The surrogate nuclear reaction method can be used to determine neutron-induced reaction cross sections from measured decay properties of a compound nucleus created using a different reaction and calculated formation cross sections. The reliability of $(n,\ensuremath{\gamma})$ cross sections determined using the Weisskopf-Ewing and ratio approximations are explored for the $^{155,157}\mathrm{Gd}$$(n,\ensuremath{\gamma})$ reactions. Enriched gadolinium targets were bombarded with 22-MeV protons and $\ensuremath{\gamma}$ rays were detected in coincidence with scattered protons using the Silicon Telescope Array for Reaction Studies/Livermore-Berkeley Array for Collaborative Experiments (STARS/LiBerACE) silicon and germanium detector arrays. The $\ensuremath{\gamma}$-emission probabilities for the $^{154,156,158}\mathrm{Gd}$ compound nuclei were measured at excitation energies up to 12 MeV. It is found that the approximations yield results that deviate from directly measured $^{155,157}\mathrm{Gd}$$(n,\ensuremath{\gamma})$ cross sections at low energies. To extract reliable cross sections, a more sophisticated analysis should be developed that takes into account angular-momentum differences between the neutron-induced and surrogate reactions.

/pdf/measurement-of-g-emission-branching-ratios-for-154-156-158-3hoj3u694h.pdf

Measurement of γ -emission Branching Ratios for 154,156,158 Gd Compound Nuclei: Tests of Surrogate Nuclear Reaction Approximations for ( n,γ ) Cross Sections

Nuclear Data Sheets for A=158

Prompt-fission γ-ray spectra (PFGS) have been measured for the U238(n,f) reaction using fast neutrons produced by the LICORNE directional neutron source. Fission events were detected with an ionization chamber containing actinide samples placed in the neutron beam, and the coincident prompt-fission γ rays were measured using a number of LaBr3 scintillation detectors and a cluster of nine phoswich detectors from the PARIS array. Prompt-fission γ rays (PFGs) were discriminated from prompt-fission neutrons using the time-of-flight technique over distances of around 35 cm. PFG emission spectra were measured at two incident neutron energies of 1.9 and 4.8 MeV for U238(n,f) and also for Cf252(sf) as a reference. Spectral characteristics of PFG emission, such as mean γ multiplicity and average total γ-ray energy per fission, as well as the average γ-ray energy, were extracted. The sensitivity of these results to the width of the time window and the type of spectral unfolding procedure used to correct for the detector responses was studied. Iteration methods were found to be more stable in low-statistics data sets. The measured values at En=1.9MeV were found to be the mean γ multiplicity Mγ=6.54±0.19, total released energy per fission Eγ,tot=5.25±0.20 MeV, and the average γ-ray energy ϵγ=0.80±0.04 MeV. Under similar conditions, the values at En=4.8MeV were measured to be Mγ=7.31±0.46, Eγ,tot=6.18±0.65 MeV, and ϵγ=0.84±0.11 MeV.

Statistical study of the prompt-fission γ -ray spectrum for U 238 (n, f) in the fast-neutron region STATISTICAL STUDY of the PROMPT-FISSION ... L. QI et al.

The surrogate nuclear reaction method is being applied in many effortsto indirectly determine neutron-induced reaction cross sections on short-lived isotopes. This technique aims to extract accurate (n,γ) cross sections from measureddecaypropertiesofthecompoundnucleusofinterest(createdusingadifferentreaction).Theadvantages and limitations of a method that identifies the γ-ray decay channel by detecting any high-energy (“statistical”) γ ray emitted during the relaxation of the compound nucleus were investigated. Data collected using the Silicon Telescope Array for Reaction Studies and Livermore-Berkeley Array for Collaborative Experiments silicon and germanium detector arrays were used to study the decay of excited gadolinium nuclei following inelastic proton scattering. In many cases, this method of identifying the γ-ray decay channel can simplify the experimental data collection and greatly improve the detection efficiency for γ-ray cascades. The results show sensitivity to angular-momentum differences between the surrogate reaction and the desired (n,γ) reaction similar to an analysis performed using low-lying discrete transitions even when ratios of cross sections are considered.

/pdf/statistical-g-rays-in-the-analysis-of-surrogate-nuclear-582ndxe6le.pdf

Statistical γ rays in the analysis of surrogate nuclear reactions

The Surrogate nuclear reactions method, an indirect approach for determining cross sections for compound‐nuclear reactions involving difficult‐to‐produce targets, is reviewed. The underlying formalism is outlined, the challenges involved in carrying out a complete Surrogate treatment are detailed, and the present status of the theory is summarized. The approximations employed in the analyses of Surrogate experiments are discussed and their validity is examined.

