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Register a new userExperimental Constraint on Stellar Electron-Capture Rates from the ${}^{88}text{Sr}(t,{}^{3}text{He}+gamma){}^{88}text{Rb}$ reaction at 115 MeV/u
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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.
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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.
Our understanding of the low-lying resonance structure in $^{12}$C remains incomplete. We have used the $^{11}text{B}(p,3alpha)gamma$ reaction at proton energies of $E_p=0.5-2.7$ MeV as a selective probe of the excitation region above the $3alpha$ threshold in $^{12}$C. Transitions to individual levels in $^{12}$C were identified by measuring the 3$alpha$ final state with a compact array of charged-particle detectors. Previously identified transitions to narrow levels were confirmed and new transitions to broader levels were observed for the first time. Here, we report cross sections, deduce partial $gamma$-decay widths and discuss the relative importance of direct and resonant capture mechanisms.
In the late stages of stellar core-collapse, prior to core bounce, electron captures on medium-heavy nuclei drive deleptonization and simulations require the use of accurate reaction rates. Nuclei with neutron number near $N=50$, just above atomic number $Z=28$, play an important role, but rates used in astrophysical simulations rely primarily on a relatively simple single-state approximation. In order to improve the accuracy of astrophysical simulations, experimental data are needed to test the electron-capture rates and to guide the development of better theoretical models. This work presents the results of the $^{86}$Kr($t$,$^{3}$He+$gamma$) experiment at the NSCL, from which an upper limit for the Gamow-Teller strength up to an excitation energy in $^{86}$Br of 5 MeV is extracted. The derived upper limit for the electron-capture rate on $^{86}$Kr indicates that the rate estimated through the single-state approximation is too high and that rates based on Gamow-Teller strengths estimated in shell-model and QRPA calculations are more accurate. The QRPA calculations tested in this manner were used for estimating the electron capture rates for 78 isotopes near $N=50$ and above $Z=28$. The impact of using these new electron-capture rates in simulations of supernovae instead of the rates based on the single-state approximation is investigated, indicating a significant reduction in the deleptonization that affects multi-messenger signals, such as the emission of neutrinos and gravitational waves.
In this Letter we report on the first inverse kinematics measurement of key resonances in the ${}^{22}text{Ne}(p,gamma)^{23}text{Na}$ reaction which forms part of the NeNa cycle, and is relevant for ${}^{23}$Na synthesis in asymptotic giant branch (AGB) stars. An anti-correlation in O and Na abundances is seen across all well-studied globular clusters (GC), however, reaction-rate uncertainties limit the precision as to which stellar evolution models can reproduce the observed isotopic abundance patterns. Given the importance of GC observations in testing stellar evolution models and their dependence on NeNa reaction rates, it is critical that the nuclear physics uncertainties on the origin of ${}^{23}$Na be addressed. We present results of direct strengths measurements of four key resonances in ${}^{22}text{Ne}(p,gamma)^{23}text{Na}$ at E$_{{text c.m.}}$ = 149 keV, 181 keV, 248 keV and 458 keV. The strength of the important E$_{{text c.m.}}$ = 458 keV reference resonance has been determined independently of other resonance strengths for the first time with an associated strength of $omegagamma$ = 0.439(22) eV and with higher precision than previously reported. Our result deviates from the two most recently published results obtained from normal kinematics measurements performed by the LENA and LUNA collaborations but is in agreement with earlier measurements. The impact of our rate on the Na-pocket formation in AGB stars and its relation to the O-Na anti-correlation was assessed via network calculations. Further, the effect on isotopic abundances in CO and ONe novae ejecta with respect to pre-solar grains was investigated.
Transfer reactions provide information about the single-particle nature of nuclear levels. In particular, the differential cross sections from these measurements are sensitive to the angular momentum of the transferred particle and the spectroscopic factor of the populated level. However, the process of extracting these properties is subject to uncertainties, both from experimental and theoretical sources. By integrating the distorted wave Born approximation into a Bayesian model, we propagate these uncertainties through to the spectroscopic factors and orbital angular momentum values. We use previously reported data of the proton pickup reaction $^{70}$Zn$(d, ^3!text{He}) ^{69}$Cu as an example. By accounting for uncertainties in the experimental data, optical model parameters, and reaction mechanism, we find that the extracted spectroscopic factors for low lying states of $^{69}$Cu are subject to large, asymmetric uncertainties ranging from $35 %$ to $108 %$. Additionally, Bayesian model comparison is employed to assign probabilities to each of the allowed angular momentum transfers. This method confirms the assignments for many states, but suggests that the data for a state lying at $3.70$ MeV is better characterized by an $ell = 3$ transfer, rather than the previously reported $ell = 2$.
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