Spectroscopy of 120Sn Homologous
Levels via the 123Sb(pJa)120Sn Reaction
P. Guazzoni (1), L. Zetta (1), A. Covello (2), A. Gargano (2), Y.
Eisermann (3), G. Graw (3), R. Hertenberger (3), H.-F. Wirth (3), M.
Jaskola (4), B. Dayman (5), and W.E. Ormand(6)
(1) Dipartimento di Fisica dell'Universita andLN.F.N, 1-20133 Milano, Italy
(2) Dipartimento di Fisica dell'Universita and IMF.NJ-80126Napoli, Italy
(3) Sektion Physik der UniversitaetMuenchen, D-85748 Garching, Germany
(4) Soltan Institute for Nuclear Studies, Warsaw, Poland
(5) Physics and Astronomy Department, University of Minnesota, Minneapolis
(6) Lawrence Livermore National Laboratory, CA-94551 Livermore, USA
Abstract. In order to investigate the spectator role of the 1 §7/2 unpaired proton outside the Z=50
closed shell in the 123Sb nucleus and to test in this region the validity of the concept of
homology, which we already tested in the Z=40 and Z=82 regions, the reaction 122 Sn(p,a)119 In
was measured and the reaction l23Sb(p,a)l2() Sn is currently being studied. In the present
contribution the multiple! of states of 120Sn, homologous to the 9/2+ 119In G.S. is described in
details.
THE EXPERIMENT
The (p,a) reactions on nuclei around closed or semiclosed shells display several
properties that make it a useful spectroscopic tool for supplementing level structure
information obtained by other charged-particle reactions. In our previous work
concerning Z=40 and Z=82 regions [1-4] we have shown that an interesting behavior
can be observed for a number of transitions induced by (p,a) reactions on near magic
target nuclei having one nucleon outside a completely filled magic shell. In this case
the unpaired nucleon, slightly bound, may act as spectator in the process. Some
distinctive features are displayed: a) weak population of residual nucleus levels below
an excitation energy strictly related to the energy gap in the nucleon state spacing at
the filling of the magic shell, b) excitation of homologous states (i.e. of states with a
close structural relationship) of residual nuclei from (p,a) on adjacent target nuclei
one magic (leading to parent state transition) and the other near-magic (leading to a
multiplet of corresponding daughter states) with one more nucleon outside the magic
shell.
In this framework, in order to complete spectroscopic study of 120Sn, a high
resolution experiment was carried out with the 24 MeV polarized proton beam of the
CP675, Spin 2002:15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. L Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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Munich MP Tandem accelerator, using the new Stern-Gerlach source [5], the Q3D
magnetic spectrograph and the new light ion focal plane detector [6] to study
l23
Sb(p,a)l20Sn, to be compared with 122 Sn(p,a) n9 In [7]. The beam current intensity
was up to 1.5 uA and the beam polarization 60%.
Angular distributions for cross section and asymmetry have been measured from 6°
to 52.5°. High resolution and very low background allowed study of 61 transitions to
final states of 120Sn up to an excitation energy of- 4700 keV, with a precision of ± 3
keV. This has allowed a remarkable increase of the knowledge of 120Sn nucleus,
because 19 new levels have been identified, with the attribution of energy, spin and
parity. To 16 levels spin and parity have been attributed, while the only parity has
been attributed to 11 levels. For the attribution of spin and/or parity the methodology
introduced by our group for homologous states has been applied. In such a way
multiplets of states of l °Sn homologous to the low lying states of 119In have been
identified. In particular we found the octet of states [1+, 2+, 3+, 4+, 5+, (6+ fragmented
in two levels), 7+, 8+ ] homologous to the G.S. (9/2^) of 119In, the doublet of states [3~
(fragmented in three levels), 4~] homologous to the level 0.311 l/2~ MeV of 119In , the
quadruplets of states [2~, 3~, 4~, (5~ fragmented in four levels)] homologous to the
level 0.604 MeV 3/2~ of 119In and the sestet of states [1, 2~ 3~ 4~ 5~, (6~ fragmented
in four levels)] homologous to the level 1.044 MeV 5/2~ of 119In.
In fig.l the a-spectrum measured at 6iab=10° is shown. It is possible to recognize
two different regions: the homologous region (Eexc> 3.6 MeV) intensively populated
and the low excitation energy region poorly populated by the pickup of the spectator
proton lg.7/2 together with a neutron pair. The configuration of the homologous states
results from the coupling of the spectator proton (not involved in the process) with the
one-proton-hole-two-neutron-hole states of the 119In core.
123
Sb(pTa)120Sn
8lab=10°, spin up
channels
FIGURE 1. The a-spectrum measured at 0iab=10°. It is possible to recognize two different regions: the
homologous region (Eexc>3.6 MeV) intensively populated and the low excitation energy region, poorly
populated.
