Radiative proton capture and exchange currents in light nuclei

Nuclear Physics A$$3 (1993) 543c-552c North-Holland, Amsterdam

NUCLEAR PHYSICS A

RADIATIVE PROTON CAPTURE AND EXCHANGE CURRENTS IN LIGHT NUCLEI B. H6istad, S. Isaksson, E. Nilsson and J. Thun Department of Radiation Sciences, Uppsala University, S-75121 Uppsala, Sweden G. S. Adams and C. Landberg Physics Department, Rensselaer Polytechnic Institute, Troy, New York 12180 T. B. Bright and S. R. Cotanch Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-8202

Abstract A new type of high resolution pair spectrometer for m e d i u m energy photons has been used for studies of the exclusive (P,7) and (p,e"e-) reactions on light nuclei in the energy region 100 to 200 MeV. From the current experiments we present angular distributions of the differential cross section for transitions to the ground state and first three excited states in 12C. The distributions from the ground state and first excited state are compared with predictions from a microscopic continuum shell model calculation which uses a realistic finite-range effective interaction with tensor components. Based on preliminary data, the salient features of the (P, 7) reaction on 12C and the (p,e÷e-) reaction on l i b are also discussed. 1. INTRODUCTION The identification of non-nucleonic degrees of freedom in the nucleus continues to be a topic of great urgency in nuclear physics. The presence of nonnucleonic particles in the nucleus is generally incorporated in the Hamiltonian for the many-body system. Mesons and resonances are thus present in the nucleus to the extent that they are explicitly used to generate the nuclear interaction. The occurrence of mesons in the nucleus is then manifested in the meson exchange part of the total nuclear electromagnetic current. These exchange currents 1, generated by the strong interaction, are generally detected by the electromagnetic interaction in photo-nuclear or electro-nuclear reactions. By choosing a suitable region with respect to energy and m o m e n t u m transfer for these reactions, it s h o u l d be possible to isolate the effects due to exchange currents in the experimental data. The radiative proton capture reaction at intermediate energies, which has been a subject of great interest 2-7, is expected to be partic~jlarly sensitive to these two-body currents, since this reaction always involves a large momentum transfer to the residual nucleus. This large m o m e n t u m is more likely to be absorbed on a correlated nucleon pair allowing momentum sharing, rather than on a single nucleon. The photon can then couple not only to the individual nucleon, but also to the mesons being exchanged. At intermediate energies delta 0375-9474/931506.00 © 1993 - Elsevier Science PublishersB.V. All rights reserved.

544c

B, H6istml et al. t Radiative prototz capture and exchange currents

isobar effects should also come into play in the emission process. Even at threshold energies exchange current effects give a I0% contribution to the elementary radiative n-p capture 4,5,8. At higher energies this reaction is dominated by exchange currents 9. The present research program aims for a proper understanding of medium energy photonuclear reactions and a quantitative description of the exchange currents in light nuclei. Experimentally only limited data is available at intermediate energies due to the difficulties associated with the use of bremsstrahlung beams for the (y,p) reaction and energy resolution for the (p,¥) reaction. In particular, there is a lack of data resolving the excited states in the residual nucleus. At energies below 80 MeV, however, high resolution (¥,p) data is becoming more abundant due to new tagged photon facilitiesT, 10. The goal of our research program is to provide and analyze high quality (p,¥) data at energies from 100 to 200 MeV where exchange currents are expected to be important. In particular, we want data from transitions to different individual states in the residual nucleus. Such data can demons~r.~te the detailed nuclear structure dependence in this reaction and thus facilitate the understanding of the interplay between nuclear structure and reaction mechanism. 2. EXPERIMENTAL APPARATUS In order to resolve transitions to different excited states, the photon detection must be made with an energy resolution compatible with the level spacing in the residual nucleus, i.e. much better than 1 MeV. Since such an energy resolution is not attainable with scintillation detectors, it has been necessary to develop a high resolution pair spectrometer for intermediate energy photons. A comprehensive d e s c r i p t i o n of the pair spectrometer, w h i c h is named PACMAN, and its performance is given in our instrumental report 11 A schematic picture of PACMAN and its detector system is shown in fig. I. i The photons are converted in a 0.1 mm thick 12 x 20.6 cm 2 gold converter, which is placed in the fringe field 58.5 cm from the target. The magnetic field is generated by a clamshell type pole configuration which yields focusing properties such that the whole available field volume is used efficiently. Fig. 1. A schematic picture of the pair spectrometer. The electron and positron are bent I80 degrees where

