UCLEAR PHYSICS

PROCEEDINGS SUPPLEMENTS

F2LSE\,'IER Nuclear Physics B (Proc. Suppl.) 44 (1995) 695-700

The three-spectrometer setup for (e,e'x) coincidence experiments at the Mainz microtron MAMI

R. Neuhausen a

~Institut ffir Kernphysik, Johannes Gutenberg-Universit~it Mainz, D-55099 Mainz, Germany

A setup of three high-resolution magnetic spectrometers has been built up as the central facility for the precise determination of electron scattering cross sections in coincidence with hadrons. The operational experiencies with two of the spectrometers, A and B, demonstrate that both spectrometers exceed the specifications. With A and B, a series of (e, e%) coincidence experiments were successfully performed. The third spectrometer, C, will be ready for operation in late autumn 1994.

1. T H E M A I N Z M I C R O T R O N

1.1 . P r o p e r t i e s o f t h e c . w . e l e c t r o n b e a m The continuous wave electron accelerator

MAMI [1] consists of three cascaded racetrack mi- crotrons with a 3.5 M e V linear accelerator as an injector. The last stage delivers electron beams from 180 M e V to 855 M e V in 15 M e V steps with currents up to 100 #A. The stability of the ma- chine and the quality of the beam are excellent. In the horizontal plane, the beam emittance of 1 • 10 - s • 7r • m • rad is mainly determined by the stochastieal emission of synchrotron radiation, whereas the vertical emit tance decreases continu- ously to the very low value of 0.7.10 -9 . ~. m . rad due to pseudodamping.

The beam energy in the third stage was mea- sured with an absolute accuracy of :L160 keV by exact determination of the beam position on the accelerator axis and in several higher return paths. The long- te rm energy stability, limited by small thermal drifts of the bunch phase, is about 40 keV. Due to the stable non-isoehronous longitudinal motion in microtrons, the residual rf phase and ampli tude fluctuations have only little influence on the beam energy, so that the spec- t rum at 855 M e V is mainly dominated by syn- chrotron radiation effects with a width of about 50 keV (FWHM).

A source of polarized electrons [2] is at tached to MAMI. The spin direction can be oriented in any direction by means of a spin ro ta tor [3] in the 100 keV line behind the source. The analysis of

0920-5632/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0920-5632(95)00604-4

a polarized 855 M e V beam by means of a Moller polarimeter demonstrated that the initial degree of polarisation of about 307o, given by the GaAsP photocathode, was preserved after more than 100 spin precessions during acceleration.

MAMI has delivered its first beam in August 1990, and is used for an extensive physics pro- gramme since summer 1991.

1.2. P h y s i c s P r o g r a m m e a t M A M I The physics programme, carried out by a large

international community organized in collabora- tions, ranges from particle and nuclear physics topics, like coincidence experiments with elec- trons, experiments with real photons produced by energy tagged Bremsstrahlung photons, measure- ment of the electric form factor of the neutron by the use of polarized electrons, and pari ty violat- ing electron scattering experiments, to the devel- opment of radiation sources of high brilliance in the soft and hard x - ray regime for applications in several fields of physics, material sciences, bi- ology, and medicine.

The present paper concentrates on (e,e~x) co- incidence experiments, where x stands for any particle or several particles. The large variety of these experiments, like quasi-free nucleon knock- out reactions, A resonance production in nuclei, pion production from nucleons and nuclei, rr ° and 7/° threshold production and others, require dif- ferent specifications for the experimental setup. An optimal solution could, of course, be devised for each of these experiments. However, the most

696 R. Neuhausen/Nuclear Physics B (Proc. SuppL) 44 (1995) 695-700

'/885 ~ - 8400 pw

QSDD ( \ \ CLAM DIPOLE / \ P2m= 73,5 bhlV/c I 1 , / I : I I~ I~ I - I ~ P.--= B70 MoV/c \ ~ \

% p _: 220 L I r \ , . , . : ,5 = \ \ \

I1 °'IT o, I /

nooi- I / Figure 2. Layout of spectrometer B.

