Investigation of the tungsten isotopes via thermal neutron capture

Abstract

Total radiative thermal neutron-capture $\ensuremath{\gamma}$-ray cross sections for the ${}^{182,183,184,186}$W isotopes were measured using guided neutron beams from the Budapest Research Reactor to induce prompt and delayed $\ensuremath{\gamma}$ rays from natural and isotopically-enriched tungsten targets. These cross sections were determined from the sum of measured $\ensuremath{\gamma}$-ray cross sections feeding the ground state from low-lying levels below a cutoff energy, ${E}_{\mathrm{crit}}$, where the level scheme is completely known, and continuum $\ensuremath{\gamma}$ rays from levels above ${E}_{\mathrm{crit}}$, calculated using the Monte Carlo statistical-decay code dicebox. The new cross sections determined in this work for the tungsten nuclides are ${\ensuremath{\sigma}}_{0}{(}^{182}\mathrm{W})=20.5(14)$ b and ${\ensuremath{\sigma}}_{11/{2}^{+}}{(}^{183}{\mathrm{W}}^{m},5.2\phantom{\rule{0.28em}{0ex}}\mathrm{s})=0.177(18)$ b; ${\ensuremath{\sigma}}_{0}{(}^{183}\mathrm{W})=9.37(38)$ b and ${\ensuremath{\sigma}}_{{5}^{\ensuremath{-}}}{(}^{184}{\mathrm{W}}^{m},8.33\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s})=0.0247(55)$ b; ${\ensuremath{\sigma}}_{0}{(}^{184}\mathrm{W})=1.43(10)$ b and ${\ensuremath{\sigma}}_{11/{2}^{+}}{(}^{185}{\mathrm{W}}^{m},1.67\phantom{\rule{0.28em}{0ex}}\mathrm{min})=0.0062(16)$ b; and, ${\ensuremath{\sigma}}_{0}{(}^{186}\mathrm{W})=33.33(62)$ b and ${\ensuremath{\sigma}}_{9/{2}^{+}}{(}^{187}{\mathrm{W}}^{m},1.38\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s})=0.400(16)$ b. These results are consistent with earlier measurements in the literature. The ${}^{186}$W cross section was also independently confirmed from an activation measurement, following the decay of ${}^{187}$W, yielding values for ${\ensuremath{\sigma}}_{0}{(}^{186}\mathrm{W})$ that are consistent with our prompt $\ensuremath{\gamma}$-ray measurement. The cross-section measurements were found to be insensitive to choice of level density or photon strength model and only weakly dependent on ${E}_{\mathrm{crit}}$. Total radiative-capture widths calculated with dicebox showed much greater model dependence; however, the recommended values could be reproduced with selected model choices. The decay schemes for all tungsten isotopes were improved in these analyses. We were also able to determine new neutron-separation energies from our primary $\ensuremath{\gamma}$-ray measurements for the respective ($n,\ensuremath{\gamma}$) compounds: ${}^{183}$W [${S}_{n}=6190.88(6)$ keV]; ${}^{184}$W [${S}_{n}=7411.11(13)$ keV]; ${}^{185}$W [${S}_{n}=5753.74(5)$ keV]; and, ${}^{187}$W [${S}_{n}=5466.62(7)$ keV].