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Register a new userExperimental searches for rare alpha and beta decays
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The current status of the experimental searches for rare alpha and beta decays is reviewed. Several interesting observations of alpha and beta decays, previously unseen due to their large half-lives ($10^{15} - 10^{20}$ yr), have been achieved during the last years thanks to the improvements in the experimental techniques and to the underground locations of experiments that allows to suppress backgrounds. In particular, the list includes first observations of alpha decays of $^{151}$Eu, $^{180}$W (both to the ground state of the daughter nuclei), $^{190}$Pt (to excited state of the daughter nucleus), $^{209}$Bi (to the ground and excited states of the daughter nucleus). The isotope $^{209}$Bi has the longest known half-life of $T_{1/2} approx 10^{19}$ yr relatively to alpha decay. The beta decay of $^{115}$In to the first excited state of $^{115}$Sn (E$_{exc} = 497.334$ keV), recently observed for the first time, has the $Q_beta$ value of only $(147 pm 10)$ eV, which is the lowest $Q_beta$ value known to-date. Searches and investigations of other rare alpha and beta decays ($^{48}$Ca, $^{50}$V, $^{96}$Zr, $^{113}$Cd, $^{123}$Te, $^{178m2}$Hf, $^{180m}$Ta and others) are also discussed.
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Results of long-term investigations of variation of cobalt beta decay rate from 28.12.2010 till 08.02.2012 are presented. The scintillation spectrometer with two LaBr3 detectors is used to register of gamma-quanta with energy 1.173 and 1.332 MeV accompanying cobalt beta decay. Counting rate of each detector and their gamma-quanta coincidence are collected in successive time intervals 10 s. The statistical Kolmogorov-Smirnov method for data analysis is used. Temperature influence on experimental results is also analyzed. Deviations of beta decay counting rate from constant distribution during the days were detected in those decades: from 11.03 to 21.03 with significance level a = 0.1; from 22.04 to 02.05 with a=0.0125; from 24.06 to 04.07 with a=0.05; from 04.08 to 14.08 with a=0.05.
Beta-delayed proton emission may occur at very low rates in the decays of the light nuclei $^{11}$Be and $^8$B. This paper explores the potential physical significance of such decays, estimates their rates and reports on first attempts to detect them: an experiment at ISOLDE/CERN gives a branching ratio for $^{11}$Be of $(2.5 pm 2.5) cdot 10^{-6}$ and an experiment at JYFL a 95% confidence upper limit of $2.6 cdot 10^{-5}$ for $^8$B.
The radioactive contamination of a BaF$_2$ scintillation crystal with mass of 1.714 kg was measured over 101 hours in the low-background DAMA/R&D set-up deep underground (3600 m w.e.) at the Gran Sasso National Laboratories of INFN (LNGS, Italy). The half-life of $^{212}$Po (present in the crystal scintillator due to contamination by radium) was measured as $T_{1/2}(^{212}$Po) = 298.8$pm$0.8(stat.)$pm$1.4(syst.) ns by analysis of the events pulse profiles. The $^{222}$Rn nuclide is known as 100% decaying via emission of $alpha$ particle with $T_{1/2}$ = 3.82 d; however, its $beta$ decay is also energetically allowed with $Q_beta = 24pm21$ keV. Search for decay chains of events with specific pulse shapes characteristic for $alpha$ or for $beta/gamma$ signals and with known energies and time differences allowed us to set, for the first time, the limit on the branching ratio of $^{222}$Rn relatively to $beta$ decay as $B_beta < 0.13$% at 90% C.L. (equivalent to limit on partial half-life $T_{1/2}^beta > 8.0$ y). Half-life limits of $^{212}$Pb, $^{222}$Rn and $^{226}$Ra relatively to $2beta$ decays are also improved in comparison with the earlier results.
We report an experimental search for an exotic spin-spin-velocity-dependent interaction between polarized electrons of Rb atoms and polarized electrons of a solid-state mass, violating both the time-reversal and parity symmetries. This search targets a minute effective magnetic field induced by the interaction. A spin-exchange relaxation-free (SERF) magnetometer based on an optically polarized Rb vapor is the key element for both a source of polarized electrons and a high-sensitivity detector. A dysprosium iron garnet (DyIG) serves as the polarized mass, with an extremely small magnetization at the critical temperature around 240 K and a high spin density. To reduce the magnetization, one of major systematic effects, a home-built cooling system controls the mass temperature. To our knowledge, this is the first search for an exotic spin-dependent interaction using the compensated ferrimagnet DyIG as a polarized mass. The experiment set the most stringent limit on the electron-electron coupling strength in the centimeter interaction range, in particular $g_V^e g_V^e <10^{4}$ at $lambda=2$ cm.
Alpha decays in the EXO-200 detector are used to measure the fraction of charged $^{218}mathrm{Po}$ and $^{214}mathrm{Bi}$ daughters created from alpha and beta decays, respectively. $^{222}mathrm{Rn}$ alpha decays in liquid xenon (LXe) are found to produce $^{218}mathrm{Po}^{+}$ ions $50.3 pm 3.0%$ of the time, while the remainder of the $^{218}mathrm{Po}$ atoms are neutral. The fraction of $^{214}mathrm{Bi}^{+}$ from $^{214}mathrm{Pb}$ beta decays in LXe is found to be $76.4 pm 5.7%$, inferred from the relative rates of $^{218}mathrm{Po}$ and $^{214}mathrm{Po}$ alpha decays in the LXe. The average velocity of $^{218}mathrm{Po}$ ions is observed to decrease for longer drift times. Initially the ions have a mobility of $0.390 pm 0.006~mathrm{cm}^2/(mathrm{kV}~mathrm{s})$, and at long drift times the mobility is $0.219 pm 0.004~mathrm{cm}^2/(mathrm{kV}~mathrm{s})$. Time constants associated with the change in mobility during drift of the $^{218}mathrm{Po}^{+}$ ions are found to be proportional to the electron lifetime in the LXe.
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