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ergamon

Prog. Part. Nucl. Phys. Vol. 34, pp. 295-296, 1995

Copyright 0 1995 Elsevier Science Ltd

Printed in Great Britain. AIJ rights reserved

0

146-64 10 95)00024-O

0146+410/95 29.00

Giant Resonance Spectroscopy of 40 Ca in Electron

Scattering Coincidence Experiments

P. van NEUMANN-COSEL

Institutfiir Kernphysik. Technische Hochschule Darmstadt Schlossgarlenstr. 9.

D-64289 Durmstadt Germany

With the development of continuous wave accelerators electron induced coincidence experiments

have become a powerful tool for giant resonance spectroscopy (11. The present work reports on some

results of a recent study of the

Ca(e,ex) reaction [2]. The experiments were performed at the

MAMI A microton in Maina and at the superconducting accelerator S-DALINAC in Darmstadt.

Data were taken at four momentum transfers in the range Q = 0.25 - 0.66 fm-. The main decay

channels (protons and alpha particles) were detected with a set of up to 10

AE E

telescopes covering

an angular range of 0, = 0 - 200 relative to the recoiling excited nucleus. For details see [2].

In the Ca resonance region of about 10 - 25 MeV excitation energy one is contronted with

strongly mingled isoscalar quadrupole (GQR)

and isovector dipole (GDR) strength. Also some

isoscalar monopole (GMR) strength is found at lower energies. A multipole decomposition of GDR

and GQR contributions in the 4x-integrated cross sections is presented and the resulting E2 strength

distribution is compared to recent microscopic calculations. The multipole separation is based an

the variation of the momentum transfer under the assumption that the form factor is excitation

energy independent and no additional multipoles (X 2 3) contribute [3]. Since the method of [3]

does not lead to unique results an additional constraint is imposed by minimizing the deviation of

the resulting energy-integrated GDR form factor from a MSI-RPA calculation.

The resulting strength distributions are displayed as histograms in fig. 1. The El strength is

compared to the difference of a total photoabsorption [4] and (r,n) data [5] (open circles). One

can state

a

very good agreement which ensures the reliabilty of the decomposition method. The E2

strength is strongly fragmented with maxima around 12, 14 and 17 MeV. Similar to the observations

of a cCa(a,ax) experiment [6] a significant part resides below 15 MeV in contrast to most RPA

predictions. However, the EWSR exhaustion up to 16 MeV is only 33(S) as compared to 61(7)

deduced from the hadron data. Note that the (e,ex) value includes monopole contributions which

cannot be separated from the quadrupole strength within this approach because of the almost equal

momentum transfer dependence of the form factors. However, it is clear from high resolution (p,px)

and inclusive (e,e) data [7] that the major part of the strength below about 17 MeV is of E2 character.

The solid line in the lower part of Fig. 1 represents a recent microscopic calculation including

lplp@phonon configurations and coupling to the continuum [8]. For a description of the approach

see (91. The quality of reproduction of the experimental results is remarkable, in particular for the

region below 15 MeV which could not be described in previous RPA calculations. A detailed analysis

reveals that the low-lying E2 strength can be traced back to 2p2h ground state correlations which

are treated beyond the usual RPA level in this model [8].

Another interesting problem addressed here is the interpretation of (e,ep) angular correlation

functions (ACF). While the multipole decompostion of a decay ACF has been successfully performed,

no model independent analysis is possible for proton decay, if more than two multipoles contribute.

It has been demonstrated in [2] that the shape of ACF of the pc and pi decay channels strongly

variing with excitation energy and momentum transfer can be consistently described by a HF-RPA

continuum approach [lo]. This finding came as a surprise since a far too simple lplh model is used

to describe the excitation process (cf. the discussion of E2 strength above). One must conclude that

the ACF shapes of ~0.1 decay are largely independent of the underlying nuclear structure.

This is further demonstrated in Fig. 2 where p. and no (taken from [ll]) decay are compared

for the same excitation energy bin. Despite the very similar dominant &/,-hole structure of the

final states the angular correlations look completely different. The dashed lines are the model [lo]

Supported by the German Federal Minister for Research and Technology (BMFT) under contract number

06 DA 665 I.

295

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296 P. von Neumann-Cosel

I

1 12 14 16 18 2

Excitation Energy MeV)

Fig. 1. El and EZ ,EO)strength distributions

resulting from a

Ca e,ex) multipole decom-

position.

, < I . & I *

90 160

270

360

Qv.~ degrees)

Fig. 2. Angular correlations of the Ca e,epa)

and Ca e,eno) reactions for excitation ener-

gies 19 - 20 MeV and comparable momentum

transfer.

results taking only the Hartree-Fock part, thus simulating a direct knock-out reaction. Obviously

the data are not described well. Only if the RPA-type multistep contributions are fully taken into

account solid lines) a simultaneous description of decay into both channels is achieved. The relative

HF contributions are small and the cross sections were scaled with factors 3 po) and 400 no) with

respect to the full calculations. Thus, the ACF shapes are largely defined by the RPA correlations

included in the model [lo]. A detailed analysis shows that the theoretical cross sections are dominated

by charge-exchange rescattering terms in the final state interaction. Because of the predominance

of these multistep contributions the oversimplified model of the excitation step does not affect the

ACF shapes, but rather reflects itself in a large variation of the normalization constants needed to

describe the absolute cross sections.

Further results of the present investigations including, e.g.,

a discussion of the relative role of

semi-direct and statistical decay contributions, the analysis of a0 ACF and a detailed comparison of

electron and hadron induced coincidence experiments can be found in [2, 7, 121.

References

[l] K.T. KnGpfle and G.J. Wagner, in

Electri c and Magnetic Giant Resonances i n Nuclei

ed.

J. Speth World Scientific, Singapore, 1991) 234.

[2] H. Dieaener et al., Phys. Rev. Lett. 72 1994) 1994.

[3] Th. Kihm et al., Phys. Rev. Lett. 56 1986) 2789.

[4] J. Ahrens et al., Nucl. Phys. A251 1975) 479.

[5] A. Veysierre et al., Nucl. Phys. A227 1974) 513.

[S] F. Zwarts et al., Phys. Lett. B125 1983) 123.

[7] P. von Neumann-Coeel et al., Nucl. Phys. A589 1994) 373~.

[B] S. Kamerdzhiev et al., Phys. Rev. Lett. submitted).

[9] S. Kamerdzhiev et sl., Nucl. Phys. A589 1994) 313~.

[lo] J. Ryckebusch et al., Nucl. Phys. A503 1989) 604.

[ll] C. Takakuwa et al., Phya. Rev. C50 1994) 845.

1121P. von Neumann-Cosel, Proceedings of the IV. International Conference on Selected Topics

in Nuclear Structure, Dubna, July 5-9, 1994 in press).