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A 90 cm scattering chamber with novel design features
NUCLEAR INSTRUMENTS AND METHODS IO4 (I972) I97-203; © NORTH-HOLLAND PUBLISHING CO.
A 90 em S C A T T E R I N G C H A M B E R W I T H N O V E L D E S I G N F E A T U R E S R. VERBRUGGEN and W. R. McMURRAY Southern Universities, Nuclear Institute, Faure, Cape, South Africa
Received 4 May 1972 A 90 cm scattering chamber has been designed and constructed for general use in a Van de Graaff accelerator laboratory. The design had to be simple, inexpensive and large enough to allow for charged particle time of flight experiments. To enable its construction in a small workshop it was made with flat aluminium plate in an octagonal shape. Unusual O-ring seals have proved their reliability over 3 y of operation. The chamber lid is opened easily on a hinge arrangement. The target and detector assembly
is supported separately on a rigid structure to maintain positional precision. A simple vacuum lock enables targets to be changed in minutes without breaking the main vacuum. Other features include external adjustment of radial and angular positions of detector supports on two arms, external adjustment of target position and angle and provision for automation of these functions. This low budget chamber has proved to be robust and simple to operate.
I. Introduction
180 ° and 0 ° for beam entry and exit also a c c o m m o d a t e the bearings and seals which allow for rotation o f the chamber about the beam axis. This was not necessary but is sometimes a convenience. Its elimination would make a very minor simplification in the construction o f the chamber. The chamber rotation bearings are m o u n t e d on an A-frame structure which rests on a base fixed to the floor. M o v e m e n t between the subframe and the base is provided to allow for the alignment o f the chamber with the beam from the accelerator. The essential elements in the chamber structure are shown in the simplified drawings in figs. l(a) and
The scattering chamber described in this report has been in use over a period o f three years. Both physics and chemistry groups have used the chamber for a variety o f experiments involving single detector 1-3) and multi-detector 4) techniques, correlated particle techniques 5) and particle identification using time o f flight 6) in studies of particle reactions and for purposes o f elemental and isotpic analysis. As the available workshops could not a c c o m m o d a t e work on a large circular chamber, it was decided to construct an octagonal chamber using flat plate o f aluminium alloy. To avoid the difficulties o f aluminium welding and the distortions resulting f r o m the welding process, the plates were assembled using O-ring type seals. The other consideration which has dictated the basic structural design o f the chamber is the flexibility of the cover plate material. Precise alignment of the detector system is ensured by m o u n t i n g the detectortarget assembly on a separate framework which is fixed relative to the beam positioning collimators.
2. Description of the chamber The basic structure o f the chamber consists o f an octagonal chamber ring 25 cm high and 90 cm across, bolted to a flat b o t t o m cover. The top cover or lid is identical to the b o t t o m cover but is detachable and hinged to the chamber ring. All these parts are made from aluminium plate, 25 m m thick. The eight sides of the octagon have ports o f 125 m m diameter centered on the reaction plane. Six ports are used for windows .and vacuum gauges or re-entrant insertions (e.g. o f bulky detectors outside the vacuum). The two ports at 197
1(b). The chamber lid can only be opened when the c h a m b e r is in a vertical position (see fig. 2) when the hinge pin is pushed into a location on the A-frame structure so as to support the entire weight o f the lid. The two detector arms inside the chamber are supported on a four legged structure shown in outline in fig. l(a) and in the p h o t o g r a p h in fig. 3. Additional detectors can be supported in fixed positions in any desired array on a disc attached inside the chamber lid. Twelve electrical feed-throughs are provided for connections to detectors or other apparatus, such as a beam pulse pick up for pulsed beam time o f flight measurements. The beam pulse pick up is supported in the entrance port o f the chamber. A gate valve m o u n t e d directly outside the chamber at the beam entrance port is used to isolate the chamber volume f r o m the beam line vacuum and a cold finger attached on the beam line side of the gate valve. Two beam line collimators, 75 cm apart, are fixed with respect to the scattering chamber body. Each collim a t o r housing accepts an electrically insulated and
198
R. V E R B R U G G E N
A N D W. R. M c M U R R A Y
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(a) Fig. 1 (a). A simplified drawing of the scattering chamber. The main components are numbered: 1, A-frame sub-structure; 2, vacuum pumping system; 3, outer shaft which supports the detector and target assembly and provides a sliding seal through the bottom cover of the chamber; 4, legs supporting the detector and target assembly; 5, upper arm for a detector support; 6, lower arm for a detector support; 7, Faraday cup; 8, hinge of the top cover or lid; 9, vacuum lock for target changing; I0, target holder; 11, gate valve; 12, cold finger; 13, insulated four position collimator. water cooled head with a choice of four different collimators selected by rotating the head to one of four positions fixed by a centering pin.
