Proton Pandemonium: A first look at the $^{31}$Cl($βp γ$)$^{30}$P decay scheme

TL;DR

This work tackles the pandemonium challenge in β-delayed decays of proton-rich nuclei by performing a high-statistics, β-delayed proton decay study of $^{31}$Cl in coincidence with γ rays using the GADGET detector array and SeGA. The authors construct the first detailed $^{31}$Cl($\beta p \gamma$)$^{30}$P decay scheme, uncovering up to 29 proton transitions and many feedings to $^{30}$P excited states, thereby enabling a more accurate extraction of the $^{31}$S Gamow-Teller strength distribution in the region $6.0<E_x(^{31}$S$)<9.5$ MeV. They show that β-p coincidence data broaden the feeding pattern and improve agreement with shell-model predictions, particularly for $7.5<E_x<9.5$ MeV, and they refine astrophysically relevant resonance parameters for $^{30}$P($p$,$\gamma$)$^{31}$S. The study demonstrates the essential role of proton-γ coincidences in constructing reliable decay schemes and highlights the potential of GADGET II for even more sensitive measurements in future FRIB-era experiments.

Abstract

Positron decays of proton-rich nuclides exhibit large $Q$ values, producing complex cascades which often involve various radiations, including protons and $γ$ rays. Often, only one of the two are measured in a single experiment, limiting the accuracy and completeness of the decay scheme. An example is $^{31}$Cl, for which protons and $γ$ rays have been measured in detail individually but never with substantial sensitivity to proton-$γ$ coincidences. The purpose of this work is to provide detailed measurements of $^{31}$Cl $β$-delayed proton decay including $β$-$p$-$γ$ sequences, extract spectroscopic information on $^{31}$S excited states as well as their $β$ feeding, and compare to shell-model calculations. A fast, fragmented beam of $^{31}$Cl provided was deposited in the Gaseous Detector with Germanium Tagging (GADGET) system. GADGET's gas-filled Proton Detector was used to detect $beta$-delayed protons, and the Segmented Germanium Array (SeGA) was used to detect $β$-delayed $γ$ rays. Up to 18 previously unobserved $β$-delayed proton transitions have been discovered, most of which populate excited states of $^{30}$P. Here present the first detailed $^{31}$Cl($βp γ$)$^{30}$P decay scheme and find improved agreement with theoretical calculations of the Gamow-Teller strength distribution for $^{31}$S excitation energies $7.5 < E_x < 9.5$ MeV. The present work demonstrates that the capability to detect $β$-delayed protons and $γ$ rays in coincidence is essential to construct accurate positron decay schemes for comparison to theoretical nuclear structure calculations. In some respects, this phenomenon for $β$-delayed protons resembles the pandemonium effect originally introduced for $β$-delayed $γ$ rays.

Figures (12)

• Figure 1: Particle identification spectrum for the $^{31}$Cl cocktail beam. The energy losses $\Delta E$ of beam ions are plotted against their time-of-flight as measured between two plastic scintillators in the A1900 Fragment Separator.
• Figure 2: Drift times of sub-200-keV, $\beta$-particle energy depositions detected in coincidence with 2234-keV $\gamma$ rays. This serves as a rough approximation of the longitudinal distribution for $^{31}$Cl $\beta^+$ decays throughout the active volume of the Proton Detector.
• Figure 3: Reconstructed beam spot representing the transverse distribution of $^{31}$Cl ions overlaid on the segmented anode plane. Active detector pads are labeled A-E, and the outer veto pads are labeled F-M. The dashed circle shows the projection of the beam entrance aperture onto the MICROMEGAS board. Arrow sizes correspond to the track-lengths of ionization electrons produced by 275-, 500-, and 900-keV proton decays constrained to the $x-y$ plane in an 800-Torr, P10 fill gas, according to the Stopping and Range of Ions in Matter (SRIM) calculator Ziegler2010.
• Figure 4: The $^{31}$Cl $\beta$-delayed, $\gamma$-ray singles spectrum, gated on all coincident charged-particle detections, including $\beta^+$ particles. Any $\gamma$ events recorded in this spectrum must be observed within the same 7.5-$\mu$s window as a charged-particle event detected in the Proton Detector.
• Figure 5: Relative detection efficiency plot for SeGA as a function of $E_{\gamma}$.