Ba138(n, γ)Ba139Reaction and Evidence for Direct Capture

The $\gamma$ radiation emitted following thermal-neutron capture in ${\mathrm{Ba}}^{138}$ has been studied both in singles and coincidence with 9-${\mathrm{cm}}^3$ and 20-${\mathrm{cm}}^3$ Ge(Li) detectors and a 9-${\mathrm{cm}}^3$ Ge(Li)-NaI detector combination at the Brookhaven research reactors. $\gamma$ rays of the following energies (in keV) and relative intensities (given in parentheses) were assigned to ${\mathrm{Ba}}^{139}$: 295 (<1), 352 (<1), 454.67 (27), 627.26 (100), 666.5 (1.5), 1047.4 (3), 1076 (<1), 1105 (2), 1420.1 (5), 1500 (<1), 1558.0 (2.5), 1854.0 (3), 1952.3 (3), 2242.0 (5), 2522.5 (2.5), 2537.0 (3), 2567 (3), 2594.1 (8), 3432 (<1), 3641.4 (20), and 4096.1 (60). All but one of these transitions were incorporated in a level scheme for the ${\mathrm{Ba}}^{139}$ nucleus with levels at the following energies (in keV): 627.26±0.10, 1081.9±0.2, 1292.6±1.0, 1420.1±1.0, 1952.3±1.0, 2129.3±1.2, 2156.4±2.0, 2186.4±1.9, and 2481.4±1.2. The level at 1292 keV which is excited in the ${\mathrm{Ba}}^{138}(d, p)$ reaction is shown to be a doublet of levels at 1284.0 and 1292.6 keV, of which only the 1292.6-keV member is observed to be populated in the ${\mathrm{Ba}}^{138}(n, \gamma )$ reaction. The neutron separation energy was found to be 4723.4±0.7 keV. The strengths of the primary $\gamma$ rays and proton groups in the ($n, \gamma$) and ($d, p$) reactions, respectively, have been compared, and a strong correlation, with a correlation coefficient of 0.95, was observed. This correlation is discussed in terms of the common unique parent assumption of Lane and Wilkinson. It is seen to be consistent with this assumption and yields evidence for direct capture. It is shown that the theory of radiative capture of Lane and Lynn predicts a cross section for direct capture in ${\mathrm{Ba}}^{138}$ which is comparable to the observed total thermal-neutron-capture cross section ${\sigma }_{\mathrm{th}}=0.35\mathrm{}\pm 0.15$ b.