Recent developments in neutron capture on actinides using the DANCE detector

Transcription

1 Recent developments in neutron capture on actinides using the DANCE detector J.L. Ullmann Los Alamos National Laboratory Sixth Workshop on Nuclear Level Density and Gamma Strength. Oslo, Norway, May 8 12, 2017 LA-UR

2 Other DANCER s B. Baramsai, T.A. Bredeweg, A.J. Couture, R.C. Haight, M. Jandel, T. Kawano, A.L. Keksis, S. Mosby, J.M. O Donnell, R.S. Rundberg, G. Rusev, D.J. Vieira, C. Walker, J.B. Wilhelmy Los Alamos National Laboratory C.-Y. Wu, J.A. Becker Lawrence Livermore National Laboratory A. Chyzh, G.E. Mitchell North Carolina State University M. Krticka Charles University, Prague Slide 1

3 Capture cross-sections and spectra important Neutron capture cross sections and gamma-emission spectra of importance to basic science and many applied fields Stellar capture cross sections calculated with the statistical model code NON-SMOKER compared with available experimental data. (From Rauscher and Thielemann, Atomic and Nuclear Astrophysics, IOP, 1998) Capture cross sections difficult to calculate, rely on measurements Careful measurements have been made on most common stable nuclides Must rely on calculations for rare or unstable nuclides Benchmark calculations against measurements Cross sections Gamma-ray spectrum Slide 2

4 Detector for Advanced Neutron Capture Experiments Nearly 4π coverage ( 3.5π) 160 BaF 2 crystals - 4 different shapes High efficiency - milligram samples Highly segmented - radioactive targets 6 LiH inner sphere to absorb scattered neutrons m from water moderator LANSCE spallation source 800 MeV 0 µa Neutron Flux Φ = 550E n/cm 2 /ev/to at 22.6 m Slide 3

5 DANCE Calorimeter Calorimetric => Summed Gamma Energy Q value (S n ) Permits ID of target! E x = T n (1+M n /M A )+Q T neut = 1 to kev 138 Ba(n,γ) Q = 4.72 MeV 134,136 Ba(n,γ) Q = 6.9 MeV 135,137 Ba(n,γ) Q = 9.11,8.62 MeV Au(n,γ) Q = 6.51 MeV Slide 4

6 Calculations some formula s Hauser-Feshbach Gamma transmission coefficient Normalized γ transmission (s-wave) DICEBOX Slide 5

7 238 U(n,γ) Spectrum Simulations Kopecky and Uhl (Phys. Rev. C 41, 1941 (1990) ) f = GLO + SLO(SF) + SLO(E2) 6 5 Level density von Egidy CT PRC72 SLO (Standard Lorentzian) 2 f SLO 1 σ (ε γ ) = R ε γ Γ R 3π 2 (!c) 2 (ε 2 γ E 2 R ) 2 +ε 2 2 γ Γ R GLO (Generalized Lorentzian) f GLO (ε γ ) = σ R Γ R ε γ Γ K (ε γ,t ) 3π 2 (!c) 2 (ε 2 γ E 2 R ) 2 +ε 2 γ Γ 2 K (ε γ,t ) Γ K (0,T ) 3 E R ) -1 ρ (MeV PSF (MEV^-3) γ 6 = Q - E x (MeV) 238 U E1 Photon Strength Function Γ K (ε γ,t ) = (ε γ 2 + 4π 2 T 2 ) Γ R E R 2 T = S n ε γ a 8 E1 SLO Berman Egam (MeV) E1 GLO Berman S n = 4.8 MeV

8 238 U(n,γ) M=2 Gamma-ray spectrum Kopecky and Uhl (Phys. Rev. C 41, 1941 (1990) ) U(n,γ) M cl =2 21 ev res 36 ev res 66 ev res 2 ev res GLO No M1 GLO with M1 f XL = GLO(E1) + SLO(SF) + SLO(E2) counts 00 (SF and E2 Parameters from RIPL) 500 DICEBOX calculation γ-ray Energy (MeV) M cl =2 GLO No M1 GLO with M1 300 MC cascades -> GEANT model ( forward modeling ) counts Realizations band γ-ray Energy (MeV) Slide 7

9 Oslo-method strength function Fit to GDR (photoneutron yield) Oslo method f xl 2 component M1 Guttormsen, Phys. Rev. C 89, (2014)