Figure 1. Layout of spectrometer A.

effective approach is the realization of a multi- purpose setup, as long as one does not compro- mise too much. Having this in mind as a gen- eral guideline, the A1 collaboration has built up a complex of three high--resolution magnetic spec- trometers as the central facility for the precise de- termination of electron scattering cross sections in coincidence with hadrons.

2. THE SPECTROMETERSETUP

2.1. Magnet-optical design The spectrometers A and C with point-to-

point optics in the dispersive plane and parallel- to-point optics in the non-dispersive plane have large solid angles and large momentum accep- tances, and consist each of a quadrupole, a sextu- pole and two dipole magnets (Fig. 1). The disper- sion to magnification ratio D / M s = 10.8 ore/% leads to a first-order resolving power of 22,000 for a beam spot size of 0.5 ram. The spectrometer

C is essentially a scaled version of spectrometer A, where the scaling factor is given by the ra- tio of the mean bending radii, 11/14. The spec- trometer B (Fig. 2) has a moderate solid angle and a somewhat reduced momentum acceptance, but it reaches a maximum momentum larger than that of the MAMI electron beam (855 MeV/c ) . It consists of a single clamshell dipole magnet with point-to-point optics in both planes. The D / M s ratio of 9.6 c m / % gives a first-order re- solving power of 19,000 for the above given beam spot size. Due to its slim construction, the spec- trometer B reaches very small sattering angles, a capability which allows a precise separation of longitudinal and transverse structure functions.

The most important properties of the three spectrometers are compiled in table 1.

A particularly important option in coincidence experiments with electrons is the capability for out-of-plane measurements in order to deter- mine the longitudinal-transverse and transverse- transverse structure functions. A detailed study of the range of the out-of-plane angle demon-

R. Neuhausen/Nuclear Physics B (Proc. Suppl.) 44 (1995) 695-700 697

Table 1 Main Parameters of the Three Spectrometers spectrometer A B C configuration QSDD D* QSDD max imum m o m e n t u m [MeV/c] maximum induction IT] momen tum acceptance [~0] solid angle [msr] long-target acceptance [mm] scattering angle range [o] length of central t ra jec tory [m] dispersion (central t ra jectory) [cm/%] magnification (central t ra jectory) dispersion to magnification [cm/%] momentum resolution angular resolution at target [mraaq position resolution at target [mm]

735 870 551 1.51 1.50 1.40 20 15 25 28 5.6 28 50 50 50

18 - 160 7 - 62 18 - 160 10.75 12.03 8.53 5.77 8.22 4.52 0.53 0.85 0.51 10.83 9.64 8.81

-<10 -4 -<10 -4 -<10 -4

_<3 -<3 _<3 3 - 5 <1 3 - 5

* clamshell dipole

s t rated the part icular need for small angles. For a range of 0 ° to 10 °, Spectrometer B can be moved ou t -o f -p lane by means of a mechanical driving system.

The spectrometers , which are rotatable around a common pivot, are installed in a new experi- mental hall with a size of 30 m × 20 m and a height of 17 m from the floor to the crane hook. The arrangement can be complemented with fur- ther magnetic or non-magnet ic detectors, as, e.g., a BGO crystal ball and a neutron detector sys- tem.

2.2. D e t e c t o r sys tems Each spectrometer is equipped with a position

sensitive detector system consisting of four planes of vertical drift chambers, two planes of plastic scintillators and a threshold gas Cherenkov de- tector. The s tar t signal for the drift t ime mea- surements is provided by the scintillators with an accuracy of 200 ps. Identification of pions and protons is obtained by comparing energy losses in the scintillators. The Cherenkov detector with an efficiency of be t te r than 99.9~0 serves as a ve to - detector for electrons. The particle momentum, the emission angles in both planes and the ver- tex are deduced by tracking the t ra jectory of a particle back to the target using the four position

measurements of the drift chambers.

