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A simple versatile cryostat for encapsulated detectors
NUCLEAR
INSTRUMENTS
AND
METHODS
72 (1969)
210-212;
©
NORTH-HOLLAND
PUBLISHING
CO.
A S I M P L E VERSATILE CRYOSTAT FOR ENCAPSULATED D E T E C T O R S R. E. T U R C O T T E
and R. B. M O O R E
Foster Radiation Laboratory, McGill University, Montreal, Canada Received 19 March 1969 A simple, inexpensive a n d versatile cryostat for encapsulated detectors is described. T h e cryostat operates at liquid nitrogen temperatures with molecular sieve p u m p i n g . S t a n d a r d perfor-
m a n c e was: pressure less t h a n 5 x 10-5 turn, t e m p e r a t u r e of detector less t h a n 80"K, c o n s u m p t i o n of liquid nitrogen a b o u t 1 liter per day.
1. Introduction
along its vertical section and another set along its horizontal section. For assembly purposes, the horizontal section of the cold finger is detachable from the vertical section and art opening directly above the latter is required to remove the bolt fastening the two sections. With a simple adapter, this opening serves as a port for a discharge type ion vacuum gauge (Consolidated Vacuurn Corporation) which can monitor the pressure, or be used as a clean-up pump whenever there is an accumulation of gas which has not been adsorbed by the Zeolite. (Zeolite does not adsorb all types of molecules equally well.) This port can also be used to attach a diffusion pump for initial testing purposes. A standard 3.8cm dia. copper plumbing Tee is used to join the external horizontal and vertical sections and provide the vacuum port. A small rough-out valve is placed on the Tee for pumping down immediately after a detector has been installed. A length of standard copper pipe (3.8 cm o.d.) is used for the external horizontal section. The nominal length of this section is 38 cm, but satisfactory performance is predicted for lengths at least twice as great. The detector end of the cold finger is sealed with a thin (0.25 ram) polished aluminum cap with an axial O-ring. The aluminum cap is normally threaded onto the end of the horizontal section, but in one design atmospheric pressure alone holds the cap in place.
The Ge(Li) diode which has revolutionized gamma ray spectroscopy is generally supplied by commercial manufacturers ready mounted in its own cryostat assembly. This is certainly the best way to guarantee that large expensive detectors will operate reliably soon after delivery. However for smaller detectors used in experimental setups, such as with cooled FET input stages or in high radiation areas near targets, there is a need for a versatile cryostat design which allows easy access for servicing or changing components. In most designs 1"3) this is not possible. Such a simple inexpensive cryostat for canned planar detectors has been built and tested in use in this laboratory for the past two years4), for work where frequent changes of detector are necessary. 2. Construction
The basic shape of the cryostat is that of two concentric cylinders in an inverted " L " - s h a p e d configuration, with the vertical section immersed in a dewar of liquid nitrogen (LN2) (fig. 1). The vacuum in the space between the cold finger and the outer shield is maintained by using a porous molecular sieve (Linde "Zeolite" Molecular sieve, type 13x, 1.5 mm pellets) which has been purged by pumping and trapping to pressures less than 20 x 10 -3 rnm at temperatures in excess of 200°C. This is in an elongated threaded copper cup which is sealed to the bottom of the main body of the cryostat by means of a thin (1.5 ram) indium O-ring. This enables changing of the Zeolite quickly and easily whenever desired. Only the O-ring must be changed each time. To prevent heat leaks to the LN2 supply, the outer vertical wall is made of stainless steel (type 304) with a 5 cm section machined to a thickness of 0.4 ram. All internal copper surfaces are silver plated and polished to minimize radiative heat absorption. The cold finger itself is made of copper. For support, the cold finger has set one of three pointed nylon stand-offs
210
The detector can is mounted on a copper pad lightly smeared with silicone grease for good thermal contact and is held in place with a threaded aluminum ring. All electrical connections are effected by pressurized Microdot connectors, helium tested for vacuum. Immediately behind the mounting pad is a section that has some mass removed to minimize backscattering. Following this is a hollow section which can be used to contain the input components of a FET preamplifier. The temperature of these components can be controlled by means of a small heating wire. The components must be covered with a polished aluminum shield to prevent excessive radiative heat absorption.
A SIMPLE S-~ ALUMINUM (FOUR ~"~"
CAP
VERSATILE
211
CRYOSTAT
(O.OI"THICK)
MfCRODOT
'~
CONNECTORS (WITH
r NYLON
I~1~1~1~}1
'.
.
.
