Present status of polarized targets for high intensity photon and electron beams

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PRESENT STATUS OF POLARIZED TARGETS FOR HIGH INTENSITYPHOTON AND ELECTRON BEAMS. Werner MEYER PhysikalischesInstitutder UniversitatBonn, Federal Republic of Germany The discovery of mania as polarized target material and its very good polarizationresistance to radiation damage makes improved ex~riments with photon and electron beams possible. Dilution refrigeratorsare now standard equipment in most laboratorieswhere polarized targets are in operation. The frozen spin technique can be employed in tagged photon beams. A first measurement of the electromagneticformfactorsof the deuteron by means of electron scattering from a tensor polarized ND3 target was performed in Bonn. 1.

INTRODUCTION In the last few years the study of polarizationphenomena in photon induced

reactions has been concentratedon the deuteron. The motivation for such measurements has been the possible existence of exotic states. C~pared to the n~ber of observables in the vd + pn reaction (23 observables)the number of experiments is still deplorably small, too small to allow reliable analyses. Looking at polarized target experimentsonly measurementswith a vector polarized deuteron target have been performed 'g*. These measurementsbecame possible after the developmentof 3He-refrigeratorsin the early seventies. In the meantime dilution refrigeratorsare used for photoinducedreactions3,as in dilution refrigeratorsthe highest polarization values can be obtained (protons: Pol. > 85%; deuterons: Pol. * 35%). In addition, the new target material ammonia could considerably improve the experimentalsituation. The present status of polarized deuteron targets allows us to perform experimentswith tensor polarized deuteron targets. The polarized deuteron is not only a good polarized neutron target, but also the study of its own properties - such as form factors - is of great interest. Polarizationexperiments in electron deuteron scatteringare expected to play a central role

form factors of the deuteron (neutron), One main problem of the polarized target bombarded with electrons - the polarization resistance to radiation damage - could be considerablyreduced by using in studies of the electric

ammonia as polarized target material. Measurementsof the electromagneticformfactors of the deuteron by means of electron scattering from a tensor polarized ND3 target

have been started

in Bonn.4

W. Meyer /Present status of polarized targets

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2.

NEW POLARIZED TARGETMATERIAL-AMMONIA Up to now butanol and propanediol have been the standard materials for po-

larized targets in high or intermediateenergy physics experiments.The method of dynamic nucleon polarization (DNP) requires the presence of paramagneticradicals. In the case of alcohol materials best results were achieved with chemically doped samples. A proton polarization higher than 90% and a deuteron polarization in the range of 25% to 45% are measured. These polarizationvalues were achieved only after a considerable improvement in cryogenics:the development of high power dilution refrigerators5. It turned out that their use is even more important for polarized deuteron target than for proton targets. In the meantime dilution refrigerators have become more and more the standard equipment of a polarized target system. In the late seventies it became clear that further improvementfor polarized target experimentscould only be obtained by new target materials with a higher content of polarizablenucleons. Especiallyfor experimentswith high intensity beams, such as electron-, proton- or photon beams, target materials with high polarizationresistance to radiation damage were urgently needed. In 1979 a breakthrough occured when it was discoveredthat high proton polarization in NH3 could be obtained using paramagnetic radicals, generated by irradiation6.Some time later it was demonstrated that this preparation technique works in ND37, too. A comparison of the maximum proton and deuteron polarization polarization in the target, measured at 2.5 T and-200 mK, for butanol and ammonia is shown in Table 1. Table 1: Comparisonof the proton and deuteron polarizationin butanol and ammonia ("200 mK; 2.5 T) Material butanol ammonia

maximum Protonpol.% 80 92

Material d-butanol d-ammonia

maximum Deuteronpol.% 27 44

However, the biggest advantage of ammonia is its extremely good polarization resistance to radiation damage which is mainly a result of the radiation induced radicals. It turned out that the depolarizingdose in NH3 is by an order of magnitude higher compared with that in butanol (fig. 1). As the experience shows the ND3 target preparation for the DNP by irradiation is more difficult compared to that of NHS. Decisive is the temperature at which the radicals are produced. Highest deuteron polarizationvalues can only be achieved by means of 'low temperature' irradiation(performed in the polarized target cryostat)*.

