CONFIRMATION OF THE L-FORBIDDEN GAMOW-TELLER DECAY BRANCH OF CA-39

A search for Gamow-Teller decay branches of Ca-39 to excited states in K-39 was conducted with a large HPGe detector which observed the deca ys of Ca-39 samples delivered by a helium-jet transport system. A pair of plastic scintillators in front of the HPGe detector were used to r eject events due to bremsstrahlung from the Ca-39 positrons. With this setup, weak gamma-rays could be observed with a degree of sensitivity not previously achieved. The 1-forbidden 1d3/2 --> 2s1/2 transition t o the first excited state in K-39, was observed with a branching ratio of (2.50 +/- 0.27) x 10(-5). This result agrees with but is more accu rate than the only previous measurement. Thus, the ratio of the l-forb idden Gamow-Teller and isovector M1 matrix elements in mass-39 nuclei continues to disagree with theoretical values obtained by Towner and K hanna from calculations involving core-polarization and meson-exchange currents. This leaves very few possibilities to explain the discrepan cy between theory and experiment.