Revalidation of the isobaric multiplet mass equation for the $A=20$ quintet

TL;DR

This study precisely measured the lowest T=2 state in $^{20}$Na, resolving previous discrepancies and confirming the validity of the isobaric multiplet mass equation for the $A=20$ quintet.

Contribution

The paper provides a highly precise measurement of the $^{20}$Na excitation energy, revalidating the isobaric multiplet mass equation for the $A=20$, $T=2$ quintet.

Findings

New excitation energy measurement differs by 1.9 sigma from previous value.

Measurement is 28 times more precise than earlier data.

Revalidation of the isobaric multiplet mass equation for $A=20$ quintet.

Abstract

An unexpected breakdown of the isobaric multiplet mass equation in the $A=20$, $T=2$ quintet was recently reported, presenting a challenge to modern theories of nuclear structure. In the present work, the excitation energy of the lowest $T = 2$ state in $^{20}$Na has been measured to be $6498.4 \pm 0.2_{\textrm{stat}} \pm 0.4_{\textrm{syst}}$ keV by using the superallowed $0^+ \rightarrow 0^+$ beta decay of $^{20}$Mg to access it and an array of high-purity germanium detectors to detect its $\gamma$-ray deexcitation. This value differs by 27 keV (1.9 standard deviations) from the recommended value of $6525 \pm 14$ keV and is a factor of 28 more precise. The isobaric multiplet mass equation is shown to be revalidated when the new value is adopted.