2.1
Observation of excited state in
41
Si
D. Sohler, Zs. Dombrádi, B. Bastin b) , S. Grévy a) , O. Sorlin a) , N.L. Achouri b) , J.C. Angélique b) , F.
Azaiez c) , D. Baiborodin d) , R. Borcea e) , C. Bourgeois c) , A. Buta e) , A. Burger f,g) , R. Chapman h) , J.C.
Dalouzy a) , Z. Dlouhy d) , A. Drouard f ) , Z. Elekes, S. Franchoo c) , S. Iacob e) , B. Laurent b) , M. Lazar e) ,
X. Liang h) , E. Lienard b) , J. Mrazek d) , L. Nalpas f ) , F. Negoita e) , N.A. Orr b) , Y. Penionskhevitch i) ,
Zs. Podolyak j) , F. Pougheon c) , P. Roussel-Chomaz a) , M.G. Saint-Laurent a) , M. Stanoiu c,k) , I. Stefan a) , F. Nowacki l) , A. Poves m)
be 2014 keV. Comparing with that of 47 Ca the
low energy value of the first excited state obtained in 41 Si can be interpreted as a further
indication of the disappearance of the N=28
spherical shell closure at Z=14.
Recently, the collapse of the Z=14 and
N=28 shell closures has been revealed in very
neutron-rich Si and P nuclei [1]. In order to further explore the behaviour of these shell gaps,
we studied the excited states of 41 Si by in-beam
γ-ray spectroscopy from fragmentation of radioactive beams.
The experiment was carried out at the
GANIL facility, France. First a stabil 4µA
48 Ca beam at 60 MeV/u was fragmented on
a 12 C target in the SISSI device. The primary reaction products were selected by measuring their energy loss and time-of-flight at
the ALPHA spectrometer. The cocktail beam
impinged onto a secondary 9 Be target placed
in the dispersive focal plane of the SPEG spectrometer which was tuned to maximise the
transmission of 42 Si. The secondary reaction products were unambiguously identified
by their ∆E and positions determined by ionisation and drift chambers, furthermore by their
TOF and residual energies measured by a plastic scintillator at the focal plane of SPEG. To
detect γ rays, the secondary target was surrounded by an 4π array of 74 BaF2 scintillators. The γ-ray spectra were obtained by selecting event-by-event the incoming nuclei and
the ejectiles after the secondary target. The γ
rays were corrected for Doppler shifts due to
the in-flight emission by the fragments.
As it can be seen in figure 1, a clear peak
on a low background at 659±14 keV energy
appears in the γ-ray spectrum of 41 Si. This
finding is in accordance with the 770 keV energy of the first 2+ excited state in 42 Si with
N=28 and Z=14 [1]. Along the N=28 isobaric
line the next double magic nucleus is 48 Ca. In
the neighbouring odd-N nucleus 47 Ca the energy of the first excited state was measured to
659(14)
20
16
41
Counts
Si
12
8
4
0
0
1000
2000
3000
4000
Eγ (keV)
Figure 1. Gamma-ray energy spectrum detected
in coincidence with 41 Si nuclei.
a) GANIL, Caen, France
b) LPC, Caen, France
c) IN2P3-CNRS, Orsay, France
d) NPI, Rez, Czech Republic
e) IFIN-HH, Bucharest-Magulere, Romania
f) DAPNIA/SPhN, CEA Saclay France
g) Universität Bonn, Germany
h) University of Paisley, Scotland, UK
i) FLNR, JINR, Dubna, Russia
j) University of Surrey, Guildford, UK
k) GSI, Darmstadt, Germany
l) IreS, Strasbourg, France
m) Universidad Autónoma de Madrid, Spain
[1] B. Bastin et al., Phys. Rev. Lett. 99, 022503
(2007)
10