 1401 Hall 210 Derouleur Hall 110 Q8 Q1 Q6 Target Q3 Q7 Focal plane Q2 Q5 Q4 Deuterons Fig. 2. Schematic view of the ISOL device PARRNe 2 at the Tandem of IPN Orsay. 3. INVESTIGATION OF PHOTOFISSION in a thick target eventually lead to a fission through PRODUCTION MODE the resulting photon produced. In the same manner, the neutrons produced by (γ,n)and(γ,2n) reactions It has recently appeared that photofission could as well as the (γ,f) itself can also induce fission, this be an alternative to neutron-induced fission [11, 12]. time by the regular (n, f) high cross section (0.5 b for We have therefore initiated a study of photofission fast neutrons). Therefore, in a thick target, photofis- induced by bremsstrahlung generated by electrons. sion may be a rather interesting way of creating ra- With an electron driver, electron interaction with dioactive fission fragments. matter will radiate bremsstrahlung photons inside the Unfortunately, no efficient monochromatic target. Fission will then be induced by those photons sources of 15-MeV photons are available. The most exciting the giant dipolar resonance (GDR) of the common way for producing high gamma fluxes is nucleus at the right energy. This well-known process the bremsstrahlung radiated by passage of electrons is called photofission. through matter. This process has a cross section The GDR cross section for 238U is shown in Fig. 4. rising linearly with energy. It will dominate the ioniza- Amaximumfission probability of 160 mb is ob- tion process above a critical energy (around 20 MeV). tained for photons having energy around 15 MeV. But the resulting bremsstrahlung spectrum is widely At that energy, the photoelectric and the Compton spread in energy from zero up to the full initial electron and Rayleigh scattering cross sections start to fall off energy (Fig. 4). Although each single electron may rapidly, so the main contributions to gamma absorp- ultimatelyproduceasmanyas20photons,only e+e− a small fraction of them (0.5 to 0.7 gamma per tion are pair production and the photonuclear − reactions (γ,f), (γ,n), and (γ,2n). Although the ab- e )are“useful” photons lying in the GDR range solute fission cross section is rather small (compared (15 ± 5 MeV). to normal fission with neutrons), its contribution is A simple calculation including the main electron not negligible as even a pair production reaction may interactions (bremsstrahlung and ionization) and the PHYSICS OF ATOMIC NUCLEI Vol. 66 No. 8 2003 1402 IBRAHIM φ, 1010 µC–1 sr–1 Nγ/(MeV sr e) Fission cross section, mb 103 101 250 Target W (d = 2 mm) 2 10 100 200 GDR 101 150 10–1 Ee = 45 MeV 100 100 –2 10 = 25 MeV Ee 50 10–1 Ee = 10 MeV 0 50 100 150 200 E , MeV d –3 0 10 1 11 21 31 41 Eγ, MeV Fig.