Search for the excitation of the giant dipole state by direct inelastic 3He scattering

Nuclear Physics A202 (1973) 557 -560; @ North-boiling

Pubiishin~ Co., Amsterdam

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SEARCH

FOR THE EXC~A~O~

OF THE GIANT DIPOLE BY DIRECT INELASTIC

STATE

3He SCATTERING

R. J. PETERSON Department of Physics and Astrophysics, University of Colorado, Boulder, Colorado, 80302, USA * Received 16 October 1972 Abstract: The direct inelastic scattering of 41 MeV 3He ions was studied for nuclear the vicinity of the giant dipole resonance (around 20 MeV). The targets 24Mg, Z6Mg, S°Cr, 60Ni and 9oZr. An upper limit of only several pb/sr was differential cross sections - lower than the yield observed for 182 MeV protons 103. By the use of DWBA calculation, this striking decrease was ascribed to the absorption of the 3He projectiles. E

excitations in studied were placed on the by a factor of strong nuclear

NUCLEAR REACTIONS z4Mg, 26Mg, s°Cr, 60Ni, 90Zr(3He, 3He’), E = 41 MeV; measured o(E(3He’). 0); deduced upper limit on u(8) to giant dipole resonance.

1. Introduction

The giant dipole resonance in nuclei is best known from interactions with an electromagnetic field. Common sources of information on the location, strength and decay of this degree of freedom are (p, y) reactions ‘), {y, n) and (y, p) reactions 2), and inelastic electron scattering “). This giant state accounts for essentially all the cross section allowed by sum rules, and is characterized by being excited by the change of one unit of isospin from the target ground state. The electromagnetic field senses the isospin by the proton distribution. A drawback of photon-induced reactions is that it is dithcult to impart enough moments transfer to populate states with spins higher than 1-. A heavy particle with isospin greater than zero is also permitted to populate the AT = 1 giant resonance. Some years ago proton spectra “) at 182 MeV showed a broad peak near the appropriate excitation energy in 40Ca. It is less clear today that this represents the giant dipole resonance in photon reactions, but it was felt that a search for the giant resonance would be profitable with a 3He beam. The heavy 3He tends even more than a proton to populate states of high angular momentum, but is identical to a proton in its isospin. The data shown by Tyren have a magnitude of several mb/sr [ref. “)I. + Work supported in part by the US Atomic Energy Commission. 5.57

R. J. PETERSON

558

2. The data and analysis A beam of 41 MeV 3He from the University of Colorado AVF cyclotron was used to bombard targets of 24Mg, 26Mg, “Cr, 60Ni and “Zr. For the two heaviest targets [refs. 1, ‘)I t h e g’lan t resonance is known to be a broad pair of states, separated by the two possible isospins, T = 2 and T = 3 for 60Ni [ref. ‘)I. The state in 50Cr is expected to be similar. For the magnesium isotopes, the giant resonance is found as a bundle of sharper, discrete states, with known excitation energies and strengths “). The cross section for populating the AT = 1 state in a target with isospin T scales as l/(T+ l),

5000

60

3 NI ( He, ‘He’ 1 12.5 deg.

4000

330

kcV

bins

3000

2000

T=3 19. 6

1000 Channel

T=2 16.6

McV

80

Fig. 1. A spectrum of 3He ions scattered from a 60Ni target covering the region of the known giant dipole state. The known structure from the 59Co(p, y) reaction [ref. I)] is sketched.

so both heavy and light isotopes were studied to cover a range of target isospins. Thicker foils were used for the heavier targets where the energy resolution could be sacrificed to the width of the giant state. Particle identification using cooled silicon detectors was used. The 4He intensity at the relevant reaction product energy was found to be much larger than the 3He intensity. Discrete states of 12C and I60 appear in the spectra because of surface contamination on the foils. These were identified by their kinematic shift with angle and by combarding a mylar foil at each angle. Fig. 1 shows a spectrum obtained from a 60Ni target and fig. 2 shows a spectrum from 26Mg. The known structure of the giant state is compared to each. It is evident that the known giant state is not being populated, and moreover, no other structure is noted. A state is found at 17.8 MeV

GlANT

DIPOLE

SEARCH

559

f

t --Mg 35cxL

I

-tie,-tie1

I5

deg.

