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Fission Cross Secons

Paul Lisowski Guest Scienst, Los Alamos Naonal Laboratory

1 Outline

• Introducon – events leading to experimental evidence of nuclear fission

• Fission Phenomenology and Observables

• Fission Cross Secons Using

• Fission Cross Secons from Charged Parcle Measurements

• The Future of Fission Cross Secon Measurements and Possible Improvements

2 Introducon – Some Key Events

1896: Becquerel accidentally discovers radioacvity from ore and discovers that deflecon in a magnec field led to posive, negave, and neutral classes of radioacvity. 1905: Einstein publishes - equivalence paper. 1906: Rutherford discovers α parcles and develops scaering apparatus – results lead him to conclude that atoms have a ‘nucleus’ of posive charge. 1920: Rutherford postulates that the nucleus has a bound ‘electron- pairs’ to explain the disparity between element’s atomic mass and number.

3 Introducon – Some Key Events

1896: Becquerel accidentally discovers radioacvity from uranium ore and discovers that deflecon in a magnec field led to posive, negave, and neutral classes of radioacvity. 1905: Einstein publishes mass-energy equivalence paper. 1906: Rutherford discovers a parcles and develops scaering apparatus – results lead him to conclude that atoms have a ‘nucleus’ of Einstein, A. (1905), "Ist die Trägheit eines posive charge. Körpers von seinem Energieinhalt abhängig?", 1920: Rutherford postulates that the nucleus Annalen der Physik, 18: 639–643 has a bound ‘electron-proton pairs’ to explain Or: the disparity between element’s atomic mass and number. Does the inera of a body depend upon its energy-content? 4 Introducon – Some Key Events

1896: Becquerel accidentally discovers radioacvity from uranium ore and discovers that deflecon in a magnec field led to posive, negave, and neutral classes of radioacvity. 1905: Einstein publishes mass-energy equivalence paper. 1906: Rutherford discovers α parcles and develops scaering apparatus – results lead him to conclude that atoms have a ‘nucleus’ of posive charge. 1920: Rutherford postulates that the nucleus has bound ‘electron-proton pairs’ to explain the disparity between element’s atomic mass and number.

5 Introducon – Some Key Events

1932: Chadwick bombards Beryllium with α parcles: proposes that the ‘electron-proton pairs’ were really a new parcle – the . 1934: Fermi finds a wide variety of from irradiang uranium using a Po-Be source including lighter elements Paraffin target 1934: Ida Noddack publishes a paper crical of Fermi’s radiochemistry and suggests ‘It is conceivable that the nucleus breaks up into n p several large fragments …” 1938: Oo Hahn and irradiate uranium and show that the lighter elements seen by Fermi were about ½ the mass of the uranium, demonstrang fission 1939: and Oo Frisch explain the process as leading to uranium spling, calculang a 200 MeV energy release. 6

Introducon – Some Key Events

1932: Chadwick bombards Beryllium with α parcles: proposes that the ‘electron-proton pairs’ were really a new parcle – the neutron. 1934: Fermi finds a wide variety of radionuclides from irradiang uranium using a Po-Be source including lighter elements 1934: Ida Noddack publishes a paper discussing Fermi’s radiochemistry and suggests ‘It is conceivable that the nucleus breaks up into several large fragments …” 1938: Oo Hahn and Fritz Strassmann irradiate uranium and show that the lighter elements seen by Fermi were about ½ the mass of the uranium, demonstrang fission 1939: Lise Meitner and Oo Frisch explain the process as neutron capture leading to uranium spling, calculang a 200 MeV energy release. 7

Introducon – Some Key Events

1932: Chadwick bombards Beryllium with α parcles: proposes that the ‘electron-proton pairs’ were really a new parcle – the neutron. 1934: Fermi finds a wide variety of radionuclides from irradiang uranium using a Po-Be source including lighter elements 1934: Ida Noddack publishes a paper crical of Fermi’s radiochemistry and suggests ‘It is conceivable that the nucleus breaks up into several large fragments …” 1938: Oo Hahn and Fritz Strassmann irradiate uranium with neutrons and idenfy Barium, showing that the lighter elements earlier seen by Fermi were about 1/2 the mass of the uranium, demonstrang fission.

