Experimental study of isomeric intruder 12+ states in At197,203

Abstract

A newly observed isomeric intruder 1 /2 + state [T1 /2 = 3.5(6) ms] is identified in 203 At using a gas-filled recoil separator and fusion-evaporation reactions. The isomer is depopulated through a cascade of E3 and mixed M1/E2 transitions to the 9 /2 − ground state, and it is suggested to originate from the π (s1 /2 ) −1 configuration. In addition, the structures above the 1 /2 + state in 203 At and 197 At are studied using in-beam γ -ray spectroscopy, recoil-decay tagging, and recoil-isomer decay tagging methods. The 1 /2 + state is fed from 3 /2 + and 5 /2 + states, and the origin of these states are discussed.

Charge radii and electromagnetic moments of At195–211

“…In particular, a drop in the excitation energy of the 2 + states when going from 200 Po to 198 Po and a bigger decrease between 198 Po and 196 Po are analogous to the behavior observed for 11/2 − and 13/2 − states in 201,199,197 At (see Refs. [15,75] and references therein). These reductions in state energies have been interpreted as evidence for an increase in collectivity and an onset of deformation in the even-even polonium core.…”

“…Furthermore, the ground states in 191,193,195 At are seen to possess spins and parities of I π gs = 1/2 + [8,9]. These 1/2 + states in the odd-A astatine isotopes have been interpreted as spherical-shell model intruder states, created by π (4p-1h) proton excitations across the Z = 82 shell gap, resulting in a proton hole in the s 1/2 orbital [10][11][12][13][14][15]. Alternatively, in the deformed shell-model approach, such 1/2 + states are associated with the population of oblately deformed Nilsson orbitals.…”

Isomeric 13/2+ state in Fr201

“…In the odd-mass nuclei a low-spin isomeric intruder 1 /2 + state, interpreted to arise from a π (s −1 1 /2 ) configuration, has been observed in bismuth (see Ref. [2] and references therein), astatine [3][4][5][6][7], and francium nuclei [8][9][10][11][12]. The energy of this state decreases with decreasing neutron number along an isotopic chain, providing an experimental signature of the onset of oblate deformation and increasing collectivity.…”

Prompt and delayed spectroscopy of At203 : Observation of a shears band and a 29/2+ isomeric state

“…Partial level scheme of 203 At. The level scheme of the levels connected to the isomeric intruder 1 /2 + state is shown in Ref [45]. The purpose of the inset is to clarify the low-energy transitions below the isomeric 29 /2 + state.…”