Isospin Properties of Nuclear Pair Correlations from the Level Structure of the Self-Conjugate Nucleus ${}^{88}\mathrm{Ru}$

The low-lying energy spectrum of the extremely neutron-deficient self-conjugate ($N=Z$) nuclide ${{}_{44}{}^{88}\mathrm{Ru}}_{44}$ has been measured using the combination of the Advanced Gamma Tracking Array (AGATA) spectrometer, the NEDA and Neutron Wall neutron detector arrays, and the DIAMANT charged particle detector array. Excited states in ${}^{88}\mathrm{Ru}$ were populated via the ${}^{54}\mathrm{Fe}({}^{36}\mathrm{Ar},2n\gamma ){{}^{88}\mathrm{Ru}}^{*}$ fusion-evaporation reaction at the Grand Accélérateur National d’Ions Lourds (GANIL) accelerator complex. The observed $\gamma$-ray cascade is assigned to ${}^{88}\mathrm{Ru}$ using clean prompt $\gamma \text{−}\gamma$-2-neutron coincidences in anticoincidence with the detection of charged particles, confirming and extending the previously assigned sequence of low-lying excited states. It is consistent with a moderately deformed rotating system exhibiting a band crossing at a rotational frequency that is significantly higher than standard theoretical predictions with isovector pairing, as well as observations in neighboring $N>Z$ nuclides. The direct observation of such a “delayed” rotational alignment in a deformed $N=Z$ nucleus is in agreement with theoretical predictions related to the presence of strong isoscalar neutron-proton pair correlations.

Figure 1

(a) Gamma-ray energy spectrum detected in coincidence with the 616, 800, 964, and 1100 keV $\gamma$ rays, with the additional requirement that two neutrons and no charged particles were detected in coincidence. (b) Expanded part of the unsubtracted gated spectrum around the new $\gamma$-ray transitions at 1063 keV ($10^{+}\to 8^{+}$), 1153 keV ($12^{+}\to 10^{+}$), and 1253 keV [$(14^{+})\to 12^{+}$] is drawn in red together with the background spectrum (black) used to produce the spectrum shown in (a). Gamma-ray peaks due to contaminant reactions on oxygen leading to the population of excited states in ${}^{49,50}\mathrm{Cr}$ and ${}^{49}\mathrm{Mn}$ are indicated. (c) Level scheme of ${}^{88}\mathrm{Ru}$ deduced from the present work. Relative intensities are proportional to the widths of the arrows.

Figure 2

Experimental values for the kinematical moment of inertia ($J^1$) for the low-lying yrast bands of the $N=44$ isotones ${{}_{44}{}^{88}\mathrm{Ru}}_{44}$ (this work), ${{}_{42}{}^{86}\mathrm{Mo}}_{44}$ [40, 41], and ${{}_{40}{}^{84}\mathrm{Zr}}_{44}$ [42]. The black dashed vertical line indicates the approximate rotational frequency of the first isovector-paired band crossing due to $g_{9/2}$ protons as predicted by standard cranked shell model calculations [43, 44]. The red dotted vertical line indicates the band crossing frequency for the ground-state band in ${{}_{44}{}^{88}\mathrm{Ru}}_{44}$ observed in this work.