Eur. Phys. J. A (2026) 62: 29
https://doi.org/10.1140/epja/s10050-026-01798-4

Regular Article - Experimental Physics

Search for a shape isomer in $_{}^{233}$Th

¹ Institut für Kernphysik, Universität zu Köln, 50937, Köln, Germany
² Ludwig-Maximilians-Universität München, 85748, Garching, Germany
³ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117, Heidelberg, Germany

^a This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 November 2025
Accepted: 20 January 2026
Published online: 12 February 2026

Abstract

A search for a shape isomer in $_{}^{233}$Th via delayed $\gamma$ decay was performed using the Heidelberg–Darmstadt Crystal Ball spectrometer together with a $\Delta E$–E telescope for light charged particle identification. Excited states in $_{}^{233}$Th were populated through the $_{}^{232}$Th(d, p) reaction. The search for isomeric $\gamma$ decay was based on a total-energy reconstruction method: prompt energy deficits $E_{\text{Miss}}$ indicated possible long-lived isomeric states. Corresponding delayed summed energies $E_{\text{del}}^{\text{Sum}}$ were recorded with the Crystal Ball. A two-dimensional analysis of $E_{\text{Miss}}$ versus $E_{\text{del}}^{\text{Sum}}$ was carried out for various prompt and delayed multiplicities and for delayed time intervals up to 200 ns within the excitation-energy range $E^x=3$–7 MeV. No evidence of isomeric $\gamma$ decay was observed. The branching ratio for the population of a potential shape isomer in the second minimum of $_{}^{233}$Th relative to the first minimum is constrained to $<{10}^{-6}$, for isomeric states with energies $E^x=2$–4 MeV and half-lives in the range $t_{1/2}=2$ ns–1 $\mu$s. These results establish stringent experimental limits on the existence and population probability of a second minimum in the potential-energy surface of $_{}^{233}$Th, thereby providing valuable constraints for theoretical models of shape isomerism in actinide nuclei.