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Journal ArticleDOI

1.6 GeV/c charged particle spectrometer facility at the Stanford Linear Accelerator Center

R. L. Anderson1, D. Gustavson1, R. Prepost1, D. M. Ritson1 
15 Dec 1968-Nuclear Instruments and Methods (North-Holland)-Vol. 66, Iss: 2, pp 328-335

AbstractA 1.6 GeV/ c spectrometer has been constructed at SLAC incorporating an n = 0, 90° bend, 254 cm radius magnet with second-order corrections. The magnet is of the window frame type allowing invariant focal properties up to 21 kG. It has a momentum resolution of ± 0.08% and an angular resolution of ± 0.4 mrad. It simultaneously focusses production angle and momentum from a 20 cm long target onto a single focal plane orthogonal to the beam direction allowing a considerable simplification in detecting a high energy scattering process.

Summary (1 min read)

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Summary

  • The magnet is of the window frame type allowing invariant focal properties up to 2lkg.
  • It simultaneously focusses production angle and momentum from a 20cm long target onto a single focal plane orthogonal to the beam direction allowing a considerable simplification in detecting a high energy scattering process.
  • The SLAC 1.6 GeV/c spectrometer is mounted on a common pivot with two other SLAC spectrometers, the 8 and 20 GeV/c spectrometers.
  • Angled entrance and exit faces introduce first-order focussing conditions which make the production angle and momentum focal planes coincident in space.
  • This feature simplifies the detector arrays required for the analysis of high energy physics experiments.
  • The production angle versus momentum display in the focal plane has a linear dispersion of 4>19cm per percent in momentum and 0.808cm per milliradian in angle, with a resolution of 2 0.0896 in momentum and 2 0.4 milliradian in angle.
  • This degree of resolution is required to kinematically separate processes differing by the production of only one a-meson.
  • This display is convenient since the kinematics of e-body processes give an approximately linear relation between production angle and momentum over the small region seen by the spectrometer at any one setting.
  • The focus of particles from a particular two body process is approximately a straight line in the foea,1 plane, and can be selectively detected by appropriate scintillation counters.
  • In practice, the focal plane is divided into strips by the hodoscope counters which can be rotated into alignment along the appropriate kinematic curve.
  • This technique thus eliminates the complex decoding necessary for SySteRS which use separate hodoscopes for both angle and momentum measurements, and makes it possible to count particles at 100 megacycle rates with relatively simple.

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I
SLAC-m-450
mm
c
1.6
GeV/c CHARGED l?ARTICL.E SPECTROKEX'ER FACILITY
AT TX3 STAJXFORD LIXEAR ACCELFRATOR CE'NTW*
f
R. Anderson, D. Gustavson, R. Prepost, D. Ritson
Stanford Linear Accelerator Center
Stanford, California
ABSTRACT
A
1.6
GeV/c spectrometer has been constructed at SLAC '
incorporating an n = 0, 90' bend, 25&m radius magnet with
second-order corrections.
The magnet is of the window frame
type allowing invariant focal properties up to 2lkg. It has
a momentum resolution of f 0.08s and an angular resolution
of
+ 0.4nrrad. It simultaneously focusses production angle
and momentum from a 20cm long target onto a single focal
plane orthogonal to the beam direction allowing a consider-
able simplification in detecting a high energy scattering
process.
(TO
be printed in Nuclear Instr. and Methods)
-E
Work supported by the U. S. Atomic Energy Commission.
-f-
Now at Department of Physics, University of Wisconsin,
Madison, Wisconsin

1.6
GeV/c CRARGRD PARTICLE
AT THE STAXFORD LINEAR
SFFCTRCMEX'RR FACILITY
ACCEXXRATOR CEWlXR
.
INTRODUCTION
The SLAC
1.6
GeV/c spectrometer is mounted
on
a common pivot with
two other SLAC spectrometers,
the
8
and 20 GeV/c spectrometers. It is a weak
focussing (n
= 0) 90' vertical bend device with a maximum acceptance exceeding
3-millisteradians of solid angle, 2Ocm of target length and $ 5$ in momentum.
Angled entrance and exit faces introduce first-order focussing conditions which
make the production angle and momentum focal planes coincident in space. This
feature simplifies the detector arrays required for the analysis of high energy
physics experiments.
The production angle versus momentum display in the focal plane has
a linear dispersion of 4>19cm per percent in momentum and
0.808cm
per milli-
radian in angle, with a resolution of 2 0.0896 in momentum and
2 0.4 milliradian
in angle.
This degree of resolution is required to kinematically separate
processes differing by the production of only one a-meson. This display is
convenient since the kinematics of e-body processes give an approximately
linear relation between production angle and momentum over the small region
seen by the spectrometer at any one setting. The focus of particles from a
particular two body process is approximately a straight line in the foea,1
plane, and can be selectively detected by appropriate scintillation counters.
In practice, the focal plane is divided into strips by the hodoscope counters
which can be rotated into alignment along the appropriate kinematic curve.
This technique thus eliminates the complex decoding necessary for
SySteRS
which use separate hodoscopes for both angle and momentum measurements, and
makes it possible to count particles at 100 megacycle rates with relatively simple
-l-

