DSAM lifetime measurements for the chiral pair in 194Tl

Abstract

Abstract. Most important for the identification of chiral symmetry in atomic nuclei is to establish a pair of bands that are near-degenerate in energy, but also in B(M 1) and B(E2) transition probabilities. Dedicated lifetime measurements were performed for four bands of 194 Tl, including the pair of four-quasiparticle chiral bands with close near-degeneracy, considered as a prime candidate for best chiral symmetry pair. The lifetime measurements confirm the excellent near-degeneracy in this pair and indicate that a third band may be involved in the chiral symmetry scenario.Chiral systems can exist in nuclei with triaxially deformed shape. Such nuclei rotate collectively predominantly around their intermediate axis. Should the valence nucleons have both particle and hole nature, their singleparticle angular momenta would align along the short and long nuclear axes, respectively. Then the total angular momentum of the nucleus will have large projections along the three major nuclear axes, forming a left-handed or a right-handed systems and exhibiting chiral symmetry.Nuclear chiral symmetry generates a pair of rotational bands with the same parity and with near-degenerate properties; for instance they have similar excitation energies, alignments, and reduced B(M 1) and B(E2) transition probabilities [1]. Most of the chiral bands known to date have been identified based on similarity in the excitation energy and alignments, but very often the most crucial chirality test (see ref.[2]), i.e. the similarity in the B(M 1) and B(E2) transition rates remain outstanding, because it needs dedicated lifetime measurements.The formation of more than one chiral system in the same nucleus is a very rare event. To date chiral multiplets were proposed in only two nuclei, Tl [5], including the only fourquasiparticle chiral pair known to date. This is also the only chiral pair for which chirality persists through a backbend. Furthermore, this chiral pair shows perhaps the best near-degeneracy found to date [6].The 181 Ta( 18 O, 5n) reaction was employed at a beam energy of 91 MeV. The target was a 1 mg/cm 2 181 Ta foil with a thick Bi layer evaporated on the back. The recoils were completely stopped in the Bi backing. The emitted γ-rays were detected with the AFRODITE array [7,8] at iThemba LABS, comprising 9 Compton-suppressed clover detectors, and 6 LEPS detectors. The trigger required 3 coincident γ-rays, with at least two detected in the clovers. Four clover detectors were arranged at 45• , another four were placed at 135• , while the remaining detectors were situated at 90• with respect to the beam direction. Data were sorted into two asymmetric matrices, with the γ-ray energies detected at 45• and at 135• , respectively, stored into one axis, while the coincident γ-ray energies registered at any angle were stored into the second axis. Gated, background-subtracted, forward (45 • ) and backward (135 • ) spectra were used to analyze the Doppler

Near-degenerate Rotational Bands in the Tl Isotopes

“…2. In addition, the calculations showed that all four bands correspond to an angular momentum that has large projections on all three nuclear axes, thus indicating chiral geometry for all four bands [12]. Therefore, these calculations highlight a strong possibility that two chiral systems are formed in 194 Tl.…”

“…Or perhaps only one chiral system is formed, while the third bands has a different, non-chiral nature. Further study of 194 Tl through DSAM analysis revealed deeper similarities of the observed bands [12], but could not resolve the open questions on their nature. To understand better the πh 9/2 ⊗ νi −n 13/2 bands in this mass region the neighbouring nucleus, 193 Tl, was studied.…”

“…In order to study further the sets of three bands, in particular to investigate whether the bands might exhibit differences in their transition probabilities, as expected if the bands were associated with different nuclear shapes, DSAM lifetime measurements were performed [12]. The results on the transition probabilities ascertained further the excellent near-degeneracy of bands 4qpB and 4qpC.…”

Tilted precession bands in Nd135

“…Nuclear chirality [2] has been intensively studied, and many chiral nuclei have been identified in the A ≈ 130 mass region (e.g., 131 Ba [3], 133 La [4], 133 Ce [5], 135,136,137,138 Nd [6][7][8][9][10][11], 124,126,128,130 Cs [12][13][14][15][16]), as well as in the A ≈ 190 mass region (e.g., 193,194,195,198 Tl [17][18][19][20][21][22]), in the A ≈ 100 mass region (e.g., 103,105,111,113 Rh [23][24][25][26], 105,107 Ag [27,28]), and in the A ≈ 80 mass region (e.g., 78,80 Br [29,30]), making this motion a more common phenomenon. For a recent review on chirality over the nuclear chart see Ref.…”

Chiral Bands in $^{193}$Tl

“…At iThemba LABS, a new chiral symmetry region, the thallium isotopes with mass A ≥ 190 was found. Candidate chiral bands were suggested in 198 Tl [2,3] and then in 194 Tl [4][5][6]. Most importantly, a comparison of the properties in the chiral partner bands in 194 Tl showed that this nucleus is one of the best chiral candidates to date [4].…”