III.B.3. Magnetic Spectrograph

Topics covered for Magnetic Spectrographs

• III.B.3.1. Introduction
• III.B.3.1.2. Properties of the Spectrograph and Detection System
• III.B.3.1.3. Target Chamber
• III.B.3.1.4. References

III.B.3.1. Introduction

As indicated in the layout of the ATLAS facility (Figure III.1.), there are two magnetic spectrographs available. One in Target Area II and the second in Target Area III. They are both of the Enge split-pole type^1-3 and are virtually identical in all major components, such as target chamber, magnetic field, ion optics and detection systems. Both will be equipped with an all cryogenic pumping system. Particles are usually detected with an ionization-chamber type focal-plane detector developed at Argonne.^4,5

III.B.3.1.2. Properties of the Spectrograph and Detection System

The general properties of the split-pole spectrograph are described in a number of papers by Enge.^1-3 A schematic layout of the spectrograph used at Argonne is shown in Figure III.6. Since its actual use is largely determined by the focal-plane detector, Table III.1 lists the properties of the respective spectrograph-detector systems.

Figures III.7a and III.7b shows a cross section through the two ionization-chamber detectors available. Since the particles enter the detector at 45deg. the effective dimensions are a factor square root two larger than shown in the figure 3. Detector 1 has a length of 60 cm with a verticle acceptance of 1.5 cm. The position- and time-information is obtained from the parallel-plate avalanche counter and intrinsic resolutions of 0.75 mm and 250 psec, respectively have been achieved. The Bragg-curve detector provides an energy and Z-signal. Details about this detector can be found in Ref. 4.

Detector 2 is a x-y position sensitive parallel grid avalanche counter with the dimension 45 cm (x) and 10 cm (y), providing position and time signals. The intrinsic resolutions measured for this detector are 1.8 mm and 450 psec, respectively. Clear Mylar foils of 1, 2.5 and 6.4 mm thickness, respectively are used for the entrance window. The counter gases are Isobutane (PPAC) and Freon (IC). An electronically controlled gas-supply system keeps the pressure constant to +/-0.2%. Detector #1 allows the movement of three blocks in front of the PPAC detector to shield unwanted regions of the focal plane. Signals from the detectors are processed in a dedicated electronics setup and fed into the data acquisition computer via CAMAC interface. Computer programs are available to calculate magnetic field settings and kinematic corrections, and to perform multiple filtering of on-line data.

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III.B.3.1.3. Target Chamber

The target chamber is made of stainless steel 31.75 cm in diameter and 15 cm high. A sliding seal allows the rotation of the spectrograph around the fixed chamber. The whole chamber is insulated from the ground and can be used as a Faraday cup. There are four side ports available between -60deg. and -120deg. which can be used for various purposes (target transfer under vacuum, viewing window, insert for GeLi detector, etc.). Two monitor detectors are located 10 deg. below the target-spectrograph plane and symmetric with respect to the beam axis. They can both be remotely rotated behind a fixed aperture to avoid rapid deterioration with heavy-ion beams. The target holder is inserted through the top of the chamber and holds four targets of the standard 1" x 1" size. A vacuum in the low 10^-6 Torr range has been obtained in the spectrograph target chamber and detector box, respectively. Twenty-seven (27) 50 Ohm BNC cables and six (6) high voltage SHV cables run from the spectrograph to the data room.

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III.B.3.1.4. References

1. H. A. Enge, Nucl. Instrum. Meth. 28, 119 (1964).

2. J. E. Spencer and H. A. Enge, Nucl. Instrum. Meth. 49, 181 (1967).

3. H. A. Enge, Nucl. Instrum. Meth. 162, 161 (1979).

4. J. R. Erskine, T. H. Braid and J. C. Stolzfus, Nucl. Instrum. Meth. 135, 67 (1976).

5. H. W. Fulbright and J. R. Erskine, Nucl. Instrum. Meth. 162, 355 (1979).

6. J. R. Erskine, Nucl. Instrum. 162, 311 (1979).

Solid angle:

• ≤5.4 msr. Presently available are spectrograph entrance apertures with 0.038, 0.252, 1.00, 5.43, and 5 x 0.20 msr (5-slit aperture).

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