Statisticalγrays in the analysis of surrogate nuclear reactions

Abstract

The surrogate nuclear reaction method is being applied in many efforts to indirectly determine neutron-induced reaction cross sections on short-lived isotopes. This technique aims to extract accurate (n,γ) cross sections from measured decay properties of the compound nucleus of interest (created using a different reaction). The advantages 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 STARS/LiBerACE 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.

Spin differences in the Zr90 compound nucleus induced by (p,p′) , Ota1, Burke2, Casperson3 et al. 2015Phys. Rev. CSelf Cite233280View full textAdd to dashboardCite

“…This, in turn, leads to a competition between γ decay and neutron emission that is quite sensitive to the J π distributions of the decaying compound nuclei and can be expected to be visible in the measured discrete γ-ray emission probabilities [7,14,17]. The effect is smaller in well-deformed nuclei with high level densities, such as rare-earth and actinide nuclei that have been studied in [15,23]. Additionally, investigating the 90 Zr nucleus also has the advantage that the nuclear structure of 90 Zr and neighboring nuclei have been extensively studied.…”

“…While this has a smaller effect on the extraction of fission cross sections [9], the γ-ray emission of the compound nucleus can be quite sensitive to the initial J π distribution [14,15,[17][18][19][20][21]. Therefore, in order to extract (n, γ) cross sections from surrogate data it becomes necessary to take into account the J π distribution of the decaying compound nucleus.…”

“…While the applicability of the surrogate reaction approach has been successfully demonstrated for (n, f ) cross sections (e.g. * ota2@llnl.gov [8][9][10][11][12][13]), it has been difficult to determine (n, γ) cross sections (e.g., [14][15][16]). …”

“…Specifically, we compare P i for γ-ray transitions in 90,89 Zr, following the production via inelastic (p, p ), (p, p n) scattering, and (p, d), (p, dn) and (p, t), (p, tn) transfer reactions. Earlier studies suggest that the direct reactions typically used in surrogate experiments transfer more units of angular momentum (∆L) to the compound nucleus than the neutron does in the desired reaction [15,21,24]. For 158 Gd, it was estimated that (p, d) and (p, t) reactions transfer ∆L = 4 and ∆L = 5 , respectively, in the quasi-continuum region (E x =3.5-5 MeV) [24], and describing the data obtained in inelastic proton scattering on gadolinium seems to require similarly large ∆L transfers.…”

“…When this distribution is known, the decay of the nucleus can be modeled and constraints on the desired (n, γ) cross sections are obtained by fitting the decay model to observables from the surrogate experiment. Preliminary work on the A=155-158 gadolinium isotopes, for which structure information, as well as high quality directly-measured cross sections are available (see [15] and references therein), indicates that accounting for J π differences between the desired (n,γ) and surrogate reactions can yield significant improvements for the extracted cross section [22].…”