3. Vacuum system
I
J
The octagonal shape of the c h a m b e r ring a n d the problems associated with welding necessitated a n u n u s u a l form of O-ring seal. A T-type O-ring j o i n t has been developed for this purpose. The assembly of the c h a m b e r begins with the eight sections of the octagonal c h a m b e r ring. Each section is given a n O-ring groove o n one end a n d a fiat edge at the other. O-ring filler is placed in each groove a n d the eight sides are bolted together. The octagonal ring so formed is Fig. 1 (b). A d i a g r a m m a t i c presentation o f the detail in the
(b)
detector and target support assembly. The rotary movements of the detector arms and target holder are supported on thrust bearings. The positions of the O-ring vacuum seals are also shown.
90 c m S C A T T E R I N G
CHAMBER
WITH
NOVEL DESIGN
FEATURES
199
VERTICAL POSITION FOR ACCESS TO CHAMBER ROTATED POSITON
90cm SCATTERING CHAMBER
BASE PLATE FOR MOUNTING DETECTORS AT FIXED ANGLE
ONE OF SIX WINDOWS
SEPARATE~ EVACUATATED TARGET CHANGING CHAMBER
TARGET SELECTOR
-
-
HIGH VACUUM PUMP
FORE VACUUM PUMP
/
EXPLODED SECTION OF
"T"
TYPE "O" RING JOINT
Fig. 2. A simplified oblique angle drawing o f the scattering c h a m b e r to s h o w its structural s h a p e a n d the lid-open position. T h e insert below is a blow-up o f the T-seal between sections o f the octagon a n d the retainer or b o t t o m plates which f o r m the c h a m b e r box.
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R. VERBRUGGEN AND W. R. McMURRAY
bolted permanently to the bottom cover plate using a normal O-ring across the ends of the O-ring filler thus creating a T-type O-ring to O-ring joint at each corner of the octagon. A 12 mm thick retainer plate with a 870 mm diameter circular cut-out is used on the top side to complete the charfiber box and provide a sealing surface for the removable lid (top cover). The construction of the T-joint seal is shown in the insert in fig. 2. The 16 seals of this kind are held permanently between the bottom plate and the retaining plate under the chamber lid. No leaks have developed in three years of regular usage by staff and students. The same kind of seal has been employed by us in demountable vacuum parts used inside the accelerator tank at over-pressures of 220 lbs/sq.inch. Initial development work on these T-joints showed that an efficient vacuum seal was achieved when (1) a sufficiently hard grade of O-ring materials is used in
the octagonal side junctions, (2) a slight overpressure is ensured by making a shallow groove for the side junction O-ring material, (3) the octagonal side pieces are bolted together first and (4) the excess O-ring material at the ends of the side joints is machined or cut away with a sharp tool to leave it flush with the bottoms of the O-ring grooves for the other part of the T-joint. The pumping system on the scattering chamber consists of a 4" oil diffusion pump (pumping speed of 6001/s) separated from the chamber by a butterfly valve and a large high speed cold trap containing liquid nitrogen. An additional vapour pump is provided by the cold finger at the entrance port of the chamber [see fig. l(a)]. A vacuum of less than 10-4torr is obtained half an hour after start of pumping. The ultimate vacuum is about 10 - 6 tort. Both the rate at which the high vacuum is achieved and the ultimate
Fig. 3. A photograph of the underside of the chamber showing the four-legged structure which supports the detector and target assembly. Angle adjustments are performed by means of the slip-on knob which operates on the detector and target supports through worm gearing.
90 c m SCATTERING CHAMBER WITH NOVEL DESIGN FEATURES v a c u u m itself d e p e n d on the contents o f the c h a m b e r e.g. detectors, cables a n d insulation material. R e p l a cement o f the oil diffusion p u m p by a t u r b o m o l e c u l a r p u m p is being u n d e r t a k e n to i m p r o v e on the p u m p i n g reliability a n d reduce the possible c o n t a m i n a t i o n o f target a n d detectors b y oil vapours.
