This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440

Becchetti, F.D.

Berg, G.P.A.

Cianciolo, V.

Harakeh, M.N.

Janecke, J.

Nadasen, A.

Roberts, D.

Sawafta, R.

Stephenson, E.J.

van Werf, S.Y.

English

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17. T. Taddeucci, et al., Phys. Rev. C 33,746 (1986). 
I 18. C. Kalbach, et al., Phys. Rev. C 23, 122 (1981). 
19. Y. Holler, et al., Nucl. Phys. A442, 79 (1985). 
20. P. Schwandt, et al., Phys. Rev. C 26, 55 (1982). 
21. S. Kosugi, et al., Phys. Lett. 127B, 389 (1983). 
A STUDY OF THE (3He,t) CHARGE-EXCHANGE REACTION AT E(3~e)=200 MeV 
J. Janecke, F.D. Becchetti, V. Cianciolo, A. Nadasen, D. Roberts 
University of Michigan, Ann Arbor, Michigan 4 8128 
G.P.A. Berg, R. Sawafta, E.J. Stephenson 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 74 05 
M.N. Harakeh 
Vrije Universit ei t ,  Amsterdam, The Netherlands 
S.Y. van der Werf 
Kernfysisch Versneller Instituut, Groningen, The Netherlands 
The (3He,t) charge-exchange reaction has been studied at a bombarding energy of 
200 MeV on 12 targets ranging from 12C to 2 4 4 P ~  including 'OZr, 120Sn, 2 0 8 ~ b  and five 
radioactive actinide targets. The objective of this work was to establish the feasibility of 
taking data at zero degrees with the K600 spectrometer and to determine the sensitivity 
of this charge-exchange reaction to spinflip and non-spinflip transitions. The results could 
then be compared to the (3He,t) charge-exchange reaction at higher bombarding energies1 
and to (p,n) charge exchange2. The selectivity to non-spinflip L=O transitions is of partic- 
ular interest for the observation of isobaric analog states (IAS) and certain isovector giant 
resonances. In a particular application, properties of IAS were measured with emphasis 
on heavy nuclei. 
The experiment was carried out with the K6OO magnetic spectrometer at zero degrees. 
The 3He++ beam entered the spectrograph and hit a beam stop inside the first dipole 
magnet. The beam stop was electrically insulated so that the beam current could be 
read directly from it. Background in the focal plane detector due to particles originating 
from the beam stop and from beam halo was low when proper precautions were taken. 
Singly-ionized 3 ~ e +  particles produced in the target with an intensity on the order of 
lo-* compared to the incident beam reached the focal plane where they increased the 
event rate by approximately 30% unless eliminated with an 'absorber in front of the focal 
plane detector. Spectra were measured mostly in singles mode, but preliminary coincidence 
spectra were also obtained in a few instances. In this case, decay protons from unbound 
states were detected in the scattering chamber at backward angles with two large-area 
Li-drifted Si detectors. 
Figures 1, 2 and 3 display spectra obtained for targets of natural carbon, 120sn and 
208 P b, respectively. All measurements were made at identical magnetic field settings. The 
energy resolution is better than 50 keV. Spectra for the light targets 12c, 160 and 2 8 ~ i  w th 
very negative Q-values were used primarily for energy calibration but have also intrinsic 
interest. The spectra for 'OZr, 120sn, l 8 l ~ a ,  208Pb and 5 radioactive actinide targets were 
obtained to observe IAS. Energies, hence Coulomb displacement energies, total widths, 
and cross sections were determined. Information about the IAS of the actinide nuclei is 
quite limited. The widths I' are of particular interest in the context of the various decay 
modes. Figure 3 shows as an example the IAS from 2 0 8 ~ b  which has a Lorentzian line 
shape with 7 -- 300 keV. 
Isobaric analog states decay by isospin-allowed and isospin-violating decay modes. 
The latter account for the so-called spreading width which is due to isospin mixing via the 
Coulomb force with T <  doorway states mediated by coupling to the (To-1)-component of 
the giant isovector monopole resonance3. Only recently has it become possible to quanti- 
tatively describe the dependence of the experimentally observed spreading widths of IAS 
on nucleon number and neutron excess4 by introducing global trends for certain properties 
of isovector monopole resonances. In particular, mixing with the isovector quadrupole res- 
onance had to be postulated for deformed nuclei. Information about the total and partial 
widths of IAS beyond 2 0 8 ~ b  is very limited, and additional measurements have therefore 
been carried out in the meantime5 at E(3He)=75 MeV on 5 radioactive actinide targets. 
Proton decay from the IAS has been measured which will permit us to separate the ob- 
served total widths into the two components for comparison with the predicted values. 
0 5 10 0 5 10 
Excitation Energy (MeV) 
Figure 1. Triton energy spectrum for 12C(3~e , t )  12N, 0=0°, E(3He)=200 MeV. 
49 

