Temperature effect on the decay periods of long-lived $^{\rm 180m}$Hf and $^{\rm 87m}$Sr isomers

Experiments on measuring the decay periods of long-lived $^{\rm 180m}$Hf and $^{\rm 87m}$Sr isomers at room temperature and at $77$ K in massive samples of HfO$_2$, Sr(NO$_3)_2$, and SrCO$_3$ are reported. The isomeric nuclear states were excited by irradiating the samples with neutrons from a Pu-Be source. According to the theory of V. I. Vysotski $et~al$., the $T_{1/2}$ value must increase if a $\gamma$-active nucleus is surrounded by many identical ground-state nuclei, because these distort the spectrum of electromagnetic vacuum oscillations near the nuclear energy level. As the temperature of the sample decreases, $\gamma$-ray lines narrow, especially for the low-energy Mössbauer transitions, thereby enhancing the resonance effect on the spectrum of vacuum oscillations. For the $^{\rm 180m}$Hf isomer, whose upper $\gamma$ transition carries away 57.55 keV, the $T_{1/2}$ value was found to increase by $2.99\pm 0.87$% upon sample cooling. For $^{\rm 87m}$Sr, whose decay scheme has no Mössbauer lines, the relative change in $T_{1/2}$ was found to be $0.77\pm 0.53$%.