Dewar cryopumping using molecular sieve

A non-evacuated dewar 10 advantageously employs a molecular sieve 30 that serves to adsorb gasses in the dewar when cooled during operation of the detector 24 thereby preventing liquid formation onto the detector. The effects of outgassing and permeation during storage are substantially eliminated because the dewar package is in partial pressure equilibrium with its environment since the interior of the dewar is backfilled with the same inert gas as is in the surrounding outside environment. A second molecular sieve 40 may be used to adsorb moisture which may permeate into the housing.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to hermetically sealed packages and, more 
particularly, to dewars containing infrared detectors. 
2. Discussion 
Some sensors, particularly mercury-cadmiumtelluride infrared detectors, are 
most sensitive when operating at approximately 77.degree. K. These 
detectors are typically used in conjunction with an evacuated dewar in 
which the detector is placed. The evacuation of the dewar is used to 
remove gasses which would otherwise occupy the region surrounding the 
detector so that the potential heat loss through convection and conduction 
during operation is minimized, as well as to eliminate the formation of 
liquid onto the detector. The detector is generally mounted onto the tip 
of a coldfinger which is in communication with a cryoengine assembly. 
During operation the cryoengine serves to expand a fluid such as helium in 
the coldfinger which, in turn, adsorbs thermal energy causing the detector 
to be cooled. 
While the traditional evacuated dewar has generally operated 
satisfactorily, it does have some drawbacks. For example, the choice of 
materials that are used to fabricate the dewar is somewhat limited and 
expensive because it is necessary to choose materials having special 
characteristics such as low diffusivity, low outgassing and other 
properties. Furthermore, implementing the necessary closure techniques 
required to create the vacuum inside the dewar is often costly and it is 
sometimes difficult to ensure that the vacuum is maintained over a long 
period of time. 
U.S. Pat. No. 4,719,353 discloses a non-evacuated dewar in which polymeric 
foam is disposed between the expander or coldfinger and the housing. While 
the above document discloses a dewar which has its advantages, it also has 
its own set of shortcomings and can be further improved. 
SUMMARY OF THE INVENTION 
In accordance with the teachings of the preferred embodiment of this 
invention, cryopumping means include a molecular sieve which is mounted to 
the dewar coldfinger adjacent the detector. When the coldfinger is cooled 
by the cryoengine, it also cools the molecular sieve causing it to adsorb 
gas in the dewar housing next to the detector. As a result, the pressure 
in the dewar is reduced to prevent liquid formation on the detector as 
well as minimizing convection and conduction losses. These advantages are 
economically obtained while avoiding the problems of the traditional 
evacuated dewar construction.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The aforementioned U.S. Pat. No. 4,719,353 discloses many of the details of 
a dewar of the general type to which the present invention pertains. The 
'353 patent is hereby incorporated by reference and the reader's attention 
is drawn to that patent for background information. The following 
specification accordingly focuses on a concise description of the 
contribution to the art made by this invention. 
Briefly, the dewar 10 includes a housing 12 with a lens cap assembly 14 at 
one end thereof and a mounting flange 16 at an opposite lower end thereof. 
Flange 16 is suitably connected to a mounting plate 18 which, in turn, 
carries a suitable cryoengine 20. Cryoengine 20 is coupled to a coldfinger 
22 which projects upwardly through the major extent of housing 12. An 
infrared detector 24 is mounted to the tip 26 of coldfinger 22. A cold 
shield 28 surrounds detector 24 and includes an aperture in an upper 
portion thereof acting as a field stop to restrict the field of view of 
detector 24 in a known manner. 
A molecular sieve 30 is in thermal contact with the tip 26 of coldfinger 
22. As will appear, the purpose of molecular sieve 30 is to remove gasses 
from the area adjacent detector 24 when it is operating. When detector 24 
is operating, the cryoengine 20 is energized to cause fluid contained 
within coldfinger 22 to expand thereby absorbing thermal energy to cool 
detector 24 to the preferred 77.degree. K. Molecular sieve 30 has a 
particular affinity for the type of gasses in the area adjacent detector 
24. Preferably, the dewar housing 12 is backfilled with an inert gas such 
as nitrogen at one atmosphere or atmospheric pressure. Thus, the gas 
adjacent .detector 24 is predominately nitrogen in the preferred 
embodiment. However, other gasses such as argon and xenon can be 
alternatively used to backfill the package. 
