Speaker
Description
Isospin symmetry arises from the charge independence of nucleon–nucleon interactions, but it is broken by the Coulomb interaction. Although a small effect, its precise determination is essential for understanding isobaric analogue states and their role in β decay.
In self-conjugate N = Z nuclei, electromagnetic selection rules forbid electric dipole (E1) γ decay. Therefore, the observation of E1 strength provides a sensitive probe of isospin mixing. The giant dipole resonance (GDR), an E1 mode, is widely used to quantify this effect [1-3].
Following early measurements in $^{28}$Si and $^{32}$S [4-6], which indicated an increase of isospin mixing with mass number A, our Milano group carried out a systematic study in N = Z nuclei in the range A = 60–80 [1-3]. In particular, $^{80}$Zr and $^{60}$Zn were investigated at LNL-INFN using GARFIELD, AGATA and GALILEO arrays coupled to BaF$_2$ or LaBr$_3$:Ce detectors, allowing the extraction of isospin mixing and its temperature dependence.
As the most recent case within this program, we studied $^{72}$Kr at a nuclear temperature of ~1.3 MeV via GDR γ decay. Measurements were performed at the Bucharest Tandem Laboratory using the ELIGANT array [7] for the reactions $^{32}$S + $^{40}$Ca (I = 0) and $^{31}$P + $^{40}$Ca (reference, I ≠ 0).
Statistical-model analysis yields an isospin mixing parameter of (3.5 ± 0.8)%, confirming the predicted temperature dependence. By combining all results, we present a systematic study of isospin mixing as a function of mass and temperature. Finally, the extracted isospin-symmetry-breaking correction δ$_C$, relevant for superallowed Fermi β transitions, is consistent with β-decay data, theoretical predictions, and previous measurements [8].
References
[1] A. Corsi et al., “Measurement of isospin mixing at a finite temperature in $^{80}$Zr via giant dipole resonance decay,” Physical Rewiev C, vol. 84, p. 041304(R), 2011
[2] S. Ceruti et al., “Isospin mixing in $^{80}$Zr: From finite to zero temperature,” Physical Rewiev Letters, vol. 115, p. 222502, 2015.
[3] G. Gosta et al., “Probing isospin mixing with the giant dipole resonance in the $^{60}$Zn compound nucleus,” Physical Rewiev C, vol. 103, p. L041302, 2021.
[4] M. Harakeh, et al., “Role of isospin in the statistical decay of the giant dipole resonance built on excited states” Phys. Lett. B 176 (1986) 297
[5] J. Behr, et al., “Restoration of isospin symmetry in highly excited compound nuclei” Phys. Rev. Lett. 70 (1993) 3201.
[6] M. Kicińska-Habior, et al., “Giant dipole radiation and isospin purity in highly excited $^{32}$S nuclei”, Nucl. Phys. A 731 (2004) 138.
[7] P.-A. Söderström et al., “Eligant-Gn-ELI gamma above neutron threshold: The gamma-neutron setup,” Nuclear Inst. and Methods in Physics Research, A, vol. 1027, p. 16617, 2022.
[8] A. Giaz et al., “Probing the Isospin Mixing in the $^{72}$Kr Compound Nucleus via GDR γ Decay” Physics Letters B, 868 (2025) 139653.