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THE ANALYSIS
Spin and Parity Assignment Via The Homologous State Methodology
In fig.2 the comparison between the measured 0(6) and Ay(6) for the transition to
the multiplet of 120Sn states (dots) homologous to the G.S. (9/2*) of 119In and the
measured 0(6) and Ay(6) for the transition to the G.S. (9/2*) of 119In is shown. The
last cross section is scaled for each level by the proper factor (2Ji+l)/Ei(2Ji+l) (solid
line). In the same figure the cumulative cross section and asymmetry for the octet of
levels (dots) homologous to the G.S. (9/2+) of 119In are compared with cross section
and asymmetry for the transition to the G.S. (9/2^) of 119In. It is possible to observe the
good agreement both in shape and absolute value.
FIGURE 2. Comparison between the measured o(0) and Ay(0) for the transition to the multiplet of
120
Sn states (dots) homologous to the G.S. (9/2+) of 119In and the measured o(0) and Ay(0) for the
transition to the G.S. (9/2+) of 119In. The last one cross section is scaled for each level by the proper
factor (2Ji+l)/2i(2Ji+l) (solid line).
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In case of weak coupling the cross section of a homologous state with spin Ji in a
given multiplet can be related to that of the corresponding parent state by the
following expression:
=aparent (119In)
In fig.3 the quantities [(2J.+1) * aparent(119In)]/ Z(2Ji+l) are reported for each
member of the multiplet vs. Ji, together with the straight (2J,+1) line.
25
1
1
I
1
1
1
8
9
20
+ .*'"
10
FIGURE 3. Plot of the quantities [(2Jj+l) * oparent(119In)]/E(2Ji+l) for each member of the
multiplet vs. Ji(dots), together with the straight (2Jj+l) line
Microscopic DWBA Calculations
The 123Sb ground state is taken to be a lg7/2 proton outside filled proton shells,
whereas the 22 valence neutrons move in the lg7/2, 2d5/2, 2d3/2, 3si/2 and lhn/ 2 shells.
These neutrons interact via a neutron-neutron pairing force, which spreads the
neutrons over the valence shells, with a total neutron angular momentum of zero. In
this same picture, the 120Sn ground state consists of filled proton shells, with 20
valence neutrons. Thus the pickup reaction involves the transfer of a lg?/2 proton, and
lg7/2, 2d5/2, 2ds/2, 3si/2 and lhn/2 neutron pairs. Since the assumed interaction is
between the incident proton and the mass-center of the three transferred nucleons, it
cannot change the relative motion of the three transferred nucleons. Thus we must
project from shell-model states such as [lg?/2 (2d5/2,2d5/2)], the part in which the three
nucleons have the same relative motion as they will have in the outgoing alpha
particle. This is done for each of the possible triples of shell-model states that can
connect the initial and final states, and then a coherent sum must be taken to obtain the
total transfer form factor.
For the population of the homologous states, one removes a lgg/2 proton, as well as
a neutron pair. In this way one can reach states with total angular momenta J=l,2,...,8,
which consist of a lg7/2 proton coupled to a lg9/2 proton hole. The form factor is the
same for each value of J, except for a factor of (2J+1)172. Thus apart from Q-value
effects, the calculated angular distribution for each of the J=l,2,...,8 final states will be
the same, with a 2J+1 factor of proportionality. Figure 4 shows the comparison
between experimental and microscopically calculated angular distributions of cross
section and asymmetry for the transition to 120Sn 0 + G.S. together with the cumulative
689
angular distributions for the 119In G.S. homologous multiplet. The calculations were
done in finite range approximation, with the previously described microscopic
configurations. The overall multiplicative factor, used to give a reasonable fit to the
experimental data, is the same for the cumulative and G.S. cross sections.
0.5
102
0
-0.5
fe 10
0.5
10
0
-0.5
20
40
60
20
40
60
Figure 4. Comparison between experimental and microscopically calculated angular distributions of
cross section and asymmetry for the transition to 120Sn 0+ G.S. (bottom) together with the comparison
for cumulative angular distributions of the 119In G.S. homologous multiplet (top), obtained in finite
range approximation with the previous microscopic configurations (dots represent experimental values,
solid lines microscopic calculations).
ACKNOWLEDGMENTS
This work was performed in part under the auspices of the Italian Ministry for
University and Research, under contract CRUI-Progetto Vigoni, the DFG under Grant
No. C4-Gr894/2, and the U.S. Department of Energy by the University of California,
Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
REFERENCES
1.
2.
3.
4.
5.
Guazzoni, P., et al., Z. Phys. A 356, 381-391 (1997).
Guazzoni, P., et al., Eur. Phys. J. A 1, 365-378 (1998).
Guazzoni, P., et al., Phys. Rev. C 49, 2784-2787 (1994).
Gu, J.N., et al., Phys. Rev. C 55, 2395-2406 (1997).
Hertenberger, R., et al., AIP Conference Proceedings 570, New York: American Institute of Physics,
2001, pp. 825-829.
6. Wirth, H.-F., Ph.D. Thesis, Technische Universitat, Miinchen, 2001.
7. Guazzoni, P., et al., AIP Conference Proceedings 570, New York: American Institute of Physics,
2001, pp. 664-668.
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