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Register a new user$text{Direct}~Qtext{-Value Determination of the}~beta^-~text{Decay of} ~^{187}text{Re}$
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The cyclotron frequency ratio of $^{187}mathrm{Os}^{29+}$ to $^{187}mathrm{Re}^{29+}$ ions was measured with the Penning-trap mass spectrometer PENTATRAP. The achieved result of $R=1.000:000:013:882(5)$ is to date the most precise such measurement performed on ions. Furthermore, the total binding-energy difference of the 29 missing electrons in Re and Os was calculated by relativistic multiconfiguration methods, yielding the value of $Delta E = 53.5(10)$ eV. Finally, using the achieved results, the mass difference between neutral $^{187}$Re and $^{187}$Os, i.e., the $Q$ value of the $beta^-$ decay of $^{187}$Re, is determined to be 2470.9(13) eV.
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We search for a dependence of the lifetime of $^{20}text{Na}$ nuclei on the nuclear spin direction. Such a directional dependence would be evidence for Lorentz-invariance violation in weak interactions. A difference in lifetime between nuclei that are polarized in the east and west direction is searched for. This difference is maximally sensitive to the rotation of the Earth, while the sidereal dependence is free from most systematic errors. The experiment sets a limit of $2times 10^{-4}$ at 90 % C.L. on the amplitude of the sidereal variation of the relative lifetime differences, an improvement by a factor 15 compared to an earlier result.
We report the first direct measurement of the $^{14}text{O}$ superallowed Fermi $beta$-decay $Q_{EC}$-value, the last of the so-called traditional nine superallowed Fermi $beta$-decays to be measured with Penning trap mass spectrometry. $^{14}$O, along with the other low-$Z$ superallowed $beta$-emitter, $^{10}$C, is crucial for setting limits on the existence of possible scalar currents. The new ground state $Q_{EC}$ value, 5144.364(25) keV, when combined with the energy of the $0^+$ daughter state, $E_x(0^+)=2312.798(11)$~keV [Nucl. Phys. A {bf{523}}, 1 (1991)], provides a new determination of the superallowed $beta$-decay $Q_{EC}$ value, $Q_{EC}(text{sa}) = 2831.566(28)$ keV, with an order of magnitude improvement in precision, and a similar improvement to the calculated statistical rate function $f$. This is used to calculate an improved $mathcal{F}t$-value of 3073.8(2.8) s.
Background: The understanding and description of forbidden decays provides interesting challenges for nuclear theory. These calculations could help to test underlying nuclear models and interpret experimental data. Purpose: Compare a direct measurement of the $^{138}$La $beta$-decay $Q$ value with the $beta$-decay spectrum end-point energy measured by Quarati et al. using LaBr$_3$ detectors [Appl. Radiat. Isot. 108, 30 (2016)]. Use new precise measurements of the $^{138}$La $beta$-decay and electron capture (EC) $Q$ values to improve theoretical calculations of the $beta$-decay spectrum and EC probabilities. Method: High-precision Penning trap mass spectrometry was used to measure cyclotron frequency ratios of $^{138}$La, $^{138}$Ce and $^{138}$Ba ions from which $beta$-decay and EC $Q$ values for $^{138}$La were obtained. Results: The $^{138}$La $beta$-decay and EC $Q$ values were measured to be $Q$ = 1052.42(41) keV and $Q_{EC}$ = 1748.41(34) keV, improving the precision compared to the values obtained in the most recent atomic mass evaluation [Wang, et al., Chin. Phys. C 41, 030003 (2017)] by an order of magnitude. These results are used for improved calculations of the $^{138}$La $beta$-decay shape factor and EC probabilities. New determinations for the $^{138}$Ce 2EC $Q$ value and the atomic masses of $^{138}$La, $^{138}$Ce, and $^{138}$Ba are also reported. Conclusion: The $^{138}$La $beta$-decay $Q$ value measured by Quarati et al. is in excellent agreement with our new result, which is an order of magnitude more precise. Uncertainties in the shape factor calculations for $^{138}$La beta-decay using our new $Q$ value are reduced by an order of magnitude. Uncertainties in the EC probability ratios are also reduced and show improved agreement with experimental data.
The Gamow-Teller strength distribution from ${}^{88}$Sr was extracted from a $(t,{}^{3}text{He}+gamma)$ experiment at 115 MeV/$u$ to constrain estimates for the electron-capture rates on nuclei around $N=50$, between and including $^{78}$Ni and $^{88}$Sr, which are important for the late evolution of core-collapse supernovae. The observed strength below an excitation energy of 8 MeV was consistent with zero and below 10 MeV amounted to $0.1pm0.05$. Except for a very-weak transition that could come from the 2.231-MeV $1^{+}$ state, no $gamma$ lines that could be associated with the decay of known $1^{+}$ states were identified. The derived electron-capture rate from the measured strength distribution is more than an order of magnitude smaller than rates based on the single-state approximation presently used in astrophysical simulations for most nuclei near $N=50$. Rates based on shell-model and quasiparticle random-phase approximation calculations that account for Pauli blocking and core-polarization effects provide better estimates than the single-state approximation, although a relatively strong transition to the first $1^{+}$ state in $^{88}$Rb is not observed in the data. Pauli unblocking effects due to high stellar temperatures could partially counter the low electron-capture rates. The new data serves as a zero-temperature benchmark for constraining models used to estimate such effects.
The bremsstrahlung flux-averaged cross sections for the photoneutron reactions $^{text{93}}$Nb($gamma $,xn;x=1-5)$^{text{(93-x)m,g}}$Nb were measured in the range of boundary energies of bremsstrahlung $gamma $-quanta $E_{gamma text{max}}$=33-93 MeV with a step $Delta E_{gamma text{max}}approx $ 2 MeV. The isomeric ratios of the average cross-sections of the products of the reactions $^{text{93}}$Nb ($gamma $ ,4n)$^{text{89m,g}}$Nb and $^{text{93}}$Nb($gamma $,5n)$^{text{88m,g}}$ Nb were determined in the energy ranges $E_{gamma text{max}}$ = 50-93 and 70-93 MeV, respectively. The experiments were carried out on the beam of the linear electron accelerator LU-40 of the Science and Research Establishment (SRE) Accelerator at National Science Center Kharkov Institute of Physics and Technology (NSC KIPT) using the method of induced activity. Calculations of the cross sections, average cross sections, and isomeric ratios of the reaction products were performed using the TALYS 1.9 code with default parameters and the GEANT4 code. The tendency of a more successful description of the average cross sections of photoneutron reactions with the formation of final odd-even Nb nuclei than odd-odd Nb nuclei is revealed. The experimental average cross sections for the reactions ($gamma $,2n) and ($gamma $,4n) are in good agreement with theory, while in the case of reactions ($gamma $,n), ($gamma $,3n), and ($ gamma $,5n), some discrepancies are observed. The results obtained for the reactions ($gamma $,n), ($gamma $,3n) and ($gamma $,4n) are in satisfactory agreement with the known literature data. The average cross sections for the reactions ($gamma $,2n) and ($gamma $,5n) and the isomeric ratios of the reaction products $^{text{93}}$Nb($gamma $,5n)$^{text{88m,g}}$Nb were measured for the first time.
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