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

Background: Adouble-K-electron capture is a rare nuclear-atomic process inwhich twoK 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 twoK-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 P-KK of Kr-81, as well as to measure the half-life of Kr-78 relative to K-2.2 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 Kr-81. The probability for this decay was found to be P-KK = (5.7 +/- 0.8) x 10(-5) with a systematic error of (Delta P-KK)(sys)t = +/- 0.4 x 10(-5). For the Kr-78(2 nu 2K) 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-life T-1/2(2.2K) (g.s. -> g.s.) = [1.9(-0.7)(+1.3) (stat) +/- 0.3(syst)] x 10(22) 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 Kr-78. 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-KK(exp) for the EC decay of Kr-81, which are in satisfactory agreement with Z-2 dependence of P-KK 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 Kr-78. The general procedure of data analysis allowed us to determine the half-life of Kr-78 relative to 2 nu 2K transition with a greater statistical accuracy than in our previous works.