SURROGATE REACTIONS. An overview of papers by Jason Burke from LLNL

Transcription

1 SURROGATE REACTIONS An overview of papers by Jason Burke from LLNL

2 Compound Nuclear Reaction cross sections Cross sections for compound-nuclear reactions are required input for astrophysical models and nuclear energy simulations Astrophysics Explain origin of heavy elements (A>56) s-process, r-process Nucleosynthesis in stellar evolution Nuclear Energy Advanced reactor designs that may recycle actinides Capture and fission reaction cross sections on Th, U, Pu, and actinides National Security Nuclear explosive device performance Escher et al. 2012

3 The Surrogate Idea Provide indirect information on cross sections that are difficult to measure or calculate accurately. n a 237 U t 1/2 =6.75 days 238 U 238 U t 1/2 =4.47x10 9 yr 238 U a fission fission The desired compound nucleus is produced by means of an alternative direct reaction and the desired decay channel is observed in coincidence with the outgoing particle. Assumes formation and decay of the compound nucleus are independent. Escher et al. 2012

4 Surrogate Methods Weisskopf-Ewing limit approximation Absolute surrogate method Assume branching ratios are independent of angular momentum and parity. σ E a = σ CN E ex P δ (E ex ) The cross section of forming the compound nucleus is estimated with an appropriate optical model Branching ratios are calculated directly my measuring coincidence probabilities Ratio Approach Requires approximate validity of Weisskopf-Ewing limit Determine the ratio of two cross sections from two separate surrogate experiments R E = C N δ1 N δ2 = σ 1 σ 2 Deduce one cross section from a well known cross section First order violations of W-E cancel No need to measure total surrogate reactions

5 ΔE (Counts/7 kev) Experimental Approaches to Studying the Fission Process Using the Surrogate Technique SARS - LiBerACE target fission ΔE-E1-E2 HYDRA chamber beam Burke et al E1 (Counts/20 kev)

6 Utilizing (p,d) and (p,t) reactions to obtain (n,f) cross sections Tested SRM for reactions on 236 U and 238 U using internal and external surrogate ratios, found to be in good agreement with ENDF data Removed systematic errors from light particle contamination by extracting from particle-γ coincidence instead of particle-single coincidence Implies the surrogate ratio method can extend to using different reaction channels with the same target Hughes et al U(p,df) 237 U/ 236 U(p,df) 235 U 238 U(p,df) 237 U/ 238 U(p,tf) 236 U

7 Structure of U nuclei Particle γ coincidence measurements used for structure studies of selected states populated in excited nuclei ( 235 U showed here) The spectra below show expected 646 kev and 659 kev transitions as well as new gamma rays By combining these with previously observed lines provides a powerful tool for spin and level assignments expanding the partial decay scheme Hughes et al. 2012

8 Surrogate measurement of the 238 Pu(n,f) cross section Used 239 Pu(α,α f) reactions and SRM to determine 239 Pu(n,f) cross section 235 U(α,α f) and 236 U(α,α f) used as references Both measurements yield similar cross sections although the different references produce different spins in the compound nucleus Indicates the ratio method reduces discrepancies due to deviations from the W-E limit Difference observed at low energies are expected as the W-E approximation breaks down. Provides continuous data from 5 MeV to 20 MeV Ressler et al. 2011

9 Gamma-ray multiplicity measurement of the spontaneous fission of 252 Cf in a segmented HPGE/BGO detector array Used highly segmented LIBERACE array to perform gamma coincidence and measure multiplicity of SF Overall multiplicity in good agreement with previous experiment Suggests broader high multiplicity tail Specific fission products are identified in coincidence by observing known gamma rays. No discernible difference in the gamma ray multiplicity between 2n and 4n fission emissions Bleuel et al. 2010