Hahn Strassmann 8 Introducon – Some Key Events

1938: Lise Meitner and Oo Frisch explain the process as neutron capture leading to uranium spling, calculang a 200 MeV energy release. 1939: Frish provides experimental confirmaon of of the energy release, confirming Einstein’s 1905 paper. 1939: Ladenberg, Kanner, Barschall, and Van Voorhies publish the first measurements of the reacon probability of 2-3 MeV neutrons of Uranium.

9 Introducon – Some Key Events

1939: Lise Meitner and Oo Frisch explain the process as neutron capture leading to uranium spling, calculang a 200 MeV energy release. 1939: Frish provides experimental confirmaon of of the energy release, confirming Einstein’s 1905 paper. 1939: Bohr and Wheeler publish the first theorecal analysis of the mechanism of nuclear fission based on the liquid drop model 1939: Ladenberg, Kanner, Barschall, and Van Voorhies publish the first measurements of the reacon probability of 2-3 MeV neutrons of Uranium.

10 Introducon – Some Key Events

1939: Lise Meitner and Oo Frisch explain the process as neutron capture leading to uranium spling, calculang a 200 MeV energy release. 1939: Frish provides experimental confirmaon of of the energy release, confirming Einstein’s 1905 paper. 1939: Bohr and Wheeler publish the first theorecal analysis of the mechanism of nuclear fission based on the liquid drop model 1939: Ladenberg, Kanner, Barschall, and Van Voorhies publish the first measurements of the reacon probability for 2-3 MeV neutrons of Uranium. 11 Fission Phenomenology and Observables Liquid Drop Model The liquid drop model developed by Gamow and formulated by Weizsäcker in 1935 treats the nucleus as a uniformly charged incompressible liquid drop with the total of the nucleus given as:

2/3 2 1/3 2 B = avA – aSA + aCZ(Z-1) /A - aa(A-2Z) /A ± δ(A,Z) Surface Coulomb Isospin Pairing

In 1939, Bohr and Wheeler applied this model to explain fission which occurs when the nucleus stretches and oscillates. For a liquid drop, of constant volume, deformaon informaon maers, so only the Coulomb and terms are important as the nucleus is stretched. 12 Fission Phenomenology and Observables Bohr and Wheeler Model of a Neutron-Induced Fission Event With the deformaon energy in the liquid drop as the difference between the surface and Coulomb terms, Bohr and Wheeler described the nuclear deformaon by a Legendre expansion about the

nuclear radius R(θ) with coefficients βn. In the (β2 – β4) plane they found a saddle point. Beyond that the minimum energy path slopes downward ≈10-21 s allowing the nucleus to break apart. The fissioning nucleus just has to have enough energy

Saddle to overcome the fission barrier Eb. β4 Scission Fission Path Ground State Eb is the height of the fission barrier Eb above the ground state.

E g.s. Energy

Fission Eb Barrier

Bjornholm, 1980 Path of deformaon Deformaon 13 Fission Phenomenology and Observables Fission Barrier Heights and Diagrams for 235U and 238U Formaon of 236U* and 239U* can lead to fission.

