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A dual chamber vacuum cryostat for germanium lithium drifted diodes
NUCLEAR
INSTRUMENTS
A DUAL
CHAMBER
AND
METHODS
VACUUM
94
(1971) 395-396; ~L) N O R T H - H O L L A N D
CRYOSTAT
FOR G E R M A N I U M
PUBLISHING
CO.
LITHIUM DRIFTED DIODES*
J. F. D E T K O
Division o f Biophysics, Sloan-Kettering Institute for Cancer Research, Neu" York, N.Y. 10021, U.S.A. Received 26 February 1971 A dual c h a m b z r cryostat is described which has a p e r m a n e n t charge o f cryosorption material that can be isolated from the diode c h a m b e r when it is opened to air.
The operation of a lithium drifted germanium diode requires a suitable vacuum cryostat. A simple and effective design, reported previously~), employs a rigid "cold finger" (usually a copper rod) which extends into an evacuated chamber to cool the diode. An adsorption agent such as charcoal is contained in the lower section of the diode chamber. The segment of the rod protruding from the vacuum chamber and most of the vacuum jacket are immersed in liquid nitrogen. The pumping capacity of the cooled activated charcoal eliminates the need for an external pump to sustain adequate vacuum in the cryostat. A limitation of the single chamber cryostat is that since the adsorption agent is depleted each time the diode chamber is opened to air, use of an external pump such as an ion pump is usually desirable in place of the adsorption agent when frequent opening of the diode chamber is necessary. For a moderate increase in the fabrication cost, this type of cryostat can be converted to a dual chamber unit which permits external valve control of communication between the charcoal and the diode chamber while still preserving the main advantages of the original unit. As a result of such an arrangement, there is no need to use an external pump to maintain vacuum during frequent cycling since the diode chamber can be opened to air repeatedly, where necessary, without depleting the charcoal's pumping capacity each time. Also, the germanium diode is not exposed to possible contamination during outgassing of the charcoal charge when the cryostat is finally closed for an extended time.
Such a cryostat can also be transported unloaded with an active charcoal charge or stored for extended periods while awaiting use. Of a number of possible dual chamber configurations considered, the one adopted employs two concentric chambers. This is shown schematically in the accompanying figure. In this unit, the charcoal is contained in a permanently sealed outer annular chamber surrounding the cold finger and is connected with the diode chamber through an external high vacuum valve t. Separate valves of the same type permit independent evacuation of the diode chamber and charcoal chamber. A prototype unit was constructed according to this design and tested. Despite the added impedance of the g1 tt bore vacuum valve and ~I t t tubing between the charcoal and diode chamber, the terminal pressure in the diode chamber of the dual chamber unit was within a factor of two of a unit constructed according to the original single chamber design +. Ion gauge measurement indicated a base pressure of around 0.9 x 10 -4 torT. Liquid nitrogen consumption with the dual chamber unit held in a commercial dewar + was measured to be around 1.5 liters a day for the unloaded cryostat. A loss of around 0.5 liters a day occurs from the capped dewar. The pumping capacity of the charcoal charge (25 cm 3) on a single outgassing has a demonstrated storage life at room temperature of over three months with no sign of exhaustion. The author gratefully acknowledges the skillful instrumentation work of M. Rossi in the fabrication of this device.
* Work supported in part by A E C (30-1)-910and NCI CA 08748. t Model 4551Q4M Hoke, Inc., Cresskill, N.J. + Assembly drawings provided by the I n s t r u m e n t a t i o n G r o u p of Brookhaven National Laboratories, U p t o n , N.Y. + Model LD 31, Cryogenic Service Division, T.W. Smith Corporation.
395
Reference l) C. C h a s m a n and R . A , Ristinen, Nucl. Instr. and Meth. 34 (1965) 250.
396
J.F.
DETKO
diode chamber copper cold "finger" o- ring groove~
.196 cm.
-- to interconnect valve
to charcoal chamber pumpdown valve
interconnect valve~ , ~ o n e --~
screen
diode chamber interconnect hole .635 cm. on a 4,52 cm bolt , ,'~ >~circle \o.~N~ 6 - 3 2 clearance holes / ~ ,_\ \ s~i x~e q u a l l y s p a c e d o n _ / ~__ ) _ _ _ ~ m . boltcircle ]
1
~-
'
( Section A-A)
~//~7 charcoalchamber umpdown valve
~tcgm!~
"\" .'~. diode chamber
2.54 " - cm,
o- ring groove !1
1
- charcoal chamber
i 5.31 cm. --~t~ i .279 cm!
30.48 cm.
/
--
7.62 cm.
~,~ i
! i
- s s screen *.041 cm. ss wall
%031 cm. ss wall
Fig. I. Cross section of dual chamber cryostat.