electronic systems. To maintain good resolution for this mode of operation,
the first-order corrections were adjusted to make the momentum and production
angle planes coincide,
and second-order corrections were made so that the
focal planes were normal to the central ray.
The use of a homogeneous n = 0 magnetic field
(1)
made it possible
to utilize a window frame design and thus to operate the magnet without any
appreciable saturation or change in focussing properties up to fields of 21
kilogauss.
The second-order corrections were introduced by shaping the pole faces
slightly in three "beta lens"
regions to produce /3 # 0 but leaving n = 0.
(59
The boundaries of the correction region and the entrance and exit regions
were shaped to minimize saturation effects.
The appropriate values of the
design parameters were found with the aid of the results of K. Brown
(3)
for
the second-order transfer matrix of a homogeneous field magnet and the SLAC!
TRANSPORT program.
(4)
The measured magnet properties were within the design
specifications without need for any shimming of the field.
(1) J. V. Allaby and D. M. Ritson, Rev.
Sci. Instr. 3, No. 5, 607(May, 1-:!65)
notation of SLAC-75 by K. L. Brown
(3)
c.f.,
K. Brown, SLAC Report No.
75, 1.967
and K. Brown, "Advances in
High Energy Physics",
Cool and Marshak, Vol.1 in press for reviews
of rel.evant formalae and methods.
(4) SLAC-DOC-12, TRANSPORT by S. HoTfry, C. H. Moore, S. H. Butler,
October,
1963..
-2-

I
THEORY AND DESIGN PARAMETERS
The choice of design characteristics was made in the following
way.
Edge focussing was introduced at the entrance face of the magnet to
obtain parallel-to-point focussing in the non-be& plane.
However, the
introduction of such focussing in the non-bend plane decreases the focussing
in the bend plane and results in a momentum focal plane at an inconvenient
distance from the magnet.
By introducing compensating focussing at the exit
to the magnet it is possible to mke the focal planes in both the horizontal
and vertical dimensions coincide at 254cm from the exit to the magnet.
Fig. 1
shows diagrammatically the first-order focussing properties of the magnet in
the horizontal and vertical dimensions.
Second-order corrections were made in drder to minimize the "circle
of confusion" in a focal plane orientated at right angles to the direction of
the incident particles.
The numerical derivatives of the various aberrations
with respect to the stren&hs of the second-order correcting elements were
obtained from the SLAC TRANSPORT program and fed into the set of minimal
equations which were then solved for the appropriate corrections.
This
procedure led to the values chosen for our design. Had the only criterion
been to minimize the "circle of confusion",
one second-order correction on
the front face of the magnet would have been the optimum design choice. Since
two second-order correction regions
aid
not allow us to simultaneously achieve
a reasonably small circle of confusion and a focal plane orthogonal to the
beam direction, three second-order correction regions were required. These
three correction regions are referred to as ."B-lenses" since in the lens
region B is non-zero.
(2)
The B*lenses are shown in Figs. 2a add 2b.
-3-

I
CONSTRUCTION
Fig. 3 shows a view of the complete spectrometer including the
magnet, carriage,and shielding.
The magnet is mounted on a carriage which
is pivoted on the target support and rolled on a 7.6m diameter circular
rail so the particle trajectories are bent upwards.
An angle readout driven
by a chain mounted on the rail makes it possible to have a remote digital
readout of the laboratory angle to .OOl degrees.
The magnetic field of the
spectrometer is measured using an NMR probe that swings into the center of
the magnetic gap where the uniform magnetic field gives very clean NMR signals.
The space traversed by the particles from target to counters is enclosed by
a vacuum chamber.
An aperture stop made of four moveable tungsten lined jaws
makes it possible to define both the vertical and horizontal acceptance angles.
The horizontal aperture stop is especially useful since it allows the target
length to be clearly defined. The spectrometer angle, magnet current, aper-
ture stop and NMR probe are all remotely controlled. The carriage also
supports a cylindrical concrete and steel shielding house which protects the
detectors from room background.
Two moveable lead doors 2-f%. thick, each
weighing 14-tons,
allow access to the detector cave.
The concrete shielding
is made in circular segments to simplify construction and assembly. Their
size was dictated by the SO-ton capability of the crane servicing the
spectrometer. The counter assembly may be rotated remotely in order to align
the hodoscope counters along the desired direction in the focal plane. The
counters are also accessible by removing the top concrete shielding block.
This is the primary access route for ihstalling and removing the large
counter assemblies.
The access afforded by the doors is used for minor
repairs and adjustments,
-4-

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References
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Journal ArticleDOI
Abstract: A 90° bend n=0 magnetic spectrometer has been constructed which uses curved field boundaries to achieve second order correction of the focal properties and to make the image plane normal to the emerging particles. The spectrometer has a 112 cm radius of curvature and a maximum momentum capability of 725 MeV/c. The window frame yoke, with sloping edge profiles to minimize edge saturation, enables the focal properties to remain invariant up to 21 kG. The solid angle acceptance is 8×10−3 sr with 0.1% resolution and the maximum momentum acceptance is ±5%.

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