201
4. Detector supports and adjustments The two d e t e c t o r a r m s a n d the target h o l d e r are m o u n t e d on an a s s e m b l y which is s u p p o r t e d independently o f the b o t t o m plate o f the chamber. The s u p p o r t structure is shown in fig. l(a). M o r e detail o f the supp o r t assembly is p r o v i d e d in fig. l(b). A n O-ring
Fig. 4. The inside of the chamber showing the detector support arms with the screwed rods which are used to position the detector supports. The shape of the one support makes it possible to put both detector supports in a straight line at the same angle.
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R. VERBRUGGEN AND W. R. Mc MURRAY
between the support assembly a n d the c h a m b e r b o t t o m plate allows an inward m o v e m e n t of the b o t t o m plate when it is evacuated. The detector assembly is f o u n d to move by less t h a n 0.05 m m with respect to the beam axis. The detector arms a n d target holder can be indepen-
dently rotated via worm gears external to the chamber. The movements are transmitted through three hollow shafts rotating in a stationary outer shaft [ c o m p o n e n t 3 in fig. l(b)] which passes through the b o t t o m cover on a sliding seal. Fitted to each shaft is a 300 m m diameter disc divided in degrees. The angle settings
Fig. 5. A view of the chamber in position showing the adjustable A-frame structure on base with chamber lid hinge pin engaged, pumping system, Faraday cup, target holder support rod housing, three discs for detector and target angle setting, vacuum gauge, two windows, target changing chamber with line to fore-vacuum pump and at the top the detector radial positioning control knob.
90 c m S C A T T E R I N G C H A M B E R W I T H N O V E L D E S I G N F E A T U R E S are precisely reproducible to within the minimum scale divisions of 0.1 ° read on verniers on the three angle scales (refer to fig. 3). The design makes allowance for future remote adjustment of detector and target angles and positions. This is indicated by incremental motors dotted into fig. l(a). Adjustment of the radial position of the detector supports on each arm is effected by rotating the upper arm to 90 ° and the lower arm to 270 ° . At these positions an external knob can be pressed in to engage the end of a screw-rod (visible in fig. 4) which runs the length of the detector arm and engages a threaded hole in the detector support. External rotation of the control knob (see fig. 5) can thus change the radial position of the detector by an amount shown on a mechanical counter attached to the knob. Releasing the knob allows a spring to disengage the control knob from the detector arm which can then be repositioned at the required angle.
5. Target support and changing system The same hollow shaft which allows for the rotary movement of the target also enables target selection and changing by a sliding movement of the insulated rod supporting the target holder. An earth connection for the target can be supplied at an external socket. The target holders are 120 m m long and contain mountings for four separate targets. The target holder itself is mounted by a snap-on fitting onto its support rod. The target support rod is partly threaded as shown in figs. l(a) and l(b). A geared nut attached to the threaded part of the rod at a fixed position in the supporting shaft is used to adjust the vertical position of the target and give a reading of target position in m m on a mechanical counter. Targets or target holders can be changed without affecting the vacuum of the scattering chamber by moving the target up into a small vacuum lock situated
203
in the lid of the chamber (see fig. 5). An O-ring then isolates the lock volume from the chamber volume. The pressure in the lock is then raised slowly to atmospheric pressure through a fixed leak which is adjusted to protect very thin targets from damage. The can forming the lock volume is screwed off and the target or target holder removed and replaced by another as required. After replacing the can and pumping out the lock volume through the same leak, the target support is withdrawn back into the scattering chamber. 6. Conclusion The chamber described here has been used extensively over the last three years. It has proved to be easy to use and sturdy in adverse handling conditions by inexperienced workers. In the design and construction of the chamber, the emphasis has been placed on simplicity. It is believed that simple solutions have been found in the design of its component parts. The chamber provides all the functions available in more complex and more expensive instruments. The authors are grateful to their colleagues, most particularly Dr. J. J. Kritzinger, for helpful discussions of the chamber design and assistance with its installation which have contributed materially to its successful utilisation.
References 1) C. Olivier and M. Peisach, J. Radioanal. Chem. 5 (1970) 391. 2) R. Pretorius and P. P. Coetzee, J. Radioanal. Chem., in press. a) W. R. McMurray, L. M. Spitz, L J. van Heerden and W. J. Naude, S U N I Ann. Res. Reports Item 2.3.1 (1969) and Item 2.3.1 (1970). 4) W. R. McMurray, D. M. Holz, I. J. van Heerden and G. Wiechers, Z. Physik 247 (1971) 453. 5) R. Pretorius, Radiochem. Radioanal. Letters, in press. 6) E. Blignaut, W. R. McMurray and A. Bottega, Nucl. Instr. and Meth. 51 (1967) 102.
