A magnetic multipurpose spectrometer

EEE TRANSACTIONSON MAGNETICS,VOL.28.NO.l, JANUARY 1992

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A Magnetic Multi Purpose Spectrometer S. Frullani, F.Garibaldi, F. Ghio, M. Jodice, G.M. Urciuoli Physics Laboratory,Istituto Superiore di Sanita' Istituto Nazionale di Fisica Nucleare Sezione Sanita' Viale Regina Elena 299,00161 Roma, Italy

R. De Leo

Physics Department, Universita' di Lecce Istituto Nazionale di Fisica NuclSezione di Lecce

K. I. Blomqmt

Institut fiir Physik, Maim University, 6500 Maim, Germany

M. Nervi, M. Repetto

Dipartirnento di Ingegneria Elettrica Universita' di Genova, Via Opera Pia 1 la, 16145 Genova, Italy

Abstract - The very high quality and precision of the CEBAF beam will allow the investigation of hypemuclear systems with high accuracy and resolution provided the appropriate experimental equipment is implemented. A possible design of a 1.3 GeV/c, high resolution, short-orbit kaon spectrometer, to be used in combination with one of the two 4 GeV/c CEBAF Hall A spectrometer is presented. The facility is a multipurpose one, serving also as a second hadron arm with the capability of going out of the scattering plane and also serving as an electron spectrometer for large scattering angles.

I. INTRODUCTION The Continuous Electron Beam Accelerator Facility

(CEBAF),currently under construction in Newport News, Virginia is a basic nuclear physics research facility, dedicated to study nuclear structure using electron and photons as probes. It will provide a low emittance, 200 pA electron beam with energies up to 4 GeV and 100% duty factor in the three experimental halls simultaneously. The core experimental equipment for Hall A is a pair of identical superconducting, 4 GeV/c, High Resolution Spectrometers ( H R S ) [l]. HRS with their optical length of 23 meters are not suitable for detect unstable hadrons as kaons (CZk = 3.7 meters) and low momentum pions, and to measure non-coplanar reactions. Both these requirements imply the use of a relatively short spectrometer whose maximum analysable momentum must be lower than HRS, if the high resolution characteristics have to be preserved. Moreover, a third magnetic spectrometer allows the possibility to perform high resolution, high luminosity triple coincidence experiments in the case that two hadrons in the final state are angularly correlated. The presented Multi Purpose Spectrometer ( M P S ) could give to Hall A all these three capabilities making possible the study of new fields in electromagnetic nuclear physics as the out-of-plane reactions, particle correlations and electromagnetic kaon physics probing strange quark behaviour in nucleons and nuclei. The basic layout of the M P S is shown in Fig. 1. It is a vertical QDQ design including a mom temperature Collins type quadrupole followed by a 3.2 meters long dipole magnet with focusing entrance and exit faces. Subsequent to the dipole is another room temperature conventional type

quadrupole. The main design characteristics for the M P S are listed in Table I below. Table I. Main Characteristics of the 1.3 GeV/c QDQ Design QDQ

Configuration Bending angle Optical length (m) Transverse focusing Momentum range (Gev/c) Momentum acceptance (%) Momentum dispersion @, cm/%) Momentum resolution (est.) Radial linear magnification I D/M I (cm/%) Angular acceptance : horiz. (mr) Angular acceptance : vert. (mr) Angular resolution : horiz. (mr) Angular resolution : vert. (mr) Solid angle (msr) Transverse length acceptance (cm) Transverse position resolution (cm)

-9

i

vertical 750 10.6 point-to-point 0.2 to 1.3 15 9.1 1 term goes to zero. Both features ensure an optimal use of the 33 centimeters gap for horizontal angular acceptance and horizontal target length acceptance. The fact that the cyly> term remains small (~2.5)all the way through the spectrometer makes the use of extended targets (16 cm. at 900) possible, with no substantial loss in solid angle. -,- I

~-,--T-~-l-r-r-l,-l-,-,

-'-I-

I-'-

D

In the dispersive plane, the c x l b term is kept large inside the dipole to make efficient use of the dipole width to build resolving power. Radial focusing is given b a -21.30 tilt angle of the entrance pole face and by a -19.dtiIt angle of the exit pole face. Due to practical considerations some constraints have been put on the development of the present optical design: - the beam envelope has been fixed not to exceed 0.33 m. (1.2 m.) in the dipole gap (width), and 0.5 m. (0.8 m.) diameter for the useful aperture for Q1 (42). These size constraints help keep the cost of the magnets under control. - The total angle of bend has been chosen at 750, a value selected as a reasonable compromise for a high resolution, short length device. - The overall optical length was constrained not to exceed 10 meters as an acceptable limit to provide a reasonable survival rate for the kaons. The fmt order transport matrix at the focal plane is the following:

(i ) ( I

y

I=I

-1.04

0.0

0.0

0.0

9.1

-5.69

-0.95

0.0

0.0

25.6

0.0

0.0

-1.22

-0.01

0.0

0.0

0.0

5.09

-0.74

0.0

0.0

0.0

0.0

0.0

1.0

0 ;

'

/

A

ky/+l

-25 -

- 50

x

yo

b

I

QZ

5 1I

Higher order aberrations were studied with RAYTRACE [3], and hardware corrections were determined. These corrections were applied in the first quadrupole, in the curvature of the effective field boundary at the entrance and exit faces of the dipole, and in the second quadrupole. Furthermore a slot was introduced in the middle of the dipole to allow for higher order corrections where the radial extent of the beam is largest. The focal surface, Fig. 3, has been determined using standard RAYTRACE 14-ray bundles from a point source at 5 different momenta ranging from 6=-7.692% to 6=7.692%.

E D

I

/

50

100

z,, (4

-50

" " " " ' ~ " " ~ " " ~ " " ~ "

10

L (m)

Fig. 2. First order coupling coefficients in the dispersive (x) and transverse (y) planes. Units are meters x radians.

Fig. 3. RAYTRACE [3] generated focal plane representation for the QDQ design. Ray bundles are from a point source spanning -100% of the full solid angle. Each bundle represents a distinct momentum ranging from 1.2 GeV/c to 1.4 GeV/c. The dashed line connects the intersections of the axial and paraxial rays at each momentum.

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The focal plane is quite curved because the compactness of the spectrometer makes it hard to eliminate all the aberrations and especially to obtain a straight focal plane. The focal plane angle with respect to the central trajectory is -470. The full width of those ray bundles, spanning a solid angle of -20 msr (-100% of the full acceptance of the spectrometer), at the west ranges from 0.03% at 6 = 0 to 0.125% at 6 = 7.692%. 111. MOMENTUM RESOLUTION In order to evaluate the spectrometer performances, the following procedure was used. A set of 2000 randomly generated rays spanning the full acceptance (-7.5%