First search for $$\alpha $$ decays of naturally occurring Hf nuclides with emission of $$\gamma $$ quanta

Abstract

The first ever search for α decays to the first excited state in Yb was performed for six isotopes of hafnium (174, 176, 177, 178, 179, 180) using a high purity Hf-sample of natural isotopic abundance with a mass of 179.8 g. For 179 Hf, also α decay to the ground state of 175 Yb was searched for thanks to the β-instability of the daughter nuclide 175 Yb. The measurements were conducted using an ultra low-background HPGe-detector system located 225 m underground. After 75 d of data taking no decays were detected but lower bounds for the half-lives of the decays were derived on the level of lim T 1/2 ∼ 10 15 − 10 18 a. The decay with the shortest half-life based on theoretical calculation is the decay of 174 Hf to the first 2 + 84.3 keV excited level of 170 Yb. The experimental lower bound was found to be T 1/2 ≥ 3.3 × 10 15 a.

A search for rare and induced nuclear decays in hafnium

“…No evidence of any alpha decay of Hf isotopes to the first excited state was observed. The half-life results presented here improve the 174 Hf limit from [10] by a factor of 8.5, which can attributed to the increased detection efficiency and counting time. A search for the decay of 177 Hf based on the decay of the daughter 175 Yb (T 1/2 = 4.185 d) at 396 keV yields a less stringent limit at 3.6×10 17 a than from 177 Hf directly.…”

“…Limits are given at 90% C.L. Measurement of the decay of 174 Hf to the first excited state of Yb with the modified HPGe detector is within reach of the theoretically-predicted rate with a factor of 10-100 reduction in backgrounds, which can be achieved with the purification technique of electron beam melting (EBM) as mentioned in [10,21]. Measurement of the alpha decays from other Hf isotopes, however, remains unattainable at present with ULB HPGe detectors.…”

“…Table 4: Hf α decays to first excited states of Yb daughter nuclides investigated in this analysis. Gamma energies (E γ ) of, and yield (y) to, the first excited state used to set the limit for each specific decay, experimental lower limits from the analysis presented herein, previous limits [10], and the theoretical evaluation of half-lives as stated in [10]. Limits are given at 90% C.L.…”

“…Recently, rare α decays of hafnium isotopes to excited states of ytterbium using a highly-purified Hf sample were investigated with two conventional HPGe detectors in coincidence [10]. This measurement was conducted in the HADES underground laboratory.…”

“…One isotope, 174 Hf, can undergo double electron capture (2ε) and electron capture with positron emission (εβ + ) [12]. The only experimental search for such decays [13] was performed by the authors of [10] with the same sample and detector technique. Again, no evidence of such ββ decays were observed and limits on the corresponding half-lives were placed in the range of 10 16 − 10 18 a.…”