A 90 em S C A T T E R I N G C H A M B E R W I T H N O V E L D E S I G N F E A T U R E S R. VERBRUGGEN and W. R. McMURRAY Southern Universities, Nuclear Institute, Faure, Cape, South Africa
Received 4 May 1972 A 90 cm scattering chamber has been designed and constructed for general use in a Van de Graaff accelerator laboratory. The design had to be simple, inexpensive and large enough to allow for charged particle time of flight experiments. To enable its construction in a small workshop it was made with flat aluminium plate in an octagonal shape. Unusual O-ring seals have proved their reliability over 3 y of operation. The chamber lid is opened easily on a hinge arrangement. The target and detector assembly
is supported separately on a rigid structure to maintain positional precision. A simple vacuum lock enables targets to be changed in minutes without breaking the main vacuum. Other features include external adjustment of radial and angular positions of detector supports on two arms, external adjustment of target position and angle and provision for automation of these functions. This low budget chamber has proved to be robust and simple to operate.
I. Introduction
180 ° and 0 ° for beam entry and exit also a c c o m m o d a t e the bearings and seals which allow for rotation o f the chamber about the beam axis. This was not necessary but is sometimes a convenience. Its elimination would make a very minor simplification in the construction o f the chamber. The chamber rotation bearings are m o u n t e d on an A-frame structure which rests on a base fixed to the floor. M o v e m e n t between the subframe and the base is provided to allow for the alignment o f the chamber with the beam from the accelerator. The essential elements in the chamber structure are shown in the simplified drawings in figs. l(a) and
The scattering chamber described in this report has been in use over a period o f three years. Both physics and chemistry groups have used the chamber for a variety o f experiments involving single detector 1-3) and multi-detector 4) techniques, correlated particle techniques 5) and particle identification using time o f flight 6) in studies of particle reactions and for purposes o f elemental and isotpic analysis. As the available workshops could not a c c o m m o d a t e work on a large circular chamber, it was decided to construct an octagonal chamber using flat plate o f aluminium alloy. To avoid the difficulties o f aluminium welding and the distortions resulting f r o m the welding process, the plates were assembled using O-ring type seals. The other consideration which has dictated the basic structural design o f the chamber is the flexibility of the cover plate material. Precise alignment of the detector system is ensured by m o u n t i n g the detectortarget assembly on a separate framework which is fixed relative to the beam positioning collimators.
2. Description of the chamber The basic structure o f the chamber consists o f an octagonal chamber ring 25 cm high and 90 cm across, bolted to a flat b o t t o m cover. The top cover or lid is identical to the b o t t o m cover but is detachable and hinged to the chamber ring. All these parts are made from aluminium plate, 25 m m thick. The eight sides of the octagon have ports o f 125 m m diameter centered on the reaction plane. Six ports are used for windows .and vacuum gauges or re-entrant insertions (e.g. o f bulky detectors outside the vacuum). The two ports at 197
1(b). The chamber lid can only be opened when the c h a m b e r is in a vertical position (see fig. 2) when the hinge pin is pushed into a location on the A-frame structure so as to support the entire weight o f the lid. The two detector arms inside the chamber are supported on a four legged structure shown in outline in fig. l(a) and in the p h o t o g r a p h in fig. 3. Additional detectors can be supported in fixed positions in any desired array on a disc attached inside the chamber lid. Twelve electrical feed-throughs are provided for connections to detectors or other apparatus, such as a beam pulse pick up for pulsed beam time o f flight measurements. The beam pulse pick up is supported in the entrance port o f the chamber. A gate valve m o u n t e d directly outside the chamber at the beam entrance port is used to isolate the chamber volume f r o m the beam line vacuum and a cold finger attached on the beam line side of the gate valve. Two beam line collimators, 75 cm apart, are fixed with respect to the scattering chamber body. Each collim a t o r housing accepts an electrically insulated and
198
R. V E R B R U G G E N
A N D W. R. M c M U R R A Y
~\\\\\\\\\\\\\~.~ 11 I
2
J'l'[ 7
7
L [
j J
/ - - J
"\ ,\
/" /
/
/
/
/
/
/
/
/
(a) Fig. 1 (a). A simplified drawing of the scattering chamber. The main components are numbered: 1, A-frame sub-structure; 2, vacuum pumping system; 3, outer shaft which supports the detector and target assembly and provides a sliding seal through the bottom cover of the chamber; 4, legs supporting the detector and target assembly; 5, upper arm for a detector support; 6, lower arm for a detector support; 7, Faraday cup; 8, hinge of the top cover or lid; 9, vacuum lock for target changing; I0, target holder; 11, gate valve; 12, cold finger; 13, insulated four position collimator. water cooled head with a choice of four different collimators selected by rotating the head to one of four positions fixed by a centering pin.
