The Joy of CEX: Sharpening the (t,^3He) probe at 345 MeV for the charge-exchange knife drawer

by Howard, Meredith E.

This data set represents the first step in a campaign of (t,^3He) charge-exchange experiments at the National Superconducting Cyclotron Laboratory designed explicitly to measure Gamow-Teller strength distributions in the electron capture direction for stable medium-heavy atomic nuclei, with particular interest in reaching the nuclei in the pf-shell or masses up to A=112. The 115 MeV/nucleon secondary triton beam with an average intensity of 4×10^6 pps on 10 mg/cm^2 thick target foils produces ^3He that are measured in the focal plane detectors of the S800 magnetic spectrometer. From these data, the scattering angle of the tritons is reconstructed to within 7 mrad and the energy of the recoil nucleus is reconstructed to better then 250 keV. Since there is no comprehensive study of the (t,^3He) probe for triton energies of 100-400 MeV/nucleon, this data is the first step in evaluating the advantages of extracting B(GT) from the (t,^3He) probe over other hadronic probes. The first target is a CD2 target used for calibrating cross section. This is the third measurement for the (t,^3He) probe on ^12C above 100 MeV/nucleon. The present ^12C(t,^3He) cross section for the ^12B ground state (J?=1^+) is lower than a previous measurement at the NSCL by more than one standard deviation in uncertainty but agrees with ^12C(^3He,t)^12N. This (t,^3He) measurement for ^24Mg, the second target, is the first above 100 MeV/nucleon. The B(GT) distribution for ^24Na is extracted from differential cross sections as a function of residual nucleus excitation energy up to 7 MeV. For each peak in excitation energy, the differential cross section as a function of reconstructed scattering angle is extrapolated to zero degrees using angular distrubutions calculated with the distorted wave calculations from the code FOLD and transformed to q=0 zero momentum transfer. Uncertainties in the B(GT) include a calculation of interference to J?=1^+ expected from ?L=2, ?S=1 reactions. Comparisons of B(GT) distributions of ^24Na with that of modern calculations using an improved interaction for the sd-shell space are discussed. In particular, a recently improved hamiltonian for the sd-shell model space is compared with the quarter-century-old USD interaction. The experimental measurements, both the present data and those of the competing charge-exchange probe (d,^2He), are in good agreement with theoretical calculations. As a result of this work, the NSCL has dramatically improved the availability of the secondary triton beam and the resolution for charge-exchange experiments and completed experiments for (t,^3He) on several nuclei above A=45.