- Email: [email protected]
A scattering chamber using junction counters
NUCLEAR
INSTRUMENTS
AND
METHODS
22 (1963)
333-338;
NORTH-HOLLAND
PUBLISHING
CO.
A SCATTERING CHAMBER USING JUNCTION COUNTERSt E. J. F E L D L , J. R. M E R I W E T H E R ,
G. R. C H O P P I N a n d J. D. F O X
Departments of Chemistry and Physics, Florida State University, Tallahassee, Florida R e c e i v e d 11 J u n e 1962
T h e specifications a n d characteristics of a v e r s a t i l e s c a t t e r i n g c h a m b e r are presented. T h e c h a m b e r is designed for use w i t h solid s t a t e detectors. I t b e c a m e n e c e s s a r y in t h e course of a s t u d y of t h e fission of lJ23s a n d T h 2sa b y a p r o t o n b e a m f r o m t h e F l o r i d a S t a t e U n i v e r s i t y T a n d e m V a n de G r a a f f to design a small s c a t t e r i n g c h a m b e r in which solid s t a t e j u n c t i o n counters could be positioned precisely a t v a r i o u s angles a r o u n d t h e t a r g e t . T h e following f e a t u r e s were desirable in t h e s c a t t e r i n g c h a m b e r : (a) precise positioning of t h e t a r g e t ; (b) precise positioning of counters b o t h w i t h respect to t h e d i s t a n c e f r o m t h e t a r g e t
a n d t h e angle to t h e b e a m ; (c) ease of r o t a t i o n of t h e counters to different angles; (d) provision for b o t h m o v a b l e and fixed counters ; (e) precise positioning of t h e c h a m b e r a b o u t t h e b e a m t u b e ; (f) precise defining slits for b o t h t h e b e a m a n d t h e particles to be counted ; (g) f r e e d o m f r o m trailing lead wires as t h e counters are r o t a t e d ; (h) a F a r a d a y cup which p r o v i d e d a c c u r a t e m e a s u r e m e n t s of t h e b e a m current. T h e c h a m b e r which waa designed possesses all of these features a n d has p r o v e n to be of sufficient usefulness to w a r r a n t j o u r n a l descriptiofi.
Fig. 1. S c a t t e r i n g c h a m b e r , t h e t o p lid is r e m o v e d to s h o w j u n c t i o n c o u n t e r m o u n t s in position.
Figures 1 and 2 show the chamber both photographically and ill schematic form. For the most part the chamber is constructed of aluminum because of the ease of machining and the low density 1" S u p p o r t e d in p a r t b y t h e A t o l n i c E n e r g y C o m m i s s i o n a n d t h e A i r Force Office of Scintific Research. 333
of aluminum. The light weight is convenient for alignment and for permitting reduction in the size of the mounting supports. The tmiquness of this chamber lies in the method of rotation of the counter about the target. Most previous scattering chambers have used either a
334
E. J. F E L D L et al.
@
Fig. 2. A. Ceramic lead through to Faraday cup.
H. Chamber mounting block.
]3. Tantalum F a r a d a y cup.
I. Teflon bearing buttons (3).
Q. Tantalum defining slits.
C. Electron suppressor ring.
J. Teflon bearing.
R. Removable slit assembly.
P. Tantalum anti-scattering slit.
D. Glass lead through to suppressor ring.
K. Port for UG-657/U connector (6).
S. Ball joint for alignment.
E. Foil holder (to use chamber as a gas cell).
L. Black nylon insualtor.
T. "Dependex" couping to beam tube.
M. Vernier scale.
U. UG-657/U connectors (16).
1% Leadthrough to allow biasing of the foil.
N. Handle for rotating counters.
V. Main pump-out port.
G. Lucite insulators around foil holder.
O. Pyrex viewing window.
W. F a r a d a y cup pump-out port.
Fig. 3. Side view of chamber housing showing side windows and verniers.
A SCATTERING
(/~t~ x
CHAMBER
USING
/f~Screws
JUNCTION
COUNTERS
335
10-32(6)
COUNTER MOUNT FOR MOVEABLEPLATE
3
° ,l{~ i I
Vczg~//A
~
II, i
f
.i degrees are etchedon outerdla of disk0° is ¢enlereao~ones{o~_
I-' -T4 "
~
~-
,_.~ ~.7~,..
I" I,Ihl
~egrees !1 ,ii
7620 . . . . . . . .