235U + n 239U* 236U* 238U + n

EB 5.6 MeV

Eb E Eex 6.3 MeV ex 6.5 MeV 4.8 MeV Fission Barrier (MeV)

236U 239U 235U + n has a higher 238U + n has a lower energy than the energy than the lowest fissionable state, so it lowest fissionable Fission Barrier Height variaon with mass number can fission with a zero- state, so it requires calculated from LDM for the most stable energy neutron addional 1.5 MeV at each mass number. The curve includes of kinec energy to shell structure. Myers Nucl Phys, 81, 1 (1966) fission 14 Fission Phenomenology and Observables A Simple Picture of a Neutron-Induced Fission Event

M(A,Z)

En mn

≈10-21 Time Scale in s ≈10-14 ≈10-14 ≈10-12 10-3 - ≈ ∞ Prompt Prompt Delayed Neutron Ground Saddle Scission Neutron Gamma and Gamma State Emission Emission Emission

2 Total available energy: Et = [M(Z,A) – ML(ZL,AL) – MH(ZH,AH) + mn] c + En

The fission fragments are generally le in an excited state and the energy is distributed between the kinec energy and excitaon energy, denoted as ETKE and ETXE.

Where ETXE is the total fragment resulng from the fragments changing to their equilibrium deformaon aer scission.

15 Fission Phenomenology and Observables Fission Energy Release 235 U thermal neutron fission

Approximate Prompt Energy Release MeV Fission Fragments 170 Fission Neutrons 5 Prompt γ emission 7 γ emission from fission fragments 7 β emission from fission fragments 8 ν from fission products 12 Total Energy per Fission 209

When resulng from more energec parcles the energy released by fission fragments has an energy dependence, shown here for post .

Excitaon energy dependence of the total kinec energy release in 235U(n,f) Yanez, 2014

See also Madland, 2006 16 Fission Phenomenology and Observables Fission has a complex set of observables, measurements of each of which provide informaon that helps us understand the overall process

• Energy Release • Fission cross secons and probabilies for different incident parcles and • Mulplicity, Energy, and Angle Distribuons of Neutrons and Gamma Rays from Fission • Fragment Kinec Energy, Mass, and Charge Yields and Angular Distribuons • Fragment Delayed Neutrons • Fragment Beta and Gamma Decay Processes • • Spin and parity distribuons of fission resonances • Parity violaon in fission resonances • Fission Time Scales • …

17 Fission Phenomenology and Observable Cross Secons t The interacon rate R for incident parcles on a area A surface of area A and thickness t depends on the intensity and the number of nuclei exposed to the incident parcles:

The intensity I (number of parcles/s) is defined in terms of a flux φ, or number of parcles/cm2/s flux φ striking an area A

3 I = φ . A (number of parcles/s) For Nt target nuclei with number density ρ (#/cm ) exposed to the incident parcles, then . . Nt = ρ A t The interacon rate R will be proporonal to the number of incident parcles (I) and . number of atoms (n) R ∝ φ Nt R ∝ φ . ρ . A . t R = σ . ρ . A . t . φ

σ = event rate per atom/incident flux with units of area It is the probability that that a parcle will interact with a nucleus, or the effecve cross seconal area of the nucleus. Units are barns or 10-24 cm2 Note: R = . N . σ t φ 18 Fission Phenomenology and Observables Neutron Induced Fission Cross Secons have a complex energy dependence

Thermal Region The cross secon generally varies as 1/v of the Resonance Region neutron. The accepted definion of the thermal Unresolved Resonance Region cross secon is for the value at 1/40 eV, 0.0253 Fast Region eV, or 2200 m/s.

235U(n,f) 239Pu(n,f)

582 b 743 b Cross Secon (b)

hp://www.nndc.bnl.gov/sigma/ ENDF/B-VII.1 19 Fission Phenomenology and Observables Neutron Induced Fission Cross Secons have a complex energy dependence

Thermal Region Characterized by narrow structure associated with Resonance Region energy levels in the compound nucleus. Experiments can resolve individual resonances and Unresolved Resonance Region the data is generally represented by resonance Fast Region parameters based on R-matrix theory.