545c

B. HOistad et al. I Radiative proton capture and exchange currents

they reach the detectors which give the event trigger and the direction information. The detectors are placed in two identical groups which are positioned symmetrically above and below the scattering center. Each detector group consists of three driftchamber packages (x and z) and three planes of trigger scintillators, where each scintillator plane consists of four separate detector blocks. The trajectories from the e*e- pair leaving the magnet are determined from the three (x,y,z) coordinates given by the driftchambers. Knowledge of the magnetic field allows these trajectories to be traced back to a common source point on the converter. The energy and direction of the incoming photon are thereby determined. The measurements of internal pair production, (p,e*e-), which involves an internal conversion of a virtual photon inside the target nucleus can also be studied with PACMAN. The only necessary modification in the experimental set up for the (p,e+e -) measurement is to remove the converter. The momentum determination of the e*e- pair is made by raytracing the measured ou~,going trajectories from PACMAN back to the target point. The separation between (P,T) and (p,e*e-) events, i.e. determining weather the e+e- pair originates from the converter or the target, can easily be done owing to the placement of the converter in the fringe field. 3. EXPERIMENTAL RESULTS FROM THE IIB(p,~/)I2C REACTION The experiments were performed at the The Svedberg Laboratory using the proton beam from the Gustaf Werner cyclotron. A subset of the (p+ F) data on 11B is presented in ref. 12. The photon energy spectrum from the ~B (p, T)12C reaction, obtained at 98 MeV is shown in fig. 2, plotted as a function of excitation energy of the residual nucleus.

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546c

B. HOistad et al, I Radiative proton captnre and exchange currents

This spectrum shows strong excitation of several levels in 12C, notably the ground state, ~he 4.4 and 9.6 MeV levels, as well as clusters of states at 12.7-15.1 MeV, 16.116.6 MeV and around 19 MeV. The spectrum is very similar to those previously measured 3 in the same reaction but at much lower energies. However, one clear difference is the smaller relative contribution from the peak at 19 MeV. This is in agreement with the resonance nature of the low energy reaction. Differential cross sections for transitions to individual levels as well as groups of levels in 12C have been extracted at lab. angles between 37.5 and 140 degrees. The peaks corresponding to the 0 + ground state and 2 + first excited state are well separated and no background exists under the ground state peak. The angular distributions for the transitions to the first two levels in 12(= are shown in fig. 3. This figure also includes one datum point from earlier (p,¥) measurements 3 of the ground stat¢ transition obtained at 100 MeV. The peaks corresponding to the transitions to the 0 + state at 7.6 MeV and the 3" state at 9.6 MeV are also dearly separated although the background from radiative tails due to states at lower excitation energies is significant. The angular distributions of the differential cross section for these transitions are presented in fig. 4. A comparison of the ~! fferent experimental angular distributions of the differential cross section reveals that they are forward peaked and of similar shape. The angular distribution of the ground state transition stands out as being significantly different from the others because it falls more rapidly with increasing scattering angle. 103 t •