10 234,236,238 U(n,γ) Gamma Ray Spectra

11 Cross-section calculation with CoH 3 Cross section (barns) U(n,γ) Cross section 4 Energy (ev) 238 U(n,γ) 5 PRC 89 Full calculation Full calc, default ρ Kopecky-Uhl (GLO + GT) K-U, Norm to Γ γ / D 0 6 GDR: Herman/EMPIRE (CoH 3 ) ρ: Gilbert-Cameron (Kawano, J. Nucl. Sci. Tech. 43, 1 (2006)) Scissors: E and Γ from Guttormsen, Phys. Rev. C 89, (2014); Strengths adjusted M1(SF), E2: RIPL-3 Cross section (barns) Cross section (barns) Cross section (barns) U(n,γ) ENDF/B_VII.1 CoH U(n,γ) ENDF/B_VII.1 CoH 3 Adamchuk Buleeva Carlson 238 U(n,γ) Ullmann CoH 3 with SM CoH 3 No SM ENDFB/VII-1 4 Energy (ev) 5 Slide

12 Sensitivity of <Γγ> to parameters Calculate <Γ γ > with different ρ, strength function parameters Vary ρ Von Egidy CT PRC 72 Von Egidy CT PRC 80 Kawano GC JNST 43 Vary strength fcn Mughabghab CoH 3 parameters from M. Herman, EMPIRE INDC-0603 (2013) Vary E1 GDR parameters (NLD, M1, E2 fixed) CoH 3 Veyssiere (Nucl.Phys. A227, 513 (1974) ) Gurevich (Nucl. Phys. A273, 326 (1976) ) Dietrich (At. Data Nucl. Data Tab. 28, 299 (1988) ) E1 = 6 mev M1 + E2 = 16 mev

13 Generalization to all nuclei E sm = 80 β 2 A 1/3 MeV σ sm Γ sm = K β 2 2 mb-mev (Γ = 1.5 MeV) K = 42.4 ± 5.0 (Fit to 6 FP s A < 200) Kawano, APS DNP 2015 Mumpower, Kawano, et al., submitted to Phys. Rev. C Slide 12

14 Compare measured <Γγ> to Calculated <Γγ> calculated with parameters on previous slide Actinide region was not used in determining K Kawano, APS DNP 2015 Mumpower, Kawano, et al., submitted to Phys. Rev. C Slide 13

15 239 Pu capture gamma-ray spectra ESum Mult 2 239Pu ev 1+ Egamma (Q-Gated) Mult 2 239Pu ev All Fission Empty All Fission Empty Energy (MeV) 239 Pu(n,γ) S n = 6.53 MeV Fission tagging with PPAC Analysis underway ev 1 + resonance Energy (MeV) Egamma Subtracted Mult 2 239Pu ev Energy (MeV) Slide 14

16 Results Shape of capture cross section vs neutron energy is not sensitive to exact form of strength function (although magnitude is) Kopecky-Uhl prescription (GLO (E1) + SLO (M1SF) + SLO (E2) ) is not sufficient to describe shape of observed gamma-ray spectra Additional strength at low energies (2 ~ 3 MeV) likely M1 Scissors Mode- is required Accurate calculations of capture cross section for 234,236,238 U can be made when including SM with proper choice of ρ and PSF Extending study to odd-mass actinides ( 235 U, 239 Pu) and additional nuclei

17 Extra slides Slide 16

18 Los Alamos Neutron Science Center (LANSCE) 17

19 Neutron Research facilities at LANSCE Weapons Neutron Research Facility Target-4 High-energy neutron research Proton Radiography Linear Accelerator NRAD Line B UCN Area A (inactive) Line D Lujan Center Target-2 - Proton-induced reactions - Single-pulse experiments DANCE Lujan Center Low-energy neutrons - Material science - Nuclear science 90R 5 60R 30R 15R Target 4 15L PSR 30L ER L 90L 9 11A/B ER-2 Target 2 Target 1

20 Uranium resonances 234 U(n,γ) U(n,γ) l=0 ½ + Counts 238 U(n,γ) Neutron Energy (ev) Slide 19

21 Cross Sections [mb] Caldwell (g,n) (g,2n) (g,3n) (g,f) (g,xn) Guttormsen ENDF/B-VII Photon Energy [MeV] 0.2 discrete levels below 1 MeV σ = 4.19 σ = U Spin Distribution Spin [h-bar]