235U(n,f) 239Pu(n,f) Cross Secon (b)

hp://www.nndc.bnl.gov/sigma/ ENDF/B-VII.1 20 Fission Phenomenology and Observables Neutron Induced Fission Cross Secons have a complex energy dependence

Thermal Region 235 Resonance Region Expansion of resonance region for U showing ENDF/ B-VII.1 evaluaon(solid line) and one set of Unresolved Resonance Region experimental data from Wagemans (1979) Fast Region

235U(n,f) 239Pu(n,f) Cross Secon (b)

hp://www.nndc.bnl.gov/sigma/ ENDF/B-VII.1 21 Fission Phenomenology and Observables Neutron Induced Fission Cross Secons have a complex energy dependence

Thermal Region In this region the cross secon sll fluctuates, but Resonance Region levels in the compound nucleus are too closely spaced to be resolved. The cross secon is Unresolved Resonance Region approximated by average resonance parameters Fast Region using stascal and level density models.

235U(n,f) 239Pu(n,f) Cross Secon (b)

hp://www.nndc.bnl.gov/sigma/ ENDF/B-VII.1 22 Fission Phenomenology and Observables Neutron Induced Fission Cross Secons have a complex energy dependence

Thermal Region Levels overlap so strongly that there are no longer Resonance Region fluctuaons, cross secons are smooth. They are Unresolved Resonance Region represented by stascal, intra-nuclear cascade, Fast Region pre-equilibrium, and evaporaon models.

235U(n,f) 239Pu(n,f) Cross Secon (b)

hp://www.nndc.bnl.gov/sigma/ ENDF/B-VII.1 23 Fission Phenomenology and Observables Mulple Chance Fission hp://www.nndc.bnl.gov/sigma/ ENDF/B-VII.1

Threshold 1.5 MeV 238U(n,f)

238U(n,f) 235U(n,f)235U(n,f) Cross Secon (b)

Second chance fission Second chance fission

Fast neutron induced fission is generally smooth with regular steps as the incident neutron energy increases enough to allow the compound nucleus to emit one neutron and sll have enough energy to fission (n,n’f). This is ‘second chance’ fission. At higher incident energies, ‘third chance’ fission becomes possible. 24 Fission Phenomenology and Observables Subthreshold fission Resonances

The simple fission barrier model of Bohr and Wheeler does not allow fission for neutron energies below a ‘threshold’ for nuclei like 237Np.

In reality, subthreshold fission resonances are observed, but with substanally lower cross secon values than for fissile nuclei.

235U(n,f) 237Np(n,f) Cross Secon (b)

hp://www.nndc.bnl.gov/sigma/ ENDF/B-VII.1 25 Fission Phenomenology and Observables Subthreshold fission Resonances

In 1967 Strunsky pointed out that the true fission barrier is more complicated than the simple LDM shape and added shell model correcons giving a double-humped barrier.

Secondary Potenal Ground State Minimum Potenal

Energy Minimum 237Np(n,f)

Deformaon

Having a double-humped fission barrier changes the fission barrier penetrability calculaon giving transmission resonances at energies corresponding to vibraonal states in the second Cross Secon (b) well and giving rise to subthreshold fission resonances

ENDF/B-VII.1 hp://www.nndc.bnl.gov/sigma/ 26 Fission Phenomenology and Observables Fission induced by other parcles than neutrons provide complimentary informaon Cross Secons from proton, neutron, and deuteron-induced Photon Induced Fission fission induced fission

) mb

“Nuclear fission studies with the IGISOL method and JYFLTRAP”, D. Gorelov Dissertaon (2015)

Cross Secon ( 27 Berman, 1986 Eγ (MeV) Neutron-Induced Fission Cross Secon Measurement Techniques Typical Fission Neutron Cross Secon Arrangements

Monoenergec Neutron Source

White Neutron Source

28 Fission Cross Secon Measurement Techniques Neutron Sources – mono-energec or quasi mono-energec

Mono-energec or quasi mono-energec neutron sources are generally made through light-ion reacons such as T(d,n), T(p,n), D(d,n), 7Li(p,n) and 9Be(p,n) and are generally good sources for fast neutron measurements. Inverse kinemacs such as H (7Li,n) can be used.