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547c

B. HiJistad et aL / Radiative proton capture and exchange currents

4. MODEL CALCULATIONS The present data are compared with predictions based on a microscopic continu u m shell model description of the nuclear system. The many-body Hamiltonian for 12C is diagonalized rigorously in a model space spanned by products of continuum single-particle states and the mass A=11 bound states. Two different manybody wavefunctions are used for the A = l l and A=12 bound states to document the sensitivity to sophistication in the shell model configuration, namely the extreme 1-particle 1-hole and the multi-particle multi-hole models. With these choices our overall continuum shell model approach is identical to that first used by Buck and Hill 13 for the 1-particle 1-hole case, and later by Ramavataran et al.14 for the multiparticle multi-hole case. However, they employ zero-ranged two-body forces and omit all non-local terms. In the present calculation, our Hamiltonian contains a realistic two-body, finite range interaction with spin, isospin and tensor components. The effective strength and range were determined by describing the giant dipol E1 resonance for the ~IB (p, 7) ~ZC reaction. Moreover, all non-localities due to anti-symmetrization are rigorously retained. The resulting large set of coupled integral-differential equations is solved numerically for the continuum scattering wavefunction. The basic approximations in the present calculation consist of omitting the exchange current effects and restricting the model space to one nucleon in the continuum, it should be remarked that our coupled channel treatment can be regarded as an extensive multi-step reaction process, and as such, transcends other schemes like the semi-direct or the quasi-deuteron methods. For more specific details see ref. 6. 4.1 Comparison with data on UB

In fig. 3 we compare the experimental angular distributions for the transitions to the ground state and first excited state with predictions based upon the above model. The model space for the mass A=11 system is spanned by the low lying negative parity states of 11B and 11 C h a v i n g s p i n / p a r i t y assignments of (~)~. (~)~. (~-)i for the reaction to the ground state and (~)~.(~)~.(~)~,(~)~.(~), for the reaction to the first excited state of 12C. This model space (ground state solid line, 4.4 MeV dot-dashed line) involves multi-particle multi-hole configurations, since the A=11 and A=12 bo, md wavefunctions are obtained from a separate shell model calculation that includes multi-nucleon excitations from the filled fermi level. The near agreement at forward angles is expected since these angles correspond to low m o m e n t u m transfer which is sensitive to initial state interactions that are reasonably well described by a continuum shell model treatment. For comparison we include a prediction based on the extreme 1-particle 1-hole limit (ground state dashed line, 4.4 MeV dotted line), which treats the A=11 bound states as pure holes relative to the 12C ground state. Especially for the excited 2 + state transition, the multi-particle multi-hole calculation vrovides a better description of the data than does the 1-particle 1-hole result, which is consistent with 12C not being a closed shell nucleus. It would appear, however that further extending the model space by including additional multi-particle multi-hole configurations with more excited states in the A=11 system would not provide complete quantitative agreement 3

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B. H6istad et ~zl. I Radiative proton capture and exchange currents

548c

with the data. Rather, the disagreement with data, especially at back angles involving large momentum transfer, is most likely due to an incomplete treatment of the short range physics, most importantly exchange current effects. In fact, preliminary calculations 15 for the reaction 160(¥,n)150 at 150-250 MeV, which is computationally more tractable, show significant delta isobar exchange current contributions. Incorporating these delta isobar effects into our microscopic model provides a good theoretical description of those data 16. Efforts are currently underway to include exchange currents also in the model prediction for 12C. 5. PRELIMINARY RESULT FROM THE 12C(p,¥)13N REACTION

Preliminary data from (p, 7) on 12C have been obtained at 98 MeV for several angles between 40 and 120 degrees. The spectra recorded at 40 and 120 degrees are shown in fig. 5. I

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549c

B. HOistad et al. I Radiative proton capture and exchange currents

he nuclear structure dependence in the (p. T) reaction appears clearly in the 12C ata. From those data one can therefore use the nuclear structure as a filter to nderstand the reaction process in a more direct way than from the liB data. Ill the 0° spectrum, transitions to the single particle states Ip~/2 (ground state), 2sl/2 (2.36 leV) and lds/2 (3.55 MeV) seem to be preferred compared to the 2particle-lhole tares at larger excitation energies. The spectrum at 120°, however, does not indicate ny preference for transitions to the single particle states. This pattern can in fact be nderstood qualitatively by noting that the momentum transfer involved is much lrger at 120 ° (460 MeV/c) than at 40° (340 MeV/c), and assuming that momentum hating is increasingly important with larger momentum transfer.