3. O P E R A T I O N A L E X P E R I E N C E S

3.1. M e a s u r e m e n t s In summer 1992, spectrometer A was com-

pleted, and detailed measurements with electron beams in an energy range from 180 MeV up to the highest MAMI energy have been performed to s tudy the properties of the spectrometer. Exper- imental raytracing with elastically scattered elec- trons were used to determine the transfer mat r ix with significant elements up to fifth order. To se- lect well determined angles in both planes a sieve collimator with 11 × 7 holes, 1.5 m m in diame- ter each, was installed in a distance of 568 m m from the target in front of the quadrupole mag- net. As targets, we used 12C and lSlTa foils with thicknesses beetween 30 and 90 mg/cm 2.

The second magnetic spectrometer, B, was ready for operation in June 1993, and, again, de- tailed measurements with beam have been per- formed in a similar way.

3.2. M o m e n t u m r e s o l u t i o n As a result, Fig. 3 shows a spectrum of elec-

trons with a pr imary momentum of 495 MeV/c scattered on 12C at a scattering angle of 65.2 °. The spectrum was measured with spectrometer A

698 R. N e u h a u s e n / N u c l e a r Physics B (Proc. Suppl.) 44 (1995) 6 9 5 - 7 0 0

400

350

300

ffl -~ 250 ¢-

o 200

150

100

~=495 MeV

'~C, 90 r a g / e r a ~ b~

0,= 65 .4 "

&~}=28 m s r

~: >, .

~ 6 ~ ~ :~ -

, I , , ~ , 50

, g_7

!

• J . t ] /~ .~ . [~_1..1~ .i.d.l 465 467.5 470 472.5 475 477.5 480 482.5 485

po (MeV/c)

Figure 3. 12C(e,e') spectrum measured with spectrometer A at full solid angle, and corrected for spherical aberrations and kinematical broad- ening.

using the full solid angle of 28 msr. The elastic line yields a width of 117 keV/c (FWHM), cor- responding to a momentum resolution of @/p 2 • 10 -4. Lines of inelastically scattered electrons are clearly resolved. Measurements with spec- trometer B have yielded FWHMs of 56 keV/c for 1~C and 51 keV/c for ~8~Ta, respectively, at a primary momentum of 495 MeV/c, correspond- ing to a momentum resolution 5p/p ~ 1 • 10 -4.

3 .3 . Angula r r eso lu t ion For each hole, the sieve collimator defines

primary angle distributions with a a-width of 1.0 mrad. For spectrometer A, Fig. 4 shows the events of elastically scattered electrons, back- traced from the detector system to the target. The various holes are clearly separated. The pro- jections of the events to the angles O0 and @0 in the dispersive and non-dispersive plane, re- spectively, show distributions with FWHMs of approximately 3.0 mrad, corresponding to a ~r- width of 1.3 mrad. Unfolding of the primary width, under the assumption of Gaussian-like dis- tributions, yields a an angular resolution of better

80 60 40

0

-20 -40 -60 -80

. , . . , ~ . . . , t r. ". ~. , . .

*:::; V! : : - ' , : " ' ' , " . I ' I " - - I . " " l .

: " ~ ~:'::~L,:." . L . ~ :.B.

'-5.;.--;~.::.~,:..'-:.-:~- .t " • -" "';..

-100-75-50-25 0 25 50 75 100 ¢Po (mrad)

700 -

600 .

500 _-

400 :- : J1 oo OO oo

0-80 -60 -40 -20

A®o=2.8 mrad

0 @ (mrad)

20 40 60 80

1000

800

600

400

200

m

o t -100-75-50-25 0 25 50 75 100

dP o (mrad)

ArPo=3. 0 mrad

Figure 4. Measurement of the angular resolution of spectrometer A. The events are back-traced from the detector system to the target (top), and projected to the Oo-axis (center) and the ~So-axis (bottom), respectively.

