Development of the recoil-beta tagging method and recoil-beta tagging studies of ⁶⁶As and ⁶⁶Se

Abstract

The structure of self-conjugate odd-odd N = Z nucleus 66As has been experimentally studied in detail. Prompt and isomeric excited states have been observed utilising recoil-β tagging and recoil-isomer tagging methods. Most importantly, candidates for the excited states in the T = 1 band were identified up to spin Jπ = 6+. Three new detectors i.e., a charged-particle veto detector, a highly-pixelated DSSD and a phosphor sandwich device have been constructed and tested to enhance the sensitivity of the recoil-β tagging approach. The newly developed charged-particle veto detector in conjunction with the recoil-β tagging allowed excited states in the exotic neutron-deficient N = Z − 2 nucleus 66Se to be identified up to spin Jπ = 6+. The newly obtained spectroscopic results both in 66As and 66Se permitted the derivation of energy differences between isobaric analogue states in the full A = 66 triplet. These results show that both the charge-symmetry and charge-independence characteristics of the strong nucleon-nucleon interaction are violated. Moreover, the experimental energy differences are larger than predicted by the shell-model calculations. This points towards a need for an additional isospin non-conserving interaction, whose fundamental origin cannot be explained on the basis of existing nuclear models.