The surrogate-reaction method: status and perspectives. Beatriz Jurado, CENBG, France

Transcription

1 The surrogate-reaction method: status and perspectives Beatriz Jurado, CENBG, France 1"

2 Nuclear data for waste incineration and innovative fuel cycles Minor actinides incineration Th/U cycle Neutron-induced fission and capture cross sections of shortlived nuclei needed! Very difficult! 2"

3 Neglecting fluctuations: Compound nuclear reactions π σ E σ + ( E*) E J ) A A 1, ( *). form π ndecay CN Pneutr on( E*, J ) Pdec ay ( *, π J Formation cross section Can be calculated with the optical model, < 10% uncertainty Decay probability Difficult to calculate, structure information, level densities, strength functions, fission barriers Weisskopf-Ewing limit, high E*: π P ( E*, J ) P ( E*) decay σ σ + decay A A 1 ndec, ay ( E*) CN ( E*). Pde cay ( E* ) 3"

4 Neutron-induced reaction Surrogate-reaction method Cramer and Britt (Los Alamos 1970!!) w Surrogate reaction Transfer n + A (A+1)* X + Y Inelastic scattering Y A+1 + Y A A 1 surro" ndec, ay ( E*) CN ( E*). Pde cay ( E* ) σ σ + 4"

5 Validity of the surrogate method: limiting cases P Neutron-induced decay probability neutron d form π π, ecay ( E*) = Pn eutron ( E*, J ) Gdeca y ( E*, J ) Surrogate decay probability P surro J π form π π, decay ( E* ) = Ps ur ro( E*, J ) Gdecay ( E*, J ) J π form form P ( E*, J π ) = ( E*, J π ) neutron P surro P n,decay (E*) =P surro,decay (E*) if or π G ( E*, J ) = G ( E*) decay decay 5"

6 Validity of the surrogate method: intermediate situation 236U* Spin distrib. n+235u Jutta Escher, et al., Rev. Mod. Phys. 84 (2012) 353 If the surrogate reaction populates a spin distribution around 0-3 ħ it will give results that are very close to the n-induced results! We do not know a priory spin distrib. or the E* where WE starts --> comparison of surrogate results with n-induced data is required 6"

7 How to measure the decay probability in a surrogate experiment 3He + X p + (X+2)* d + (X+1)* 3He + X* 4He + (X-1)* P ( E*) = surro, decay singles ejec N coin ejec decay ( E*) N ( E*)Eff ( E*) decay Identification of ejectile E kin and θ of ejectile " Detection of decay particles Particle telescopes Fission or gamma detectors with known efficiency 7"

8 Experimental difficulties Precise ejectile E kin calibration Target quality : backing, contaminants (oxygen) (Polluted singles spectrum) Determining the eff. for detecting a gamma cascade Suppressing the gammas from fission " 8"

9 RE ( *) The surrogate ratio method A A+ 1 A+ 1 σndecay, ( E*) σcn ( E*) Psurrodecay, ( E*) = σ ( E*) σ ( E*) P ( E*) B B+ 1 B+ 1 ndecay, CN surrodecay, If nuclei A and B are close enough The same type of surrogate reaction N N B+ 1 singles A+ 1 singles ="F" Can"be"determined"from"beam" current,"target"thickness"and" experiment"live"ame" σ ( E*) = R( E*) σ ( E*) A B ndecay, ndecay, 9"

10 Comments on the surrogate ratio method Advantages: Solves problem of light target contaminants for fission J. Escher et al. predicted that it is less sensitive to spin/parity mismatch but It requires two targets of similar mass Ref. cross section should be well known It does not solve the problem of target contaminants for capture because contaminants may also emit gammas Experiments show that it does not attenuate the spin sensitivity at low E* Introduces additional uncertainties (for obtaining F, and reference cross section) " 10"

11 Modern surrogate experiments Jutta Escher, et al., Rev. Mod. Phys. 84 (2012) Th(n,γ)""""""""""""""""0I1.2"""""""""""" 232 Th(d,p)"""""""""absolute""""""J."Wilson"et"al."(2012)" 175 Lu(n,γ)""""""""""""""""0I1""""""""""""""" 174 Yb( 3 He,p)"""""absolute""""""G."Boutoux"et"al."(2012)" 172 Yb(n",γ)"""""""""""""""0I1""""""""""""""" 174 Yb( 3 He,α)"""""absolute""""""G."Boutoux"et"al."(2012)" " 11"

12 Stars-liberace set-up (USA) 12"

13 Experimental set-up at IPN Orsay Fission Detectors Fission Fragment Fission Detector Beam Target (x= p,d,t,α) Si Telescope E ΔE 13"

