Discrete line -ray spectroscopy in the „50 – 60… spin domain of
161,162
Er
J. Simpson,
1
A. P. Bagshaw,
2
A. Pipidis,
3,4
M. A. Riley,
3
M. A. Bentley,
5
D. M. Cullen,
6,
* P. J. Dagnall,
2
G. B. Hagemann,
7
S. L. King,
6
R. W. Laird,
3
J. C. Lisle,
2
S. Shepherd,
6
A. G. Smith,
2
S. To
¨
rma
¨
nen,
7
A. V. Afanasjev,
8,9,10
and I. Ragnarsson
10
1
CLRC, Daresbury Laboratory, Daresbury, Warrington WA4 4AD, United Kingdom
2
Schuster Laboratory, University of Manchester, Manchester, M13 9PL, United Kingdom
3
Department of Physics, Florida State University, Tallahassee, Florida 32306
4
Department of Physics, School of Physical Sciences, University of Surrey, Guildford, Surrey GU2 5XH, United Kingdom
5
School of Sciences, Staffordshire University, Stoke on Trent ST4 2DE, United Kingdom
6
Oliver Lodge Laboratory, Department of Physics, University of Liverpool, Liverpool L69 7ZE, United Kingdom
7
The Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100 Copenhagen, Denmark
8
Physik-Department der Technischen Universita ¨t Mu ¨nchen, D-85747 Garching, Germany
9
Laboratory of Radiation Physics, Institute of Solid State Physics, University of Latvia, LV-2169, Salaspils Miera Str. 31, Latvia
10
Department of Mathematical Physics, Lund Institute of Technology, Box 118, S-221 00, Lund, Sweden
Received 6 April 2000; published 25 July 2000
Very high spin states ( I =50–60 ) have been observed in the transitional nuclei
161
Er and
162
Er using
the Euroball -ray spectrometer. In
161
Er, three bands are observed well above spin 50 . In the positive
parity, positive signature ( +, +
1
2
) band a discontinuity in the regular rotational behavior occurs at
109
2
+
and a splitting into two branches occurs at
97
2
-
in the negative parity, positive signature ( -, +
1
2
) band. The
( -, -
1
2
) band continues in a regular fashion to
115
2
-
, tentatively (
119
2
-
). In
162
Er the positive parity, even spin
+,0 yrast band is observed to continue smoothly up to 58
+
(60
+
) and the negative parity, even spin
( -,0) and odd spin ( -,1) bands are extended from 30
-
to 34
-
and from 31
-
to 47
-
(49
-
), respectively. The
high spin experimental spectra are compared with both a simple model involving the occupation of specific
single neutron states in the absence of neutron pair correlations and with more detailed cranked Nilsson-
Strutinsky calculations in which both proton and neutron pairing correlations are neglected. The very high spin
domain is found to comprise a series of unpaired rotational bands. Unpaired band crossings between bands
with different neutron and proton configurations are identified in
161
Er. There is no evidence for aligned oblate
or terminating states being close to the yrast line in
161,162
Er up to spin 60 in contrast to the lighter Er
isotopes.
PACS numbers: 21.10.Re, 23.20.Lv, 27.70.+q
I. INTRODUCTION
A persistent theme in science is to investigate the behav-
ior of physical systems under extreme conditions. The quest
to observe increasingly high angular momentum states in
atomic nuclei has driven the field of high spin nuclear spec-
troscopy for many years. With each step forward in detector
technology the observation limit for discrete nuclear states
has been pushed upward in spin and an increasingly rich
variety of new phenomena have been discovered. It is in the
light mass A 160 Dy and Er nuclei that the highest spin
states in normal deformed nuclei have been observed spin
60 and E
excit
30 MeV1–12. Aside from the question
of the limiting spin at which discrete states in nuclei exist,
other fundamental issues concern the effect of rotation on the
nuclear equilibrium shape, on the nuclear pairing correla-
tions and charting the correct single-particle spectrum of
states at ultrahigh spins.
The nucleus displays well-established superfluid proper-
ties at low angular momentum values but collective rotation
of the nucleus tends to destroy such correlated fermion mo-
tion and can lead to a superfluid to normal phase transition
the Mottelson-Valatin effect 13. Thus, with increasing ro-
tational frequency spin and valence particle alignments, a
change from a regime dominated by strong superfluid prop-
erties ‘‘static pairing regime’’ to one where the effects of
pairing correlations on the nuclear excitation spectrum will
be greatly weakened 14 is expected. It is now realized how-
ever, that because of dynamic fluctuations, a complete
quenching of the pair field will not occur in the finite particle
number system of the nucleus 15–19. In this new, often
called ‘‘unpaired’’ regime the static pairing gap has vanished
and the pair field consists of essentially dynamic contribu-
tions. As a result nuclear structure phenomena become very
much more sensitive to the underlying single-particle spec-
trum of states. Band crossings can occur 20,21, but they are
of a different nature to those at lower spins where the Cori-
olis and centrifugal forces break apart and align specific pairs
of correlated nucleons 22. In the high spin regime where
pairing is not dominant, an ‘‘unpaired’’ band crossing can
occur when a rearrangement of nucleons conserving the par-
ity and signature quantum numbers of the original configu-
ration becomes energetically favorable due to changes in
energy of particular single-particle orbits with rotational fre-
quency or spin 20,21. Such changes give rise to band cross-
ings at high angular momentum that are not correlated in
*Present address: Schuster Laboratory, University of Manchester,
Manchester M13 9PL, UK.
PHYSICAL REVIEW C, VOLUME 62, 024321
0556-2813/2000/622/0243218/$15.00 ©2000 The American Physical Society 62 024321-1