High-spin states in Al29 and Mg27

Abstract

The structure of 29 Al and 27 Mg was investigated using the reactions 18 O( 14 C,p2n) and 18 O( 14 C,α n) at 40 MeV. The charged particles were detected and identified with a ∆E-E telescope in coincidence with γ radiation detected in the Florida State University (FSU) Compton suppressed γ detector array. The level and decay schemes of both nuclei have been expanded at higher spins and excitation energies. The positive-parity states up to 3.5 to 4.5 MeV agree well with shell model calculations using the USDA interaction. The negative-parity states in 27 Mg are reproduced relatively well by one-particle-hole calculations with the WBP-a interaction. Three 27 Mg states unbound by 0.4 to 1.4 MeV to neutron decay were observed to decay radiatively. One of these states had been previously observed to γ decay in a (d,pγ) experiment along with a surprising 16 other neutron unbound states. The competition between neutron and gamma decay in these states is discussed in terms of angular momentum barriers and spectroscopic factors.

Possible onset of multifaceted excitation modes in Al29

“…This motivated us to undertake the detail investigations on the level structure of 29 Al using a heavy-ion induced fusion evaporation reaction. Recently, Dungan et al [12] have reported results on the level structure of 29 Al using the 18 O( 14 C, p2n) reaction at an incident beam energy of 40 MeV. Indeed, the level scheme of 29 Al as deduced from the present measurement is similar to that of Dungan et al [12].…”

“…5/2 (J=5/2) (s 1/2 d 3/2 ) 3 (J=5/2) ] J=10/2 ). As mentioned earlier (see Table III), Dungan et al [12] placed a level at E x = 7263 keV which decays to the lower level members of Band-I through 1411-and 3688-keV transitions. From the present study, it is not possible to place the level in the level scheme because of the limited statistics in our data.…”

“…Recently, Dungan et al [12] have reported results on the level structure of 29 Al using the 18 O( 14 C, p2n) reaction at an incident beam energy of 40 MeV. Indeed, the level scheme of 29 Al as deduced from the present measurement is similar to that of Dungan et al [12]. However, the use of a larger array in the present work, with the detectors distributed at different angles about the beam direction, facilitated observation of Doppler shapes / shifts in the γ-ray spectra.…”

“…Thus, till now the available information on the level structure of 29 Al has primarily been deduced from the decay studies and light-ion induced reactions. The only available spectroscopic work for 29 Al involving heavy ion induced fusion-evaporation reaction is that from the recent study of Dungan et al [12].…”

Cross-shell excitations inSi31

“…values in the range of 10 −2 to 10 −4 or less. This is consistent with previous observations [35,36]. As a test of these estimates, the neutron decay spectroscopic factor for the 6661 keV state was calculated in the shell model to be 0.0012, a factor of 2 above the upper limit in A bigger challenge is understanding the structure of the negative-parity states which must involve an odd number of particles outside the sd shell.…”