Physics Letters B 732 (2014) 161–166
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Experimental investigation of the 0
+
2
band in
154
Sm as a β-vibrational
band
J. Smallcombe
a,∗
, P.J. Davies
a
,C.J.Barton
a
,D.G.Jenkins
a
, L.L. Andersson
b
,P.A.Butler
b
,
D.M. Cox
b
,R.-D.Herzberg
b
, A. Mistry
b
,E.Parr
b
,T.Grahn
c
, P.T. Greenlees
c
,
K. Hauschild
c,d
,A.Herzan
c
, U. Jakobsson
c
,P.Jones
c
,R.Julin
c
, S. Juutinen
c
,S.Ketelhut
c
,
M. Leino
c
,A.Lopez-Martens
c,d
, P. Nieminen
c
, J. Pakarinen
c
, P. Papadakis
c,b
, P. Peura
c
,
P. Rahkila
c
, S. Rinta-Antila
c
, P. Ruotsalainen
c
, M. Sandzelius
c
,J.Sarén
c
,C.Scholey
c
,
J. Sorri
c
, J. Uusitalo
c
a
Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom
b
Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 9ZE, United Kingdom
c
Department of Physics, University of Jyväskylä, FI-40014, Finland
d
CSNSM-IN2P3-CNRS, Université Paris-Sud, 91406 Orsay, France
article info abstract
Article history:
Received 10 January 2014
Received in revised form 20 February 2014
Accepted 18 March 2014
Available online 21 March 2014
Editor: D.F. Geesaman
Keywords:
Internal conversion electrons
Gamma ray
Spectroscopy
Electric monopole
Collective models
Rare-earth
Astudyof
154
Sm through γ -ray and internal conversion electron coincidence measurements was per-
formed using the Silicon And GErmanium spectrometer (SAGE). An upper limit for the
ρ
2
(E0; 2
+
2
→ 2
+
1
)
and measurement of the ρ
2
(E0; 4
+
2
→ 4
+
1
) monopole transitions strengths were determined. The ex-
tracted transition strength for each is significantly lower than that predicted by either the Bohr and
Mottelson
β-vibration description or the interacting boson model. Hence, the long standing interpreta-
tion of these states as a collective band built on the 0
+
2
state, which is conventionally assigned as a Bohr
and Mottelson
β vibration is questionable.
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/3.0/). Funded by SCOAP
3
.
1. Introduction
The first excited 0
+
states in the vast majority of even–even
rare-earth nuclides have long been interpreted as collective excita-
tions. This is due, in part, to these states being observed below
the pairing gap. Collective behaviour in nuclei that are known
to have significant quadrupole deformation can be described by
a geometric model of an axially symmetric rotor. The most suc-
cessful and long-standing theoretical description of such collective
behaviour involve solutions to the Bohr and Mottelson Hamilto-
nian [1]. Solutions to this Hamiltonian show that collective excita-
tion modes may arise from shape oscillations parallel to (
β vibra-
tion) or perpendicular to (
γ vibration) the symmetry axis. A sig-
nificant amount of experimental evidence exists for
γ vibrations,
and typically the first rotational structure identified as being built
*
Corresponding author.
E-mail address: james.smallcomb
e@outlook.com (J. Smallcombe).
on a K = 2 state is labelled as the γ -band. Hence, it is common
practise to label the first excited 0
+
state as a β vibration and the
rotational structure built upon this state as a
β-band. However,
a significant amount of theoretical [2–5] and experimental [6–11]
work has questioned this interpretation.
The historical approach of identifying these
β-bands simply by
the energy spacing of the 0
+
2
,2
+
2
and 4
+
2
states is not sufficiently
rigorous [2]. This method belies the evidence provided by modern
measurements such as B
(E2) and ρ
2
(E0) strengths. An enhanced
decay strength should be seen in the case of E0decaysfroma
β-band to the ground state band (GSB). These E0 transition rates
can be related to B
(E2) values by [12,13]:
ρ
2
(E0; n
β
= 1 → n
β
= 0) =
9
8π
2
Z
2
β
4
0
E(2
+
1
)
E(0
+
β
)
=
B(E2; 0
+
1
→ 2
+
β
)4β
2
0
e
2
r
4
0
A
4/3
, (1)
http://dx.doi.org/10.1016/j.physletb.2014.03.034
0370-2693/
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). Funded by
SCOAP
3
.