The most common accelerators used for mono-energec neutron producon are Van de Graaff machines, examples used for neutron physics are:

Facility Parcles Max Energy Pulse Width Typ. Beam Current Triangle Universies Nuclear Laboratory H,D 20 MeV 1 ns 1 – 5 µA (TUNL) at Duke University

Edwards Accelerator Laboratory H, D, 3He, 4He 9 MeV 1 ns 1 – 3 µA at Ohio University

Instute for Reference Materials and H,D 14 MeV 2 ns 1 – 3 µA Measurements (IRMM), Geel Belgium 29 Fission Cross Secon Measurement Techniques Neutron Sources – White Neutron Sources and Reactors

Mul-spectral neutron sources generally use energec beams of or electrons striking neutron producon targets. Neutron energies are then determined using me- of-flight techniques over relavely long flight paths to allow studies as a funcon of neutron energy.. Examples of neutron sources available to users for fission research:

Facility Parcles n Energy n Pulse Width Flight Paths LANSCE-WNR 800-MeV p 0.1- 600 MeV 0.5 ns 6-25 m

LANSCE-Lujan 800-MeV p Thermal – 0.1 MeV 125 ns 6-25 m

GELINA 100 MeV e- Subthermal – 20 MeV 1 ns 10 – 400 m

RPI 100 MeV e- 0.01 eV to 1 keV 6 ns 25 m

nTOF (CERN) 20 GeV p meV – GeV 6 ns 185 m

Facilies such as the Lohengrin spectrometer at the ILL reactor allow unique spectroscopic measurements of mass and isotopic yields for fission products. 30 Fission Cross Secon Measurement Techniques Types of Fission Fragment Detectors Proporonal Counters - most common technique Parallel plate chamber for fragment detecon Gridded to improve S/N and for angular informaon

Parallel-plate avalanche counter (PPAC) Low energy resoluon Very fast response (sub ns)

Gas scinllaon chambers Noble gas mixtures (90% 3He, 10% Xe) at pressure Tovesson, 2007 Fast response, Large solid angle

Solid-state silicon detectors PPAC Good pulse height informaon and ming Wu 2015 Good energy resoluon, Small solid angle

Time Projecon Chamber 31 Fission Cross Secon Measurement Techniques

Neutron Beam and Flight Path Consideraons Fission Fragment Detecon

• Time marker for pulse generaon t0 • Accurate determinaon of fission foil • Adequate Collimaon -Illuminaon of , isotopics, and uniformity fission mass uniformly without having • Adequate electronics for pulse height and neutrons striking structure ming and gain stability • Removal of potenal neutron beam • Adequate fission-fragment to decay alpha contaminants pulse height separaon Charged parcles, gamma rays • Cooling if needed to reduce Doppler Frame overlap for pulsed beam broadening in resonance region measurements • Incident neutron beam quality Uniformity, extent at experiment • Adequate shielding Tovesson, 2014 neutron beam dump Adjacent flight paths (sky shine) • Appropriate neutron flight path length

Parallel Plate ion Ionizaon Chamber Fission Fragment Response 32 Neutron Flight Path at WNR Fission Cross Secon Measurement Techniques Lead Slowing Down Spectrometer (LSDS)

Neutrons are produced by a mul-spectral pulsed neutron source inside a large block of lead. Useful for measuring fission cross secons of quanes of acnides to small for other techniques. This technique is especially useful for small (ng to µg)samples and highly radioacve materials over the energy range 0.1 eV to 100 keV

LSDS sources are possible at RPI, j-PARC, and WNR (Target-2):

LSDS at RPI Linac • Ta target at center produces neutrons • Neutrons slow down by elasc and inelasc scaering • Neutrons produce a broad high- intensity, 30-40% resoluon pulse • Neutrons traverse the detector mulple mes so the flux is 103-104 higher that that on a flight path of equivalent me-of-flight resoluon (5.6 m) 33 Fission Cross Secon Measurement Techniques Absolute fission cross secon data require an accurate neutron flux measurement • Use of reference cross secon standards H(n,p), 6Li (n,α) (www-nds.iaea.org/ standards/) • Black detector • Acvaon Foils • Associated parcle technique (monoenergec sources)

