Robert Haight LANSCE-NS Workshop on Statistical Nuclear Physics

1. LA-UR-08-4399 Statistical Neutron-Induced Reactions Studied by Neutron, Proton, and Alpha-Particle Emission Robert Haight LANSCE-NS Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology Ohio University July 8-11, 2008

2. Outline • Context • Neutron-induced reactions • Charged particle emission • Neutron emission • (Gamma-ray emission) • Concentrate around A ~ 56

3. Context – history, data bases, previous works • Lots of data on charged-particle induced reactions • Protons, 3He, alphas, heavy ions, etc. • Emission spectra, angular distributions, etc. for charged particles and neutrons • Major experimental efforts in the 1960’s, 1970’s; continuing at lower intensity through the present time • Major analyses of data • Gilbert & Cameron • Backshifted Fermi Gas • E.g. Vonach, Dilg, etc. • Superfluid models • Neutron-induced reactions • Lots at 14 MeV incident energy • Some at other energies • Evaluated data files – ENDF, JEFF, JENDL, BROND, etc.

4. Why then study more neutron-induced reactions? • Applications • Neutron transport for many applications • Radiation damage in fast fission reactors (AFCI, GNEP) and fusion reactors of the future from (n,H) and (n,He) reactions– a.k.a. “Gas Production” • Requirements on accuracy of data • Basic physics • Learn more about reaction models, level densities • Other data (e.g. total cross sections, known very well) constrain reaction models • Reactions can be studied over a wide range of incident energies in the same experiment – use “white” neutron source

5. 11.388 r p (J ,E ) x 59 Ni + 2n n r p (J ,E ) x p r a p (J ,E ) x E n + 4 2.506 2.042 + 2 1.332 60 Co + p + 0 0.000 -1.354 60 Ni + n a 57 Fe + (target) -7.820 61 Ni (n,xp) and (n,xalpha) reactions are in competition with neutron emission (n,n’), (n,2n), etc. • Physics: • Optical model for transmission coefficients • Nuclear levels • spectroscopy • level densities • We measure as a function of incident neutron energy • Traces out competition with excitation energy • Insights into non-statistical reactions, e.g. direct and pre-equilibrium

6. Los Alamos Neutron Science Center - LANSCE

7. LANSCE Neutron Sources cover 16 orders of magnitude in neutron energy To Areas A, B and C LINAC Proton storage ring • Lujan - ultra-cold to epithermal neutrons up to ~500 keV • Target 2 - UCN to fast neutrons; also protons • Target 4 – Fast neutrons from 0.1 to 800 MeV PSR Target-2 “Blue Room” Lujan center En < 500 keV Target-4 (Fast neutrons)

8. We use the 30-degree flight path at Target 4 (WNR) Proton beam Charged-particle emission @ 15. 1 meters “NZ” Neutron emission @ 22.7 meters “FIGARO”

9. Our incident neutron energy range is from 1 to 100 MeV • Neutron energy range can be studied in one experiment • Covers energies of statistical reactions – up to ~ 10 MeV • Covers much higher energies where direct and other pre-equilibrium reactions become important Fast neutron source spectra at LANSCE High-energy tail Fission spectrum

10. Light charged-particle emission p, d, t, 3He, alpha

11. Many approaches have been used to measure charged-particle emission in reactions induced by fast neutrons • Gas accumulation: irradiate and then measure by mass • spectrometry • only for Helium (hydrogen contamination is everywhere) • need a monoenergetic neutron source or the result is an • average over the spectrum • Activation: e.g. 56Fe(n,p)56Mn (2.579 hours) • Need monoenergetic source (as above) • Need a radioactive product – e.g. not 56Fe(n,alpha)53Cr(stable) • Not complete when other channels are open, e.g. • 56Fe(n,n’p)55Mn (stable); 56Fe(n,n +alpha)52Cr(stable) • Detect protons, deuterons, tritons, 3He and alpha particles • Monoenergetic source • White source and time-of-flight techniques LANSCE

12. Charged particles emitted in the reactions are identified by DE detectors and their energies are determined by stopping detectors of silicon or CsI(Tl)

13. We choose detectors to give information on the complete charged-particle spectra A large dynamic range of particles is detected. The range is defined by low energy helium ions and high energy protons • Low pressure proportional counters allow identification of helium ions to below 3 MeV • Silicon detectors stop alpha particles up to 33 MeV • CsI(Tl) scintillators – 3 cm thick -- stop 100 MeV protons

14. 59Co(n,xalpha) angle-integrated emission spectra are described well by calculations Ref: S. M. Grimes et al., Nucl. Sci. Eng. 124, 271 (1996)

