Measurement of (n, xn γ) reaction cross sections in W isotopes

Transcription

1 Measurement of (n, xn γ) reaction cross sections in isotopes Greg Henning IPHC/CNRS 1

2 Context: Nuclear data evaluations for applications 4 th generation nuclear reactors and new fuel cycles, currently understudy, will met safety, sustainability, environment and economic challenges. Development and risk analysis are done with numerical simulations that use evaluated nuclear data bases. Evaluations are built from experimental data and theoretical predictions. These databases still present large uncertainties, because of a lack of data and limits of models. 238 U(n,n') Generation IV International Forum, % of total Uncertainty on k eff in GFR σ keff = 1.88 % (NEA/PEC-26) This leads to uncertainty on calculations of control parameters in nuclear applications( k eff, power,...) 238 U 239 Pu 2

3 (n, xn) : reactions of interest for evaluations Impact on reactors: Residu Change the number and energy of neutrons, create new isotopes. Nucleus Neutron Fundamental interest: The mechanism probes different reaction regimes Direct nucleon-nucleon interaction, Compound nucleus, Pre-equilibrium. Compound nucleus Pre-equilibrium Emitted neutrons Direct Experimental study: σ Detection of scattered neutrons, Activation, (n, xn γ) reactions strong constrains on models Reaction mechanism, Nuclear de-excitation, Nucleus level structure. Reaction time E out 3

4 hy study Tungsten? Practical interests: Present in many alloys in reactors (fission and fusion). High melting point, strong mechanical resistance, low thermal expansion, resist to oxidation, acids, alkalines. Theoretical interests: Deformation similar to uraniums. No fission channel (B f LD ~ 20 MeV) simpler description by models. Easier experiment to setup (compared to actinides): No radioactivity, chemical toxicity, 3 times less expensive than nat U (raw material price). Defomation (beta_2) 0,26 0,24 0,22 0,2 Applications use natural tungsten. Need to study 182 (26.5 %), 183 (14.3 %), 184 (30.6 %), (28.4 %) 4

5 hy study Tungsten? Only a few measurements available today, the majority of points comes from one experiment only. Most on (n,2n), some (n,3n) 182 (n,n') (n,2n) (n,3n) ENDF/B-VII.1 JEFF-3.2 JENDL-4.0 FENDL/E-2.1 Available data (J. Frehaut et al., 1980) 184 Level production cross sections exist for (n,n') 5

6 Neutron beam at GELINA (IRMM, Geel, BE): Electrons accelerated on Uranium target, Neutron production by fission, Pulsed beam (800 Hz), E n between kev and 20 MeV, Our experiment set up at 30 m. Experimental study of (n, xn γ) reactions GELINA HPGe detectors pulsed neutron beam γ γ Fission chamber to measure neutron flux, Large sample (diameter > 55 mm), Detection of γ rays emitted in (n, xn) reactions using 4 planar Ge detectors, Connected to a digital acquisition recording time and energy in list mode. Neutron beam fission chamber γ sample 6

7 Analysis principle: vs. ToF Target : 232 Th Radioactivity 232 Th(n,n ) 232 Th [arb.un.] 232 Th(n,2n ) 231 Th 232 Th(n,3n ) 230 Th Time of Temps Flight de [arb.un.] vol [arb.un.] Flash E n (ToF )=[ 1 1 ( DoF/ToF c 2 1]m ) 2 nc 7

8 Analysis principle: (n, xnγ) cross section extraction The intensity of the gamma line is determined for different time intervals kev Number of hits 110 deg kev Time Of Flight [ns] Neutron energy [MeV] No correction for electronic conversion «true (n, xn γ)», The ratio to the flux gives the production cross section. d σ d Ω ( E n, γ ; θ)= N γ(e n, γ ;θ) ε( ) 1 σ (E 235 U (n, f ) n)ε CF N target N CF ( E n ) 8

9 Structure of Tungsten Focus on even-even isotopes: 182, 184, Studied at the same time in nat sample Rotational band 2 + (γ), 0 + (β) bands

10 Ground state rotational band PRELIMINARY Experiment TALYS-1.2 (P. Romain, 2011) 182 =100 kev 184 =111 kev =122 kev 2 + g.s. Cross section [barn] Correction for 183 (n,2n) TALYS over estimates gsb contribution at high E n =253 kev =273 kev =229 kev Incident neutron energy [kev] 10

11 Experiment TALYS-1.2 (P. Romain, 2011) 182 =1121 kev Interband transitions 184 =792 kev PRELIMINARY =615 kev Mixed multipolarity M1+E2 Cross section [barn] PRELIMINARY =1221 kev TALYS under estimates inter band transitions =903 kev =738 kev g.s. Incident neutron energy [kev] 11

12 Effect of reaction mechanism? PRELIMINARY Decomposition of (n,n') cross section into contribution of individual levels reproduces well (2011) experimental values (w/ +15 % scaling) up to ~ 4 MeV. Cross section [mbarn] 2 + level, direct inelastic 2 + level, CN inelastic Contribution of scattering off individual states Contribution of scattering off continuum Experimental 2 + cross section D.Lister et al., 1967 P.T.Guenther et al., 1977 TALYS 2 + level cross section Incident neutron energy [MeV] P. Romain (CEA Bruyères-le-châtel) 12

13 Effect of reaction mechanism? Bad reproduction above ~5 MeV.? Decay channels competition,? Effect of spin distribution in pre-equilibrium,? Separation and coupling between continuum and discrete level scheme. PRELIMINARY Cross section [mbarn] Cross section [mbarn] P. Romain (CEA Bruyères-le-châtel) Incident neutron energy [MeV] Incident neutron energy [MeV] 13

14 Conclusion and perspectives First results from preliminary analysis on 182,184, in nat : TALYS over estimates population of high spin states in the gsb and under estimates inter band transitions. Also, bad reproduction of cross section shape in 2 + g.s. at high E n. Possible effects of structure and/or pre-equilibrium model. Extraction of level cross section would help to distinguish BR effect (and comparison with existing data), but only an upper limit is possible. Perspectives: 5 consistent data sets ( nat,182,183,184, ) to analyze cross check and normalization, transitions to study per isotope, Covariant analysis to reflect correlations between the measurements, will produce a very rich and constraining set of experimental values to compare with the models. Part of larger (n, xn γ) study program on isotopes from 7 Li to 238 U in collaboration with IRMM, IFIN HH, CEA/DAM, CEA/DEN. 14

15 Collaborators Greg Henning, A. Bacquias, Ph. Dessagne, M. Kerveno, G. Rudolf, P. Scholtes, IPHC/CNRS & Université de Strasbourg, France A.J.M. Plompen, F. Belloni, M. Nyman, E. Pirovano, J.C. Drohé, R. ynants,. Motta, G. Sibbens, A. Moens, S. Melis, EC/ JRC-IRMM, Geel, Belgique C. Borcea, A. Negret, A. Olacel G. Suliman Nat. Inst. Of Phys. And Nucl. Eng., Bucharest, Roumanie P. Romain, M. Dupuis, S. Hilaire, B. Morillon CEA Bruyères-le-châtel D. Bernard, P. Lecomte, C. de Saint Jean CEA Cadarache NUDATRA EUFRAT NUDAME ERINDA ANDES CHANDA GEDEPEON NEEDS 15