W.Meyer /Present status of polarized targets

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A comparison of the deuteron polarization behaviour of d-butanol and ND3 is plotted in fig. 2. Latest results on the target material developmentwere presented on the '4th Workshop on Polarized Target Materials and Techniques' taking place in Bonn-Bad Honnefg. Especially, where experiments with high intensity beams are performed ammonia has replaced the alcohol materialslO*ll. Prto/b1

T = 1.0K fig. 1: Polarizationbehaviour of protons in NH3 and butanol in dependenceof the electron irradiationdose at 1 K and 2.5 T 1

2

3

Dose C10'5e'/cm21

I

I

T=O.ZK 8.2.5.T I

Pot%31

fig. 2: Polarizationbehaviour of the deuterons in ND and d-butanol in dependence of 2he photon flux at 0.2 K and 2.5 T.

1

3.

2 3 Dose C10'5photonslcm21

POLARIZED TARGET EXPERIMENTSWITH PHOTONS In recent years attention has been concentratedon the deuteron. In spite of

the fact that the reactionyd + pn is an important reaction in the field of the two-nucleon physics, most of our experimentalknowledgeof the photodisintegration reaction is limited to measurements of the differential cross section. Only few experiments have been performed to investigate single polarization quantitiesas well as double polarizationobservables12.Due to the complicated spin structure of the deuteron photodisintegrationreaction, 12 complex helicity amplitudes are required to characterize completely the yd + pn process. Hence 23 different observables have to be measured as a function of the photon energy and the proton c.m.s. angle.

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W. Meyer /Present status of polarized targets

Recent target asymmetry measurements13 of the deuteron photodisintegration reactionud -+ pn suffer from the fact that the maximum attainable polarization cannot be maintained in the course of the experiment. Using ammonia as target material the radiation damage problems are considerably reduced. However, the beam heating, a serious problem for all experiments with high intensitybeams operating with 3He/4He dilution refrigerators, is responsible for a reduction of the polarizationll. All these problems are not relevant to tagged photon beams as the photon flux is low (106-lo7 photons/set).In this case, however, a large solid angle detection is necessary. The optimum solution for such experimentalconditions offers the frozen spin target which allows arbitrary spin orientation by appropriate setting of the holding magnet. These different spin orientationsare needed for double polarization experiments performed with polarized photons. Using a crystal radiator in the tagging system linearly polarized photons become available. The productionof circularly polarized photons can be done with polarized electrons. With such -a facility [tagged (polarized) photon beam - polarized target (frozen spin type) - 4~ detection system)] seven (or more) independentexperiments on YN + IIN, necessary for a complete determinationof the helicity amplitudes, can be performed. However, it is unlikely that a complete measurement of the deuteron photodisintegrationreactionvd + pn (23 observables)will ever be performed. At higher energies (KY'

700 MeV) it will certainly be possible

to measure differential cross sections. Due to the low cross section the measurement of polarization quantities will be difficult. New techniques like the use of a tensor polarized deuteron target, however, can increase the number of possible experiments in the near future. The installationof such a facility is planned for measurements at the new 3.5 GeV electron stretcher accelerator (ELSA) in Bonn. 4.

POLARIZED TARGET EXPERIMENTSWITH ELECTRONS All previous electron scattering experiments with a polarized target were

limited by the relative low polarizationresistanceto radiation damage of the alcohol materials. Using ammonia as target material the beam heating is the more serious problem. An electron beam with a current of 20 nA deposits about 100 mW in a 3 cm long target, which can easily be handled by 4He-coolingat 1 K (4He-refrigerator).However, it is obvious that 3He-4He dilution refrigerators must be used to achieve very high polarization. This is especially valid if high tensor polarizationof the deuteron is required.

IV.Meyer / Fresent status ofpdwized

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targets

The measurement of the elastic electron deuteron scattering off a tensor polarized deuteron target can be used to separate the charge monopole factor Fc and the charge quadrupole formfactor FQ14. This experimentwas recently started at the 2.5 GeV electron synchrotron in Bonn. A first measurement with a ND3 target shows that an electron (2 GeV) beam current of 0.4 nA could be tolerated without considerable loss of polarization.An average deuteron tensor polarization of 0.17 f 0.02 could be obtained. The cooling power of the used dilution refrigerator is 6 mW at 200 mK and 20 mW at 270 mK3. The target is located in the center of a large su~rconducting split pair coil producing a 3.5 Tesla ~gnetic

field over the target volume of 25x16~16 mm3, The spin quantization

axis is oriented in the ed-scattering plane, ~rpendicular to the

virtual

photon direction. The scattered electrons and deuterons are analysed in two magnetic spectrometers with charged particle identification

and

tracking

capabilities. This first measurement was performed at a four momentum transfer Q2 = 0.5 GeV2. The data are in evaluation and a further run is necessary to improve the statistics. It is planned to extend this measurement at ELSA to higher Q2-values

(Q2

x 2 Gev2), where the sensitivity to the differences between theoretical

models becomes higher. However, at these large Q2-values the cross section is considerably reduced. Therefore a large solid angle detection and a highly tensor polarized deuteron target, which can withstand some nA electron beam current are decisive for the success of the experiments in the Q2-region of about 1 GeV2. Then the maximum luminosity (target nuclei per cm2 x incident electrons per set) which can be tolerated by a large solid angle detector in electron scattering experiments can become the limited factor. Fortunately, background problems should be reduced by the significantlyimproved duty cycle of the ELSA machine15. 5.