R. Neuhausen/Nuclear Physics B (Proc. Suppl.) 44 (1995) 695-700 699

than 2 mrad for both planes, m30000 t- For spectrometer B, the angular resolution was

also measured to be bet ter than 2 mrad for both O o planes.

3.4. V e r t e x r e s o l u t i o n Due to the para l le l - to-point optics in the non-

dispersive plane the vertex resolution is ra ther poor for spectrometer A. The measurements have shown a vertex resolution of 6 ram. A much bet- ter resolution of ~ 1 r am is obtained with spec- t rometer B, due to the po in t - to point optics of this spectrometer.

4. P E R F O R M A N C E O F C O I N C I D E N C E E X P E R I M E N T S

As the first coincidence experiment, the mul t i - hadron electroproduction in the quasielastic, dip and A resonance region was investigated, where the scattered electrons were detected with spec- t rometer A and the emit ted hadrons were mea- sured with a BGO bM1 detector covering a solid angle of almost 4~r.

After the completion of spectrometer B, a se- ries of (e,e'x) coincidence experiments with two magnetic spectrometers were successfully started. Measurements of the (e, e'p) reaction on 12C and 160 were first performed at kinematics of earlier NIKHEF measurements [4], subsequently kine- matics, where up to now no experimental da ta exist, were investigated. Data with missing mo- menta p,~ = 700 M e V / c and missing energies E,~ = 200 M e V were taken. The data, corrected for different pa th lengths of the electrons and pro- tons relative to the central t ra jectory in each of the spectrometers, show an excellent ratio of true to accidental events (Fig. 5). First results on the h igh - momen tum components in the lp orbitals of 160 [5] show that , at high momenta , the rise above the mean-f ield prediction is modest.

Missing momen ta as high as p,~ = 950 M e V / c corresponding to two nucleons at 0.2 f m were reached in a D(e, e~p) experiment making use of a l iquid-hydrogen/deuter ium target which sustains current up to 37 ttA.

First results for the ~'0 electroproduction on the proton near threshold were obtained by the study

20000

10000

0

<pro,=590 MeV/c corrected

~At=1.2 ns

uncorrected °°°~° c~°~o coo

900 1000 1100 1200 1300 1400 tcoin c (chan)

Figure 5. Coincidence time spectrum for the 160(e,e'p) reaction at a missing momentum of 590 MeV/c , taken with a beam current of 25 #A and a target thickness of 25 mg/e m 2.

of the p(e,e'p)~ro reaction at a four -momentum transfer squared q2 = -0 .1 (GeV/c) 2. The data were taken with a l iquid-hydrogen target cell with a diameter of 2 cm and beam currents up to 22 #A, corresponding to a luminosity of 1.2.1037. cm -2. s -1 . The scattered electrons were detected with spectrometer B, and the recoil pro- tons with a mean kinetic energy of 55 M e V were detected with spectrometer A. The energy loss of the protons in the target could easily be cot- rected for, due to the good vertex resolution of spectrometer B. All kinematical variables of the reaction were measured, and the 7r0 was identified via the missing mass (Fig. 6).

Further experiments were successfully per- formed (i) for the separation of the longitudi- nal and transverse structure functions of the 1H(e, e'Tr +) taken in parallel kinematics at an in- variant mass of 1125 M e V , (ii) for the determina- tion of the magnetic form factor of the neutron via the measurement of the ratio of cross sections for the D(e, e'n) and D(e, e'p) reactions, and (iii) for the forward-angle electroproduction as a means

700 R. Neuhausen/Nuclear Physics B (Proc. Suppl.) 44 (1995) 695-700

5lIlJ[~

413130

2 ~ D

Figure

o

50 80 100 120 1413 150

missing mass / MeV

6. Missing mass spectrum of the p(e, e'p)Tro reaction. The 7r0 peak with a width of 2 M e V centers at 135 M e V .

of investigating preformed A's in the 3He ground state.

