Investigation of background sources associated with the production of radioactive isotopes in the DEVIS experiment (double-beta decay)

Measurements performed in searches for the double-beta decay of ¹³⁶Xe showed an excess of events in exposures of a target enriched in the isotope ¹³⁶Xe in relation to exposures of a target depleted in this isotope. The activation of xenon in the detector used under the effect of background neutron and muon radiation and the possible contribution of these processes to the results of relevant measurements are studied.     

Measurement of the <Superscript>222</Superscript>Rn background in the DEVIS experiment

In the DEVIS tracking experiment that is being performed at the Institute of Theoretical and Experimental Physics (ITEP, Moscow, Russia) and which is devoted to searches for ¹³⁶Xe double-beta decay, the ²²²Rn concentration is measured in xenon samples with different content of the isotope ¹³⁶Xe. The number of probable double-electron events from ²¹⁴Bi decay mimicking the double-beta decay of ¹³⁶Xe is estimated.     

Search for Neutrinoless Double-Beta Decay in ^{136}Xe with EXO-200

We report on a search for neutrinoless double-beta decay of ^{136}Xe with EXO-200. No signal is observed for an exposure of 32.5?kg?yr, with a background of ～1.5×10^{-3} kg^{-1}?yr^{-1}?keV^{-1} in the ±1σ region of interest. This sets a lower limit on the half-life of the neutrinoless double-beta decay T_{1/2}^{0νββ}(^{136}Xe)>1.6×10^{25} yr (90% C.L.), corresponding to effective Majorana masses of less than 140-380?meV, depending on the matrix element calculation.     

Double-beta decay of 48Ca in the TGV experiment

The low-background, high-sensitivity Ge multidetector spectrometer TGV is used to study the double-beta decay of ⁴⁸Ca. Additional suppression of the recorded background is achieved with neutron shielding and a method for distinguishing β particles from γ rays by detector-pulse rise time. The estimates T _1/2 ^γγ2ν = (4.2 ± 2.4) × 10¹⁹ yr and T _1/2 ^γγ0ν > 1.5 × 10²¹ yr (at a 90% C.L.) for the double-beta decay of ⁴⁸Ca are obtained.     

New limit on the half-life of 78Kr with respect to the 2K(2ν)-capture decay mode

The features of data accumulated for 1817 h in an experimental search for the 2K(2ν)-capture mode of ⁷⁸Kr decay are discussed. A new limit on the half-life for this decay is found: T _1/2≥2.3×10²⁰ yr (at a 90% C.L.).     

Search for two-neutrino 2<Emphasis Type="Italic">K</Emphasis> capture in <Superscript>124</Superscript>Xe: the method and preliminary results

The technique used in the experiment in the search for the process of 2K2ν-capture in ¹²⁴Xe using large high-pressure proportional counters is briefly described. The measurements with a xenon sample enriched to 21% (or ~57 g) of the ¹²⁴Xe isotope carried out for 3340 hours yielded a zero signal. The resultant upper limit on the ¹²⁴Xe half-life with respect to the 2ν-mode of the nuclear capture of two K-shell electrons amounts to T_1/2>4.6 10²¹ yr at the 90% confidence level.     

Recent results of the IGEX 76Ge double-beta decay experiment

The International Germanium Experiment (IGEX) has now analyzed 117 mol yr of data from its isotopically enriched (86% ⁷⁶Ge) germanium detectors. Applying pulse shape discrimination (PSD) to the more recent data, the lower bound on the half-life for neutrinoless double-beta decay of ⁷⁶Ge is deduced: T _1/2(0ν)>1.57×10²⁵ yr (90% C.L.). This corresponds to an upper bound on the Majorana neutrino mass parameter, 〈m _ν〉, between 0.33 eV and 1.35 eV depending on the choice of theoretical nuclear matrix elements used in the analysis.     

