β-delayed proton pandemonium: A first detailed 31Cl(βp γ)30P decay scheme
Abstract
Positron decays of proton-rich nuclides exhibit large Q values, producing complex cascades which frequently involve various radiations, including protons and γ rays. Often, only one of the two is measured in a single experiment, limiting the accuracy and completeness of the decay scheme. An example is 31Cl, for which protons and γ rays have been measured separately in detail but never with substantial sensitivity to proton-γ coincidences. We provide detailed measurements of 31Cl β-delayed proton decay including β-p-γ sequences, extract spectroscopic information on 31S excited states as well as their β+ feedings, and compare to shell-model calculations. A fast fragmented beam of 31Cl provided by the National Superconducting Cyclotron Laboratory (NSCL) was deposited in the Gaseous Detector with Germanium Tagging (GADGET) system. GADGET's gas-filled Proton Detector was used to detect β-delayed protons, and the Segmented Germanium Array (SeGA) was used to detect β-delayed γ rays. As many as 20 previously unobserved β-delayed proton transitions are reported, most of which populate excited states of 30P. The first detailed 31Cl(βp γ)30P decay scheme is presented, including updated β-delayed proton energies and intensities, as well as several new 31S levels. Improved agreement is found with theoretical calculations of the Gamow-Teller strengths B(GT) for 31S excitation energies 7.5 < Ex < 9.5 MeV. The present work demonstrates that the ability to detect β-delayed protons and γ rays in coincidence is essential for accurate positron decay schemes to compare with nuclear structure theory. This phenomenon for β-delayed protons resembles the pandemonium effect originally introduced for β-delayed γ rays.