Cross-shell excitations near the ``island of inversion'': Structure of $^{30}\mathrm{Mg}$

Cross-shell excitations near the “island of inversion”: Structure of ${}^{30}{Mg}$

A. N. Deacon, J. F. Smith, S. J. Freeman, R. V. F. Janssens, M. P. Carpenter, B. Hadinia, C. R. Hoffman, B. P. Kay, T. Lauritsen, C. J. Lister, D. O’Donnell, J. Ollier, T. Otsuka, D. Seweryniak, K.-M. Spohr, D. Steppenbeck, S. L. Tabor, V. Tripathi, Y. Utsuno, P. T. Wady, and S. Zhu

Phys. Rev. C 82, 034305 – Published 7 September 2010

Abstract

Excited states in ${}^{30}{Mg}$ have been populated to $~$ $6 ℏ$ and 5 MeV excitation energy with the ${}^{14}C$ ( ${}^{18}O$ , $2 p$ ) reaction. Firm spin assignments for states with $J > 2 ℏ$ have been made in this nucleus. The level scheme is compared to shell-model calculations using the Universal $sd$ effective interaction and the Monte Carlo shell model method. Calculations employing a full $sd$ model space fail to reproduce the observed levels. The results indicate that excitations across the $N = 20$ gap are required at relatively low excitation energy to achieve a description of the data. The incorporation of the $f_{7 / 2}$ and $p_{3 / 2}$ orbitals into the model space gives improved results but indicate the need for further refinement of the models to reproduce the observed spectra.

Received 18 June 2010

DOI:https://doi.org/10.1103/PhysRevC.82.034305

Authors & Affiliations

A. N. Deacon^1,*, J. F. Smith², S. J. Freeman¹, R. V. F. Janssens³, M. P. Carpenter³, B. Hadinia², C. R. Hoffman^4,†, B. P. Kay³, T. Lauritsen³, C. J. Lister³, D. O’Donnell^2,‡, J. Ollier^2,‡, T. Otsuka^5,6, D. Seweryniak³, K.-M. Spohr², D. Steppenbeck^1,§, S. L. Tabor⁴, V. Tripathi⁴, Y. Utsuno⁷, P. T. Wady², and S. Zhu³

¹Schuster Laboratory, University of Manchester, Manchester M13 9PL, United Kingdom
²University of the West of Scotland, High Street, Paisley PA1 2BE, United Kingdom
³Argonne National Laboratory, Argonne, Illinois 60439, USA
⁴Department of Physics, Florida State University, Tallahassee, Florida 32306, USA
⁵Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁶Center for Nuclear Study, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁷Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan

^*alick.deacon@manchester.ac.uk
^†Current address: Argonne National Laboratory, Argonne, IL 60439, USA.
^‡Current address: STFC Daresbury Laboratory, Warrington, WA4 4AD, UK.
^§Current address: RIKEN Nishina Center, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan.

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Vol. 82, Iss. 3 — September 2010

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Figure 1
(a) Intensity of the transitions feeding directly the ground states of ${}^{26, 30}{Mg}$ , ${}^{30}{Al}$ , and ${}^{30}{Si}$ , plotted as a function of the energy loss in the first ion chamber, $Δ E$ . (b) Two-dimensional histogram of $Δ E$ versus time of flight between the first and third PGACs, $T_{PGAC1-3}$ . The approximate locations of recoil groups of different $Z$ are indicated by dashed lines.Reuse & Permissions
Figure 2
$γ$ -ray singles spectra formed by gating on different groups in the $Δ E$ vs $T_{PGAC1-3}$ plot. (a) The coincidence gate corresponds to the expected location of ${}^{30}{Mg}$ (note the change in intensity scale at 350 keV). (b) Normalized subtraction of ${}^{30}{Al}$ -gated spectrum from spectrum (a). Photopeaks are labeled by their energy in keV. Peaks labeled with an asterisk are known to originate from ${}^{30}{Al}$ , while those observed in ${}^{30}{Si}$ are marked with a $\oplus$ . See text for further details.Reuse & Permissions
Figure 3
Sample $γ$ -ray coincidence spectra for ${}^{30}{Mg}$ events, formed from a gate in the plot of $Δ E$ vs $T_{PGAC1-3}$ . The spectra are additionally gated on $γ$ -ray energies of (a) 1481 keV, with the inset showing the region around the 895/990-keV doublet on a larger scale; (b) 1898 keV; (c) 1060 keV; and (d) 985 keV.Reuse & Permissions
Figure 4
Excited states and transitions in ${}^{30}{Mg}$ . Energies are given to the nearest keV. The thickness of the arrows is proportional to the intensity of the $γ$ rays. Parentheses indicate tentative spin and parity assignments. The apparent intensity mismatch to and from the state at 2541 keV is not significant given the experimental error, as discussed in the text.Reuse & Permissions
Figure 5
Shell-model calculations for ${}^{30}{Mg}$ resulting from the USD and SDPF-M interactions, shown alongside the experimental levels deduced in this work, with the addition of an excited $0^{+}$ state observed in Ref. [21]. Calculated energies are given to the nearest 10 keV. For the USD interaction and positive-parity SDPF-M calculations, the lowest two states of each spin are given. For the negative-parity levels, only the lowest of each spin up to $3^{-}$ have been calculated. The numbers on the right-hand side represent the average number of neutron excitations to the $fp$ shell. See text for details.Reuse & Permissions
Figure 6
$E (2_{1}^{+})$ and $R (4_{1}^{+} / 2_{1}^{+})$ values as a function of neutron number for the even-even isotopes $^{22 - 34}$ Mg. The $E (2_{1}^{+})$ and $R (4_{1}^{+} / 2_{1}^{+})$ scales are offset by 2 and 600 keV, respectively. The brackets indicate tentative assignments; see text for comments on the sources of the data shown.Reuse & Permissions

Physical Review C

covering nuclear physics

Cross-shell excitations near the “island of inversion”: Structure of ${}^{30}{Mg}$

A. N. Deacon, J. F. Smith, S. J. Freeman, R. V. F. Janssens, M. P. Carpenter, B. Hadinia, C. R. Hoffman, B. P. Kay, T. Lauritsen, C. J. Lister, D. O’Donnell, J. Ollier, T. Otsuka, D. Seweryniak, K.-M. Spohr, D. Steppenbeck, S. L. Tabor, V. Tripathi, Y. Utsuno, P. T. Wady, and S. Zhu

Phys. Rev. C 82, 034305 – Published 7 September 2010

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Physical Review C

covering nuclear physics

Cross-shell excitations near the “island of inversion”: Structure of Mg30

A. N. Deacon, J. F. Smith, S. J. Freeman, R. V. F. Janssens, M. P. Carpenter, B. Hadinia, C. R. Hoffman, B. P. Kay, T. Lauritsen, C. J. Lister, D. O’Donnell, J. Ollier, T. Otsuka, D. Seweryniak, K.-M. Spohr, D. Steppenbeck, S. L. Tabor, V. Tripathi, Y. Utsuno, P. T. Wady, and S. Zhu

Phys. Rev. C 82, 034305 – Published 7 September 2010

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Cross-shell excitations near the “island of inversion”: Structure of ${}^{30}{Mg}$