5. C O N C L U S I O N S A N D O U T L O O K

The first coincidence experiments have demon- strated that the thre~spect rometer setup is, in connection with the high-quality continuous wave electron beam of MAMI, a very effective facility for investigating the structure of the nucleon and nuclei with electron scattering.

The physics programme will be extended to triple coincidence experiments of the type (e, e'prr) and (e,e 'pp), when spectrometer C will be operational in late autumn 1994. As a further step, a proton polarimeter for spectrometer A is under construction to open the possibility of in- cluding polarization degrees of freedom to the ex- perimental programme.

A1 C O L L A B O R A T I O N

Many colleagues have contributed to the real- ization of the three-spectrometer setup and are involved in the physics programme. This is illus- trated by the following list of A1 collaborators:

W. Bertozzi 1~, L. de Bever 3, K.I. Blomqvist 1, W.U. Boeglin 1, R. B6hm 1, E. Brash 4, J.R. Ca- larco 2, J.P. Chen 12, D. Dale 12, O. Denhard 1, M. Distler I , R. Edelhoff 1 , A. Felthman 3, R. Flori- zone 12, J. Priedrich 1, D. pritschi 3, R. Geiges 1, S. Gilad 12, R. Gilman 4, P. Gitzel 1 , C. Glashaus- ser 4, F. Heinemann 1, M. Jones 4, J. Jourdan a, M. Kahrau 1, M. Klein 1, M. Korn 1, H. Kramer 1, K.W. Krygier 1, V. Kunde 1 , M. Kuss 5, J. Ldc 4, J.M. Laget 6, A. Liesenfeld 1 , M. Loppacher a, G. Masson 3, K. Merle 1, C.L. Morris 7, R. Neu- hausen 1, E.A.J.M. Offermann 1, Th. Pospischil 1, M. Potokar s, C. Rangacharyulu 9, R.D. Ran- some 4, A. Rokavee 8, A. Richter 5, A.W. Rich- ter 1, B.G. Ritchie ~°, S. Robinson 3, G. Rosner 1, P. Rutt 4, P. Sauer 1, A. Sarty r2, S. Schardt 1, C.G. Schilling 1, J.P. Schiffer 11, G. Schrieder 5, Ch. Schrimpf 1, I. Sick 3, M.D. Solano Ros 1, Ph. Steiner a, P. Trfib 3, Th. Veit 1, B. Vode- nik s, A. Wagner 1, Th. Walcher 1, W. Wilhelm 1 , S. Wolf I , M. Yadav 4, B. Zihlmann 3

l Institut fiir Kernphysik, Universitgt Mainz, D-55099 Mainz; 2Department of Physics, Univer- sity of New Hampshire, Durham, USA; 3Institut fiir Physik, Universit/it Basel, CH-4056 Basel; 4Physics Department, Rutgers University, Piscat- away, USA; 5Institut ffir Kernphysik, TH Darm- stadt, D-64289 Darmstadt; 6CEN Saclay, Prance; 7LAMPF, LANL, Los Alamos, USA; SInstitut "Jo~ef Stefan", University of Ljubljana, Slovenia; 9University of Saskatchewan, Saskatoon, Canada; 1°Physics Department, Arizona State University, Tempe, USA; 11Physics Department, ANL, Ar- gonne, USA; r) Massachusetts Institute of Tech- nology, Cambridge, USA

R E F E R E N C E S

1. H. Herminghaus et al., Proc. LINAC Conf. 1990, Albuquerque, New Mexico, USA

2. W. Hartmann et al., Nucl. Instr. and Meth. A286 (1990) 1

3. K.-H. Steffens et al., Nucl. Instr. and Meth. A325 (1993) 378

4. G. van der Steenhoven et al., Nucl. Phys. A480 (1988) 547; M. Leuschner et al., Phys. Rev. C49 (1994) 955

5. K.I. Blomqvist et al., Phys. Lett. B (in press)