A scattering chamber using semiconductor detectors

NUCLEAR INSHRUMENTS AYD Mf~THODS 47 (I967) 137-140; © NORTH-HOLLAND PU~ClSHIYG CO.

A SCATTERINGCHAMBERUSING SEMICONDUCTORDETECTORS M.A. H A Q U E , O . H . H O L Z E R , A. H O S S A I N and A. SABIR

Atomic Energy Centre, Dacca, Paki,stan and A.K.M. SIDDIQ

Department ~f Physics, Dacca Universit), Dacca, Pakistan Received 4 July 1966 The design and construction of a 24" dia. scattering chamber which will be used primarily for angular distribution studies of neutron induced reactions are presented. Five semiconductor detectors can be used at a time to detect charged nuclear reaction products at difl'erent angles and at different distances with respect

to the incident beam. The detectors are mounted on a rotating table which is graduated in degrees and is rotated externally. A liquid air trap is used in the vacuum system to prevent deposition of p u m p oil on thc surfacc of the detectors.

1. Introduction

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Semiconductor detectors and improved techniques of ~-particles discrimination in emulsion"2), and the availability of intense and inexpensive sources of monoenergetic neutrons, have made it possible to investigate spectral and angular distributions of the charged particles in neutron induced reactions. The study of the energy and angular distributions in these reactions throw light on the mechanism governing these interactions. A number of scattering chambers "s-5) has been designed for this purpose with solid state detectors. The availability of a 3 MeV Van de Graaff accelerator created an interest in some experimental nuclear physicists of the Dacca Atomic Energy Centre and Dacca University to construct a scattering chamber to study neutron induced reactions. The main purpose of the design of the chamber is to study the angular distribution of charged particles in neutron induced reaction, but it can be useful for many other experiments. The chamber is simple to operate which will facilitate its use. The p u m p system °) is suitabl} designed to provide low pressure, The increase in size of a chamber no doubt improves the angular distribution studies, but poses several mounting and alignment problems which are not important in small chamber design. As the wire from the solid state detector to preamplifier should be as short as possible to reduce the capacitance due to cables, the height of the chamber becomes important for the signal output from the detectors, 2.

Descriptionof the chamber

The assembled chamber on its stand together with the pumping system, the vacuum measuring equipments, the plate driving system and the connected preamplifier

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Fig. I. Assembled chamber with stand, the plate driving system, charge sensitive preamplifier and amplifier.

is shown in fig. 1. The internal diameter of the chamber is 23" and the height 18". The side wall is ½" thick and the top and the bottom lids are made of 4" thick brass sheet. There are two ports 2~-~" i.d. and I" i.d. respectively in the chamber wall opposite to each other for beam entry and exit as shown in fig. 2. As we do not need very high vacuum we used mylar film (or a thin aluminium foil) arrangements to separate the chamber from the high vacuum accelerator tube. The mylar film (or Al-foil) arrangements are simple. There is a 2~" long and 2~6-6"i.d. brass tube. One end of the tube is connected to the beam entry port (left side) of the chamber with O-ring arrangement and on the other end, mylar film or Al-foil) is put. On the beam exit port (right side) a Faraday cup is fitted with O-ring for alignment purposes and beam current measurement. 137

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JCJc c o n n e c ~ l o n

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Fig. 2. Cross sectional diagram of the scaltcring c h a m b e r with target holder.

There are two more ports of 2" i.d. opposite to each other in the chamber wall. They are view ports and are covered with ~" thick and 4" dia. perspex glass flanges with O-rings. Later, they will be provided with closedcircuit TV camera arrangements, One rotating plate of 19" dia. and 3,, thickness is provided for inserting the detector mounts as shown in fig. 3. The table is machine graduated at an interval of 5':' and is held by deep grove ball bearings and screwed to the bottom lid. It can be rotated both ways with reasonably adjusted speed by a small motor which is fitted inside the chamber touching the plate and is

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driven from outside by dry batery celts. When the motor is switched off. the table can be firmly locked into position by brake. The position of the turntable (i.e. the detector mount position) can be read easily from the dials through perspex glass flanges. With proper vernier arrangements good precision can bc obtained. Five grooves each 7" long, ~" wide and separated by 25' are provided for inserting the detector mounts. Metric scales are placed along each groove so that the detectors may be set at a given distance from the target. The detector mounts can be easily slided

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Fig. 3. P h o t o g r a p h or" the inside of c h a m b e r showing rotating plate with detectors and deteclor mounts,

Fig. 4. Picture of underside o f top lid showing target holder and target position control.

