GSI Proposal for the Rising Stopped Beam campaign
Structure of the odd-odd N=Z nuclei in the vicinity of 100Sn: high
spin isomers in 90Rh studied through GT -decay
L.Cáceres1,2, A. Gadea3, M. Górska2, P. H. Regan4, G. de Angelis3,
A. Bracco5, G. Benzoni5, N. Blasi5, P. Boutachkov2, F. Camera5, F. Crespi5,
F. Della Vedova3, P. Doornenbal2, E. Estevez6, E. Farnea7, J. Gerl2,
A. B Garnsworthy4, R. Hoischen8,2, A. Jungclaus1, S. Leoni5, S. Lunardi7,
R. Menegazzo7, D. Mengoni7, B. Million5, V. Modamio1, D. R. Napoli3,
R. Orlandi3, M. Pfützner9, S. Pieti4, Z. Podolyák4, F. Recchia3,
D. Rudolph8, S. Steer4, S.Tashenov2, J. Walker1, J. J. Valiente-Dobón3,
O. Wieland5, H-J.Wollersheim2.
1
Universidad Autónoma de Madrid, Madrid, Spain.
GSI, Darmstadt, Germany.
3
INFN-Laboratori Nazionali di Legnaro, Padova, Italy.
4
University of Surrey, Surrey, UK.
5
University and INFN-sezione di Milano, Italy.
6
Universidad de Santiago de Compostela, Santiago de Compostela, Spain.
7
University and INFN-sezione di Padova, Italy.
8
Institute of Experimental Physics, Warsaw University, Warsaw, Poland.
9
Lund University, Lund, Sweden.
2
Abstract: The goal of the proposed experiment is to study Gamow-Teller -decay of
the high spin isomer in 90Rh determining its spin and parity. 90Rh belongs to the family of
odd-odd N=Z nuclei with T=1 J=0+ ground state. Two β-decaying states are known in
this nucleus and shell model calculations assuming a spherical core and a the g9/2 g9/2
model space, predict the existence of a 5+ spin-gap isomer which decays by a GamowTeller transition to the excited states in the daughter nucleus, 90Ru.
Introduction
The structure of very proton-rich N=Z nuclei, approaching the double shell closure at
N=Z=50, has been one of the main subjects of interest for the nuclear structure. The
reasons are the consequences derived from the two different types of nucleons occupying
the same shell model orbitals which have large spatial overlap of their wave functions,
with impact on the pairing modes (possible existence of proton-neutron isoscalar pairing)
as well as the behavior of the nuclear system in which protons and neutrons contribute
coherently to build, especially, the collective phenomena.
Odd-odd heavy N=Z nuclei present aspects that characterize them as important targets for
the structure studies. In nuclei heavier than mass A=40, with the exception of 58Cu [1],
contrary to what has been observed in lighter ones, the T=1, J=0+ state becomes the
ground state.
The evolution of collectivity in the pfg shell indicates a maximum of deformation around
78
Y. Due to limitation in the model space, shell model calculations fail to reproduce the
recent experimental data. In fact isomers observed in 82Nb, 86Tc [2] are better described
in the framework of deformation. Moreover maximum aligned T=0 J=9+ (proton-neutron
pairing) isomeric state predicted by the shell model calculation to decay by GamowTeller beta-decay was not observed [3] in these nuclei. However, going towards the
doubly magic 100Sn the deformation seems to vanish and in fact shell model calculations
reproduce successfully the level scheme of 94Pd and 94Ag [4]. The most relevant
predictions indicate the presence of high spin-gap isomers caused by the T=0 coupling of
maximum aligned nucleons. In the odd-odd N=Z nuclei from 90Rh up to 98In isomers
decaying by Gamow-Teller transitions are predicted together with the superallowed
ground state Fermi decay. Indeed, Kienle et al. [3] observed the presence of long and
short beta decay lifetimes in 90Rh, 94Ag and 98In nuclei. The same work points out a
discrepancy between the lifetimes of the Gamow-Teller transition and the predicted ones
[5]. Therefore, the measurement of the spin and parity of beta decaying isomers and their
excitation energy will provide crucial information concerning the structure of these
nuclei.
The goal of this proposal is to measure the Gamow-Teller strength of the 90Rh isomeric
state and determine its spin and parity. The spin and parity of this isomer is predicted by
shell model calculation with Gross-Frenkel empirical matrix elements [6] to be 5+ at
1.1MeV excitation energy. On the other hand Hernld and Brown [7] predict a low lying
4- state at the same excitation energy.
In addition to the interest of these studies in terms of the nuclear structure, the unitarity of
the Cabibbo-Kobayashi-Maskawa matrix by comparition of superallowed Fermi decay in
these nuclei to the muon decay lifetime can be tested. This requires a deep understanding
of the nuclear wavefunction to correct the measured ft values. The nuclear structure
corrections to these measured ft values come from isospin mixing effect as well as from
the radial overlap between the proton and neutron wavefuntions. A good knowledge of
the nuclear structure, in this region where rapid change of collectivity occurs, is of
paramount importance for the calculation of these corrections.
The 90Rh physics case:
Moving on the N=Z line from the middle of the pfg shell (78Y) towards 100Sn, 90Rh is the
first odd-odd nucleus with a beta-decaying isomeric state.