10 Channel selection of neutron-rich nuclei following fusion-evaporation reactions of light systems Fusion-evaporation with gamma spectroscopy is widely used to study nuclear structure with STARS/LIBERACE Particle gamma coincidence is used to isolate yield ratios of for specific reactions of light nuclei Experimental results are compared to fusion-evaporation codes to determine their predictive power A simple calculation for the expected incident beam energy needed to populate the 2p evaporation channel Determines necessary beam energies for gamma spectroscopic nuclear structure studies Gibelinet al. 2011

11 Statistical Gamma Rays in Surrogate Reactions The density and width of nuclear excited states increase with excited energy towards the particle separation energies creating a quasicontinuum of levels in heavier nuclei. These are best characterized using statistical quantities such as level density and photon strength function. Important when considering (n,γ) surrogate reactions Wiedeking et al. 2012

12 Statistical γ rays in the analysis of surrogate nuclear reactions Proton-proton inelastic scattering off 154,155,156,158 Gd targets to validate the surrogate approach to measure (n,γ) reactions( particle- γ coincidence) The number of p- γ coincidence was measured above a threshold to determine exit channel probability (n,γ) reactions on these Gd isotopes have been directly measured with low uncertainties The statistical gamma ray approach used with STARS produces counting statistics much higher than using discrete transitions The cross section ratio does not match direct reaction measurements better than the discrete method due to spin-parity mismatch in the compound nucleus Scielzo et al. 2012

13 Neutron-capture cross sections from indirect measurements Level of precision needed is often higher than the fission case and cross sections are needed in low energy range (few kev 200 kev). This is where the W-E approximation breaks down. The probability for a compound nucleus to decay via γ depends on the spin-parity population of the nucleus prior to decay. The range of cross sections obtained by varying spin distributions measures uncertainty of cross sections from W-E. Discrepancies are smaller for the cases where spin parity is dominated by large level densities from deformed nuclei The ratio approach was found to reduce, but not eliminate, the effect of spin-parity mismatch for energies where the W-E approximation is poor Escher et al. 2012

14 Measurement of the entry-spin distribution imparted to the high excitation continuum region of gadolinium nuclei via (p,d) and (p,t) reactions Attempt to measure the entry spin distribution in Gd nuclei populated by (p,d) and (p,t) reactions. Gate a discrete gamma ray transition and particle energy that populates that level Measure angular distribution and see which angular momentum transfer it best corresponds with from calculations By gating larger excitation energies the spin distributions into the quasicontinuum can be measured. Measurements in the quasicontinuum closely match spin distribution calculations Ross et al. 2012

15 Low-Energy Enhancement in the Photon Strength of 95 Mo Critical input to statistical reaction models are nuclear level density and the photon strength function f(eγ) Low energy enhancement can have order of magnitude effects on astrophysical calculations Experiment designed to investigate statistical feeding from the quasicontinuum to individual low-lying levels 94 Mo(d,p) 95 Mo To extract primary gamma rays involves Tagging on proton energies to determine excitation of the nucleus Tagging low-lying levels by selecting gamma rays Require the sum of discrete and primary gamma rays equal the excitation energy Wiedeking et al. 2012

16 Neutron-induced cross sections of short lived nuclei via the surrogate method Measurement performed using an array of HPGe, Liquid scintillators and silicon ΔE-E telescopes Used 174 Yb( 3 He,p) 176 Lu* and 174 Yb( 3 He, 4 He) 173 Yb These surrogate reaction data show large discrepancies with respect to the neutron-induced data Errors attributed mainly to spin parity distribution mismatches Boutoux et al. 2012

17 Neutron-induced cross sections of short lived nuclei via the surrogate method contd. Compared to neutron induced reactions, transfer reactions favor gamma emission beyond the neutron separation energy Spin distributions are determined by fitting radiative capture data and assuming a Gaussian shape of the spin distribution Spin distributions are significantly larger for these reactions (J = 3.9) than for neutron induced reactions (J = 0) Boutoux et al. 2012