SEALS)
STAND - O F F S
#
k
O-RING
.
.
FET
STAINLESS CHAMBER (WITH
HEATER)
(TYPE
STEEL 304)
~J
STA,NLESS
STEEL ROD ~
"DETECTOR
RETAINING
RING
HARD
SOLDER
..,-~/ NDIUM
SEAL
\ DETECTOR ~ CLEARANCE
= 0.04"
ff /¢ .~7 > .
MOLECULAR SIEVE
DISCHARGE
PUMPING
ARRANGEMENT
WALL THIC TO
DEWAR (LINDE
LD-25)
r,O QUID
NITROGEN
_- NEIGHT
SCALE
Fig. 1. Details o f the cryostat construction. Materials are copper unless specified otherwise. All copper p a r t s are silverplated except the molecular sieve container. T h e details o f the cryostat in the region o f its detector are s h o w n for a particular experimental a r r a n g e m e n t with a cooled F E T i n p u t stage. F o r situations in which backscattering s h o u l d be minimized the section i m m e d i a t e l y b e h i n d t h e detector is replaced by a l u m i n u m .
212
R.
E.
TURCOTTE
AND
In the fabrication of the cryostat, particular care must be taken in the hard soldering of the stainless steel-copper joint, and in the alignment of the central cold finger near the detector end of the cryostat. This means that the ends of the cold finger sections must be accurately faced. The total machining time of the finished cryostat was about 25 h. For mobility, the dewar and cryostat assembly is mounted on a bathroom scale which in turn is normally mounted on a wheeled doily. The bathroom scale serves as an inexpensive, dependable LN2 level monitor.
3. Performance and testing Normal pressures maintained in the cryostat are 5 x 10-s m m or better while the ultimate temperature of the detector was 80°K. These figures have been maintained in one cryostat for a period of 17 months. Temperatures were measured with a resistance thermometer made from a spool of copper wire (dia. ~ 0.1 ram) which was placed near the end of the cold finger. The heat load of an actual detector was approximated by an aluminum dummy. Fig. 2 shows the variation of temperature with time when the detector is subjected to three specific conditions. The first corresponds to the normal cooling cycle after immersion in LN2, with a detector mounted in place. A detector would be protected from undrifting after 15 to 20 rain. The second would occur if a vacuum leak occurred while the system was at thermal equilibrium. The detector temperature would rise by some 100°K, but it would not suffer any deterioration. The third condition corresponds to the situation when the supply of L N 2 is depleted. In such a case, noticeable deterioration of the detector would begin in about 2 h. Since the ultimate temperature was only 3 °K above the boiling temperature of LN2, it is a good approxi-
R.
B.
MOORE
103]
T-%= T,e-t/~
lo o~o' o ~oo% T°=77~K °°°Oo i0z-
\ \
d
\, \\° --g-CY----
40 TIME AFTER IMMERSION IN LN2 (MIN)
Fig. 3. Thermal characteristics of the cryostat compared to the predictions of the diffusion equation. The circles represent temperature measurements.
)
mation to assume that the cold finger behaves as though it were a uniform conductor whose sides and one end were perfectly insulated. The temperature distribution along its length would then obey the diffusion equation, with the initial conditions being that the whole length was at one temperature and at the one uninsulated end a heat sink was placed in contact with it. Then at any one point along its length, the temperature should be exponential in time, once a thermal gradient has been established. The results of this assumption are shown in fig. 3 with the solid curve representing the calculation for a final temperature of 77°K. The actual data, represented by the small circles, indicate that the thermometer was properly calibrated and in good working order. It should be pointed out that only about 2 rain are required to mount the detector, seal off the cryostat, and complete the rough-out pumping operation. It is therefore possible to mount a small planar detector several times without having to give it a cleanup drift.
Fi B. 2. Thermal behaviour ofcryostat during: 1. regular cooling cycle; 2. vacuum failure; 3. depletion of LN2 supply.
References ~) S. Buhler and L. Manous, Nucl. Instr. and Meth. 50 (1967) 170. e) R. Marten, Nucl. Instr. and Meth. 57 (1967) 274. ~) C. 1. Williamson and J. Alster, Nucl. Instr. and Meth. 46 (1967) 341. 4) R. E. Turcotte, M. Sc. Thesis (McGill University, 1968).
300
~-*--o-.,o' % \t (Z) %o~, o, °,~. . . . .
2OO ,,'/
j
f +~/ + ~ +//+ ---+,. . . .
J79° K [,
+>~+~o.
