Speaker
Description
Experimental observation of 2-phonon gamma vibration in atomic nucleus has been challenging due to their location at excitation energies close to the pairing gap. The observation of such states provides stringent constraints on nuclear models. Although such excitations were proposed earlier in a number of transitional and deformed nuclei, it was later pointed out that many of those suggested cases may not be entirely, or even predominantly two-phonon in their characters, rather these could have fragmented vibrational strength through mixing with other close-lying, noncollective two-quasiparticle states which could well be described in terms of hexadecapole phonon vibrations, or g-boson structures [1].
Collective gamma-gamma vibration have been established in several candidate nuclei in A~160 and A~100 mass region with the most recent ones in $^{164}$Dy [2] and $^{104}$Nb [3]. It is worth noting that in the rare-earth region and elsewhere in the nuclide chart, $^{164}$Dy, $^{166}$Er, and $^{168}$Er were predicted by different theoretical models to be the most favourable ones to exhibit such collective vibration. Following an earlier measurement in $^{164}$Dy, a level at 2173.1 keV was reported to exhibit a collective enhancement in its decay to the single-$\gamma$ vibration, suggesting a possibility of the K$^\pi$ = 4$^+$ double-$\gamma$ excitation [4]. However, due to the large uncertainty in the measured B(E2) value, it remained inconclusive whether the 2173.1-keV level is predominantly two-phonon in character or if it merely has a minor two-phonon component in its wave function. Following this, an experiment was performed recently to explore the possibility of observing multiple $\gamma\gamma$ states employing the $^{163}$Dy(n$_{th}$,$\gamma$)$^{164}$Dy reaction and DURGA (Dhruva Utilization in Research using Gamma Array) facility at the Dhruva reactor, BARC, India. This new data has revealed the first identification of the lowest K$^\pi$ = 4$^+$ state in the $^{164}$Dy nucleus at 1978.5 keV. The 1978.5-keV level, together with the 2113.2-keV level, and the two other newly observed levels at 2261.4 and 2463.5 keV depopulate exclusively to the states in the $\gamma$ band with deexciting properties in reasonable agreement with Alaga rules. These four states, established in the present work, have been interpreted as the members of the rotational band built on a fragment of the K$^\pi$ =4$^+$ $\gamma\gamma$ vibration in this nucleus.
Novel calculations in the framework of Triaxial Projected Shell Model (TPSM) have been carried out for the two-$\gamma$-phonon bandhead state and the band structure built above it [2]. The TPSM results were found to be sensitive to the strength of the pair correlations. The TPSM energies with full-pairing and reduced-pairing calculations have been found to be on an average 100 keV away from the now-established $4^+_3$ level at 1978.5 keV. The state $4^+_3$ (1978.5 keV) state was found to be the collective $\gamma\gamma$ vibration, and the $4^+_4$ (2173.1 keV) to have predominantly two-quasineutron nature [2]. The calculations pointed to a fragmentation of the E2 strength between the $4^+_3$ (1978.5 keV) and $4^+_4$ (2173.1 keV) states. The predicted B(E2, $4^+_3$ → $\gamma$) TPSM value has been found to be consistent with sum of the experimental B(E2, $4^+_4$ → $\gamma$) value and the estimated B(E2, $4^+_3$ → $\gamma$) value.
In $^{168}$Er, the two-phonon character of the K$^\pi$ = 4$^+$ state (E$_x$=2.055 MeV), along with the 5$^+$ member (E$_x$=2.169 MeV) of the K$^\pi$ = 4$^+$ band, were affirmed earlier [5]. Although the 2307 keV state was proposed to be the 6+ member of this band, subsequent Coulex study [6] did not report any direct observation of this. To identify the elusive K$^\pi$ = 0$^+$ 2-phonon-gamma state in $^{168}$Er, a new experiment has been performed using the DURGA facility employing $^{167}$Er(n$_{th}$,$\gamma$)$^{168}$Er reaction. Preliminary analysis of the data does not corroborate the 2307 keV state to be the 6$^+$ member of the $\gamma\gamma$ band [7]. Nevertheless, the band has now been extended up to 7$^+$ spin with the addition of two new levels. The implication of alternate decay pathway for the 4$^+$ $\gamma\gamma$ bandhead state (and also other members of this band) to the 4$^-$ isomeric band (with T$_{1/2}$ = 157 ns) will be discussed.
[1] D. G. Burke, Phys. Rev. Lett. 73, 1899 (1994)
[2] S. Mukhopadhyay $et~al.$, Phys. Rev. C 112,064325(2025)
[3] E. H. Wang $et~al.$, Phys. Rev. Lett. 136, 072501 (2026)
[4] F. Corminboeuf $et~al.$, Phys. Rev. C 56, R1201 (1997)
[5] H. G. Börner $et~al.$, Phys. Rev. Lett. 66, 691 (1991)
[6] T. Hårtlein $et~al.$, Eur. Phys. J. A 2, 253 (1998)
[7] W. D. Davidson $et~al.$, J. Phys. G: Nucl. Part. Phys. 17, 1683(1991)
| Contribution category | Experiment |
|---|---|
| Presenter status | Student |