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The signature of the simultaneous capture of two K-shell electrons on the $^{124}$Xe and $^{126}$Xe nuclei with emission of two neutrinos is the detection of two K-shell X-rays and an Auger electron. Relaxation of the excited daughter atomic shell of Te by emitting two K X-ray quanta and an Auger electron is carried out with a probability of 0.734. Selection of such a complex event is possible with measurements using a large low background high-pressure proportional counter. The experiment to search for double beta decay processes in $^{124,126}$Xe is being carried out at the "2K-CAPTURE" facility of the underground laboratory DULB-4900 of the Baksan Neutrino Observatory of the INR RAS for several years [1,2]. The working medium of the detector is a pure xenon sample enriched in $^{124}$Xe and $^{126}$Xe to 21% and 28%, respectively. The LPC was filled with working gas up to a pressure of 5.8 bar without quenching and accelerating additives. This report presents the summary results of an experimental search of 2$\nu$2K-capture for two isotopes as a result of data processing for exposure of 340 kg×day. The estimate of the total number of the triple [K$_h$$\otimes$K$s$$\otimes$e$_A$] coincidences was 29±5 events. The reaction Q-value is 2864 keV for $^{124}$Xe and 896 keV for $^{126}$Xe. The signal region will be dominated by $^{124}$Xe, due to the Q$^5$ dependence of the phase space factor. If the observed effect is attributable to 2$\nu$2K capture in $^{124}$Xe, then the appropriate value of the half-life is
T$_{1/2}$(2$\nu$KK) = [1.9 ± 0.3(stat) ± 0.2(syst)] $\times$ 10$^{22}$ years.
Even though the experimental technique used does not allow separating the responses from the decays of different isotopes will enable us to give a constraint on 2$\nu$2K on $^{126}$Xe in the same manner:
T$_{1/2}$(2$\nu$KK) $\geq$ 3.3$\times$10$^{22}$ years at 90% CL.
- Yu. M. Gavrilyuk et al. // Nucl. Instrum. Methods Phys. Res., Sect. A, 2013. V.729, 576.
- Yu.M.Gavrilyuk et al. // Physics of Particles and Nuclei. 2018. V.49, No.1, P.563.