3. Vacuum system
I
J
The octagonal shape of the c h a m b e r ring a n d the problems associated with welding necessitated a n u n u s u a l form of O-ring seal. A T-type O-ring j o i n t has been developed for this purpose. The assembly of the c h a m b e r begins with the eight sections of the octagonal c h a m b e r ring. Each section is given a n O-ring groove o n one end a n d a fiat edge at the other. O-ring filler is placed in each groove a n d the eight sides are bolted together. The octagonal ring so formed is Fig. 1 (b). A d i a g r a m m a t i c presentation o f the detail in the
(b)
detector and target support assembly. The rotary movements of the detector arms and target holder are supported on thrust bearings. The positions of the O-ring vacuum seals are also shown.
90 c m S C A T T E R I N G
CHAMBER
WITH
NOVEL DESIGN
FEATURES
199
VERTICAL POSITION FOR ACCESS TO CHAMBER ROTATED POSITON
90cm SCATTERING CHAMBER
BASE PLATE FOR MOUNTING DETECTORS AT FIXED ANGLE
ONE OF SIX WINDOWS
SEPARATE~ EVACUATATED TARGET CHANGING CHAMBER
TARGET SELECTOR
-
-
HIGH VACUUM PUMP
FORE VACUUM PUMP
/
EXPLODED SECTION OF
"T"
TYPE "O" RING JOINT
Fig. 2. A simplified oblique angle drawing o f the scattering c h a m b e r to s h o w its structural s h a p e a n d the lid-open position. T h e insert below is a blow-up o f the T-seal between sections o f the octagon a n d the retainer or b o t t o m plates which f o r m the c h a m b e r box.
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R. VERBRUGGEN AND W. R. McMURRAY
bolted permanently to the bottom cover plate using a normal O-ring across the ends of the O-ring filler thus creating a T-type O-ring to O-ring joint at each corner of the octagon. A 12 mm thick retainer plate with a 870 mm diameter circular cut-out is used on the top side to complete the charfiber box and provide a sealing surface for the removable lid (top cover). The construction of the T-joint seal is shown in the insert in fig. 2. The 16 seals of this kind are held permanently between the bottom plate and the retaining plate under the chamber lid. No leaks have developed in three years of regular usage by staff and students. The same kind of seal has been employed by us in demountable vacuum parts used inside the accelerator tank at over-pressures of 220 lbs/sq.inch. Initial development work on these T-joints showed that an efficient vacuum seal was achieved when (1) a sufficiently hard grade of O-ring materials is used in
the octagonal side junctions, (2) a slight overpressure is ensured by making a shallow groove for the side junction O-ring material, (3) the octagonal side pieces are bolted together first and (4) the excess O-ring material at the ends of the side joints is machined or cut away with a sharp tool to leave it flush with the bottoms of the O-ring grooves for the other part of the T-joint. The pumping system on the scattering chamber consists of a 4" oil diffusion pump (pumping speed of 6001/s) separated from the chamber by a butterfly valve and a large high speed cold trap containing liquid nitrogen. An additional vapour pump is provided by the cold finger at the entrance port of the chamber [see fig. l(a)]. A vacuum of less than 10-4torr is obtained half an hour after start of pumping. The ultimate vacuum is about 10 - 6 tort. Both the rate at which the high vacuum is achieved and the ultimate
Fig. 3. A photograph of the underside of the chamber showing the four-legged structure which supports the detector and target assembly. Angle adjustments are performed by means of the slip-on knob which operates on the detector and target supports through worm gearing.
90 c m SCATTERING CHAMBER WITH NOVEL DESIGN FEATURES v a c u u m itself d e p e n d on the contents o f the c h a m b e r e.g. detectors, cables a n d insulation material. R e p l a cement o f the oil diffusion p u m p by a t u r b o m o l e c u l a r p u m p is being u n d e r t a k e n to i m p r o v e on the p u m p i n g reliability a n d reduce the possible c o n t a m i n a t i o n o f target a n d detectors b y oil vapours.
