Identification of theJπ=1−state inRa218populated viaα

Abstract

The α decay of 222 Th populating the low-lying J π = 3 − state, and also a proposed 1 − state, in 218 Ra has been observed. The observations suggest an excitation energy of 853 keV for the 1 − state, which is 60 keV above the 3 − state. The hindrance factors of these α decays give a possible boundary to the region of ground-state octupole deformation in the light-actinide nuclei. The relative positions of the J π = 1 − and 3 − states suggest they are produced by an octupole-vibrational mechanism, as opposed to α clustering or rotations of a reflection-asymmetric octupole-deformed shape.

α -decay spectroscopy of the N=130 isotones Ra218 and Parr1, Smith2, Greenlees3 et al. 2019Phys. Rev. CSelf Cite

“…The energy of the α particle emitted in the 220 Th → 216 Ra ground-state-to-ground-state decay has only previously been measured once, by Häusser et al in 1973 [7], where it was reported with a value of 8790 (20) keV. In that work, the half-life of the ground state of 220 Th was measured to be 9.7(6) μs.…”

“…The calibrations were carried out using known energies of α particles emitted from evaporation residues implanted into the DSSDs or in their decay chains. For the 18 20 Ne + 208 Pb experiment, the same α decays were used, with the exception of those from 220 Ra and 219 Ra. Due to more abundant statistics from the 18 O + 208 Pb experiment, αγ coincidences were studied; for that experiment, the absolute efficiency for the detection of γ rays in the focal-plane clover HPGe detectors was determined by comparing the numbers of α particles in the DSSDs with numbers of detected αγ coincidences.…”

“…The first experiment (denoted as Experiment 1 in Table I) used an 18 O beam incident on a 208 Pb target and was optimized to study the nucleus 222 Th produced via the 208 Pb( 18 O, 4n) 222 Th fusionevaporation reaction. The second experiment (Experiment 2) essentially swapped the 18 O beam for 20 Ne and was optimized for the study of 224 U, also produced by 4n evaporation. The 218 Ra and 220 Th nuclei of interest in the present paper were produced as the α-decay daughters of the main 222 Th and 224 U reaction products, respectively.…”

“…The energies of the α particles measured by Valli et al [4] and Torgerson et al [6] were 8385(10) keV and 8392(8) keV, respectively, which are consistent with each other but that measured by Kim at al. [21] is almost 100 keV larger with a value of 8480 (20) keV. The half-life of 218 Ra was measured to be 14(2) μs by Valli et al [4] but was later consistently measured with higher values of 25.6(11) μs by Toth et al [22], 26(2) μs by Wieland et al [23], and 25.2(3) μs by Kuusiniemi et al [24].…”

“…Experimental study of the α decay of these nuclei can provide useful information about the development of octupole collectivity as a function of N and Z as discussed in Refs. [18][19][20]. However, the α-decay spectroscopy of these nuclei is difficult due to their short-lived daughter nuclei, as discussed above.…”