Physics potential of searching for 0vββ decays in JUNO

In the past few decades, numerous searches have been made for the neutrinoless double-beta decay (0vββ) process, aiming to establish whether neutrinos are their own antiparticles (Majorana neutrinos), but no 0vββ decay signal has yet been observed. A number of new experiments are proposed but they ultimately suffer from a common problem: the sensitivity may not increase indefinitely with the target mass. We have performed a detailed analysis of the physics potential by using the Jiangmen Underground Neutrino Observatory (JUNO) to improve the sensitivity to 0vββ up to a few meV, a major step forward with respect to the experiments currently being planned. JUNO is a 20 kton low-background liquid scintillator (LS) detector with 3%E(MeV) energy resolution, now under construction. It is feasible to build a balloon filled with enriched xenon gas (with ¹³⁶Xe up to 80%) dissolved in LS, inserted into the central region of the JUNO LS. The energy resolution is ～1.9% at the Q-value of ¹³⁶Xe 0vββ decay. Ultra-low background is the key for 0vββ decay searches. Detailed studies of background rates from intrinsic 2vββ and ⁸B solar neutrinos, natural radioactivity, and cosmogenic radionuclides (including light isotopes and ¹³⁷Xe) were performed and several muon veto schemes were developed. We find that JUNO has the potential to reach a sensitivity (at 90% C. L.) to T_1/2^0vββ of 1.8×10²⁸ yr (5.6×10²⁷ yr) with ～50 tons (5 tons) of fiducial ¹³⁶Xe and 5 years exposure, while in the 50-ton case the corresponding sensitivity to the effective neutrino mass, m_ββ, could reach (5-12) meV, covering completely the allowed region of inverted neutrino mass ordering.     

AMoRE: Collaboration for searches for the neutrinoless double-beta decay of the isotope of 100Mo with the aid of 40Ca100MoO4 as a cryogenic scintillation detector

The AMoRE (Advanced Mo based Rare process Experiment) Collaboration is planning to employ ⁴⁰Ca¹⁰⁰MoO₄ single crystals as a cryogenic Scintillation detector for studying the neutrinoless double-beta decay of the isotope ¹⁰⁰Mo. A simultaneous readout of phonon and scintillation signals is performed in order to suppress the intrinsic background. The planned sensitivity of the experiment that would employ 100 kg of ⁴⁰Ca¹⁰⁰MoO₄ over five years of data accumulation would be T _1/2 ^0ν = 3 × 10²⁶ yr, which corresponds to values of the effective Majorana neutrino mass in the range of 〈m _ν 〉 ～ 0.02–0.06 eV.     

Observation of two-neutrino double-beta decay in 136Xe with the EXO-200 detector

We report the observation of two-neutrino double-beta decay in (136)Xe with T(1/2) = 2.11 ± 0.04(stat) ± 0.21(syst) × 10(21) yr. This second-order process, predicted by the standard model, has been observed for several nuclei but not for (136)Xe. The observed decay rate provides new input to matrix element calculations and to the search for the more interesting neutrinoless double-beta decay, the most sensitive probe for the existence of Majorana particles and the measurement of the neutrino mass scale.     

Life-times of the 4 1 + -states in134Te and136Xe

The half-lives of the first excited 4⁺-levels in theN=82 isotones ₅₂ ¹³⁴ Te and ₅₄ ¹³⁶ Xe have been determined tot _1/2=(1.50±0.13) ns andt _1/2=(1.32±0.10) ns, respectively, through the measurement of delayedγ,γ-coincidences. The value for¹³⁴Te is in agreement with the assumption that the low lying levels of this nucleus are based mainly on theg ₇ ^{2 /2} proton configuration. In¹³⁶Xe the half-life is unexpectedly small which indicates a more strongly admixed structure of the levels involved in this case.     

New stage of search for 2K(2ν) capture of 78Kr

A technique to search for 2K capture of ⁷⁸Kr with a large low-background proportional counter filled with a sample of krypton enriched in ⁷⁸Kr up to 99.8% at a pressure of 4.51 atm is described in this paper. The results of first measurements are presented. An analysis of data collected for 159 h yielded a new limit to the half-life of ⁷⁸Kr with regard to 2K capture (T _1/2 ≥ 1.5 × 10²¹ yr (90% C.L.)). The sensitivity of the facility to the process for one year of measurement was evaluated to be S = 1.0 × 10²² yr (90% C.L.). The text was submitted by the authors in English.     

Decay properties of the new proton-rich nucleus104Sn: Mass determinations beyond the proton drip line

The new proton-rich nucleus¹⁰⁴Sn was produced in a⁵⁰Cr(⁵⁸Ni, 2p2n)¹⁰⁴Sn reaction. Its half-life was measured to be 21±1s. From a newly established partial decay scheme, aQ _EC value of 4,000 _?300 ⁺⁶⁵⁰ keV was estimated. The resulting mass excess value for¹⁰⁴Sn, combined with decay energies for α and proton emission, leads to mass excess values for the nuclei¹⁰⁸Te,¹⁰⁹I,¹¹²Xe, and¹¹³Cs. These mass values are compared to predictions of mass formulae.     