T(d,n)α Ed = 500 keV

Result for 235U(n,f) at 14.1 MeV

σf(b) = 2.080 ± 0.030

Wasson, 1982

34 Fission Cross Cross Secon Measurement White-Source Example Fission Fragment detecon Neutron flux monitor Shielded Neutron beam neutron E beam stop φ( ) Collimaon Fission Foil N . . In an ideal fission experiment the rate of events is given by: R(E) = σ (E) N φ(E) . . . . Collecng events for a fixed me: C(E) = ∫σ (E) N φ(E) dt = σ (E) N Φ(E) Where Φ(E) is the integrated neutron flux and σ (E) is the fission cross section . In a real experiment, the number of events is influenced by the system liveme (ω(E)), the efficiency with which the fission fragments are detected, η(E), and any . . . . background events Cb(E), so C(E) = ω(E) [ N σ (E) η(E) Φ(E) + Cb(E) ]

. . . For a pure sample, the cross secon is: σ (E) = [1/ω(E) C(E) - Cb(E) ] / [N η(E) Φ(E)] Any fissionable impuries in the sample contribung to the events counted. As a result:

. . . . . ι . C(E) = ω(E) [ η(E) Φ(E) N {σ (E) + ∑ (Ni σι (E)} + Cb(E)} ] where the ∑ (Ni σι (E) is the contribution from impurity fission cross sections. Then: . . . . σ (E) = [1/ω(E) C(E) - Cb(E) - ∑ (Ni σι (E)] / [N η(E) Φ(E)] 35 Fission Cross Cross Secon Measurement Rao Example Fission Fragment detecon Neutron flux monitor Shielded Neutron beam neutron beam stop φ(E) Collimaon Fission Foil N Fission Foil N1 2 In order to avoid the problem of absolute flux determinaon, fission cross secons are oen measured as raos to a standard fission cross secon, such as 235U. In that case:

. . . . σ1 (E) = [1/ω1(E) C1(E) – C1b(E) - ∑ (N1i σ1ι (E) ] / [N1 η1(E) Φ1(E)] . . . . σ2 (E) = [1/ω2(E) C2(E) – C2b(E) - ∑ (N2i σ2ι (E)] / [N2 η2(E) Φ2(E)] And the cross section ratio is: . . . . σ1 (E) (N2 η2(E) Φ2(E)) (1/ω1(E) C1(E) – C1b(E) - ∑ (N1i σ1ι (E)) = . . . . . σ2 (E) (N1 η1(E) Φ1(E)) (1/ω2(E) C2(E) – C2b(E) - ∑ (N2i σ2ι (E)) Although this looks like a complication to the absolute cross section measurement, it avoids the necessity of a completely independent determination of the neutron flux, and in a well-designed experiment, many of the quantities in the measurement result in small corrections 36 Fission Cross Secon Rao Measurements

238U(n,f) 237Np(n,f) U (n,f) Cross Secon Rao U (n,f) Cross Secon Rao 235 235 U/ Np/ 238 Paradela, 2015

237 Tovesson, 2007

• Rao measurements offer the best 234U(n,f) opportunity for low systemac error.

• For 238U/235U all recent rao data

are in good agreement U (n,f) Cross Secon Rao 235 U/ • There are sll differences of order 3 234 Tovesson, 2014 – 5% for many other rao data 37 Surrogate Cross Secon Techniques Using Charged Parcle Reacons • Many minor important to fission technology or to theorecal understanding of fission are hard to obtain in sufficient quanty or too short half-lives for a direct neutron measurement. • A light-ion charged parcle reacon is used to form the same compound system B* as in neutron-induced fission.

