Determination of neutron induced fission fragment spin distribution after neutron evaporation

Transcription

1 Determination of neutron induced fission fragment spin distribution after neutron evaporation A. Chebboubi, G. Kessedjian, C. Sage, O. Méplan LPSC, Université Grenoble-Alpes, CNRS/IN2P3, F Grenoble, France H. Faust, U. Köster, A. Blanc, P. Mutti Institut Laue-Langevin, F Grenoble, France O. Serot, O. Litaize, D. Bernard CEA, DEN, DER, SPRC, Cadarache, Physics Studies Laboratory, F Saint-Paul-lès-Durance, France 1

2 Context of the fission yield studies for nuclear reactor Impact of fission yields on the current and innovative fuel cycles Inventory of spent fuel Isotopic composition Residual power Radiotoxicity of spent fuel Modeling prompt particle emission (n/γ) foreseen material damage/heating in the reactor studies Y(A, Z): High precision measurement new methodology P E, J π Y A, Z, E, J π = Y A, Z P(E, J π ) 2

3 Excitation Energy (MeV) How to evaluate Spin Distribution? Y th A, Z, E, J π = Y A P Z A P E A,Z P J π A,Z,E Isomer States filled by fission Neutrons Statistical (prompt γ) Discrete γ emission Spin(J) To study Spin distribution, look at γ prompt, n prompt or structure effect at low excitation energy : Isomeric Ratio Isomer : state with longer half-life than neighboring states 3

4 How to evaluate Spin Distribution? Y th A, Z, E, J π = Y A P Z A P E A,Z P J π A,Z,E P N isomer N GS A,Z,E k P γ prompt A,Z,E FIPPS (Fission Product Prompt gammaray Spectrometer) objectives : Direct measurement of the prompt particles ( γ prompt, n prompt ) Fission product spectroscopy (astrophysics interest) Neutron emission Short lifetime isomers (ps,ns) 4

5 How to evaluate Spin Distribution? Y th A, Z, E, J π = Y A P Z A P E A,Z P J π A,Z,E P N isomer N GS A,Z,E k P γ prompt A,Z,E Isomeric ratio : Recent measurements on Lohengrin Min/s Isomers : I, Sb, Sb, Sb, Sn, Nb, Y µs Isomers : Xe, Te, Sn, Sb, Y, Y, Y, Br ns Isomers : almost all isotopes in heavy mass region Older experiment IR E k : J.P. Bocquet et al, Physics and Chemistry of Fission 1979, IAEA H.O. Denschlag et al, Physics and Chemistry of Fission 1979, IAEA F. Gönnenwein et al, Int. J. Mod. Phys E16, targets 88 U & U 5

6 Experimental setup Lohengrin : selection with the mass over ionic charge ratios A q and Kinetic energy over Ionic charge E k q (A 1,E 1,q 1 ) (A 2,E 2,q 2 ) (A 3,E 3,q 3 ) Experimental Setup : Ionization chamber surrounded by 2 clovers of germanium The fission fragment after loosing 1/3 of their kinetic energy are implanted into an aluminum foil 6

7 Count rate extraction from γ spectra Ungated γ spectrum GS measurement Extraction more difficult because of the on S/B ratio Coincidence between ionization chamber and γ detectors : gated spectrum Isomeric state measurement ΔT Gate = 10T1 2 7

8 µs Isomer : Analysis Path Simplified scheme used for the analysis IR(E k ) = N f ( m Y) N f ( m Y) + N f ( GSY) Correction needed to get Isomeric Ratio : I γ : intensity of the γ line Efficiency Sum effect (not done yet) Resolution of Bateman equation Time of flight (from the target to the detection system) Energy loss trough the target and the foil Limits : Nuclear structure dependent 8

9 Propagation Uncertainty : Total Monte Carlo Method Simultaneous drawing over independent parameters I γ, time, λ free path, N count, according to a Gaussian law High count rate Low count rate pdf Gaussian law (for low count rate) σ syst ~ 6 % 1 % σ stat 55 % 9

10 Building of a Correlation Matrix IR (at a given energy E i ) expressed as a function of parameters : IR E i = f {a k } Sensibility definition : S ik = f i a k f a k a k 10

11 Building of a Correlation Matrix IR (at a given energy E i ) expressed as a function of parameters : IR E i = f {a k } Sensibility definition : S ik = f i a k Covariance IR E i, IR E j k Sensibility Variance a k + Covariance a l, a m Corr IR E i, IR E j = Cov IR E i, IR E j σ IR Ei σ IR Ej Isomeric Ratio Correlation Matrix of f a k a k Br for U Thin Target (2D & 3D) 11

12 µs Isomer : Preliminary results 233 U Thick 12

13 µs Isomer : Preliminary results 233 U Thick 235 U Thin 13

14 µs Isomer : Preliminary results 233 U Thick 235 U Thin 233 U Thin 14

15 µs Isomer : Preliminary results 233 U Thick 235 U Thin Target effect : Too thick 300 μg. cm 2 average IR Thin 100 μg. cm 2 structure with E k Target Mean IR 233 U Thick 0,33 ± 0, U Thin 0,29 ± 0, U Thin 0,28 ± 0, U Thin 15

16 µs Isomer : Preliminary results U Thin U Thick 235 U Thin 16

17 Deduction of a spin distribution FIFRELIN = Monte Carlo code simulating prompt fission neutron and γ-ray emission P E, J π In this work, FIFRELIN (developed by CEA Cadarache) is used only as a nuclear de-excitation code What is required for FIFRELIN : experimental level scheme Model of nuclear density to complete the level scheme Model of γ strength function For comparison with experimental results standard spin distribution: P J 2J + 1 exp J+1 2 J2 rms 2 Fission entry state : E, J π IR calculation using FIFRELIN Bayesian comparison with experimental IR 17

18 Results of the comparison with FIFRELIN 18

19 Conclusion Open question : how to explain angular momentum [4 5 ħ] of 132 Sn? Classical model, wriggle and bending modes, assumes that the spin generation mechanism is related to the deformation of the nucleus at the scission point. Then for spherical nucleus : J rms is expected equal to Sn is commonly admitted as a spherical nucleus at the scission point Collective modes are not the only spin generation mechanism! Thermal excitation? recent work on pumping mechanism predicts a J rms 0 Remnant of the dynamical effects from the saddle point to the scission point? 19