Lifetime measurements in /sup 38/Cl and /sup 40/K. [Lifetime, transitions, strengths, J,. pi.

Abstract

Electromagnetic transition rates within the /sup 38/Cl and /sup 40/K nuclei were studied through lifetime measurements with a double-coincidence variant of the Doppler-shift-attenuation method. In /sup 40/K the excitation energies (MeV) and measured mean lives (psec) are 0.800 (0.56 +- 0.13), 0.891 (1.53/sub -0.35//sup +0.40/), 1.959 (0.7 +- 0.5), 2.047 (0.49 +- 0.10), 2.070 (0.4/sub -0.2//sup +0.3/), 2.103 (0.54 +- 0.15), 2.261 (0.05 +- 0.05), 2.290 (0.33/sub -0.20//sup +0.40/), 2.291 (0.31/sub -0.12//sup +0.18/), 2.419 (0.55/sub -0.30//sup +0.85/), 2.575 (0.22 +- 0.10), 2.626 (0.32/sub -0.16//sup +0.40/), 2.731 (less than 0.120), 2.787 (0.13/sub -0.13//sup +1.00/), 3.110 (less than 0.14), 3.228 (0.04 +- 0.04), and 3.629 (less than 0.10). In /sup 38/Cl the lifetime results are 0.755 (0.30 +- 0.60), 1.309 (0.55/sub -0.20//sup +0.40/), 1.617 (1.5/sub -0.6//sup +1.2/), 1.692 (1.3/sub -0.3//sup +0.5/), 1.746 (1.1 +- 0.5), 1.785 (0.10/sub -0.08//sup +0.10/), 1.981 (0.26 +- 0.06), and 2.743 (0.05 +- 0.03). The energies of the low-lying (..pi..d/sub /sup 3///sub 2//)/sup +-1/ (..nu..f/sub /sup 7///sub 2//) shell-model states in /sup 38/Cl and /sup 40/K are known to be accurately related by the Pandya transformation. The (d/sub /sup 3///sub 2//)(f/sub /sup 7///sub 2//) model, when used with effective magnetic moments, can also approximately reproduce the /sup 40/K ground-state magnetic moment. However, the present work demonstrates that even when effective moments are employed, the (d/sub /sup 3///sub 2//)(f/sub /sup 7///sub 2//) model fails in its predictions of electromagnetic transition rates. The intramultiplet M1 transitions are observed to have allowed strengths, whereas the l-forbidden intermultiplet transitions to the states of the (d/sub /sup 3///sub 2//)/sup +-1/(f/sub /sup 7///sub 2//) configurations have strengths that are lower by two orders of magnitude. The one exception to this picture, namely the 1.617 ..-->.. 1.309-MeV transition in /sup 38/Cl, is discussed.