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The reactor production of the nuclear high-spin isomer ${}^{178m2}$Hf ($T_{1/2}$ = 31 years) was considered. According to [1], in the inelastic scattering of neutrons by ${}^{178}$Hf and in the (n, 2n) reaction by ${}^{179}$Hf, the ${}^{178m2}$Hf isomer is not formed, because this requires the transfer of too large angular momentum to the nucleus. For the formation of the ${}^{178m2}$Hf in the radiative capture of neutrons by ${}^{177}$Hf nuclei, two different cross section values were obtained earlier. In [2] $\sigma$ = 0.2 ± 0.1 $\mu$barn when the ${}^{177}$Hf isotope was irradiated with thermal neutrons with fluence up to $10^{22}$ cm${}^{–2}$. In [3] $\sigma$ = 2.6 ± 0.7 $\mu$barn when neutron irradiating natural hafnium at the IBR-2 reactor of the Joint Institute for Nuclear Research (Dubna) with a relatively small fluence of ~$10^{18}$ cm${}^{–2}$ (the errors are at the level of one standard deviation).
We studied the formation of the ${}^{178m2}$Hf isomer in the material of the regulatory body of the RBT-6 reactor (SSC RIAR), made of 6 mm thick plate of metallic natural hafnium [4], after it's irradiation with neutron flux of ~ $10^{14}$ cm${}^{–2}$∙s${}^{–1}$ with fluence up to $10^{22}$ cm${}^{–2}$. Samples were taken at several points of the plate from the upper layer with a depth of 0.5 mm and from a depth of 2 to 4 mm. The neutron fluence at the sampling points with an accuracy of 20% was determined using a numerical model based on the MCU code (version MCU-RR). For one of the typical sampling points, the fluence of neutrons of energy less than 0.625 eV was 7.5×$10^{21}$ cm${}^{–2}$, the specific activities of the surface and deep samples were 1547 ± 92 Bq×g${}^{–1}$ and 1368 ± 82 Bq×g${}^{–1}$, respectively. If we assume that the whole ${}^{178m2}$Hf isomer was formed by the capture of thermal neutrons by ${}^{177}$Hf nuclei, then taking into account the burn-up of ${}^{177}$Hf and ${}^{178m2}$Hf, the isomer cross section $\sigma$ = 3.6 ± 0.5 $\mu$barn is obtained, which coincides with the result of [3]. Thus, we did not see the contribution of any other reactions to the formation of the ${}^{178m2}$Hf isomer other than radiative capture of neutrons by ${}^{177}$Hf nuclei.
In principle, the ${}^{178m2}$Hf isomers can be produced by the two-stage reaction ${}^{179}$Hf (n, n ’)${}^{179m2}$Hf + ${}^{179m2}$Hf (n, 2n)${}^{178m2}$Hf, which was already discussed in [5] for the interaction of 14 MeV neutrons with ${}^{179}$Hf nuclei. According to our estimates, in the irradiated hafnium studied by us, the specific activity of the ${}^{178m2}$Hf isomer obtained by this reaction is ~ 0.1 Bq×g${}^{-1}$.
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