Latest experimental results on (n,xnγ) on 235U and 238U

Transcription

1 Latest experimental results on (n,xnγ) on 235 and 238 CNRS-IPHC Strasbourg A. Bacquias, Ph. Dessagne, G. Henning, H. Karam, M. Kerveno, G. Rudolf, J.C. Thiry JRC-IRMM Geel J.C. Drohé, N. Nankov, M. Nyman, A. Plompen, C. Rouki IFIN-HH Bucharest C. Borcea, A. Negret, M. Stanoiu CEA DAM-DIF S. Hilaire, P. Romain niversité libre de Bruxelles S. Goriely niversity of Vienna A. Pavlik T Wien E. Jericha

2 Motivations 238 Reactions induced by fast neutrons RNR systems (GEN-IV, ADS...) fission neutrons in fuel (even PWR) Impact the neutron number and their Ecin reactor features (keff, βeff, Pr...) waste/dismantling (isotopic composition) 238 (n,n') Entry in HPRL for (n,n') on 238 Few data available (esp. between 1 and 14 MeV) Precision requirement for data bases (OCDE/NEA HPRL)

3 Method Three methods to measure (n,xn) cross-sections : - activation - neutron detection - prompt gamma spectroscopy Measured (n,xnγ) cross section Theoretical model for (n,xn) Spectroscopic Parameters Total (n,xn) cross section Lj E x ( Li ) E σ n ' ( E )= i=1 Observed γ The total inelastic scattering cross section is the sum of the cross sections carried by all transitions to the ground-state General case: p( Li g. s. ) σ n', γ ( E, Li L k ) p γ ( Li Lk ) Requires a good knowledge of spectroscopic parameters Practically, the deduced cross section is a lower limit for the total inelastic cross section model prediction Lk gs 0 A Li X

4 Method Three methods to measure (n,xn) cross-sections : - activation - neutron detection - prompt gamma spectroscopy Measured (n,xnγ) cross section Theoretical model for (n,xn) Spectroscopic Parameters Total (n,xn) cross section Measure as many (n,xnγ) transitions as possible (n,n') JEFF keV TALYS 103keV TALYS Total 238(n,n') cross-section 238 (n,n'γ) Eγ=45 kev (1er niv.) 238 (n,n'γ) Eγ=103 kev (2e niv.)

5 Experimental area IRMM, Geel, Belgium Pulsed electron beam on neutron production target (ranium) Bremsstrahlung (photofission; gamma flash) Wide energy-range neutron beam (few ev to 20MeV) FP16 30m

6 Experimental set-up Fission chamber (neutron flux determination) Sample (nucleus of interest) Germanium planar detectors (prompt gamma spectroscopy) Pulsed beam (800 Hz) Digital data acquisition (100 MHz): gamma energy Time of Flight (ToF) measurement 1.5 m beam

7 Data analysis Identify reaction rays Extract Nγ as a function of ToF radioactivity (n,n'γ) Gamma flash

8 Cross-section For one detector (at a given angle), for one gamma transition: N FC N 235 d n,xn = n,f d NFP 235 N sample Angle-integrated information using Gauss quadrature ncertainty sources : Number of counts (Ng, NFP)... ~1-20 % Efficiencies... ~1-2 % Neutrons loss... ~1 % Dead-time & pile-up... < 1 % Fission cross-section... < 1 % N ~ 0.5 % Samples figures : Total uncertainty budget (optimum) ~ 3 %

9 Challenges Number of target nuclei Apparent density of ranium samples ~ g/cm3 (pure metal : 19 g/cm3) Oxydation: O2, 3O8? Layers or bulk? simulations transmission measurements target laboratory (possibly destructive) methods

10 Challenges Weak transitions (low xs, high internal conversion) high statistics Over 1200 hours of beam time 238 raw data 238 corrected data Systematic gain correction

11 Challenges ToF determination for low amplitude signals «shift» corrected for Eγ < 150keV

12 Challenges 235 : peaks overlap (radio + FP) Example of 235(n,n γ) for Eγ = kev fit procedures (Root, gf3...)

13 Challenges 238 : first level de-excitation Crucial, but transition highly subject to internal conversion (α ~ 610)

14 Experimental results

15 235 (n,n'γ) Available data: Younes et al., LLNL report CRL-ID Model calculations: TALYS with various inputs & optical models (P. Romain et al.) Kerveno et al., PRC 87, (2013)

16 235 (n,2nγ) main band of 234 Data sets: Kerveno et al., PRC 87, (2013) Younes et al., LLNL report CRL-ID Hutcheson, PhD thesis, Duke niversity Theoretical calculations: TALYS for various inputs & optical models phenomenological approach works best few cross-sections, but pointing out description deficiencies

17 238 (n,xnγ) 3 transitions (n,2nγ) 4 transitions (n,3nγ) 36 transitions (n,n'γ) including 5 in the main band 6 to the ground state Wide variety of transitions Selective constraints on reaction mechanisms?

18 238 (n,n'γ) Cross-sections summed by initial level (no IC correction) ID number (GS=0) of analyzed levels spin*parity of analyzed levels

19 238 (n,n'γ) Groups of transitions can be formed... Groups by initial level: Total level deexcitation Branching ratios Relative gamma intensities (NNDC) : 635 kev : kev : kev : kev : kev : kev : kev : 72 complete level deexcitation observed for 10 levels

20 238 (n,n'γ) Groups of transitions can be formed... Groups by final level: in/out balance level production transitions feeding the first level transitions going to the ground state

21 238 (n,n'γ) Available data: 29 (n,xn γ) by Fotiades et al., PRC 69, (2004) 9 (n,xn γ) by Hutcheson et al., PRC 80, (2009) 6 (n,n' γ) by Voss et al., Kern. Karlsruhe reports, 2379 (1979) 28 (n,n' γ) by Olsen et al., Knoxville proc., p.677 (1979) 42 (n,xn γ) in our data set, comprehensive & coherent Theoretical models : TALYS & EMPIRE No systematic trend!

22 238 (n,2nγ) 238 (n,2nγ) kev 106 kev? 149 kev 155 kev peak contaminated by a fission product?

23 238 (n,3nγ) 238 (n,3nγ) kev 642 kev 160 kev 104 kev 104 kev peak still contaminated by and kev...

24 Outlook Next results : 232 Th 182,183,184,186 W Next measurements : 233 : Th-cycle dismantlement ; no (n,2n) xs data yet! 239 Pu : among CIELO's big 3 ; few (n,2n) xs measurements

25 Support Défi interdisciplinaire NEEDS

26

27 The 45 kev

28 Theoretical developments Different models, different options with impact on (n,n'γ) xs: - structure model - optical model potentials - spin distribution constraints from data - compound nucleus - direct scattering EMPIRE (R. Capote, M. Sin) new calc. coming (following latest evaluation) TALYS calc. (M. Dupuis et al. - CEA) with QRPA inelastic scattering model

29 Theoretical trends 235 : models can be ~ OK for fission & total inelastic and not for partial inelastic 235 total (n,f) xs 235 total (n,n') & (n,2n) xs

30 A few more cross-sections

31 Great agreement cases With EMPIRE From L12 at 967keV, the 2+ of the next 0+,2+,4+ sequence

32 Great agreement cases With TALYS From L17 at 1037keV, the 2+ of the next to next 0+,2+,4+ sequence