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Register a new userFirst observation of the electron capture of $^{76}$As
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For a variety of radionuclides which decay via $beta^{-}$ and a weak $beta^{+}$-channel the electron-capture (EC) is not observed yet. As the interest of exact decay characteristica increased again, not least with the need of reliable data for experiments on investigation of the neutrinoless double-beta-decay, an experiment for the investigation of the $^{76}$As-EC was performed. The first time observation of the EC of $^{76}$As by this experiment resulted in a total branching-ratio for the EC / $beta^{+}$-channel of $p_mathrm{EC} = 0.0269 pm left(0.0080(mathrm{stat.}) pm 0.0029(mathrm{sys.}) right)$ and $p_mathrm{EC} = 0.0263 pm left(0.0077(mathrm{stat.}) pm 0.0047(mathrm{sys.}) right)$ according to two different methods. For the branching of this decay-channel into the first excited- and the ground state of gess a limit was obtained.
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Two-neutrino double electron capture ($2 u$ECEC) is a second-order Weak process with predicted half-lives that surpass the age of the Universe by many orders of magnitude. Until now, indications for $2 u$ECEC decays have only been seen for two isotopes, $^{78}$Kr and $^{130}$Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance. The $2 u$ECEC half-life provides an important input for nuclear structure models and its measurement represents a first step in the search for the neutrinoless double electron capture processes ($0 u$ECEC). A detection of the latter would have implications for the nature of the neutrino and give access to the absolute neutrino mass. Here we report on the first direct observation of $2 u$ECEC in $^{124}$Xe with the XENON1T Dark Matter detector. The significance of the signal is $4.4sigma$ and the corresponding half-life $T_{1/2}^{2 utext{ECEC}} = (1.8pm 0.5_text{stat}pm 0.1_text{sys})times 10^{22};text{y}$ is the longest ever measured directly. This study demonstrates that the low background and large target mass of xenon-based Dark Matter detectors make them well suited to measuring other rare processes as well, and it highlights the broad physics reach for even larger next-generation experiments.
The photon spectrum accompanying the orbital K-electron capture in the first forbidden unique decay of 81Kr was measured. The total radiation intensity for the photon energies larger than 50 keV was found to be 1.47(6) x 10^{-4} per K-capture. Both the shape of the spectrum and its intensity relative to the ordinary, non-radiative capture rate, are compared to theoretical predictions. The best agreement is found for the recently developed model which employs the length gauge for the electromagnetic field.
We report the first observation of the parity-violating 2.2 MeV gamma-ray asymmetry $A^{np}_gamma$ in neutron-proton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. $A^{np}_gamma$ isolates the $Delta I=1$, mbox{$^{3}S_{1}rightarrow {^{3}P_{1}}$} component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless EFT. We measured $A^{np}_gamma = [-3.0 pm 1.4 (stat) pm 0.2 (sys)]times 10^{-8}$, which implies a DDH weak $pi NN$ coupling of $h_{pi}^{1} = [2.6 pm 1.2(stat) pm 0.2(sys)] times 10^{-7}$ and a pionless EFT constant of $C^{^{3}S_{1}rightarrow ^{3}P_{1}}/C_{0}=[-7.4 pm 3.5 (stat) pm 0.5 (sys)] times 10^{-11}$ MeV$^{-1}$. We describe the experiment, data analysis, systematic uncertainties, and the implications of the result.
Neutrino accompanied double beta-decay of Ge-76 can populate the ground state and the excited states of Se-76. While the decay to the ground state has been observed with a half-life of 1.74 +0.18 -0.16 10^21 years, decays to the excited states have not yet been observed. Nuclear matrix elements depend on details of the nuclear transitions. A measurement of the half-life of the transition considered here could help to reduce the uncertainties of the calculations of the nuclear matrix element for the neutrinoless double beta decay of Ge-76. This parameter relates the half-life of the process to the effective Majorana neutrino mass. The results of a feasibility study to detect the neutrino accompanied double beta-decay of Ge-76 to the excited states of Se-76 are presented in this paper. Segmented germanium detectors were assumed in this study. Such detectors, enriched in Ge-76 to a level of about 86%, will be deployed in the GERDA experiment located at the INFN Gran Sasso National Laboratory, Italy. It is shown that the decay of Ge-76 to the 1122 keV 0+ level of Se-76 can be observed in GERDA provided that the half-life of the process is in the range favoured by the present calculations which is 7.5 10^21 y to 3.1 10^23 y.
The lifetimes of the first 2$^{+}$ states in the neutron-deficient $^{76,78}$Sr isotopes were measured using a unique combination of the $gamma$-ray line-shape method and two-step nucleon exchange reactions at intermediate energies. The transition rates for the 2$^{+}$ states were determined to be $B$(E2;2$^{+}$$to 0^{+}$) = 2220(270) e$^{2}$fm$^{4}$ for $^{76}$Sr and 1800(250) e$^{2}$fm$^{4}$ for $^{78}$Sr, corresponding to large deformation of $beta_2$ = 0.45(3) for $^{76}$Sr and 0.40(3) for $^{78}$Sr. The present data provide experimental evidence for mutually enhanced collectivity that occurs at $N$ = $Z$ = 38. The systematic behavior of the excitation energies and $B$(E2) values indicates a signature of shape coexistence in $^{76}$Sr, characterizing $^{76}$Sr as one of most deformed nuclei with an unusually reduced $E$(4$^{+}$)/$E$(2$^{+}$) ratio.
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