A small medium-pressure gas target system for use with polarized and unpolarized ion beams

N U C L E A R INSTRUMENTS AND METHODS 147 ( 1 9 7 7 )

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N O R T H - H O L L A N D PUBLISHING CO.

LETTERS TO THE EDITOR A SMALL MEDIUM-PRESSURE GAS TARGET SYSTEM FOR USE WITH POLARIZED AND UNPOLARIZED ION BEAMS J. B. A. ENGLAND and R. VLASTOU

Department of Physics, Universityof Birmingham, Birmingham BI5 2TT, England Received 17 June 1977 The design and experimental performance of a small gas target cell to operate at an absolute pressure of up to 2 bar and its associated detector collimation system are reported.

The use of gas target cells in charged particle scattering experiments poses a number of special problems not encountered with solid targets. A number of these problems are discussed by England 1) and a typical high accuracy gas target system is described by Morsch et al.2). We report here a small gas target cell to operate at an absolute pressure of up to 2 bar and its associated detector collimation system. This gas target system has been specifically designed and built to have a high degree of symmetry in the scattering plane about the beam direction to enable it to be used with polarized ion beams. It has been extensively tested in a general purpose scattering chamber 3) on the University of Birmingham A.V.F. Cyclotron with low-intensity 3He and 13 beams. The target cell is compact and easily mountable. When installed in the scattering chamber it can be positioned, both in height and angular alignment, to the same accuracy as that achieved with solid targets3). The target stem and gas supply system were developed at Oxford for use with very low pressure gas cells4). Plan and section views of the cell are shown in fig. 1. The scattered particle exit windows are of 12.5 # m Melinex and cover the angular range from +12.0 ° to ±168.0 ° . This angular range is covered in three steps: ±12.0 ° to ±35.0 °, ±45.0 ° to ±105.0 ° and ±115.0 ° to 168.0° . The missing angles are obtained by rotating the cell through 180°. A novel feature of this gas cell is the quickchange design of the beam entrance and exit windows. By withdrawing the complete target cell through an air-lock on the chamber lid these windows can be changed in a typical time of 30 min. The windows are mounted on 8 . 1 m m o.d. x 5 . 5 m m i.d. x0.81 mm thick brass rings and these are then clamped on O-ring seals to the en-

trance and exit snouts by threaded brass caps. These snouts are continued inside the cell and prevent the windows from being seen by the detectors of the scattered particles. The material from which these beam windows are made is usually 6 . 4 ~ m Havar, although 12.5/~m Melinex has been used for some measurements. The total volume of gas contained in the target cell and its filling tube is approximately 40 cm 3. Since the cyclotron operates at a fixed energy for a given particle, measurements at energies lower than that of the main beam can only be ob-, tained by the use of energy degrading foils placed in the main beam. For polarized beams, these foils must be placed close to the target to avoid the loss of beam intensity due to multiple scattering which would occur if these foils were placed before the main collimation system of the scattering chamber. With this gas target cell the degrader foil holder is mounted on a tube which is a good slide fit over the retaining cap of the beam entrance window. The whole foil assembly can be removed and replaced under vacuum via a second air-lock on the side of the scattering chamber. This allows detailed changes to be made to the foil thickness to ensure that a particular energy is achieved. The operation of this system has proved to be more flexible than the more usual foil-wheel arrangement for energy degrading. The detector collimation system for use with this gas target cell is shown in fig. 2. Up to ten detector telescopes can be mounted, five on each of the two independently moveable arms which can be placed in symmetrical positions about the beam direction to within _0.01 °. The basic principle is one in which an accurately machined and aligned guide tube is set up at each detector position. Each of the two chamber arms has a single

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curved mounting plate carrying five of these guide tubes, the axes of which are horizontal, coplanar and coincident to within 0.5 mm at the centre of the target position for the chamber. These mounting plates have been accurately made and surveyed so that the locating faces for the detector mounting blocks ensure that the radial positions of all detector final collimation apertures are equal

to (307.00___0.05)mm and that the angular separations between the centres of adjacent collimation apertures are 5.000°__+0.008° . The blocks on which the detectors themselves are mounted have attached to them thin-walled tubes which are accurate sliding fits in the guide tubes. When the gas target cell is in use, long extension collimation tubes are attached to the accurately machined

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ends of thin-walled tubes on the mounting blocks. The detector collimation apertures fit into accurately machined recesses at either end of the thinwalled tube and at the target end of the extension tube. When normal solid targets are used, the separation between the main collimation aperture and the front, antiscatter, aperture is 53.5 mm. When the gas target cell is in use with the extension tubes in place, the antiscatter aperture is at a distance of 190ram from the main collimation aperture; a second antiscatter aperture may also be placed at the 53.5 mm position. The relationships quoted by Silverstein 5) are used to calculate the Gfactor for the gas target collimation system. The detector and collimators are assembled and their alignment checked on the bench away from the scattering chamber and then the completed as-

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sembly is pushed home into the chosen guide tube on the appropriate arm and secured by a single locating screw. The whole mounting plate system on each arm is thermally insulated from the arm by ceramic insulators and can be cooled to - 2 0 ° C by cold alcohol circulated through internal coolings ways. This cold alcohol is supplied via flexible stainless steel pipes from a refrigeration system external to the chamber3). Permanent magnets giving a field of 0.025 T over a length of 27 mm are fitted to each guide tube so that electron suppression is permanently installed. This design has proved to be simple, troublefree and effective in catering for the many different combinations of semiconductor detectors and collimation systems currently in use for nuclear structure experiments in a general purpose scattering chamber with both gas and solid targets. In experimental use with the gas target cell the whole system has been found to be very satisfactory. No window scattering is observed at any angle for low gas pressures. At the maximum pressure, some particles are seen which are initially scattered by the window and then multiply scattered by the target gas. This effect is only

important if beams of doubly charged particles of energies less than 20 MeV are used. The minimum useful angle at which measurements can be taken has been found to be 11.25 ° for polarization studies and 12.0 ° for cross section studies. A typical spectrum obtained for 12 MeV polarized deuterons incident on 22Ne at a pressure of 1.8 bar at 20°C is shown in fig. 3. This illustrates the excellent quality of the experimental data which can be obtained with this gas target system. We are grateful for the helpful discussions and encouragement from Dr. J. M. Nelson and Prof. G. C. Morrison during the design and testing of this system. One of us (R. V.) thanks the Greek State Fellowship Foundation for financial support.

References 1) j. B. A, England, Techniques in nuclear structure physics, vol. 1 (Macmillan, London, 1974) pp. 193-202. 2) H. P. Morsch, T. Becker and W. Fitz, Nucl. Instr. and Meth. 68 (1969) 39. 3) j. B. A. England, Nucl. Instr. and Meth. 98 (1972) 237. 4) j. M. Nelson, private communication (1975). 5) E. A. Silverstein, Nucl. Instr. and Meth. 4 (1959) 53.