CONCLUSION In the last years the study of polarization phenomena at intermediate

energies has been of increasing interest. In many cases serious experi~ntal difficultiesare arising because of the limitationsof the available target materials. NH3 and ND3 as improved polarized target materials have replaced the alcohol materials, where experiments with high intensity particle beams are performed. Up to now the best deuteron polarizationresults (Pvector*0.45; Ptensor, 0.16) could be obtained in a magnetic field of 3.5 T. In addition, the very good polarization resistance to radiation damage of ammonia is a result of the radiation induced radicals necessary for the DNP. With the use of ND3 in 3He-4He dilution refrigeratorsa new type of experi-

W. Heyer /Present status ofpolarized targets

386~

ment has been performed - the electron deuteron scatteringoff a tensor polarized deuteron target. The polarization results obtained in this experiment demonstrate that for electron beams high magnetic fields (~2.5 T) must be used. From this point of view the situation for polarized target experiments with photons is different. Using tagged photon beams the technique of the frozen spin target, which operates at much lower fields (0.25 - 0.4 T) can be employed. The cabined use of a (linearlyor circularly) polarized photon beam and a (vectoror tensor) polarized deuteron target is planned for ~asur~ents

at the

3.5 GeV ELSA accelerator in Bonn. Taking all results together, the development of the hydrogen rich target material ammonia makes new types of polarized target experiments possible, which will certainly be improvedwith the next acceleratorgeneration at intermediate energies - like stretcher rings. So the conventionalpolarized nucleon target will be with us for some time to come. ACKNOWLEDGEMENT I would like to thank K.H. Althoff for many inspiring discussions and for his steady help and interest. My special thanks go to the members of the Bonn Polarized Target Group E. Schilling, R. Dostert, E. Kohlgarth and W. Thiel. I am grateful to 6. Knop and M. Leenen for useful discussions. REFERENCES 1) K.H. Althoff et al., ZeitschriftfilrPhysik C. Particle and Fields 26 (1984) 175 2) T. Ishii et al., Phys.Lett. 1108 (1982) 441 3) W. Meyer et al., Nucl.Instr.and Meth. 204 (1982) 59 4) K.H. Althoff. B. Boden, V. Burkert, R. Dostert, T. Hewel, G. Knop, E. Kohlgarth, 6. Kroesen, M. Leenen, W. Mehnert, W. Meyer, R. Sauerwein, H. Schablittky, E. S&hilling. H.H. Schmitr, W. Thiel, contributedpaper to this conference 5) W. de Boer, M. Borghini, K. Morimoto, T.O. Niinikoski,F. Udo, J. Low Temp.Phys. 15 (1974) 249 6) T.O. Niinikoski,and J.M, Rieubland, Phys.Lett. 72A (1979) 141 7) U. Cartel, 0. Kaul, W. Meyer, K. Rennings, E, &hilling, PrOC. Conf. on High Energy Physics with Polarized Beams and Targets, eds., C. Joseph and Soffer (Birkhluser,Basel, 1981) p. 451 8) W. Meyer et al., Nucl.Instr.and Moth. 227 (1984) 35 9) Proc. of the 4th Workshop on Polarized Target Materials and Techniques,ed. W. Meyer (PhysikalischesInstitut Bonn 1985) 10) 0. Crabb, in Ref. 9, p. 7

W.Meyer 11) E. Schilling,

/Present

status of pokrized

targets

in Ref. 9, p. 13

12) W. Meyer, Proc. of the 6th Int. Symp. on High Energy Spin Physics, ed. J. Soffer (Journal de Physique) p. C2-455 13) E. Schilling,

Thesis (Bonn) in preparation.

14) M. Gourdin and C.A. Piketty, Nuovo Cimento 32, (1964) 1137 15) K.H. Althoff et al., Proc. 11th Int.Acc.Conf.,CERN 1980, p. 196 0. Husmann, BONN-IR-83-6

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