Molecular sieve 30 can be made of a variety of zeolite materials such as 
activated crystalline silicoaluminate with organic binders. This preferred 
material is commercially available from Multiform Desiccants under the 
trade designation NATRASORB 900. It is approximately 0.100 inch thick and 
about 0.400 inch in diameter. Sieve 30 is attached to the outer walls of 
coldfinger 22 adjacent tip 26 by way of adhesive. In the drawing, sieve 30 
is shown with a plurality of annular grooves 32 which are for the purpose 
of increasing surface area to enhance gas adsorption. 
Provision is made for reducing the amount of gas adjacent detector 24 that 
needs to be adsorbed by the sieve 30 during operation. To this end, much 
of the interior space within housing 12 is filled with insulation 34. 
Preferably, the insulation is made of a polymeric foam such as a 
polystyrene composite material. As can be seen in the drawing, there is no 
insulation in the space above detector 24 which could otherwise block 
thermal radiation to be sensed by the detector. The insulation serves a 
variety of functions such as reducing heat loss due to gas conduction and 
convection until the gas is adsorbed by the sieve 30, acting as a 
stiffener for the coldfinger 22 and it can also aid in positioning of 
control cables. 
The dewar 10 is typically located in an outside environment containing 
nitrogen gas at one atmosphere pressure. Although the dewar 10 is also 
backfilled with nitrogen at atmospheric pressure and is therefore at 
equilibrium with the outside environment when in a non-operational state, 
there still exists a possibility of moisture permeating the interior of 
package, for example, through seals 38 between the mounting flange 16 and 
mounting plate 18. Moisture in an appreciable amount can degrade the 
cryopumping operation of molecular sieve 30. To remove moisture within the 
housing 12 a second molecular sieve 40 with a larger surface area is 
contained within the dewar 10. It is preferably located at the lower end 
of the housing 12 adjacent the seals 38 which represent the most likely 
point of entry of moisture. Molecular sieve 40 likewise can be made of the 
same material as sieve 30. 
When the detector 24 is not operating, the dewar components are 
substantially at room or ambient temperature, i.e., the cryoengine 20 is 
not functioning to cool the detector to its operating point which is below 
80.degree. K. and preferably about 77.degree. K. In this non-operating 
condition, there exists nitrogen gas in the area above the detector 24 
since the molecular sieve 30 is at equilibrium. Any moisture that 
permeates the seals and enters the interior of the dewar 12 is adsorbed 
primarily by the molecular sieve 40. Thus, the dewar 10 can exhibit 
extended shelf life. This is important since infrared detectors of this 
type may remain in their non-operating state for some period of time. 
When it is desired to utilize the detector 24, the cryoengine 20 is 
operated to cool the coldfinger 22. As is known in the art, the tip 26 of 
coldfinger 22 is cooled more quickly than the lower portions thereof. The 
cooling of coldfinger tip 26 simultaneously cools the detector 24 and 
molecular sieve 30. The cooling of sieve 30 causes it to change from its 
equilibrium condition to a condition at which it adsorbs or getters gasses 
surrounding the detector 24. This adsorbing of gasses creates a 
"cryopumping" action in which the pressure in the dewar housing 12 is kept 
below the gas triple point, e.g., 94 torr for nitrogen, during operation 
of the detector 24. Thus, the possibility of a liquid forming onto the 
detector 24 is substantially eliminated. In addition, heat losses through 
convection and conduction are also substantially reduced. 
When the dewar returns to its non-operational mode, the molecular sieve 30 
desorbs the adsorbed gasses and the package returns to its equilibrium 
condition. Permeation during storage is kept to a minimum because the 
package is in partial pressure equilibrium with its outside environment. 
As noted above, moisture which may enter the package is removed by the 
molecular sieve 40. 
Those skilled in the art can now appreciate that the present invention 
provides an economical, yet reliable dewar package construction that 
eliminates many of the problems associated with traditional evacuated 
dewars. It should be understood that while this invention was described in 
connection with one particular example, many modifications can be made 
thereto without departing from the spirit of this invention after having 
the benefit of studying the specification, drawing and following claims.