Low-energy states of ${}_{31}^{81}{\mathrm{Ga}}_{50}$: Proton structure of the nuclei close to $^{78}\mathrm{Ni}$

Low-energy states of $_{31}^{81} {Ga}_{50}$ : Proton structure of the nuclei close to ${}^{78}{Ni}$

D. Verney, F. Ibrahim, C. Bourgeois, S. Essabaa, S. Galès, L. Gaudefroy, D. Guillemaud-Mueller, F. Hammache, C. Lau, F. Le Blanc, A. C. Mueller, O. Perru, F. Pougheon, B. Roussière, J. Sauvage, and O. Sorlin (PARRNe Collaboration)

Phys. Rev. C 76, 054312 – Published 15 November 2007

Abstract

The β decay of the neutron-rich $_{30}^{81} {Zn}_{51}$ has been investigated at the PARRNe mass separator at the IPN Orsay. The sources of ${}^{81}{Zn}$ were produced using the ISOL (Isotopic Separation On Line) technique by the fission of $^{nat}$ U exposed to the neutron flux produced by the 26-MeV deuteron beam delivered by the MP-Tandem. With γ and $γ − γ$ coincidence measurements, excited levels were attributed to $_{31}^{81} {Ga}_{50}$ for the first time. A partial decay scheme for ${}^{81}{Zn}$ is proposed. The proposed level scheme is well reproduced by shell model calculations using the most recent effective empirical interaction. We show that the structure of this nucleus is consistent with that of the heavier odd-proton $N = 50$ isotones within the assumption of strong proton $Z = 28$ and neutron $N = 50$ effective shell effects. The observed states can be associated to rather simple and clean configurations of three protons placed in the $1 f_{5 / 2}$ and $2 p_{3 / 2}$ orbits.

Received 25 January 2007

DOI:https://doi.org/10.1103/PhysRevC.76.054312

Authors & Affiliations

D. Verney^1,2,*, F. Ibrahim¹, C. Bourgeois¹, S. Essabaa¹, S. Galès^2,1, L. Gaudefroy^1,†, D. Guillemaud-Mueller¹, F. Hammache¹, C. Lau¹, F. Le Blanc¹, A. C. Mueller¹, O. Perru¹, F. Pougheon¹, B. Roussière¹, J. Sauvage¹, and O. Sorlin^2,1 (PARRNe Collaboration)

¹Institut de Physique Nucléaire CNRS-IN2P3/Université Paris Sud-XI, F-91406 Orsay Cedex, France
²GANIL, BP 55027, F-14076 Caen Cedex 5, France

^*verney@ipno.in2p3.fr
^†Permanent address: CEA/DIF/DPTA/PN, BP 12, 91680 Bruyères-le-Châtel, France.
^‡http://ipnweb.in2p3.fr/tandem-alto/

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Vol. 76, Iss. 5 — November 2007

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Figure 1
Part of the β-gated γ spectrum recorded at mass 81. The observed γ lines have been identified as transitions fed in the β decays of ${}^{81}{Ga}$ (•), ${}^{81}{Ge}$ ( $*$ ), and ${}^{81}{As}$ ( $▪$ ) and the β-n decay of ${}^{81}{Ga}$ ( $□ P_{n} = 11.9 %$ ). The activity of ${}^{132}I$ ( $○$ ) is also identified, it comes from a previous setting on mass 132, which is often used as a reference in our experiments. The new line at 351.1 keV ( $⋄$ ) is clearly visible and well isolated. The projection on the time scale of the background substracted events in this peak is shown in the insert.
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Figure 2
(a) γ-ray spectrum in coincidence with the peak at 351.1 keV. A narrow line is clearly observable at 451.7 keV ( $⋄$ ) that was barely visible in the direct spectra. The wider line around 179 keV ( $*$ ) is due to the $180^{°}$ backscatter of the 530.22 keV γ ray from the ${}^{81}{Ga}$ decay. (b) γ-ray spectrum in coincidence with the peak at 451.7 keV. The 351.1 keV peak is clearly visible (•); the negative-positive oscillation around 200 keV has a similar origin as $*$ in Part (a).
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Figure 3
Tentative experimental level scheme for ${}^{81}{Zn}$ . The proposed spin assignments come from the Discussion (Sec. 3).
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Figure 4
Experimental and calculated spectra for the last stable ( ${}^{87}{Rb}$ ) and proton-deficient odd $N = 50$ isotones; spin values are multiplied by two and only the six first $J^{π} = 1 / 2^{-}$ , $3 / 2^{-}$ , $5 / 2^{-}$ and three first $7 / 2^{-}$ and $9 / 2^{-}$ calculated levels are displayed. “Exp.” stands for experimental levels, “JW” stands for the results of shell model calculations using the interaction from Ref. [21], “JW $^{'}$ ” stands for the same with modified pairing (see text), and “jj4pna” and “jj4b” stand for those from Refs. [22] and [23], respectively. The $3 / 2$ levels marked with asterisks in the ${}^{81}{Ga}$ and ${}^{83}{As}$ calculated spectra correspond to the $π 2 p_{3 / 2}$ q.p. states.
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Figure 5
Experimental and calculated spectra for the last stable ( ${}^{86}{Kr}$ ) and proton-deficient even $N = 50$ isotones. Notations for the interactions are the same as those described in the legend to Fig. 4.
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Figure 6
Summary of the structure change in the gs and first excited state between ${}^{81}{Ga}$ and ${}^{83}{As}$ from the change of the set of parameters from JW to JW $^{'}$ . Only the two lowest proton single particle orbits of the valence space are represented, viz., $1 f_{5 / 2}$ and $2 p_{3 / 2}$ , which dominate the configurations. Occupied magnetic substates are represented by filled circles and empty ones by open circles. The wavy lines indicate symbolically the position of the Fermi level.
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Physical Review C

covering nuclear physics

Low-energy states of $_{31}^{81} {Ga}_{50}$ : Proton structure of the nuclei close to ${}^{78}{Ni}$

D. Verney, F. Ibrahim, C. Bourgeois, S. Essabaa, S. Galès, L. Gaudefroy, D. Guillemaud-Mueller, F. Hammache, C. Lau, F. Le Blanc, A. C. Mueller, O. Perru, F. Pougheon, B. Roussière, J. Sauvage, and O. Sorlin (PARRNe Collaboration)

Phys. Rev. C 76, 054312 – Published 15 November 2007

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Low-energy states of 3181Ga50: Proton structure of the nuclei close to Ni78

D. Verney, F. Ibrahim, C. Bourgeois, S. Essabaa, S. Galès, L. Gaudefroy, D. Guillemaud-Mueller, F. Hammache, C. Lau, F. Le Blanc, A. C. Mueller, O. Perru, F. Pougheon, B. Roussière, J. Sauvage, and O. Sorlin (PARRNe Collaboration)

Phys. Rev. C 76, 054312 – Published 15 November 2007

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Low-energy states of $_{31}^{81} {Ga}_{50}$ : Proton structure of the nuclei close to ${}^{78}{Ni}$