15. Excitation function for 59Co(n,xalpha) is described well by calculations up to > 20 MeV

16. However, level density parameters needed to be modified to fit the 59Co(n,xalpha) data Gilbert & Cameron systematics

17. Measurements on nickel isotopes show problems with evaluated data libraries

18. Peter Fu analyzed the differences in the evaluated cross sections for 58Ni(n,alpha) -- 1995

19. Fu’s analysis shows variations in the used level densities Uhl / Fu BSFG / GC Ratio of level densities

20. Low energy 58Ni(n,alpha) data (Tohoku) could be fit well • T. Kawano, et al., J. Nucl. Sci. Tech. 36, 256 (1999) • Baysian analysis (KALMAN)

21. The situation with 60Ni is similar with regard to evaluated data libraries

22. Recent results for iron also show a problem with the ENDF evaluation

23. Results for hydrogen production are in agreement with ENDF and also confirm LA150 evaluation up to 50 MeV

24. Neutron emission

25. With gamma-ray detectors near the sample, we trigger off the prompt gamma-rays to study neutron emission • 20 Neutron detectors • “Double time-of-flight“ experiment • Incident neutron energy from TOF from souce • En’ emitted from TOF ~ 1m • Neutron emission in coincidence with gamma rays FIGARO (n,xn+g) n x sample 22 m from WNR source Neutron emission contingent on one specific g-transition

26. Nickel data were described well with EMPIRE calculation, with modified level density 58,60Ni(n,n’) (natural elemental isotopes) Ref: D. Rochman, Nucl. Instr. Meth. in Phys. Res. A523, 102 (2004)

27. r r (J (J p p ,E ,E ) ) n' x x E n + 2 2.657 + + 4 2.085 2 + Trigger 0.847 0 + 56 Fe + n (target) 57 Iron data are obtained by triggering on the lowest 2+  ground state gamma ray • Dietrich noted that nearly all of the excited states in 56Fe decay through the 847 keV 2+ state Fe

28. 1.5-2 MeV 1–1.5 MeV 2–2.5 MeV 3.5-4 MeV 2.5–3 MeV 3–3.5 MeV 5–5.5 MeV 4–4.5 MeV 4.5–5 MeV 6–6.5 MeV 5.5-6 MeV 6.5-7 MeV 7–7.5 MeV 7.5-8 MeV 13-15 MeV Iron data are being analyzed One neutron detector, binned in incident neutron energies

29. Examples of preliminary data for 56Fe 0.847 0.847 0.847

30. Preliminary data for 56Fe are encouraging • Emission spectra are measured as a function of incident neutron energy • Only part of data are analyzed so far (one of 3 gamma-ray detectors) and better statistics are on the way • Energy resolution is good for neutrons of a few MeV • Gating on other gamma-rays is possible to test angular momentum distribution of states populated by (n,n’)

31. Systematics give an estimate of accuracy of level density inputs to calculations Show distribution of “a” Ref. Dilg et al., Nucl. Phys. A217, 269 (1973)

32. Some observations • Emission data (p, alpha, n,…) neutron-induced reactions can be described by statistical reactions with suitable parameter selection. • Can these data be predicted ab initio with confidence from global or other parameters? • Competition among reaction channels can reduce the errors in the calculated results, but, given “bad” parameters, it is hard to predict the outcome. • Users need data to some accuracy

33. Path forward • Increasingly large set of data to test reaction models • Neutron reaction data complement charged-particle data • Will the fits be physics or parameterizations? • New approach: Measure and model neutron emission spectrum contingent on the following gamma cascade going through a given level (or set of levels) -- angular momentum selection -- need new reaction model code (Monte Carlo HF)

34. Applications motivate (fund) this work • To provide data for GNEP – “Gas Production” by neutrons on structural and other materials – e.g. Fe, Cr, Ni, Zr, Ta, W etc. • The cross sections are “source terms” for assessing radiation damage of materials • Gas production is an important component of radiation damage in materials irradiated to high fluences in advanced fuel concepts. • Other applications • Neutron interrogation – transport though containers, etc. • Shielding • Fusion • Criticality safety • Detector development. For example, neutron output detection same as for fission neutrons

35. 0.5 Tmelt Effects of Helium are observed at temperatures above 0.5 Tmelt Copper Thanks to Stuart Maloy

36. Summary • Increasingly large data set for nucleon-induced reactions on nuclides with A ~ 56 can be used to test reaction model calculations • Charged- particle emission • Neutron emission • Gamma - ray emission • Model calculations can describe data if suitable parameters are used • Nuclear level densities are the largest uncertainty in the reaction model calculations • Some widely used evaluations for helium production are in disagreement with our results … and with others. • Evaluations for hydrogen production are in somewhat better shape

38. An aside -- higher energy data for iron test new evaluations – different physics at higher energies