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Fig. 6. Reaction diagrams corresponding to the a) one-nucleon and b) two nucleon processes showing examples of sharing of the momentum transfer involved. In a simple one-body process, shown in fig. 6a the photon is emitted directly tom the incident proton, and the momentum transferred to the residual nudeus las to be absorbed by the captured proton alone. In this process the captured proton s simply added to the intact target nucleus, and accordingly only the single particle ;tates can be populated. In a two-body process, shown in fig. 6b, the incident proton nteracts with a target nucleon which means that momentum sharing can take )lace. In this process the emitted photon couples either to the one-body current 'tom a correlated nucleon pair or to the exchange current between the incident ~roton and a target nucleon. In this case both the captured proton and the participating target nucleon can occupy an empty state, and consequently both lp-0h and ~.p-lh states can be populated. The selectivity in our data indicates that at momentum transfer smaller than lbout 350 MeV/c, the contribution from two body processes is small, while at larger nomentum transfer nuclear correlations a n d / o r exchange currents play an impor:ant role. This is also in agreement with our theoretical result from (p,F) on I1B, ~hich shows good agreement with the data owing to the implicit inclusion of nu:lear correlations in the model calculation. i. PRELIMINARY RESULT FROM THE 11B(p,e+e')12C REACTION The rare (p,e÷e -) reaction, which involves an internal conversion of a virtual ~amma inside the nucleus, has large similarities with the (p, T) reaction, but it ~lso exhibits some distinct differences. In the simple one-body process the virtual photon is emitted directly from the incident proton. The fact that the emitted photon is virtual, implies that th~ longitudinal part JL of the nuclear current appears in the transition matrix, as opposed to the case with real photon emission, which

only contains the transverse part Jr. Assuming contribution from the electric part only in the photon-nucleon interaction one gets the amplitude M as 17

where 8” and I$*denote the e+e-angles in the virtual photon rest system, and Q is the momentum of the photon with energy Q, and mass My. From this expression we see that, the sensitivity to the longitudinal part of the nuclear current becomes smaller with smaller photon mass My because of the factors M: /Qz and My /2Q,. The massive virtual photon decays in an electron and a positron, which are emitted with an opening angle related to the magnitude of the mass and the energy of the photon. For a photon of total energy 100 MeV, the minimum opening angle (degrees) in the lab. is approximately numerically the same as the photon mass (Met’). The large opening angle of TACMAN allows the virtual gamma emission to be studied for an invariant mass of the e’e- pair up to about 10 MeV. Fortunately, the cross section for the @.e+e-) reaction is peaked strongly towards low values of the photon mass. For example, approximately 50% of the cross section is expected to be found below a mass of 10 MeV for a photon with 150 MeV total energy r7. Since the phase space covered by PACMAN only includes small photon masses one can not expect a large sensitivity to the iongitudinal part in our data.

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Fig. 7. Energy spectrum of the efe- pair from the (p,e+c-1 reaction at 45 o plotted as a function of the excitation energy of the residual nucleus 1%.

We have measured the ttB!p,e+e-)r7C reaction at 98 MeV at 45 and 80 degrees. A spectrum of the data obtained at 45” is shown in fig.7. The similarity with the (P, Y) data is almost total in this way of presentation (compare fig. 2). In order

B. H6istad et al. I Radiative proton capture and exchange currents

551 c

to find a n y differences, if there are any, it is thus necessary to select data far a w a y from t h e (p, 7) kinematics region, i.e. at large m a s s e s of the p h o t o n or at selected a z i m u t h a l a n g l e s for the e+e - pair. Such analysis is in progress. The cross sections for the transitions to the g r o u n d state a n d first excited state respectively have been d e d u c e d f r o m these data a n d f o u n d to be about 1.1 n b / s r a n d 1.5 n b / s r at 45*, a n d 0.18 n b / s r a n d 0.61 n b / s r at 80 °. A n o t h e r e x p e r i m e n t a l result from the (p,e+e -) m e a s u r e m e n t is the i n v a r i a n t m a s s distribution of the virtual photon. This distribution can easily be calculated since the m o m e n t a of both the e + a n d e- are m e a s u r e d as well as the opening angle. The m a s s resolution obtained b y P A C M A N is about 1 MeV. U s i n g the (p,e+e -) data at 45 ° for the transitions to the g r o u n d state a n d first excited state we get the result s h o w n in fig. 8. The s h a p e of this distribution is very m u c h affected b y the acceptance of P A C M A N for different invariant m a s s e s of the e+e- pair. The solid line in the figure corresponds to the expected m a s s distribution given by the M o n t e Carlo calculation a s s u m i n g an event distribution given by t h e expression above, b u t w i t h n o contribution from the longitudinal part a n d setting JT' to a constant. The data p o i n t s are the experimental values for the photon m a s s distribution. bOO

