Abstrakt
Total radiative thermal neutron-capture gamma-ray cross sections for the W-182,W-183,W-184,W-186 isotopes were measured using guided neutron beams from the Budapest Research Reactor to induce prompt and delayed gamma rays from natural and isotopically-enriched tungsten targets. These cross sections were determined from the sum of measured gamma-ray cross sections feeding the ground state from low-lying levels below a cutoff energy, E-crit, where the level scheme is completely known, and continuum gamma rays from levels above E-crit, calculated using the Monte Carlo statistical-decay code DICEBOX.
The new cross sections determined in this work for the tungsten nuclides are sigma(0)(W-182) = 20.5(14) b and sigma(11/2+) (W-183(m), 5.2 s) = 0.177(18) b; sigma(0)(W-183) = 9.37(38) b and sigma(5)-(W-184(m), 8.33 mu s) = 0.0247(55) b; sigma(0)(W-184) = 1.43(10) b and sigma(11/2+) (W-185(m), 1.67 min) = 0.0062(16) b; and, sigma(0)(W-186) = 33.33(62) b and sigma(9/2+) (W-187(m), 1.38 mu s) = 0.400(16) b. These results are consistent with earlier measurements in the literature.
The W-186 cross section was also independently confirmed from an activation measurement, following the decay of W-187, yielding values for sigma(0)(W-186) that are consistent with our prompt 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-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 gamma-ray measurements for the respective (n,gamma) compounds: W-183 [S-n = 6190.88(6) keV]; W-184 [S-n = 7411.11(13) keV]; W-185 [S-n = 5753.74(5) keV]; and, W-187 [S-n = 5466.62(7) keV].