BETA-DECAY STUDIES AT THE N = 28 SHELL CLOSURE
S. Grévya, J.C. Angéliquea, P. Baumannb, C. Borceac, A. Butac, G. Canchelb,
W.N. Catfordd, a, S. Courtinb, J.M. Daugase, F. de Oliveirae, P. Dessagneb, Z. Dlouhyf,
A. Knipperb, K.L. Kratzg, F.R. Lecolleya, J.L. Lecoueya, G. Lehrsenneaub,
M. Lewitowicze, E. Liénarda, S. Lukyanovh, F. Maréchalb, C. Miehéb, J. Mrazekf,
F. Negoitac, N.A. Orra, D. Pantelicac, Y. Penionzhkevichh, J. Pétera, B. Pfeifferg, S. Pietria,
E. Poirierb, O. Sorlini, M. Stanoiue, I. Stefanc, C. Stodele and C. Timisa
a
Laboratoire de Physique Corpusculaire de Caen, IN2P3-CNRS, ENSICAEN et Université de Caen,
F-14050, Caen cedex, France
b
IReS, IN2P3/ULP, 23 rue du Loess, BP20, F-67037, Strasbourg, France
c
Institute of Atomic Physics, IFIN-HH, Bucharest-Magurele, P.O. Box MG6, Romania
d
Department of Physics, University of Surrey, Guildford, Surrey, GU2 7XH, UK
e
GANIL, CEA/DSM-CNRS/IN2P3, BP5027, F-14076, Caen cedex, France
f
Nuclear Physics Institute, AS CR, CZ-25068, Rez, Czech Republic
g
Institut für Kernchemie, Universität Mainz, D-6500, Mainz, Germany
h
FLNR, JINR, 141980, Dubna, Moscow region, Russia
i
Institut de Physique Nucléaire, IN2P3-CNRS, F-91406, Orsay cedex, France
The investigation of very neutron-rich
isotopes provides a fertile testing ground for
our understanding of nuclear shell structure.
As an example, the neutron-rich isotopes of Na
and Mg around N = 20 have long been known
to lie within the so-called “Island of Inversion”
[1] whereby deformed fp-shell configurations
dominate the ground state structure. However
only detailed spectroscopic measurements,
such as decay studies [2], have enabled a
complete understanding to be acquired. In the
region of N = 28, evidence has recently
accumulated of modifications in the shell
structure.
First results [3] on a -decay
spectroscopy experiment of nuclei in the
region of N = 28 are reported. New half-lives
for nuclei from Mg to Ar have been extracted.
For the heavier ones, the new periods of 48Ar
and 47Cl are of importance in understanding of
the solar abundance ratio 48Ca/46Ca. The
measured half-lives of Si isotopes have been
extended from N = 25 to N = 28 and are
discussed in the light of possible deformation
occurring in this region. Finally, preliminary
spectroscopy of 45Ar is also reported.
The present experimental half-lives are
compared with values from the literature,
when available. For the lighter nuclei (Mg to
Si), all the extracted half-lives were unknown
up to now. For the Si isotopes, we extended
the measured T1/2 up to N = 28 with the 42Si.
These results are interesting in the light of the
deformation in the neutron rich N = 28 nuclei
below 48Ca. In shell model calculations
performed in the sd+fp shells [4] concluded to
a moderate modification of the shell gap at
N = 28 since Ar and S isotopes are deformed
but on the other hand the slope of the two
neutron separation energy S1n seemed to
indicate that 42Si has the characteristics of a
doubly magic nucleus, as 48Ca. However
recently the same authors adjusted their
interaction to reproduce single particle states
in 35Si [2] interpreting the reduction of the
neutron gap between the f7/2 and p3/2 shells as
an erosion of the spin-orbit force far from
stability. This erosion is moderate and the
changes between the two calculations at N
= 28 are very small except in the case of 42Si
where the doubly closed shell character is less
pronounced compared to [4] with a 2+ energy
going down from 2.56 to 1.49 MeV. Then, 42Si
appears to be very unstable with respect to the
choice of the interaction. On the opposite,
Lalazissis [5] concluded, on the basis of
relativistic Hartree-Bogoliubov calculations,
that the shell gap N = 28 is well broken below
48
Ca with a shape coexistence in 44S and a
very deformed configuration for 42Si for
which an oblate ground state is predicted with
 = -0.4. More recently, the observation of the
N = 29 isotope 43Si has been reported. The
existence of this nucleus is in contradiction
with
the Finite Range Drop Model
(FRDM) predicting a neutron separation
energy S1n = -1.68 MeV, but can be understood
in the Extended Thomas-Fermi plus Strutinsky
Integral method (ETFSI, S1n = 0.6 MeV). The
main difference between the two approaches
lies in the degree of deformation, the ETFSI
predicting a larger deformation than the
FRDM or the Si isotopes at N  28 suggesting
that the N = 28 shell closure may have been
exaggerated too strongly in the FRDM. In this
Table 1.
Nucleus
36
Mg
37
Al
38
Al
39
Al
39
Si
40
Si
41
Si
42
Si
39
P
40
P
41
P
context, experimental results on the - halflives of Si isotopes can put more constraints on
the possible deformation.
This work has been carried out with the
partial financial support of INTAS under grant 0000463, RFBR under grant 01-02-22001 and PICS
IN2P3-1171.
Half-lives of Mg to Ar isotopes, obtained in this work.
T1/2 (msec) this work
3.9±1.3
10.7±1.3
7.6±0.6
7.6±1.6
47.5±2.0
33±1.0
20±2.5
12.5±3.5
250±80
125±25
100±5
Nucleus
42
P
43
P
44
P
43
S
44
S
45
S
46
S
46
Cl
47
Cl
47
Ar
48
Ar
T1/2 (msec) this work
48.5±1.5
36.5±1.5
18.5±2.5
282±27
100±1
69±2
50±8
232±2
101±6
1250±150
475±40
References:
1. E. K. Warburton et al., Phys. Rev. C41
(1990) 1147.
2. S. Nummcla et al., Phys. Rev. C63
(2001) 044316 and references therein.
3. C. Grevy et al., Nucl. Phys. A722
(2003) 424.
4. J. Retamosa et al., Phys. Rev. C55
(1997) 1266.
5. G. A. Lalazissis et al., Phys. Rev. C60
(1999) 01431.