*2.2. Decay Tagging of Fusion-Evaporation Channels and a Case Study: The Tz* = −<sup>1</sup> *Nucleus* <sup>66</sup>*Se*

Instead of tagging the prompt emitted *γ* rays by the prompt evaporated particles, an alternative approach to select the low cross-section neutron-evaporation channel of interest is to tag the *γ* rays by the ground-state decay emissions characteristic of the nucleus

of interest. For spectroscopy of *Z* ≥ *N* nuclei, a highly effective technique is recoil-beta tagging (RBT) [29,30], which takes advantage of cases where the ground state of the nucleus of interest (*N*, *Z*) *β*-decays to its isobaric analogue state in the *N* + 1, *Z* − 1 neighbour. Such decays are characterised by fast, superallowed, *β*-decays, with high *β* end-point energy.

In the RBT approach, outlined in Figure 2, a triggerless data acquisition system is used to enable temporal correlation between prompt *γ*-ray emission at the target and the subsequent decays of the residual nuclear ground state. The recoiling nuclei are separated using a magnetic spectrometer and implanted in a highly pixellated double-sided silicon strip detector (DSSSD). The subsequent decay of the ground state of the implanted nucleus is detected in the same position as the implantation within the DSSSD and a second detector (a planar Ge detector or plastic scintillator) is used to measure the remaining energy of the *β*-decay A correlation in time of the three events (prompt emission, implantation and *β*-decay) and in position using the pixellated DSSSD, allows the selection of the proton-rich nucleus when a short correlation time (few 10 s of ms, typically) is required as well as a high-energy *β*-decay

**Figure 2.** A schematic diagram summarising the recoil-beta-tagging technique, [29] used for identifying prompt *γ* decays, emitted from proton-rich nuclei through tagging with the characteristic superallowed *β*-decay of the residue ground state. See text for details.

The example of spectroscopy of *Tz* = −<sup>1</sup> 66Se [12] is chosen as the case study for this technique. The experiment was performed at the at the University of Jyväskylä (JYFL) using the JUROGAMII *γ*-ray array and the RITU gas filled separator [31,32], in which 66Se was populated through a 2*n* evaporation channel. The fusion products were implanted in the DSSSD, which was followed by a planar Ge detector for detection of the high-energy positrons from the fast superallowed *β*-decay. A key component of this experiment was the inclusion of a high-efficiency veto detector to measure prompt charged particles—the UoYTube [33] detector. This is essential to help identify, and remove, contamination in the final spectrum coming from reaction channels with evaporation of one or more charged particles. The resulting clean spectrum identified decays from states with *J<sup>π</sup>* = 2+, 4<sup>+</sup> and 6+, which in turn enabled the completion of the full set of *T* = 1 isobaric analogue states up to 6<sup>+</sup> in the *A* = 66 *T* = 1 triplet, allowing the TED to be extracted. The impact of this result on the understanding of isotensor isospin non-conserving interactions, within the shell model description of TED, is discussed in Section 4.1.

Since the work on 66Se, the same RBT approach, including charged-particle vetoing, has been applied successfully at JYFL to identify the excited states in the *Tz* <sup>=</sup> <sup>−</sup>1 nuclei 70Kr [13] and 74Sr [15], and a programme using the same methodology is underway

using the new MARA spectrometer [34], which additionally allows for mass selection and identification.