It is interesting to compare the zero-degree spectrum of Fig. 1 for 12c with that 
obtained at 81 MeV (Ref. 6 ) .  The absence of the 0+ state at 2.42 MeV is noteworthy 
and not understood, and an additional resonance near 10 MeV seems to be indicated. 
Equally interesting are the cross sections for the transitions to the (112)- ground and 
(312)- 3.51 MeV states in 13iV resulting from the 1.1% isotopic abundance of 13C in 
natural carbon. The ratio of these cross sections has been used by Bergquist et al.' to 
determine the ratio of the squares of the spinflip (Gamow-Teller) to non-spinflip (Fermi) 
matrix elements based on the fact that the transition to the ground state is primarily a 
non-spinflip, whereas the transition to the excited state is a spinflip, transition. Compared 
to the ( 3 ~ e , t )  results at higher energies, the ground state transition is much more favored 
at our bombarding energy leading to a greatly reduced ratio of spinflip to non-spinflip 
matrix elements on the order of unity. The enhancement of non-spinflip charge-exchange 
transitions is apparent in all observed spectra. 
The IAS at 10.27 MeV dominates the spectrum observed for the 120sn target, 
Fig. 2. Nevertheless, the spectrum also seems to show some Gamow-Teller strength near 
E,=12 Mev7. Similarly, the spectrum for the ' O Z ~  target (not shown) displays the IAS 
at 5.03 MeV but also Gamow-Teller strength to a sharp state at 2.1 MeV and a weak and 
broad resonance centered near 8.7 MeV. The 120 ~n spectrum received special attention 
because future attempts to observe components of the isovector monopole resonance will 
initially concentrate on this target. Figure 4 shows the angular distribution from 0° to 16' 
measured for the transition to the IAS. The analysis of the data point at 0" is incomplete. 
A slit aperture was employed which will make it possible to obtain by ray tracing a slightly 
Figure 4. Angular distribution for 
120~n(3He,t) 120Sb(1~S) at E(3~e)=200 MeV. 
The solid line is from a DWBA calculation 
using microscopic wave functions and an ef- 
fective interaction. 
averaged angular distribution over the entire 3" to 4" interval. The angular distribution 
displays a strongly diffractive pattern with a sharp maximum at zero degrees and the 
first minimum at 3.5". The observed distribution was found to be in excellent agreement 
with D WBA calculations using an effective interaction (Yukawa shape) and microscopic 
BCS wave functions. The absolute values for the cross sections are overestimated at our 
higher bombarding energy suggesting an energy-dependence in the strength of the effective 
interaction. 
The expected sharp maximum at 0" will be used in future attempts to observe and 
identify a broad L=O resonance near 40 MeV excitation energy. The cross sections can now 
reliably be estimated, and the need for making the measurements at 0" are quite obvious. 
The continuous background observed up to excitation energies of 28 MeV (see Fig. 2) is 
expected to increase slowly towards higher excitation energies. This background is known 
to result primarily from breakup/pickup reactions on the target which are also forward 
peaked but to a lesser extend. Additional techniques may therefore become necessary to 
isolate this giant resonance, for example by measuring neutrons (or protons) in coincidence 
as has been done in the observation of the isoscalar monopole resonance7. 
1. I. Bergquist et al., Nucl. Phys. A469 (1987) 648. 1 
2. G.F. Bertsch and H. Esbensen, The (p,n) reaction and the nucleon-nucleon force, 
Argonne National Laboratory Report PHY-4803-TH-86 (May 1986). l 
3. N. Auerbach, J. Hufner, A.K. Kerman and C.M. Shakin, Rev. Mod. Phys. 44 (1972) 
48. I 
4. J. Janecke, M.N. Harakeh and S.Y. van der Werf, Nucl. Phys. A463 (1987) 571. 
5. J. Janecke, S.Y. van der Werf, M.N. Harakeh et al., KVI (1988) to be published. 
6. W.A. Sterrenburg et al., Nucl. Phys. 405 (1983) 109. 
7. W.A. Sterrenburg et al., Phys. Rev. Lett. 45 (1980) 1839. 
8. S. Brandenburg et al., Nucl. Phys. A466 (1987) 29. 


A Study of the (3-He,t) Charge-Exchange Reaction at E(3-He) = 200 MeV

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A Study of the (3-He,t) Charge-Exchange Reaction at E(3-He) = 200 MeV

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