IO0 "~°
20
40
60
o_ _ o _ _ 80
I00
MINUTES
INSTRUMENTS
AND
METHODS
72 (1969)
210-212;
©
NORTH-HOLLAND
PUBLISHING
CO.
A S I M P L E VERSATILE CRYOSTAT FOR ENCAPSULATED D E T E C T O R S R. E. T U R C O T T E
and R. B. M O O R E
Foster Radiation Laboratory, McGill University, Montreal, Canada Received 19 March 1969 A simple, inexpensive a n d versatile cryostat for encapsulated detectors is described. T h e cryostat operates at liquid nitrogen temperatures with molecular sieve p u m p i n g . S t a n d a r d perfor-
m a n c e was: pressure less t h a n 5 x 10-5 turn, t e m p e r a t u r e of detector less t h a n 80"K, c o n s u m p t i o n of liquid nitrogen a b o u t 1 liter per day.
1. Introduction
along its vertical section and another set along its horizontal section. For assembly purposes, the horizontal section of the cold finger is detachable from the vertical section and art opening directly above the latter is required to remove the bolt fastening the two sections. With a simple adapter, this opening serves as a port for a discharge type ion vacuum gauge (Consolidated Vacuurn Corporation) which can monitor the pressure, or be used as a clean-up pump whenever there is an accumulation of gas which has not been adsorbed by the Zeolite. (Zeolite does not adsorb all types of molecules equally well.) This port can also be used to attach a diffusion pump for initial testing purposes. A standard 3.8cm dia. copper plumbing Tee is used to join the external horizontal and vertical sections and provide the vacuum port. A small rough-out valve is placed on the Tee for pumping down immediately after a detector has been installed. A length of standard copper pipe (3.8 cm o.d.) is used for the external horizontal section. The nominal length of this section is 38 cm, but satisfactory performance is predicted for lengths at least twice as great. The detector end of the cold finger is sealed with a thin (0.25 ram) polished aluminum cap with an axial O-ring. The aluminum cap is normally threaded onto the end of the horizontal section, but in one design atmospheric pressure alone holds the cap in place.
The Ge(Li) diode which has revolutionized gamma ray spectroscopy is generally supplied by commercial manufacturers ready mounted in its own cryostat assembly. This is certainly the best way to guarantee that large expensive detectors will operate reliably soon after delivery. However for smaller detectors used in experimental setups, such as with cooled FET input stages or in high radiation areas near targets, there is a need for a versatile cryostat design which allows easy access for servicing or changing components. In most designs 1"3) this is not possible. Such a simple inexpensive cryostat for canned planar detectors has been built and tested in use in this laboratory for the past two years4), for work where frequent changes of detector are necessary. 2. Construction
The basic shape of the cryostat is that of two concentric cylinders in an inverted " L " - s h a p e d configuration, with the vertical section immersed in a dewar of liquid nitrogen (LN2) (fig. 1). The vacuum in the space between the cold finger and the outer shield is maintained by using a porous molecular sieve (Linde "Zeolite" Molecular sieve, type 13x, 1.5 mm pellets) which has been purged by pumping and trapping to pressures less than 20 x 10 -3 rnm at temperatures in excess of 200°C. This is in an elongated threaded copper cup which is sealed to the bottom of the main body of the cryostat by means of a thin (1.5 ram) indium O-ring. This enables changing of the Zeolite quickly and easily whenever desired. Only the O-ring must be changed each time. To prevent heat leaks to the LN2 supply, the outer vertical wall is made of stainless steel (type 304) with a 5 cm section machined to a thickness of 0.4 ram. All internal copper surfaces are silver plated and polished to minimize radiative heat absorption. The cold finger itself is made of copper. For support, the cold finger has set one of three pointed nylon stand-offs
210
The detector can is mounted on a copper pad lightly smeared with silicone grease for good thermal contact and is held in place with a threaded aluminum ring. All electrical connections are effected by pressurized Microdot connectors, helium tested for vacuum. Immediately behind the mounting pad is a section that has some mass removed to minimize backscattering. Following this is a hollow section which can be used to contain the input components of a FET preamplifier. The temperature of these components can be controlled by means of a small heating wire. The components must be covered with a polished aluminum shield to prevent excessive radiative heat absorption.
A SIMPLE S-~ ALUMINUM (FOUR ~"~"
CAP
VERSATILE
211
CRYOSTAT
(O.OI"THICK)
MfCRODOT
'~
CONNECTORS (WITH
r NYLON
I~1~1~1~}1
'.
.
.
SEALS)
STAND - O F F S
#
k
O-RING
.