201
4. Detector supports and adjustments The two d e t e c t o r a r m s a n d the target h o l d e r are m o u n t e d on an a s s e m b l y which is s u p p o r t e d independently o f the b o t t o m plate o f the chamber. The s u p p o r t structure is shown in fig. l(a). M o r e detail o f the supp o r t assembly is p r o v i d e d in fig. l(b). A n O-ring
Fig. 4. The inside of the chamber showing the detector support arms with the screwed rods which are used to position the detector supports. The shape of the one support makes it possible to put both detector supports in a straight line at the same angle.
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R. VERBRUGGEN AND W. R. Mc MURRAY
between the support assembly a n d the c h a m b e r b o t t o m plate allows an inward m o v e m e n t of the b o t t o m plate when it is evacuated. The detector assembly is f o u n d to move by less t h a n 0.05 m m with respect to the beam axis. The detector arms a n d target holder can be indepen-
dently rotated via worm gears external to the chamber. The movements are transmitted through three hollow shafts rotating in a stationary outer shaft [ c o m p o n e n t 3 in fig. l(b)] which passes through the b o t t o m cover on a sliding seal. Fitted to each shaft is a 300 m m diameter disc divided in degrees. The angle settings
Fig. 5. A view of the chamber in position showing the adjustable A-frame structure on base with chamber lid hinge pin engaged, pumping system, Faraday cup, target holder support rod housing, three discs for detector and target angle setting, vacuum gauge, two windows, target changing chamber with line to fore-vacuum pump and at the top the detector radial positioning control knob.
90 c m S C A T T E R I N G C H A M B E R W I T H N O V E L D E S I G N F E A T U R E S are precisely reproducible to within the minimum scale divisions of 0.1 ° read on verniers on the three angle scales (refer to fig. 3). The design makes allowance for future remote adjustment of detector and target angles and positions. This is indicated by incremental motors dotted into fig. l(a). Adjustment of the radial position of the detector supports on each arm is effected by rotating the upper arm to 90 ° and the lower arm to 270 ° . At these positions an external knob can be pressed in to engage the end of a screw-rod (visible in fig. 4) which runs the length of the detector arm and engages a threaded hole in the detector support. External rotation of the control knob (see fig. 5) can thus change the radial position of the detector by an amount shown on a mechanical counter attached to the knob. Releasing the knob allows a spring to disengage the control knob from the detector arm which can then be repositioned at the required angle.
5. Target support and changing system The same hollow shaft which allows for the rotary movement of the target also enables target selection and changing by a sliding movement of the insulated rod supporting the target holder. An earth connection for the target can be supplied at an external socket. The target holders are 120 m m long and contain mountings for four separate targets. The target holder itself is mounted by a snap-on fitting onto its support rod. The target support rod is partly threaded as shown in figs. l(a) and l(b). A geared nut attached to the threaded part of the rod at a fixed position in the supporting shaft is used to adjust the vertical position of the target and give a reading of target position in m m on a mechanical counter. Targets or target holders can be changed without affecting the vacuum of the scattering chamber by moving the target up into a small vacuum lock situated
203
in the lid of the chamber (see fig. 5). An O-ring then isolates the lock volume from the chamber volume. The pressure in the lock is then raised slowly to atmospheric pressure through a fixed leak which is adjusted to protect very thin targets from damage. The can forming the lock volume is screwed off and the target or target holder removed and replaced by another as required. After replacing the can and pumping out the lock volume through the same leak, the target support is withdrawn back into the scattering chamber. 6. Conclusion The chamber described here has been used extensively over the last three years. It has proved to be easy to use and sturdy in adverse handling conditions by inexperienced workers. In the design and construction of the chamber, the emphasis has been placed on simplicity. It is believed that simple solutions have been found in the design of its component parts. The chamber provides all the functions available in more complex and more expensive instruments. The authors are grateful to their colleagues, most particularly Dr. J. J. Kritzinger, for helpful discussions of the chamber design and assistance with its installation which have contributed materially to its successful utilisation.
References 1) C. Olivier and M. Peisach, J. Radioanal. Chem. 5 (1970) 391. 2) R. Pretorius and P. P. Coetzee, J. Radioanal. Chem., in press. a) W. R. McMurray, L. M. Spitz, L J. van Heerden and W. J. Naude, S U N I Ann. Res. Reports Item 2.3.1 (1969) and Item 2.3.1 (1970). 4) W. R. McMurray, D. M. Holz, I. J. van Heerden and G. Wiechers, Z. Physik 247 (1971) 453. 5) R. Pretorius, Radiochem. Radioanal. Letters, in press. 6) E. Blignaut, W. R. McMurray and A. Bottega, Nucl. Instr. and Meth. 51 (1967) 102.