1.88cm v
INSTRUMENTS
A DUAL
CHAMBER
AND
METHODS
VACUUM
94
(1971) 395-396; ~L) N O R T H - H O L L A N D
CRYOSTAT
FOR G E R M A N I U M
PUBLISHING
CO.
LITHIUM DRIFTED DIODES*
J. F. D E T K O
Division o f Biophysics, Sloan-Kettering Institute for Cancer Research, Neu" York, N.Y. 10021, U.S.A. Received 26 February 1971 A dual c h a m b z r cryostat is described which has a p e r m a n e n t charge o f cryosorption material that can be isolated from the diode c h a m b e r when it is opened to air.
The operation of a lithium drifted germanium diode requires a suitable vacuum cryostat. A simple and effective design, reported previously~), employs a rigid "cold finger" (usually a copper rod) which extends into an evacuated chamber to cool the diode. An adsorption agent such as charcoal is contained in the lower section of the diode chamber. The segment of the rod protruding from the vacuum chamber and most of the vacuum jacket are immersed in liquid nitrogen. The pumping capacity of the cooled activated charcoal eliminates the need for an external pump to sustain adequate vacuum in the cryostat. A limitation of the single chamber cryostat is that since the adsorption agent is depleted each time the diode chamber is opened to air, use of an external pump such as an ion pump is usually desirable in place of the adsorption agent when frequent opening of the diode chamber is necessary. For a moderate increase in the fabrication cost, this type of cryostat can be converted to a dual chamber unit which permits external valve control of communication between the charcoal and the diode chamber while still preserving the main advantages of the original unit. As a result of such an arrangement, there is no need to use an external pump to maintain vacuum during frequent cycling since the diode chamber can be opened to air repeatedly, where necessary, without depleting the charcoal's pumping capacity each time. Also, the germanium diode is not exposed to possible contamination during outgassing of the charcoal charge when the cryostat is finally closed for an extended time.
Such a cryostat can also be transported unloaded with an active charcoal charge or stored for extended periods while awaiting use. Of a number of possible dual chamber configurations considered, the one adopted employs two concentric chambers. This is shown schematically in the accompanying figure. In this unit, the charcoal is contained in a permanently sealed outer annular chamber surrounding the cold finger and is connected with the diode chamber through an external high vacuum valve t. Separate valves of the same type permit independent evacuation of the diode chamber and charcoal chamber. A prototype unit was constructed according to this design and tested. Despite the added impedance of the g1 tt bore vacuum valve and ~I t t tubing between the charcoal and diode chamber, the terminal pressure in the diode chamber of the dual chamber unit was within a factor of two of a unit constructed according to the original single chamber design +. Ion gauge measurement indicated a base pressure of around 0.9 x 10 -4 torT. Liquid nitrogen consumption with the dual chamber unit held in a commercial dewar + was measured to be around 1.5 liters a day for the unloaded cryostat. A loss of around 0.5 liters a day occurs from the capped dewar. The pumping capacity of the charcoal charge (25 cm 3) on a single outgassing has a demonstrated storage life at room temperature of over three months with no sign of exhaustion. The author gratefully acknowledges the skillful instrumentation work of M. Rossi in the fabrication of this device.
* Work supported in part by A E C (30-1)-910and NCI CA 08748. t Model 4551Q4M Hoke, Inc., Cresskill, N.J. + Assembly drawings provided by the I n s t r u m e n t a t i o n G r o u p of Brookhaven National Laboratories, U p t o n , N.Y. + Model LD 31, Cryogenic Service Division, T.W. Smith Corporation.
395
Reference l) C. C h a s m a n and R . A , Ristinen, Nucl. Instr. and Meth. 34 (1965) 250.
396
J.F.
DETKO
diode chamber copper cold "finger" o- ring groove~
.196 cm.
-- to interconnect valve
to charcoal chamber pumpdown valve
interconnect valve~ , ~ o n e --~
screen
diode chamber interconnect hole .635 cm. on a 4,52 cm bolt , ,'~ >~circle \o.~N~ 6 - 3 2 clearance holes / ~ ,_\ \ s~i x~e q u a l l y s p a c e d o n _ / ~__ ) _ _ _ ~ m . boltcircle ]
1
~-
'
( Section A-A)
~//~7 charcoalchamber umpdown valve
~tcgm!~
"\" .'~. diode chamber
2.54 " - cm,
o- ring groove !1
1
- charcoal chamber
i 5.31 cm. --~t~ i .279 cm!
30.48 cm.
/
--
7.62 cm.
~,~ i
! i
- s s screen *.041 cm. ss wall
%031 cm. ss wall
Fig. I. Cross section of dual chamber cryostat.
1.88cm v