111~6 /
,
- Z: ....................... c .
-
~
FIXED COUNTER MOUNT \ ~
< - 6 , \ ~
scales,center of circle=Oc.m.
t"2 drill,(6) "~_~ " drill for screw6-4~',~1 5- drill (61
\\\ \
,
Fig. 5. S p r i n g clip to hold slits in position. ~" hole for m o u n t i n g slits. Hole for m o u n t i n g t r a n s i s t o r socket. "Dove tail". T a n t a l u m slit. " D o v e t a i l " tail t r a c k for v a r y i n g t h e c o u n t e r ' s distance f r o m t h e t a r g e t . G. " D o v e t a i l " for m o u n t i n g onto c h a m ber wall ; radius - - 4".
A. ]3. C. D. E. F.
Dove!oil slots 240 deep (6) Fig. 4. R o t a t i n g plate assembly.
rotating arm assembly or a full rotating lid. The first method suffers from a lack of accuracy in the angular position while the second suffers from difficnlty in the rotation of the counter under vacuum. F r o m the detailed views in figs. 2 and 4, it caE be seeI1 that the rotating assembly of our chamber utilizes a large rotating plate from which the angular position is read directly, However, it is not required that this large surface area be moved against a vacuum seal. An angular scale is stamped on the side of the rotating plate and the readings are taken through lucite windows
into which vernier scales have been cut. Parallax is avoided by having the vernier cut on both sides of each window (fig. 3). The angular position Call be reaG to within a quarter of a degree. The vacuum seal around the hub of the plate is made with an "O-ring" seal. The rotating assembly rides against and is held in position by teflon thrust bearings (I and J ill fig. 2). The bearing pressure is controlled by alternate pushing and pulling screws on the ring just above bearing j . The rotating assembly is installed in both the top and the bottom plates of the chamber.
336
E.J.
F E L D L e$ al.
Six equally spaced 60 ° " d o v e t a i l " grooves are cut into the r o t a t i n g plates in which counter m o u n t s m a y be placed. The " d o v e tail" groove is used because of t h e accuracy a n d reproducability with w h i c h t h e counter m o u n t s m a y be positioned. Metric scales are placed along each groove so t h a t the counters m a y be set a given distance from t h e center of t h e c h a m b e r . To alIow electrical lead t h r o u g h to t h e counters, six UG-657/U BNC connectors, w h i c h are provided w i t h "O-rings," are placed t h r o u g h t h e base of b o t h of t h e r o t a t i n g hubs. This allows t h e lead wires to be r o t a t e d along w i t h t h e counters a n d p r e v e n t s mechanical interference from t h e wires. Fig. ~ shows t h e r o t a t i n g a s s e m b l y m o u n t e d t h r o u g h one of t h e plates a n d the r o t a t i n g counter m o u n t s in position. The counter m o u n t s h o w n to t h e left is a fixed counter m o u n t which can be placed in a " d o v e t a i l " groove in t h e c h a m b e r wall. Additional BNC connectors are m o u n t e d i n the wall for connection to these fixed counters. Detail views of the counter m o u n t s m a y
be seen i n fig. 6. A detailed view of t h e c h a m b e r h o u s i n g showing t h e " d o v e tail" groove ill tile wall is s h o w n in fig. 7. Various types of t a r g e t m o u n t s call be placed into t h e c h a m b e r a n d held in one or b o t h of the sliding " O - r i n g " seals at t h e top a n d b o t t o m of tile chamber. These seMs are cut t h r o u g h black n y l o n so t h a t t h e t a r g e t rod is insulated from t h e rest of the chamber. I n practice a fixed plate above or below the c h a m b e r is required to hold t h e target rod in a fixed position w h e n the counters are r o t a t e d a n d to allow accurate positioning of t h e target. A view of the c h a m b e r with a target m o u n t in position is shown in fig. 8. Since the c h a m b e r was p u t into operation, a b r a c k e t has been a d d e d to t h e lower lid of t h e chamber, in w h i c h t h e target rod m a y be locked in place. Scales were placed on the target rod which allow t h e t a r g e t to be positioned ill t h e center of t h e c h a m b e r a n d its angle with respect to the b e a m fixed. Before e n t e r i n g the c h a m b e r proper, the b e a m of
Fig. 6. top lid assembly.