/pdf/compound-nuclear-reaction-cross-sections-from-surrogate-335q0g4h5q.pdf

Compound-Nuclear Reaction Cross Sections from Surrogate Measurements: Status and Challenges

The astrophysical s-process is responsible for the synthesis of many of the nuclei heavier than iron through a series of low-energy (n,{gamma}) reactions and {beta}-decays. For nuclei for which the neutron capture and {beta}-decay rates are comparable, the branching is crucial for tests of s-process models. Direct measurements of (n,{gamma}) cross sections for these nuclei are extremely challenging due to the inherent difficulties associated with radioactive targets and the low intensity of available neutron beams. The surrogate reaction technique can be used to circumvent these difficulties by creating the same compound nucleus through light-ion reactions on a stable target. The cross section can be determined by combining optical model calculations for the formation of the compound nucleus with the measured exit channel probability for {gamma}-ray emission. We have collected data to determine the low-energy (n,{gamma}) cross section for the unstable nucleus {sup 153}Gd by bombarding a stable {sup 154}Gd target with protons from the 88-Inch Cyclotron at the Lawrence Berkeley National Laboratory to create the desired {sup 154}Gd* compound nucleus. The STARS/LiBerACE silicon and clover germanium detector arrays were used to detect {gamma}-rays in coincidence with the scattered protons. Additional cross section measurements using {sup 156}Gd and {sup 158}Gd targetsmore » will be compared to direct measurements of the (n,{gamma}) cross sections for {sup 155}Gd and {sup 157}Gd. The current status of the analysis is summarized.« less

/pdf/determininig-the-n-g-cross-section-of-153gd-using-surrogate-4ke7rmk2ro.pdf

Determininig the (n,γ) cross section of 153Gd using surrogate reactions

Coincident {gamma} rays from a {sup 252}Cf source were measured using an array of six segmented high-purity germanium (HPGe) Clover detectors each enclosed by 16 bismuth-germanate (BGO) detectors. The detectors were arranged in a cubic pattern around a 1 {micro}Ci {sup 252}Cf source to cover a large solid angle for {gamma}-ray measurement with a reasonable reconstruction of the multiplicity. Neutron multiplicity was determined in certain cases by identifying the prompt {gamma} rays from individual fission fragment pairs. Multiplicity distributions from previous experiments and theoretical models were convolved with the response function of the array and compared to the present results. These results suggest a {gamma}-ray multiplicity spectrum broader than previous measurements and models, and provide no evidence of correlation with neutron multiplicity.

/pdf/gamma-ray-multiplicity-measurement-of-the-spontaneous-4l8jtg5p2c.pdf

Gamma-ray multiplicity measurement of the spontaneous fission decay of 252Cf in a segmented HPGe/BGO detector array

We are studying the gamma ray and neutron multiplicity of various fission processes, beginning with the spontaneous fission of {sup 252}Cf, for a variety of basic and applied science purposes. The Livermore-Berkeley Array for Collaborative Experiments (LiBerACE) consists of six high-purity germanium Clover detectors (HPGe) each enclosed by an array of 16 bismuth-germanate (BGO) detectors. These detectors were arranged in a cubic pattern around a 1 {micro}Ci {sup 252}Cf source to attempt to cover as much solid angle of gamma ray emission as possible with a high level of segmentation. The single-gamma detector response function is determined at several energies by tagging in a HPGe detector on the photopeak of one of two gamma rays in two-gamma ray calibration sources and observing the multiplicity of the remainder of the array. Summing these single-gamma responses in groups yields the response function of the array to higher multiplicity events, which are convolved with multiplicity distributions from theoretical models and compared to the measured results to test the models validity.

/pdf/gamma-ray-multiplicity-of-252cf-spontaneous-fission-using-16ugfy99ju.pdf

Gamma Ray Multiplicity of 252Cf Spontaneous Fission using LiBerACE

This paper highlights some of the current basic nuclear physics research at Lawrence Livermore National Laboratory (LLNL). The work at LLNL concentrates on investigating nuclei at the extremes. The Experimental Nuclear Physics Group performs research to improve our understanding of nuclei, nuclear reactions, nuclear decay processes and nuclear astrophysics; an expertise utilized for important laboratory national security programs and for world-class peer-reviewed basic research.

Nuclear physics and heavy element research at LLNL

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Determininig the (n,γ) cross section of 153Gd using surrogate reactions

Gamma-ray multiplicity measurement of the spontaneous fission decay of 252Cf in a segmented HPGe/BGO detector array

Gamma Ray Multiplicity of 252Cf Spontaneous Fission using LiBerACE

Nuclear physics and heavy element research at LLNL