Gamow-Teller transition strengths from Fe 56 extracted from the Fe 56 (t, He 3) reaction

Abstract

Background: Gamow-Teller (GT) transition strengths are key inputs for estimating weak reaction rates of importance for a wide variety of astrophysical applications. (n,p)-type charge-exchange reactions, such as the (t,3He) reaction used in this work, are commonly used for extracting the GT strength distribution in the β+ (electron-capture) direction. Such studies are important for testing theoretical models used to estimate weak rates for a large number of nuclei for simulations of astrophysical phenomena. Purpose: The 56Fe(t,3He) reaction at 115 A MeV was measured in order to extract GT strengths for transitions to 56Mn. The extracted strength distributions were compared with shell-model calculations in the pf -shell model space using the KB3G and GXPF1a interactions, and with calculations in the quasi-particle random-phase approximation (QRPA). Method: Differential cross sections and excitation-energy spectra for the 56Fe(t,3He) reaction were determined by measuring the trajectories of 3He ejectiles through the S800 magnetic spectrograph and deducing their momenta. Contributions corresponding to GT transitions were isolated by using a multipole decomposition analysis. A well-established proportionality between GT strength and differential cross section at zero-linear-momentum transfer was utilized to convert extracted cross sections to GT strengths. Results and Conclusions: GT transition strengths from 56Fe to 56Mn were extracted up to an excitation energy of 10 MeV. Shell-model calculations with the GXPF1a interaction reproduced the observed GT strength distribution slightly better than calculations with the KB3G interaction. The calculated strength distribution in the QRPA did not reproduce the observed strength distribution. The new experimental data have an improved precision at low excitation energies compared to previous results obtained from an 56Fe(n,p) experiment. Electron-capture rates based on the experimental and theoretical Gamow-Teller strengths were compared and deviations were included in an assessment of the validity of electron-capture rates based on theoretical models for nuclei in the pf shell.