A SCATTERING
CHAMBER
USING
II
I~
,r
;
337
",,"..,K
I I / II
\ \~\ \
_kfi/
- -
["
COUNTERS
mounting brackets are not shown but merely consist of a frame in which the set screws are mounted. A quartz plate with a hole in its center slightly larger t h a n that of the first slit is mounted just before this slit. The hole in the quartz is at the center of the radius of a ball joint (S in fig. 2). I n order to align the chamber, the beam is first centered on the quartz using the first set of aligning set screws. The quartz m a y be viewed through a pyrex window sealed in the beam tube with Apiezon W (black wax). The chamber is then moved about the ball joint until the center line of the chamber lies along the beam axis. This position is determined either b y using a quartz plate in the Faraday cup position or b y maximizing the beam current on the Faraday cup.
bombarding particles passes through a pair of defining slits approximately eight inches apart in the entrance tube. The entire slit assembly is made of stainless steel for rigidity and is able to be removed through the chamber. This increases the ease of changing slits. Holes are placed ill the slit assembly to allow free passage of air during evacuation. Just after the last defining slit and mechanically attached to it is a slightly larger antiscattering slit. This last slit prevents beam particles which are scattered b y the defining slits from entering the chamber. Tile chamber is mounted using four large set screws against block H (fig. 2) and on another block prior to the chamber. This block is not shown on fig. 2, but can be seen in either fig. 1 or fig. 8. The
/ ,?" / ,;/
JUNCTION
7. 6 4 2
3500
2.~0 ~
L.-J
Fig. 7. C h a m b e r housing.
Ill
ii
338
E.J.
F E L D L e~ al.
After leaving the chamber the beam reaches the Faraday cup (]3) which is made of tantalum and is insulated from the rest of the chamber by a ceramic insulator (A). To prevent the ejec-
plate put in its place for chamber alignment. In order to utilize the chamber for gas scattering, thin nickel foils (or other suitable foils) m a y be placed at E and in a second holder prior '3J:'
Fig. 8. Top view of chamber with target bolder in position.
tion of electrons from the Faraday cup, which would cause erroneous readings of the beam current, an electron suppressing ring (C) is biased negatively with respect to the Faraday cup. The entire Faraday cup call be removed and a quartz
to the chamber. Provision has been made to place a bias voltage on the foil if this is found necessary. The authors wish to t h a n k Dr. G. M. Temmer for his helpful suggestions.
INSTRUMENTS
AND
METHODS
22 (1963)
333-338;
NORTH-HOLLAND
PUBLISHING
CO.
A SCATTERING CHAMBER USING JUNCTION COUNTERSt E. J. F E L D L , J. R. M E R I W E T H E R ,
G. R. C H O P P I N a n d J. D. F O X
Departments of Chemistry and Physics, Florida State University, Tallahassee, Florida R e c e i v e d 11 J u n e 1962
T h e specifications a n d characteristics of a v e r s a t i l e s c a t t e r i n g c h a m b e r are presented. T h e c h a m b e r is designed for use w i t h solid s t a t e detectors. I t b e c a m e n e c e s s a r y in t h e course of a s t u d y of t h e fission of lJ23s a n d T h 2sa b y a p r o t o n b e a m f r o m t h e F l o r i d a S t a t e U n i v e r s i t y T a n d e m V a n de G r a a f f to design a small s c a t t e r i n g c h a m b e r in which solid s t a t e j u n c t i o n counters could be positioned precisely a t v a r i o u s angles a r o u n d t h e t a r g e t . T h e following f e a t u r e s were desirable in t h e s c a t t e r i n g c h a m b e r : (a) precise positioning of t h e t a r g e t ; (b) precise positioning of counters b o t h w i t h respect to t h e d i s t a n c e f r o m t h e t a r g e t
a n d t h e angle to t h e b e a m ; (c) ease of r o t a t i o n of t h e counters to different angles; (d) provision for b o t h m o v a b l e and fixed counters ; (e) precise positioning of t h e c h a m b e r a b o u t t h e b e a m t u b e ; (f) precise defining slits for b o t h t h e b e a m a n d t h e particles to be counted ; (g) f r e e d o m f r o m trailing lead wires as t h e counters are r o t a t e d ; (h) a F a r a d a y cup which p r o v i d e d a c c u r a t e m e a s u r e m e n t s of t h e b e a m current. T h e c h a m b e r which waa designed possesses all of these features a n d has p r o v e n to be of sufficient usefulness to w a r r a n t j o u r n a l descriptiofi.