Determination of the neutron binding energy of the delayed neutron emitter137Xe

The neutron binding energy of¹³⁷Xe has been deduced to be 4025.2±0.6 keV from a study of the¹³⁶Xe(n, γ)¹³⁷Xe reaction. The importance of a precise value for this quantity is due to the fact that an accurate determination of binding energies of delayed neutron emitters is possible only for⁸⁷Kr and¹³⁷Xe, neighbouring stable isotopes. Nuclear reaction.¹³⁶Xe(n,γ), enriched target; measurede89-01, deduced neutron binding energy, Ge(Li) detector.     

Double-beta decay of <Superscript>150</Superscript>Nd to the first 0<Superscript>+</Superscript> excited state of <Superscript>150</Superscript> Sm: Current state

Two-neutrino double-beta decay of ¹⁵⁰Nd to the first 0⁺ excited state in ¹⁵⁰Sm is investigated with the 400-cm³ low-background HPGe detector. Preliminary data analysis for 6843 h shows an excess of events at 333.9 and 406.5 keV. If this excess is assigned to the investigated transition, then its half-life can be estimated at [1.2 _?0.3 ^+0.5 ±0.4(syst.)]×10²⁰ yr.     

The decay of the136I isomers to the energy levels of136Xe

The gamma ray spectra associated with the decay of the¹³⁶I isomers have been measured by means of a Ge(Li) detector. The iodine samples were produced by using the technique of on-line isotope separation of the fission products of a²³⁵U target at the R2-O reactor in Studsvik, Sweden. On the basis of the measured singles and Ge-Ge coincidence spectra a new level scheme was constructed for¹³⁶Xe. A comparison with theoretical calculations is included. The half-life analysis of the gamma peaks as well as the beta andK X-rays associated with mass 136 iodine showed that there exist three separate half-lives: 48 ± 2 s, 83 ± 3 s and 100 ± 3 s, the last one being not previously reported. The possibility of the existence of three isomeric states in¹³⁶I is discussed.     

Search for the double-beta decay of <Superscript>130</Superscript>Te to excited states in <Superscript>130</Superscript>Xe

Experimental limits on the half-lives with respect to the (0ν+2ν) double-beta decay of ¹³⁰Te to excited states in ¹³⁰Xe are obtained by using low-background HPGe detectors. At a 90% C.L., they are 1.6×10²¹, 2.7×10²¹, and 2.3×10²¹ yr for transitions to the 2 ₁ ⁺ , 2 ₂ ⁺ , and 0 ₁ ⁺ levels, respectively.     

Results of a search for 2β decay of 136Xe with high-pressure copper proportional counters in Baksan Neutrino Observatory

A search for 2β decay of ¹³⁶Xe with two high-pressure copper proportional counters was carried out in the Baksan Neutrino Observatory. The experiment is based on comparison of spectra measured with natural and enriched xenon. No evidence was found for 2β(2ν) and 2β(0ν) decay. The decay half-life limit based on data measured for 8000 h is T _1/2 ≥ 8.5 × 10²¹ yr for 2ν mode and T _1/2 ≥ 3.1 × 10²³ yr for 0ν mode (90% C.L.). The text was submitted by the authors in English.     

Velocity dependence of total Penning ionization cross sections

The velocity dependence of the total Penning ionization cross sections,σ(v), is measured in the thermal relative velocity region, using a time of flight method.σ(v) curves are reported for the collision systems He(2¹ S)/Ar, Kr, Xe, N₂, Hg, He(2³ S)/Ar, Kr, Xe, N₂, Hg, Ne(³ P _{2, 0})/Kr, Hg, and Ar(³ P _{2, 0})/Hg. In a qualitative discussion it is shown that all features of the measuredσ(v) curves may be explained within the frame of the theory of Penning ionization, allowing to extract information on the physical quantities governing the process: on the interaction potentialV(R) and on the transition probabilityW(R). A theoretical calculation for the He(2³ S)/Ar system shows good agreement with our experimentalσ(v) curve. On the basis of the present results earlier data onσ(v), and on absolute cross sections and rate constants obtained at certain relative velocity distributions are discussed.