Comparative study of the double- K -shell-vacancy production in single- and double-electron-capture decay

Abstract

Background: A double K-electron capture is a rare nuclear-atomic process in which two K electrons are captured simultaneously from the atomic shell. A "hollow atom" is created as a result of this process. In single K shell electron-capture decays, there is a small probability that the second electron in the K-shell is excited to an unoccupied level or can (mostly) be ejected to the continuum. In either case, a double vacancy is created in the K-shell. The relaxation of the double K-shell vacancy, accompanied by the emission of two K-fluorescence photons, makes it possible to perform experimental studies of such rare processes with the large-volume proportional gas chamber. Purpose: The purpose of the present analysis is to estimate a double-K-shell vacancy creation probability per K-shell electron capture PKK of 81 Kr, as well as to measure the half-life of 78 Kr relative to 2ν2K capture. Method: Time-resolving current pulse from the large low-background proportional counter (LPC), filled with the krypton sample, was applied to detect triple coincidences of "shaked" electrons and two fluorescence photons. Results: The number of K-shell vacancies per the K-electron capture, produced as a result of the shake-off process, has been measured for the decay of 81 Kr. The probability for this decay was found to be PKK = (5.7 ± 0.8) × 10 −5 with a systematic error of (∆PKK )syst = ±0.4 × 10 −5 . For the 78 Kr(2ν2K) decay, the comparative study of single-and double-capture decays allowed us to obtain the signal-to-background ratio up to 15/1. The half-life22 y is determined from the analysis of data that have been accumulated over 782 days of live measurements in the experiment that used samples consisted of 170.6 g of 78 Kr. Conclusions: The data collected during low background measurements using the LPC were analyzed to search the rare atomic and nuclear processes. We have determined P exp KK for the EC decay of 81 Kr, which are in satisfactory agreement with Z −2 dependence of PKK predicted by Primakoff and Porter. This made possible to more accurately determine the background contribution in the energy region of our interest for the search for the 2K-capture in 78 Kr. The general procedure of data analysis allowed us to determine the half-life of 78 Kr relative to 2ν2K-transition with a greater statistical accuracy than in our previous works.

Two-neutrino double electron capture on 124Xe based on an effective theory and the nuclear shell model

“…Here, two K-or L-shell orbital electrons are simultaneously captured, rather than β emitted. This mode is kinematically unfavored with respect to ββ decay, and at present only a geochemical measurement of 130 Ba [11,12], and a possible detection in 78 Kr [13,14] have been claimed. Moreover, a resonant neutrinoless ECEC could be fulfilled in selected nuclei [15][16][17].…”

Status and Perspectives of 2ϵ, ϵβ+ and 2β+ Decays

“…After a total exposure of 0.343 kg × yr, the authors of Reference [168] quotes a possible hint at 2.5 σ of statistical significance with a half-life of 78 Kr relative to 2ν2K capture: T 1/2 = (9.2 +5.5 −2.6 stat ± 1.3 sys ) × 10 21 yr [168]. Later, a comparative study on double electron capture re-estimated the previous value at T 1/2 = (1.9 +1.3 −0.7 stat ± 0.3 sys ) × 10 22 yr [169] (see Table 1). We also remind the previous limits at the level of 10 20 -10 21 yr set since 1994 [170,184,185].…”

Dark-matter detector observes exotic nuclear decay

“…Such emissions pave the way for the direct observation of two-neutrino double electron capture in a nucleus. The first experimental indications of this process were obtained for krypton-78 in direct counting experiments 3,4 , in which the double electron captures are registered one by one, and for barium-130 in geochemical studies 5,6 .…”

“…CRISPR can be used to mutate, repress or activate any targeted human gene 4,5 . In functional genomics, gene function is assessed in a single experiment by growing a large number of cells and then perturbing one gene in each of the cells.…”