Fig. 1. S c a t t e r i n g c h a m b e r , t h e t o p lid is r e m o v e d to s h o w j u n c t i o n c o u n t e r m o u n t s in position.
Figures 1 and 2 show the chamber both photographically and ill schematic form. For the most part the chamber is constructed of aluminum because of the ease of machining and the low density 1" S u p p o r t e d in p a r t b y t h e A t o l n i c E n e r g y C o m m i s s i o n a n d t h e A i r Force Office of Scintific Research. 333
of aluminum. The light weight is convenient for alignment and for permitting reduction in the size of the mounting supports. The tmiquness of this chamber lies in the method of rotation of the counter about the target. Most previous scattering chambers have used either a
334
E. J. F E L D L et al.
@
Fig. 2. A. Ceramic lead through to Faraday cup.
H. Chamber mounting block.
]3. Tantalum F a r a d a y cup.
I. Teflon bearing buttons (3).
Q. Tantalum defining slits.
C. Electron suppressor ring.
J. Teflon bearing.
R. Removable slit assembly.
P. Tantalum anti-scattering slit.
D. Glass lead through to suppressor ring.
K. Port for UG-657/U connector (6).
S. Ball joint for alignment.
E. Foil holder (to use chamber as a gas cell).
L. Black nylon insualtor.
T. "Dependex" couping to beam tube.
M. Vernier scale.
U. UG-657/U connectors (16).
1% Leadthrough to allow biasing of the foil.
N. Handle for rotating counters.
V. Main pump-out port.
G. Lucite insulators around foil holder.
O. Pyrex viewing window.
W. F a r a d a y cup pump-out port.
Fig. 3. Side view of chamber housing showing side windows and verniers.
A SCATTERING
(/~t~ x
CHAMBER
USING
/f~Screws
JUNCTION
COUNTERS
335
10-32(6)
COUNTER MOUNT FOR MOVEABLEPLATE
3
° ,l{~ i I
Vczg~//A
~
II, i
f
.i degrees are etchedon outerdla of disk0° is ¢enlereao~ones{o~_
I-' -T4 "
~
~-
,_.~ ~.7~,..
I" I,Ihl
~egrees !1 ,ii
7620 . . . . . . . .
111~6 /
,
- Z: ....................... c .
-
~
FIXED COUNTER MOUNT \ ~
< - 6 , \ ~
scales,center of circle=Oc.m.
t"2 drill,(6) "~_~ " drill for screw6-4~',~1 5- drill (61
\\\ \
,
Fig. 5. S p r i n g clip to hold slits in position. ~" hole for m o u n t i n g slits. Hole for m o u n t i n g t r a n s i s t o r socket. "Dove tail". T a n t a l u m slit. " D o v e t a i l " tail t r a c k for v a r y i n g t h e c o u n t e r ' s distance f r o m t h e t a r g e t . G. " D o v e t a i l " for m o u n t i n g onto c h a m ber wall ; radius - - 4".
A. ]3. C. D. E. F.
Dove!oil slots 240 deep (6) Fig. 4. R o t a t i n g plate assembly.
rotating arm assembly or a full rotating lid. The first method suffers from a lack of accuracy in the angular position while the second suffers from difficnlty in the rotation of the counter under vacuum. F r o m the detailed views in figs. 2 and 4, it caE be seeI1 that the rotating assembly of our chamber utilizes a large rotating plate from which the angular position is read directly, However, it is not required that this large surface area be moved against a vacuum seal. An angular scale is stamped on the side of the rotating plate and the readings are taken through lucite windows
into which vernier scales have been cut. Parallax is avoided by having the vernier cut on both sides of each window (fig. 3). The angular position Call be reaG to within a quarter of a degree. The vacuum seal around the hub of the plate is made with an "O-ring" seal. The rotating assembly rides against and is held in position by teflon thrust bearings (I and J ill fig. 2). The bearing pressure is controlled by alternate pushing and pulling screws on the ring just above bearing j . The rotating assembly is installed in both the top and the bottom plates of the chamber.
336
E.J.
F E L D L e$ al.
