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Register a new userThe background simulation of experiment for searching of $2 u2K$ capture in Xe-124
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We describe the Monte Carlo (MC) simulation package of the `2K-CAPTURE setup and discuss the agreement of its output with data. The `2K-CAPTURE MC simulates the energy loss of particles in detector and components of the passive shield and generates the resulting response in working volume large proportional counter (LPC). The simulation accounts for absorption, reemission, and scattering of both photons and neutrons and tracks them until they either are absorbed. The algorithm proceeds with a detailed simulation of the electronics chain. The MC is tuned using data collected with radioactive calibration source deployed in the internal channel of the installation. The simulation reproduces the energy response of the detector corresponding to distribution of the generated pointwise clusters of a charge of primary ionization in LPC.
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Double electron capture by proton-rich nuclei is a second-order nuclear process analogous to double beta decay. Despite their similarities, the decay signature is quite different, potentially providing a new channel to measure the hypothesized neutrinoless mode of these decays. The Standard-Model-allowed two-neutrino double electron capture ($2 u ECEC$) has been predicted for a number of isotopes, but only observed in $^{78}$Kr, $^{130}$Ba and, recently, $^{124}$Xe. The sensitivity to this decay establishes a benchmark for the ultimate experimental goal, namely the potential to discover also the lepton-number-violating neutrinoless version of this process, $0 u ECEC$. Here we report on the current sensitivity of the NEXT-White detector to $^{124}$Xe $2 u ECEC$ and on the extrapolation to NEXT-100. Using simulated data for the $2 u ECEC$ signal and real data from NEXT-White operated with $^{124}$Xe-depleted gas as background, we define an optimal event selection that maximizes the NEXT-White sensitivity. We estimate that, for NEXT-100 operated with xenon gas isotopically enriched with 1 kg of $^{124}$Xe and for a 5-year run, a sensitivity to the $2 u ECEC$ half-life of $6 times 10^{22}$ y (at 90% confidence level) or better can be reached.
Radiation background studies pertaining to $0 ubetabeta$ decay in $^{124}$Sn have been carried out. A TiLES setup has been installed at TIFR for this purpose. Neutron-induced background is studied in the TIN.TIN detector materials using fast neutron activation technique. The neutron flux ($E_nleq15$ MeV) resulting from SF and ($alpha, n$) interactions for the rock in the INO cavern is estimated using MC simulations. A two layer composite shield of borated paraffin (20 cm) + Pb (5 cm) is proposed for the reduction of neutron flux.
Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shell. For $^{124}$Xe this process has not yet been observed and its detection would provide a new reference for nuclear matrix element calculations. We have conducted a search for two-neutrino double electron capture from the K-shell of $^{124}$Xe using 7636 kg$cdot$d of data from the XENON100 dark matter detector. Using a Bayesian analysis we observed no significant excess above background, leading to a lower 90 % credibility limit on the half-life $T_{1/2}>6.5times10^{20}$ yr. We also evaluated the sensitivity of the XENON1T experiment, which is currently being commissioned, and find a sensitivity of $T_{1/2}>6.1times10^{22}$ yr after an exposure of 2 t$cdot$yr.
The neutron sensitivity of the C$_6$D$_6$ detector setup used at n_TOF for capture measurements has been studied by means of detailed GEANT4 simulations. A realistic software replica of the entire n_TOF experimental hall, including the neutron beam line, sample, detector supports and the walls of the experimental area has been implemented in the simulations. The simulations have been analyzed in the same manner as experimental data, in particular by applying the Pulse Height Weighting Technique. The simulations have been validated against a measurement of the neutron background performed with a $^mathrm{nat}$C sample, showing an excellent agreement above 1 keV. At lower energies, an additional component in the measured $^mathrm{nat}$C yield has been discovered, which prevents the use of $^mathrm{nat}$C data for neutron background estimates at neutron energies below a few hundred eV. The origin and time structure of the neutron background have been derived from the simulations. Examples of the neutron background for two different samples are demonstrating the important role of accurate simulations of the neutron background in capture cross section measurements.
The results of the experimental search for two-neutrino $2K$-capture in $^{124}$Xe with a large copper proportional counter obtained by processing the data for an exposure of 37.7 kg$times$day are presented. The experimental setup is located at the Underground Low-Background Laboratory of the Baksan Neutrino Observatory at a depth of 4900 m w.e. The combination of methods of selection of useful signals with a unique set of characteristics and the event topology taken into account allowed us to suppress the background in the energy region of interest. A new half-life limit for $2K(2 u)$-capture in $^{124}$Xe was determined: T$_{1/2}geq7.7cdot10^{21}$ yrs (90% C.L.).
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