.
FET
STAINLESS CHAMBER (WITH
HEATER)
(TYPE
STEEL 304)
~J
STA,NLESS
STEEL ROD ~
"DETECTOR
RETAINING
RING
HARD
SOLDER
..,-~/ NDIUM
SEAL
\ DETECTOR ~ CLEARANCE
= 0.04"
ff /¢ .~7 > .
MOLECULAR SIEVE
DISCHARGE
PUMPING
ARRANGEMENT
WALL THIC TO
DEWAR (LINDE
LD-25)
r,O QUID
NITROGEN
_- NEIGHT
SCALE
Fig. 1. Details o f the cryostat construction. Materials are copper unless specified otherwise. All copper p a r t s are silverplated except the molecular sieve container. T h e details o f the cryostat in the region o f its detector are s h o w n for a particular experimental a r r a n g e m e n t with a cooled F E T i n p u t stage. F o r situations in which backscattering s h o u l d be minimized the section i m m e d i a t e l y b e h i n d t h e detector is replaced by a l u m i n u m .
212
R.
E.
TURCOTTE
AND
In the fabrication of the cryostat, particular care must be taken in the hard soldering of the stainless steel-copper joint, and in the alignment of the central cold finger near the detector end of the cryostat. This means that the ends of the cold finger sections must be accurately faced. The total machining time of the finished cryostat was about 25 h. For mobility, the dewar and cryostat assembly is mounted on a bathroom scale which in turn is normally mounted on a wheeled doily. The bathroom scale serves as an inexpensive, dependable LN2 level monitor.
3. Performance and testing Normal pressures maintained in the cryostat are 5 x 10-s m m or better while the ultimate temperature of the detector was 80°K. These figures have been maintained in one cryostat for a period of 17 months. Temperatures were measured with a resistance thermometer made from a spool of copper wire (dia. ~ 0.1 ram) which was placed near the end of the cold finger. The heat load of an actual detector was approximated by an aluminum dummy. Fig. 2 shows the variation of temperature with time when the detector is subjected to three specific conditions. The first corresponds to the normal cooling cycle after immersion in LN2, with a detector mounted in place. A detector would be protected from undrifting after 15 to 20 rain. The second would occur if a vacuum leak occurred while the system was at thermal equilibrium. The detector temperature would rise by some 100°K, but it would not suffer any deterioration. The third condition corresponds to the situation when the supply of L N 2 is depleted. In such a case, noticeable deterioration of the detector would begin in about 2 h. Since the ultimate temperature was only 3 °K above the boiling temperature of LN2, it is a good approxi-
R.
B.
MOORE
103]
T-%= T,e-t/~
lo o~o' o ~oo% T°=77~K °°°Oo i0z-
\ \
d
\, \\° --g-CY----
40 TIME AFTER IMMERSION IN LN2 (MIN)
Fig. 3. Thermal characteristics of the cryostat compared to the predictions of the diffusion equation. The circles represent temperature measurements.
)
mation to assume that the cold finger behaves as though it were a uniform conductor whose sides and one end were perfectly insulated. The temperature distribution along its length would then obey the diffusion equation, with the initial conditions being that the whole length was at one temperature and at the one uninsulated end a heat sink was placed in contact with it. Then at any one point along its length, the temperature should be exponential in time, once a thermal gradient has been established. The results of this assumption are shown in fig. 3 with the solid curve representing the calculation for a final temperature of 77°K. The actual data, represented by the small circles, indicate that the thermometer was properly calibrated and in good working order. It should be pointed out that only about 2 rain are required to mount the detector, seal off the cryostat, and complete the rough-out pumping operation. It is therefore possible to mount a small planar detector several times without having to give it a cleanup drift.
Fi B. 2. Thermal behaviour ofcryostat during: 1. regular cooling cycle; 2. vacuum failure; 3. depletion of LN2 supply.
References ~) S. Buhler and L. Manous, Nucl. Instr. and Meth. 50 (1967) 170. e) R. Marten, Nucl. Instr. and Meth. 57 (1967) 274. ~) C. 1. Williamson and J. Alster, Nucl. Instr. and Meth. 46 (1967) 341. 4) R. E. Turcotte, M. Sc. Thesis (McGill University, 1968).
300
~-*--o-.,o' % \t (Z) %o~, o, °,~. . . . .
2OO ,,'/
j
f +~/ + ~ +//+ ---+,. . . .
J79° K [,
+>~+~o.
IO0 "~°
20
40
60
o_ _ o _ _ 80
I00
MINUTES