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

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

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.

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

(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 61 Ni 60 Ni + n (target) 59 Ni + 2n E n Co + p 57 Fe +   (J ,E x )  (J ,E x )  (J ,E x ) n p 

Los Alamos Neutron Science Center - LANSCE

LANSCE Neutron Sources cover 16 orders of magnitude in neutron energy Lujan center E n < 500 keV PSR Target-2 “Blue Room” Target-4 (Fast neutrons) LINAC 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 Proton storage ring To Areas A, B and C

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

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 Our incident neutron energy range is from 1 to 100 MeV High-energy tail Fission spectrum Fast neutron source spectra at LANSCE

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

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. 56 Fe(n,p) 56 Mn (2.579 hours) Need monoenergetic source (as above) Need a radioactive product – e.g. not 56 Fe(n,alpha) 53 Cr(stable) Not complete when other channels are open, e.g. 56 Fe(n,n’p) 55 Mn (stable); 56 Fe(n,n +alpha) 52 Cr(stable) Detect protons, deuterons, tritons, 3 He and alpha particles Monoenergetic source White source and time-of-flight techniques LANSCE

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

We choose detectors to give information on the complete charged-particle spectra 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 A large dynamic range of particles is detected. The range is defined by low energy helium ions and high energy protons

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

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

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

Measurements on nickel isotopes show problems with evaluated data libraries

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

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

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) ParameterPriorPosterior a( 58 Ni) (/MeV) a( 58 Co) (/MeV) a( 55 Fe) (/MeV)

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

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

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

Neutron emission

With gamma-ray detectors near the sample, we trigger off the prompt gamma-rays to study neutron emission FIGARO (n,xn+  ) n x 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 sample 22 m from WNR source Neutron emission contingent on one specific  -transition

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

Iron data are obtained by triggering on the lowest 2+  ground state gamma ray Dietrich noted that nearly all of the excited states in 56 Fe decay through the 847 keV 2+ state Fe 57 (target) 4 +  (J ,E x ) 56 Fe + n  (J ,E x ) n' Trigger E n

Iron data are being analyzed One neutron detector, binned in incident neutron energies 1–1.5 MeV MeV 2–2.5 MeV 2.5–3 MeV3–3.5 MeV MeV 4–4.5 MeV 5–5.5 MeV 6–6.5 MeV 7–7.5 MeV MeV MeV MeV13-15 MeV 4.5–5 MeV

Examples of preliminary data for 56 Fe 0.847

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’) Preliminary data for 56 Fe are encouraging

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

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

Path forward 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) Increasingly large set of data to test reaction models – Neutron reaction data complement charged-particle data – Will the fits be physics or parameterizations?

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 Applications motivate (fund) this work

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

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

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