A SCATTERING

CHAMBER

USING SEMICONDUCTOR

inside the groove and kept tight firmly in any position, The detector m o u n t is L-type and holds the detector firmly. The top lid o f the scattering c h a m b e r a c c o m m o d a t e s the target mounting arrangement and special hermetically sealed Microdot connectors with O-rings for access through the chamber. The special short cables from the detectors are connected to ends which are in vacuum. This allows the lead wire to be rotated along with the detectors and prevents mechanical interference from the wires. The other ends of the Microdot connectors which are outside are connected to preamplifiers with short cables. The target mounting system is shown in fig. 4. This. consists • of7Tg i ~,, I o n g a n d 1-~" dia. hollow brass tube through which passes a rod. The upper end of the rod is threaded and the lower end is soldered to a long piece of bellow at the lower end of the brass tube. The other end o f the bellow is attached to a rectangular aluminium piece of 1" × -i~" dimension. This aluminiurn piece serves the purpose of target placing whose height can be varied by a special screw system at the threaded part of the rod. The

DETECTORS

scale is placed on the target rod which allows tile target to be positioned in the centre of the chamber. The size of the sensitive target area is ½" diameter. With the size o f the bellow available two targets can be arranged. But for the time being we have the facility of only one. When one target is damaged it is replaced by another target by breaking the vacuum. The target mounting system is positioned on the top lid through a hole of 1 ~" dia. with O-ring. The chamber is mounted on an iron block with four large set screws. 3. Component list The c o m p o n e n t list for the line drawings as given in fig. 2 a r e : 1. Mylar film (or Al-foil) tube, 2. Beam entry in the c h a m b e r wall, 3. Beam exit port, 4. F a r a d a y cup, 5. Viewing port, 6. High vacuurn pumping port, 7. Rotating plate (aluminium),

To Preamp

ORTEC 101 Preampliiier

To Vac pump Am 24~ ~.- source

S6' ORTEC detector I

:

--(D-;

3RTEC 201 Amp(itier Biased Output

512 Multichannel ........ Anolyser

139

Direct Output

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Fig. 5. High resolution arrangement.

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Scope

M.A. HAQU[- et al.

140

8. Ball bearing a r r a n g e m e n t for rotating plate, 9. 10. II. 12. 13.

Target m o u n t i n g groove, Tile target control knob, The target position, Target rod, Height a d j u s t m e n t for target

position control

assembly, 14. M o t o r for driving the plate (lig. 3), 15. The m o t o r driving assembly (fig. 1), 16. Circular scale in degree on tile rotating plate. We have been able to achieve a v a c u u m of ~ 10 '* Torr which is sufficient for the nature of work we intend to carry on. 4. Operational features The specilic operational features of the scattering c h a m b e r are summarised below:

5. Resolution of Ortec solid state detector The resohltion of Ortec surface barrier solid state detectors was o b t a i n e d by the set tip as shown in fig. 5. 2alAn1 was used for :~-source (5.477 MeV). The resohltion measurements were done in a wlcutnl) of ~ 10 3 T o r t . The following procedures determine the resohltion.

were

carried

out

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I. The main ampl. gain, post ampl. gain and the post ampl. bias level were set at 1,2 and 10 respectively, so that the ~-particle peak was within the pulse height analyser limits. 2. The pulser was calibrated with the help of tile 3(-source.

1. Precise po,~'ilion o f lhe target." The a r r a n g e m e n t provides facility to place the target accurately in the

3. The pulse height analyser was calibrated in key per channel with the pulser. From this calibration wc also obtained the zero position of tile multichannel analyser (512-channels).

centre of the chamber, so that the incident beam can strike the central area of the target,

4. The alpha particle resolution was then obtained by measuring tile full width at half maxinlum of tile

2. Precise position ~?1 dewctors. A r r a n g e m e n t is provided on the rotating table so that it can hold live detectors at a time. The detector positions tire variable with respect to the target position and tingle of tile beam and the position can accurately be read by scales placed along the grooves in which the d e t e c t o r - m o u n t s slide smoothly, 3. Ea,s3' to #'olale delgclors: The detectors can be rotated very easily from 0 to any desired angle with respect to the beam. The rotation is done externally by a small m o t o r attached to the inside c h a m b e r wall and touching firmly the rotating table.

4. Precise position of the chamber a b o u t the beam tube. 5. A k~waday cup is provided for the accurtlte beam current measurements.

spectrum peak. The resolution obtained was ~-~0.7".,. The authors pay sincere thanks to the members of the United Engineering, Dacca for the casting and machining works ol" the chamber. They thank the workshop staff" of tim Centre who helped in the constrtlction of this chamber. They also thank Dr. J.C. Dutt and Mr. M.A. Robert ( C E R N ) for construtting the vactltHll system.

References ~) w. Patzak and H. Vonach, Nucl. Physics 39 (1962) 263. e) V. Paic and M. Pate, Nucl. Instr. and Moth. 26 gl964) 42. :q E.J. Feldlet al.,Nucl, lnstr, and Mcth. 22(19631 333. 4) E.J. Feldl et al., Nucl. Instr. and Meth. 28 (19641 309. r>) R.W. Zurmuhle, Nucl. Instr. ,and Meth. 36 (1965} 16g. % J.C. Dutt ct al., A dual-purpose unit for high ;aCtlUnl pumping system and leak detection in '~acuun~ chamber, AECD./EP/10.