Within the shell model approach long lived beta decaying isomers are predicted in all
odd-odd N=Z g9/2 nuclei from 82Nb to 98In. In Fig. 2 the predictions of those isomers
based on the Gross-Frenkel interaction are shown for the whole chain.
Figure 1: The pfg shell N=Z odd-odd nuclei with T=1 ground state. The lifetime of the
Fermi decaying ground state and of the known isomers is indicated.
Figure 2: Predicted isomers with the Gross-Frenkel interaction for the odd-odd N=Z
nuclei in the pfg shell.
The 0+, T=1 90Rh ground state was measured to decay by a superallowed Fermi transition
[3] to the ground state of the daughter nucleus, 90Ru [8]. Another long-lived high-spin
isomeric state (T1/2 = 1.0 s) belonging to 90Rh was reported as well by Kienle et al. in Ref.
[3]. The spin and parity assignment of the long-lived isomeric state in 90Rh will be
extracted by the balance of the gamma-decay intensity to the levels in 90Ru (Fig 3). If the
isomer is a 5+ as predicted, the unpaired g9/2 proton will decay to a g9/2 neutron and the
gamma transitions associated with the deexcitation of the 4+ and 6+ excited states in 90Ru
will be observed. Therefore an increase in gamma intensity corresponding to the deexcitation of the 4+ state will be visible. The effect can be easily study by analyzing the
beta-delayed gamma coincidence spectrum.
The experimentally known negative parity cascade in 90Ru is interpreted to be formed by
coupling one g9/2 proton and one f5/2 proton [8]. If the observed isomeric state in 90Rh is
the 4- as predicted by Hernd and Brown [7], with p1/2 proton and g9/2 neutron assigned
configuration, then the the 5- excited state in 90Ru will not be populated. Instead new
negative parity states involving the neutron p1/2 g9/2 configuration will be directly fed.
However, the gamma spectrum of 90Ru populated in the beta-decay of 90Rh is completely
unknown; therefore we expect to populate low-lying non-yrast states of the daughter
nucleus as well as the yrast states, which will provide additional information about other
contributions to the 90Rh ground state wavefunction.
Figure 3: Level scheme of 90Ru obtained in fusion-evaporation measurement.
Experimental Method:
We propose to study the Gamow-Teller beta-decay of the 90Rh high spin isomer by
fragmentation of 750MeV/u 107Ag on a 4011mg/cm2 9Be target. The reactions products
will be separated and identified unambiguously by means of Bρ – ΔE - Bρ method in the
Fragment Separator (FRS) at GSI. The identified fragments will be implanted into a
double-sided strip detector system (DSSDs), with area of 5 x 5 cm2 with 16 x 16 strips,
placed in the S4 focal plane of the FRS. The stopper will be placed in the target position
and viewed by the standard RISING Ge detector array in close geometry (55o, 90o, 1250)
which will allows to measure beta-delayed gammas-rays. Time correlation between the
ion implanted and emitted beta particle will permit to extract the beta-decay half-life with
high accuracy. The beta branching ratio will be extracted from the gamma transition
intensities in daughter nucleus.
The implantation rate at the last focal plane is given by:
I sec 1    cm 2  d g / cm 2   A1 mol / g    sec 1  Ttot  N A mol 1 
With σ : cross section ~1.043 nb
d : target thickness ~4.011 g/cm2
A : atomic mass of the target = 9
 : primary beam current ~1 x 10 9 ions per second.
Ttot : total transmition to the stopper ~40 %
NA : Avogadro constant
We expect to produce ~6.7 iones of 90Rh per minute or approximately 9670 ions per day.
If we assume an isomeric ratio of 10 % and a beta efficiency of 100 %, then the number
of beta decays per day will be:
Rday 1   I day 1  Riso     967decays / day
Since the Q-value is more than 13 MeV [9], more than 99.5% of the decay will proceed
with positron emission. Therefore looses due to the non identification of the electron
capture decay are not expected.
Taking into account that the photopeak gamma efficiency of the RISING array in the
stopped beams configuration is approximately 10% at 800 keV, we will expect ~96 betadelayed gamma-rays detected per day.
Therefore we estimate that a beam time of 6 days will be necessary in order to achieve
the sufficient number of delayed beta-gamma coincidences, 1 day of beamtime for the
FRS settings and 2 days parasitic ( 1:10 no block mode ) for FRS calibration is requested.
References
[1] A. F. Lisetskiy et al. Phys. Rev. C 68, 034316 (2003).
[2] A. Garnsworthy, P. Regan. Private communications.
[3] P. Kienle et al., Prog. Part. Nucl. Phys. 46, 73-78 (2001)
[4] C. Plettner et al., Nucl. Phys. A 733, 20-36 (2004).
[5] H. Schatz, Phys. Rep. 294, 167 (1998).
[6] R. Gross, A. Frenkel, Nucl. Phys. A 67, 85 (1976).
[7] H. Herndl, B.A. Brown, Nucl. Phys A 627, 35-52 (1997).
[8] D. Bucurescu et al., Phys. Rev. C 69, 064319 (2004).
[9] G. Audi et al., Nucl. Phys. A 279, 337-676 (2003).