14 Experimental set-up at IPN Orsay ejectile Si Telescopes Beam Target C 6 D 6 detectors Germanium detectors 14"

15 Results"for"fission" 15"

16 Absolute surrogate method n+236u 238U(3He,α) Surrogate Ratio Method 238U(3He,α)237U n+236u 238U(3He,α) 235U(3He,α)234U B.F. Lyles, et al., Phys. Rev. C 76 (2007) "

17 Absolute surrogate method 3He(24 MeV) + 243Am (T 1/2 =7370 y)!! 241Am T 1/2 =432.6 y 242Cm T 1/2 =163 d G. Kessedjian, et al., Phys. Lett B 692 (2010) 297

18 Results"for"capture" 18"

19 Absolute 156 Gd(p,p ) N. D. Scielzo, et al., Phys. Rev. C 81 (2010) Ratio 158 Gd(p,p ) / 156 Gd(p,p ) 19"

20 Surrogate method applied to capture in rare-earth region 174Yb(3He,p)176Lu Spin distributions G. Boutoux, et al., Phys. Lett B 712 (2012) "

21 Why do we obtain such big differences? E*" S n =6.27MeV " J=7# n # 175 Lu# 15/2+#,#595#keV# 11/2+#,#251#keV# 9/2+#,#113#keV# 7/2+# 0" γ 176 Lu*# 7,# Due to the high spin of the decaying nucleus, neutron emission to the ground- and first excited states is highly improbable and gamma emission is highly enhanced! Things should get better when the level density of the nucleus after neutron emission increases --> better for actinides (See talks by Q. Ducasse and J. Wilson!) 21"

22 PerspecAves" 22"

23 Theory Complete Modelling of surrogate reactions " Objective: predict 1. Population of a highly excited unbound state in the continuum by a direct reaction 2. Damping of this state into a compound nucleus I. Thompson, F. Dietrich, J. Escher, LLNL, Livermore, USA This can be achieved in the near future for (d,p), (p,d) and (p,p ) Reactions interesting for experiments in inverse kinematics with RIBs Results already available for (d,p), important probability that the transferred neutron escapes before CN formation (See talk Q. Ducasse) I. J. Thompson, J. Phys. Conf. Ser. 312 (2011) "

24 Perspectives for experiments New targets, new nuclei Z 241 Cm 243 Bk 244 Bk 245 Bk 241 Cm 241 Cm 242 Cm 243 Cm 244 Cm 245 Cm 238 Am 239 Am 240 Am 241 Am 242 Am 243 Am 244 Am 236 Pu 237 Pu 238 Pu 239 Pu 240 Pu 241 Pu 242 Pu 243 Pu 235 Np 236 Np 237 Np 238 Np 239 Np 240 Np 241 Np 242Am, T 1/2 G.S. = 16 h 240Am, T 1/2 =2.1 d N Incineration 241Am Target 242 Pu Target 240 Pu Targets 240,242 Pu NEA high priority list Targets not available yet!! 24"

25 Perspectives for experiments Inverse kinematics High-Intensity and Energy-ISOLDE (CERN) Actinides beams (Ac, Th, Pa) at 10 A MeV (d,p),(p,d),(p,p ) surrogate reactions 25"

26 Perspectives for experiments ELISe (ELectron Ion Scatering experiment) (e,e ) with radioactive beams: Unique! e e A A* Surrogate reaction: e,e Well defined fissioning nucleus (E*,spin!!) P f (E*)!σ f (E n ) with equivalent E n =0-20MeV Highly precise fission-fragment isotopic yields Preparation experiment SOFIA (2012) 26"

27 Nuclei that can be studied at F-ELISe 27"

28 Conclusions Experiments to test the surrogate " method: Fission: USA collaboration finds good agreement for En > 1-2 MeV CENBG data are in very good agreement for En > 0.5 MeV Capture: For medium mass nuclei important discrepancies are found even with the ratio method. One should also consider systematic errors like (E calibration or contaminants) when comparing different experimental results. For n-induced cross sections that we will never be able to measure directly: If we know we can: -Try to find the appropriate surrogate reaction -Try to correct surrogate results for the spin/parity mismatch (Younnes et al.) -Use the surrogate results to constrain statistical model parameters (already done by evaluators in ENDF) 28"

29 Difficulties in measuring cross sections of very short-lived nuclei Fabrication, import and transport (Strong radioprotection constrains) Handling during the experiment (Strong radioprotection constrains) Reduced target thickness (Low statistics or long measurement times) Strong background (Alpha particles, gammas, spontaneous fission ) 29"