Meson-exchange enhancement in first-forbidden β\beta -transitions: the case of 50^{50}K and 38^{38}Ca

The $\beta$- decay of $^{50}$K and $^{38}$Ca have been investigated with the main motive of determining more accurately the first-forbidden $\beta$- branches, in particular the rank-zero, $\Delta$J = 0, $\,\beta$ -transitions. $^{50}$K and $^{38}$Ca have been produced by fragmentation of U and Ti targets respectively, with a 1 GeV proton beam and subsequent on-line mass separation. For $^{50}$K, $\gamma$-ray spectroscopy, as well as delayed neutron spectroscopy by time of flight, were carried out to obtain a detailed decay scheme to 20 (bound and unbound) levels in $^{50}$Ca. The level structur e of $^{50}$Ca can be compared to recent calculations which incorporate 1p1h excitations from the f$_{7/2}$ shell. The first-forbidden $\beta^-$ transition $^{50}$ K(0$^-$)$\,\to{}^{50}$Ca(0$^+$) g.s. has been evaluated for the first time by a direct measurement of $\beta$- and $\gamma$- activities. Its importance (61.0 $\pm$ 7.4$\%$) is interpreted as an effect of the meson-exchange current (MEC) l eading to an enhancement factor of 62(5)$\%$ in comparison with the value predicted by shell-model calculations using the impulse approximation. For the $^{38}$ Ca$\,\to{}^{38}$K decay, chemical selec tive production was obtained through separation of the molecular ion CaF$^+$ without contamination by isobars. In these conditions, the measurement of very weak $\beta$-branches, at a level of 10$^{-3}\%$ decays, could be made and a limit, at the 2$\sigma$-confidence level, has been obtained for the 0$^+\to$ 0$^-$ branch to the level at E$_x$ = 2993 keV (I$_\beta$ < 0.0046$\%$). Imp lications of these results on the general trend of meson-exchange enhancements of first-forbidden transitions within the framework of the spherical shell model are discussed

Structure of high-lying levels populated in the ⁹⁶Y → ⁹⁶Zr β decay

The nature of Jπ=1− levels of 96Zr below the β-decay Qβ value of 96Y has been investigated in high-resolution γ-ray spectroscopy following the β decay as well as in a campaign of inelastic photon scattering experiments. Branching ratios extracted from β decay allow the absolute E1 excitation strength to be determined for levels populated in both reactions. The combined data represents a comprehensive approach to the wavefunction of 1− levels below the Qβ value, which are investigated in the theoretical approach of the Quasiparticle Phonon Model. This study clarifies the nuclear structure properties associated with the enhanced population of high-lying levels in the 96Ygs β decay, one of the three most important contributors to the high-energy reactor antineutrino spectrum

Structure of high-lying levels populated in the Y-96 -> Zr-96 beta decay

The nature of $J^{\pi}=1^-$ levels of $^{96}$Zr below the $\beta$-decay $Q_{\beta}$ value of $^{96}$Y has been investigated in high-resolution $\gamma$-ray spectroscopy following the $\beta$ decay as well as in a campaign of inelastic photon scattering experiments. Branching ratios extracted from $\beta$ decay allow the absolute $E1$ excitation strength to be determined for levels populated in both reactions. The combined data represents a comprehensive approach to the wavefunction of $1^-$ levels below the $Q_{\beta}$ value, which are investigated in the theoretical approach of the Quasiparticle Phonon Model. This study clarifies the nuclear structure properties associated with the enhanced population of high-lying levels in the $^{96}$Y$_{gs}$ $\beta$ decay, one of the three most important contributors to the high-energy reactor antineutrino spectrum

Pulsed neutron interrogation with PVT plastic scintillators to detect nuclear materials

International audienceIn the framework of homeland security, the Nuclear Measurements Laboratory of CEA Cadarache is studying the detection of Special Nuclear Materials hidden in sea-going cargo containers. Proof of principle experiments done in the DANAIDES facility of CEA Cadarache shows the feasibility of detecting some dozen of grams of highly enriched uranium by means of neutron interrogation with a 14 MeV D-T pulsed neutron generator and an array of EJ-200 plastic scintillators. We show that uranium can be detected by measuring correlated induced-fission prompt neutrons and gamma rays in coincidence, which is a first attempt with such detectors without neutron-gamma PSD capabilities, or more classically by detecting fission-induced delayed gamma rays