132 keV bins

I

22 Energy

m

Exc?t:tion

16

( Me% 1

14

Fig. 2. A spectrum of 3He ions scattered from a 26Mg target over the region of the giant resonance. The cross-hatched peaks are identified as r2C and I60 contaminations. and the known structure of the giant state is sketched [ref. J)]. ’ 100.

,T J

JJ

%Mg(3He,3He’)

T

IO.

\

I.

t

IO

20.

30

40 &(dril)

50

60

Fig. 3. The observed upper limits on the differential cross sections for 3He scattering populating the entire giant state are compared to a DWBA prediction using the methods of ref. ‘). The observed cross sections are three orders of magnitude smaller than observed for 182 MeV protons.

560

R. J. PETERSON

in 24Mg but is not related to the known “) giant state. Only upper limits could be placed on the cross sections for populating the giant state in the five nuclei studied. These limits are all near several pb/sr, three orders of magnitude less than observed for I82 MeV protons, over a similar range of momentum transfers “). 3. DWBA calculations Bang and Kunz “) have been able to reproduce the magnitude and shape of the data of Tyren “). The calculation is performed in the DWBA, with a form factor taken from a hydrodynamical model, and normalization of the predictions from the appropriate collective sum rules. The code DWUCK ‘) was used to compute the differential cross sections for scattering 3He to the giant state (taken as a l- state), and were successful in accounting for the three orders of magnitude reduction in cross section compared to the proton scattering results. The strength of the z * T term in the optical potential was taken to be twice the strength used by Bang and Kunz for the proton potential, and only the real form factor with radius r0 = 1.25 fm, without Coulomb excitation, was used. The predictions and data for 26Mg are shown in fig. 3. The observed and predicted cross sections for the other targets are similar. The loss of 20 MeV to the nucleus is a relatively more important mismatch for 41 MeV 3He ions than for 182 MeV protons. If the giant state had zero excitation energy, the 3He yield would be a factor of 20-30 larger. The remaining discrepancy is related to the strong absorptioi~ of the 3He by the nuclear target. For 5oCr, the form factor of Bang and Kunz peaks at 3.2 fm, while a form factor for a traditional vibrational excitation peaks at 4.3 fm. The strong absorption radius “) of 50Cr for 3He is about 8 fm [ref. ‘)I, and little 3He flux is available to sense the n~aximum of either form factor. The 182 MeV proton flux through the target nucleus is much larger. Direct inelastic 3He scattering would have been an admirable way to study giant resonance structures at high excitations, but the minute cross sections due to the strong nuclear absorption will make such experiments very difficult. Professor P. D. Kunz provided the modified form of DWUCK and expert and willing advice on its use. References 1) E. M. Diener et al., Phys. Rev. C3 (1971) 2303 2) F. W. K. Firk, Ann. Rev. Nucl. Sci. 20 (1970) 39 3) A. Goldmann, Laborbericht Nr 41, Inst. fiir Technische Kernphysik, Technische Hochschule Darmstadt, 1969, unpublished 4) H. Tyren and Th. A. J. Maris, Nucl. Phys. 4 (1958) 622 5) B. L. Berman et al., Phys. Rev. 162 (1967) 1098 6) J. Bang and P. D. Kunz, Phys. Lett. 37B (1971) 128 7) P. D. Kunz, DWUCK, a distorted-wave Born approximation computer program, Univ. of Colorado, unpublished 8) B. Fernandez and J. S. Blair, Phys. Rev. Cl (1970) 523 9) R. J. Peterson, E. W. Stoub and R. A. Ristinen, Phys. Rev. C6 (1972) 829