New investigation of half-lives for the decay modes of V50

Abstract

A new search for the decay modes of the 4-fold forbidden non-unique decay of 50 V has been performed at the Gran Sasso Underground Laboratory (LNGS). In total an exposure of 197 kg×d has been accumulated. The half-life for the electron capture into the first excited state of 50 Ti has been measured with the highest precision to date as 2.67 +0.16 −0.18 x 10 17 yr (68% C.I.) in which systematics uncertainties dominate. The search for the β-decay into the first excited state of 50 Cr resulted in a lower limit of 1.9 x 10 19 yr (90% C.I.), which is an improvement of almost one order of magnitude compared to existing results. The sensitivity of the new measurement is now in the region of theoretical predictions.

Experimental searches for rare alpha and beta decays

“…The half-life of the 50 V was calculated in nuclear shell model by Nishimura in 1979 [185] as: T 1/2 (ε) = 1.5 × 10 16 yr, T 1/2 (β − ) = 9.04 × 10 17 yr, which is in disagreement with the experimental values. Recent calculations of Haaranen et al [186] for the electron capture branch: T 1/2 (ε) = (5.13 ± 0.07)[(3.63 ± 0.05)] × 10 17 yr given for g A = 1.00 [1.25] is in good agreement with the measured value [184]. The predicted value for β − decay T 1/2 (β − ) = (2.34 ± 0.02)[(2.00 ± 0.02)] × 10 19 yr [186] is not far from the best achieved limit 1.9 × 10 19 yr [184], that gives hope to observe the effect in near future.…”

“…Early reports on positive observations of electron capture and β − at level of T 1/2 ∼ 10 14 −10 16 yr [221,222,223] were not supported in later investigations. The detection of the 783 keV γ peak expected in 50 V β − decay by Simpson et al in 1989 [224] with T 1/2 (β − ) = (8.2 +13.1 −3.1 ) × 10 17 yr was quite uncertain and not confirmed in the subsequent investigations; the best known limit obtained in a recent experiment [184] is T 1/2 (β − ) > 1.9 × 10 19 yr.…”

“…For 184 Os, only limits were known previously: T 1/2 > 2.0 × 10 13 yr [75,104] obtained with nuclear emulsions, and T 1/2 > 5.6 × 10 13 yr [105] with proportional counter measurements of Os sample enriched in 184 Os to 2.25%. However, recently an indication on α decay of 184 Os was found in geochemical measurements [78] where an excess of the daughter 180 W was measured in meteorites and terrestrial rocks; the half-life was determined to be T 1/2 = (1.12 ± 0.23) × 10 13 yr. This experimental value is in good agreement with theoretical calculations: T 1/2 = 2.6 × 10 13 yr [106] and T 1/2 = 1.3 × 10 13 yr [107] (see also Table 1).…”

“…On the contrary, the 1554 keV γ peak, expected in the electron capture branch of the 50 V decay, is confidently observed in all experiments since 1984, with the recent, most accurate result of T 1/2 (ε) = (2.67 +0. 16 −0.18 ) × 10 17 yr [184].…”

A search for rare and induced nuclear decays in hafnium

“…Searches for these rare nuclear processes have become more practical in part due to the usage of high-purity materials and enriched isotopes, operation of cryogenic (scintillating- [5]) bolometers [6,7,8] and ultra-lowbackground high purity germanium detectors (ULB HPGe) [9], as well as the requirement that these measurements be performed in underground laboratories screening against cosmic ray backgrounds. Proper application of these techniques have allowed for sensitivity to half-lives reaching 10 16 − 10 19 a.…”