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Fig. 8. Invariant mass distribution for e+e" pairs corresponding to transitions to the sum of the ground state and the 4.4 MeV excited state. The solid line is from a Monte Carlo calculation and the points are from real data. It is interesting to compare the cross sections for the (p,e+e - ) a n d (p,y) reactions. O n e w o u l d expect that the cross section ratio, denoted by R below, between these reactions is r o u g h l y given by the coupling constant ct=t/137 because of the second electromagnetic vertex occurring in the (p,e+e - ) reaction. W e can calculate R from the e x p r e s s i o n above u s i n g that M~, m u s t be small a n d neglecting terms including JL • O n e t h e n gets the expression 2a

Q

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552c

B. If¢~istad et eft, / R~zdialive prottm capture attd exchange currents

which yields after integration the value 0.88 a for the present kinematical conditions of our data. Note that, with the assumptions above, R has no dependence on the photon angle or any sensitivity to the nuclear structure involved. However, the experimental data yield a ratio which shows both angular and nuclear structure dependencies. For the ground state transition we get the ratios (times o0 1.41+0.11 at 45 °, and 1.33+0.21 at 80 °. For the transition to the first excited state we get the corresponding ratios 1.77+0.12 and 1.52+0.14. The experimental values thus indicate that the (p,e+e - ) cross section is about twice as large as one would expect from the known ( p , y ) cross section. It is therefore possible that the reaction mechanisms yielding e+e- pairs are different, at least partly, from those for real photons. We also note that there is an indication that R is getting smaller with increasing angle. 7. REFERENCES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

A recent review article or. exchange currents is given by D.O. Riska in Phys. Report, 181 (1989) 207. See also M. Marangoni and A.M. Saruis, Phys. Lett. B 262 (1991) 193. M.A. Pickar et al., Phys. Rev. C 35 (1987) 37; ]. D. Kalen et al., Phys. Rev. C 39 (1989) 340; G. M. Lotz and H. S. Sherif, Phys. Lett. B 210 (1988~ 45. H. ]. Hausman et al., Phys. Rev. C 37 (1988) 503. J.F. Mathiot, J. Phys. G: Nucl. Phys. 14 Suppl. (1988) $357. K. Abrahams et al., ]. Phys. G: Nucl. Phys. 14 Suppl. (1988) $373. L.D. Ludeking and S. R. Cotanch, Phys. Rev. C 29 (1984) 1546, and AIP Conf. Proc. 150 (1986) 542. S.V. Springham et al., Nucl. Phys. A 517 (1990) 93, and A. C. Shotter et al., Phys. Rev. C 37 (1988) 1354. D.O. Riska and G. E. Brown, Phys. Lett. B 38 (1972) 193; M. Gari and A. Huffman, Phys. Rev. C 7 (1973) 994. J.M. Laget, Can. J. Phys. 62 (1984) 1046, and Nucl.Phys. A 312 (1978) 265. J.O. Adler et al., Nucl. Instr. Meth. in Phys. Research A 294 (1990) 15. B. H6istad et al., Nucl. Instr. Meth. in Phys. Research A 295 (1990) 172. B. H/~istad et al., Phys Lett B276 (1992) 294. B. Buck and A.D. Hill, Nucl. Phys. A 95 (1967) 271. S. Ramavataran et al., Prog. Theor. Phys. 65 (1981) 1928. T.B. Bright and S. R. Cotanch, to be published. E.J. Beise et al., Phys. Rev. Lett. 62 (1989) 2593. U. Tengblad, TSL/ISV-89/0028, ISSN 0284-2769, internal report.