Temperature effect on the decay periods of long-lived Hf-180m and Sr-87m isomers

Experiments on measuring the decay periods of long-lived Hf-180m and Sr-87m isomers at room temperature and at 77 K in massive samples of HfO2 , Sr(NO 3)(2) , and SrCO3 are reported. The isomeric nuclear states were excited by irradiating the samples with neutrons from a Pu-Be source. According to th e theory of V.I. Vysotskii et al., the T-1/2 value must increase if a gamma -active nucleus is surrounded by many identical ground-state nuclei, becau se these distort the spectrum of electromagnetic vacuum oscillations near t he nuclear energy level. As the temperature of the sample decreases, gamma -ray lines narrow, especially for the low-energy Mossbauer transitions, the reby enhancing the resonance effect on the spectrum of vacuum oscillations. For the Hf-180m isomer, whose upper gamma transition carries away 57.55 ke V, the T-1/2 value was found to increase by 2.99 +/- 0.87% upon sample cool ing. For Sr-87m, whose decay scheme has no Mossbauer lines, the relative ch ange in T-1 /2 was found to be 0.77 +/- 0.53%. (C) 2001 MAIK "Nauka/Interpe riodica".