High-resolution study of E0 internal-pair transitions from excited 0⁺ states in ⁵⁸’⁶⁰’⁶²Ni

Abstract

A new method based on a combination of an intermediate-image magnetic lens and two cooled Si(Li) detectors has been developed for in-beam studies of internal-pair transitions. Calculations of the spectrometer pair-line efficiency for EO, El, E2 and Ml transitions versus transition energy are made in the zero-order Born approximation. The main advantages gained over the previous methods used for pair-line measurements are an excellent energy resolution of the sum-coincidence pair lines (4-6 keV) and a high pair-line efficiency (s 10-3) provided by the high transmission and the broad momentum band width of the magnetic lens. With the aid of the new technique the first extensive systematic investigation of the EO transitions has been carried out in 58,60,62Ni. The present internal-pair measurements confirmed the 0+3 assignment of the 3530.9 keV state in 58Ni and the 3318.3 keV state in 60Ni, and yielded a spin and parity assignment of 0+4 for the 3588.0 keV state in 60Ni. The X(EO/E2) values are determined for these states and for the 0+2 states at 2942.3 keV, 2284.8 keV and 2048.4 keV in 58Ni, 60Ni and 62Ni, respectively. The results are combined with the available lifetimes of these states to extract the monopole strengths p2 (0+1→0+1). The results and the nature of the O+ states are discussed.