Neutron-induced reacon of Interest Surrogate Reacon d b a A D C C B* B* from Dirich, 2007 c c

• The decay of B* is detected in coincidence with the reacon product b. The energy of b fixes E*, the excitaon energy of B*. • Hauser Feshbach formalism is then used to correct for the different populaon mechanisms and produce esmated neutron-induced fission cross secons. 38 Surrogate Cross Secon Techniques Using Charged Parcle Reacons - Examples

Goldblum, 2009 Ressler 2011

Surrogate Rao Method Determinaon of Surrogate Rao Method Determinaon of 238Pu(n,f) 230Th(n,f) cross secon from 232Th(3He,αf) cross secon from 235U(α,α’f) and 236U(α, α’f) and 234U(n,f) reacons. reacons.

39 Time Projecon Chamber (TPC)

Important fast neutron fission cross secons such as those for 239Pu(n,f) have a 3-6% spread in values for experiments claiming 1-6% uncertaines. This potenally indicates unexplained systemac uncertaines.

Applicaons for both naonal defense and future fast reactor development require more confidence in the actual value.

By incorporang parcle tracking techniques developed for high-energy parcle detectors, the goal of the TPC collaboraon (NIFFTE) is to reduce systemac uncertaines Tovesson, 2010 in a credible way to 1% or less for important fission cross secons.

40 Time Projecon Chamber (TPC)

• The TPF uses a two-volume ionizaon chamber with a segmented anode plane allowing detecon of the track of an ionizing parcle

• Dri of the ionized gas to a segmented anode gives an x-y signature of the track, and arrival me provides z. im

• The ionizaon profile of the parcle track allows parcle idenficaon. neutron beam

• The geometry of the TPC allows nearly 2π coverage of parcles emied from the target. 41 Time Projecon Chamber (TPC)

• The TPC uses a two-part system for ionizaon tracking. Inially the track dris in a 500 V/cm region

• Aer driing 5 cm, the track passes through a micromesh and enters a region The amplified signal is with 5000 V/cm, detected on the anode plane creang an with 5952 hexagonal pads. electron avalanche amplificaon

42 Time Projecon Chamber

Ionizaon profiles of decay α and fission products allow parcle idenficaon and 2D analysis provides clean separaon between the fission fragments from other reacons such as with the argon-isobutane mulplying gas.

43 Time Projecon Chamber (TPC)

• Neutron energy informaon from me-of-flight

• Energy resoluon determined by 2 ns FWHM of target pulse

This gives:

En(MeV) ΔE(keV) % 1 6 0.6 5 70 1.4 10 198 2.0 15 365 2.4 20 565 2.8 44 Time Projecon Chamber (TPC)

• The TPC has the potenal to measure high-precision neutron-induced acnide cross secons with an unsurpassed ability to understand all features of the measurement in-situ in order to produce high-accuracy, precision results. • Parcle idenficaon • Target thickness and uniformity • Target isotopic content • Fragment emission angular distribuons

• The TPC has other potenal uses: • Fission cross secons for binary and ternary fission • Fission cross secon raos • Neutron-induced reacon cross secons • Improved data for cross secon standards such as H(n,p), 6Li(n,α), …

45 The Future of Fission Cross secon Measurements and Possible Improvements

• Improved confidence in our current knowledge of neutron-induced fission cross secons, including sufficient informaon to evaluate covariance data over all energy ranges, is needed for naonal security applicaons.

• Development of new fission applicaons for technology will be a driver for future fission cross secon measurements Generaon IV reactors and research on acnide burning systems require beer accuracy in both data and higher confidence in evaluaons. Some measurements may be possible with a LSDS for very small or highly radioacve samples.

• Connued use of the surrogate technique for acnide fission where samples are unsuitable for direct measurement

• The Time Projecon Chamber is a significant step forward in our ability to make accurate and precise measurements because of the increased ability to understand the target and detecon process in detail. Once fully developed, expanding the energy range of the TPC to lower energies would allow measurements in an energy range with significant naonal security and

civilian energy importance. 46