Physics Letters B 772 (2017) 703–707
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Physics Letters B
www.elsevier.com/locate/physletb
Spectroscopy at the two-proton drip line: Excited states in
158
W
D.T. Joss
a,∗
, R.D. Page
a
, A. Herzán
a
, L. Donosa
a
, J. Uusitalo
b
, R.J. Carroll
a
, I.G. Darby
a,c
,
K. Andgren
d
, B. Cederwall
d
, S. Eeckhaudt
b
, T. Grahn
b
, P.T. Greenlees
b
, B. Hadinia
d,e
,
U. Jakobsson
b,d
, P.M. Jones
f
, R. Julin
b
, S. Juutinen
b
, M. Leino
b
, A.-P. Leppanen
b
,
M. Nyman
b
, D. O’Donnell
a
, J. Pakarinen
b
, P. Rahkila
b
, M. Sandzelius
b
, J. Sarén
b
,
C. Scholey
b
, D. Seweryniak
g
, J. Simpson
h
, J. Sorri
b
a
Oliver Lodge Laboratory, University of Liverpool, Liverpool, L69 7ZE, United Kingdom
b
Department of Physics, University of Jyvaskyla, PO Box 35 FI-40014, Jyvaskyla, Finland
c
Department of Nuclear Sciences and Applications, International Atomic Energy Agency, A-1400 Vienna, Austria
d
Department of Physics, Royal Institute of Technology, Alba Nova Centre, S106 91, Stockholm, Sweden
e
Department of Physics, University of Guelph, Guelph, Ontario N1G 2W1, Canada
f
Department of Nuclear Physics, iThemba Laboratory for Accelerator Based Sciences, PO Box 722, Somerset West 7129, South Africa
g
Argonne National Laboratory, Argonne, IL, 60439, USA
h
STFC, Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, United Kingdom
a r t i c l e i n f o a b s t r a c t
Article history:
Received
2 June 2017
Received
in revised form 5 July 2017
Accepted
15 July 2017
Available
online 20 July 2017
Editor:
V. Metag
Keywords:
Multiparticle
excited states
Proton
radioactivity
Excited states have been identified in the heaviest known even-ZN= 84 isotone
158
W, which lies in a
region of one-proton emitters and the two-proton drip line. The observation of γ -ray transitions feeding
the ground state establishes the excitation energy of the yrast 6
+
state confirming the spin-gap nature of
the α-decaying 8
+
isomer. The 8
+
isomer is also expected to be unbound to two-proton emission but no
evidence for this decay mode was observed. An upper limit for the two-proton decay branch has been
deduced as b
2p
≤ 0.17% at the 90% confidence level. The possibility of observing two-proton emission
from multiparticle isomers in nearby nuclides is considered.
© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Establishing the limits of observable nuclei is a long-standing
challenge in nuclear physics. For proton-rich nuclei, theoretical
predictions suggest that these limits are determined by two-proton
emission in even- Z nuclei up to Z = 82 and by the emission of a
single proton for odd-Z nuclei [1–4]. Two-proton radioactivity is a
rare phenomenon and experimental discoveries from ground states
has been limited to a few light nuclei. For example, two-proton
emission from
19
Mg ( Z = 12) [5] has been identified by measur-
ing
the decay products in flight, while two-proton decays from
the ground states of
45
Fe ( Z = 26) [6,7],
48
Ni (Z = 28) [8],
54
Zn
(Z = 30) [9] and
67
Kr [10] have been observed at the focal planes
of fragment separators. However, extrapolations from the table of
measured masses [11] combined with advances in nuclear den-
sity
functional theory have allowed candidates where two-proton
*
Corresponding author.
E-mail
address: david.joss@liverpool.ac.uk (D.T. Joss).
radioactivity competes with α decay in heavy nuclei to be pre-
dicted [3,4].
In
most cases, two-proton emission from the ground states of
even-Z nuclei would occur much further from β stability than the
one-proton drip line for odd- Z nuclei due to the pairing interac-
tion.
The known cases of ground state two-proton emission in light
nuclei occur around two neutrons lighter than the predicted two-
proton
drip line [3]. Two-proton emission from the ground states
of heavy nuclei would only dominate in nuclides that lie ten or
more neutrons beyond the two-proton drip line [3] and are in-
accessible
using current experimental facilities. However, there is
a possibility that direct two-proton emission might proceed from
excited states in nuclei closer to stability. This would be analogous
to the first observation of direct one-proton emission, which was
from a 19/2
−
isomer in
53
Co [12–14]. This nuclide is bound in its
ground state yet its excited state at 3.2 MeV is proton unbound.
In this case, the high excitation energy (and therefore large pro-
ton
decay Q value) is sufficient to overcome the confining effect
of the centrifugal barrier, which for
53
Co results in the largest spin
change of any known proton emitter (I = 9
¯
h). The discovery of
http://dx.doi.org/10.1016/j.physletb.2017.07.031
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© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.