Six equally spaced 60 ° " d o v e t a i l " grooves are cut into the r o t a t i n g plates in which counter m o u n t s m a y be placed. The " d o v e tail" groove is used because of t h e accuracy a n d reproducability with w h i c h t h e counter m o u n t s m a y be positioned. Metric scales are placed along each groove so t h a t the counters m a y be set a given distance from t h e center of t h e c h a m b e r . To alIow electrical lead t h r o u g h to t h e counters, six UG-657/U BNC connectors, w h i c h are provided w i t h "O-rings," are placed t h r o u g h t h e base of b o t h of t h e r o t a t i n g hubs. This allows t h e lead wires to be r o t a t e d along w i t h t h e counters a n d p r e v e n t s mechanical interference from t h e wires. Fig. ~ shows t h e r o t a t i n g a s s e m b l y m o u n t e d t h r o u g h one of t h e plates a n d the r o t a t i n g counter m o u n t s in position. The counter m o u n t s h o w n to t h e left is a fixed counter m o u n t which can be placed in a " d o v e t a i l " groove in t h e c h a m b e r wall. Additional BNC connectors are m o u n t e d i n the wall for connection to these fixed counters. Detail views of the counter m o u n t s m a y
be seen i n fig. 6. A detailed view of t h e c h a m b e r h o u s i n g showing t h e " d o v e tail" groove ill tile wall is s h o w n in fig. 7. Various types of t a r g e t m o u n t s call be placed into t h e c h a m b e r a n d held in one or b o t h of the sliding " O - r i n g " seals at t h e top a n d b o t t o m of tile chamber. These seMs are cut t h r o u g h black n y l o n so t h a t t h e t a r g e t rod is insulated from t h e rest of the chamber. I n practice a fixed plate above or below the c h a m b e r is required to hold t h e target rod in a fixed position w h e n the counters are r o t a t e d a n d to allow accurate positioning of t h e target. A view of the c h a m b e r with a target m o u n t in position is shown in fig. 8. Since the c h a m b e r was p u t into operation, a b r a c k e t has been a d d e d to t h e lower lid of t h e chamber, in w h i c h t h e target rod m a y be locked in place. Scales were placed on the target rod which allow t h e t a r g e t to be positioned ill t h e center of t h e c h a m b e r a n d its angle with respect to the b e a m fixed. Before e n t e r i n g the c h a m b e r proper, the b e a m of
Fig. 6. top lid assembly.
A SCATTERING
CHAMBER
USING
II
I~
,r
;
337
",,"..,K
I I / II
\ \~\ \
_kfi/
- -
["
COUNTERS
mounting brackets are not shown but merely consist of a frame in which the set screws are mounted. A quartz plate with a hole in its center slightly larger t h a n that of the first slit is mounted just before this slit. The hole in the quartz is at the center of the radius of a ball joint (S in fig. 2). I n order to align the chamber, the beam is first centered on the quartz using the first set of aligning set screws. The quartz m a y be viewed through a pyrex window sealed in the beam tube with Apiezon W (black wax). The chamber is then moved about the ball joint until the center line of the chamber lies along the beam axis. This position is determined either b y using a quartz plate in the Faraday cup position or b y maximizing the beam current on the Faraday cup.
bombarding particles passes through a pair of defining slits approximately eight inches apart in the entrance tube. The entire slit assembly is made of stainless steel for rigidity and is able to be removed through the chamber. This increases the ease of changing slits. Holes are placed ill the slit assembly to allow free passage of air during evacuation. Just after the last defining slit and mechanically attached to it is a slightly larger antiscattering slit. This last slit prevents beam particles which are scattered b y the defining slits from entering the chamber. Tile chamber is mounted using four large set screws against block H (fig. 2) and on another block prior to the chamber. This block is not shown on fig. 2, but can be seen in either fig. 1 or fig. 8. The
/ ,?" / ,;/
JUNCTION
7. 6 4 2
3500
2.~0 ~
L.-J
Fig. 7. C h a m b e r housing.
Ill
ii
338
E.J.
F E L D L e~ al.
After leaving the chamber the beam reaches the Faraday cup (]3) which is made of tantalum and is insulated from the rest of the chamber by a ceramic insulator (A). To prevent the ejec-
plate put in its place for chamber alignment. In order to utilize the chamber for gas scattering, thin nickel foils (or other suitable foils) m a y be placed at E and in a second holder prior '3J:'
Fig. 8. Top view of chamber with target bolder in position.
tion of electrons from the Faraday cup, which would cause erroneous readings of the beam current, an electron suppressing ring (C) is biased negatively with respect to the Faraday cup. The entire Faraday cup call be removed and a quartz
to the chamber. Provision has been made to place a bias voltage on the foil if this is found necessary. The authors wish to t h a n k Dr. G. M. Temmer for his helpful suggestions.