GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany.
Published: October 2020
AI Article Synopsis
Article Abstract
The electron-capture decay followed by a prompt fission process was searched for in the hitherto unknown most neutron-deficient Md isotope with mass number 244. Alpha decay with α-particle energies of 8.73-8.86 MeV and with a half-life of 0.30_{-0.09}^{+0.19} s was assigned to ^{244}Md. No fission event with a similar half-life potentially originating from spontaneous fissioning of the short-lived electron-capture decay daughter ^{244}Fm was observed, which results in an upper limit of 0.14 for the electron-capture branching of ^{244}Md. Two groups of fission events with half-lives of 0.9_{-0.3}^{+0.6} ms and 5_{-2}^{+3} ms were observed. The 0.9_{-0.3}^{+0.6} ms activity was assigned to originate from the decay of ^{245}Md. The origin of eight fission events resulting in a half-life of 5_{-2}^{+3} ms could not be unambiguously identified within the present data while the possible explanation has to invoke previously unseen physics cases.
We give for the first time theoretical estimates of unknown rare electron-capture (EC) decay branchings of ^{44}Ti, ^{57}Co, and ^{139}Ce, relevant for searches of (exotic) dark-matter particles. The nuclear-structure calculations have been done exploiting the nuclear shell model with well-established Hamiltonians and an advanced theory of β decay. In the absence of experimental measurements of these rare branches, these estimates are of utmost importance for terrestrial searches of dark-matter particles, such as axionic dark matter in the form of axionlike particles, anapole dark matter, and dark photons in nuclear transitions.
We provide an overview of the isotopic signatures of presolar supernova grains, specifically focusing on Ti-containing grains with robustly inferred supernova origins and their implications for nucleosynthesis and mixing mechanisms in supernovae. Recent technique advancements have enabled the differentiation between radiogenic (from Ti decay) and nonradiogenic Ca excesses in presolar grains, made possible by enhanced spatial resolution of Ca-Ti isotope analyses with the Cameca NanoSIMS (Nano-scale Secondary Ion Mass Spectrometer) instrument. Within the context of presolar supernova grain data, we discuss () the production of Ti in supernovae and the impact of interstellar medium heterogeneities on the galactic chemical evolution of Ca/Ca, () the nucleosynthesis processes of neutron bursts and explosive H-burning in Type II supernovae, and () challenges in identifying the progenitor supernovae for Cr-rich presolar nanospinel grains.
Free molecules undergo processes with photons; in particular, they can undergo photoionization and photodissociation, which are relevant processes in nature and laboratory. Recently, it has been shown that in a cavity, the reverse process of photoionization, namely, electron capture becomes highly probable. The underlying mechanism is the formation of a hybrid resonance state.
Terbium-157 was radiochemically extracted from an irradiated tantalum target. Since the resulting material contained a significant impurity of Tb, Tb was isotopically purified using laser resonance ionization at the RISIKO mass separator in Mainz and then implanted on an aluminum (Al) foil. The implanted Tb was measured by two different calibrated gamma-ray spectrometers to determine photon emission rates.
Gadolinium-153 was standardized for activity by live-timed anticoincidence counting and an ampoule was submitted to the international reference system (SIR). Absolute emission intensities for the main γ rays were determined with calibrated high-purity germanium (HPGe) and lithium-drifted silicon (Si(Li)) detectors. A revised decay scheme is indicated, with no probability of direct electron capture to the Eu ground state.
