FIELD OF THE INVENTION

The present invention relates to the field of cryogenic radiation detectors, and more particularly to mechanically cooled cryogenic radiation detectors operating under vacuum conditions.

BACKGROUND OF THE INVENTION

Radiation detectors, which may be X-ray detectors, infrared radiation detectors or other types, commonly have an evacuated envelope with a vacuum space, at least one radiation detector element being mounted in the vacuum space. These devices include scanning electron microscopes (SEM), X-ray spectrometers, infrared spectrometers, and other known devices. For optimal operation, with low electronic noise and high sensitivity, these detectors are operated cryogenically. Therefore, the detector is provided with a cooling device. In order to reduce parasitic heat input to the system, the detector and cooling system are generally insulated from the environment in a Dewar, having a vacuum space within a sealed envelope or chamber. A cooling element is provided in the chamber and serves to cool the detector element during operation of the detector. The vacuum minimized heat conduction through a gas-filled space, and the surfaces are formed with low heat radiation emissivity construction. In general, the detector may be allowed to return to ambient temperature when not in operation.

A member, in thermal communication with the cooling device, protrudes into the chamber and supports the detector. As is known, the member is actively cooled by the Joule-Thomson effect (gas expansion), liquid nitrogen (or other gas), Stirling cycle cooling, Peltier junctions, or by other known means. The cooled protruding member is often termed "the cold finger" of the detector. The cold fmger is also thermally coupled to a detector to be cooled, and generally acts as a mechanical support as well.

It is known that a sigmficant cause of detector failure is the gradual degradation of the vacuum in the evacuated space due to, e.g., internal out-gassing of the

CONFIRMATION COW

various component parts of the detector exposed to the vacuum and leakage from outside the evacuated envelope. In order to reduce outgassing of the component parts of the detector in the vacuum space during the service life of the detector, these component parts are generally prebaked in known manner in vacuo before assembly, and a general bakeout of the assembly is also carried out before mounting the detector electronic element(s). The excessive outgassing which generally occurs in X-ray and infrared detectors may be due to the fact that the gases cannot be driven out by baking the whole device during vacuum pumping (in the way which is usual for other vacuum devices) because X-ray or infrared detector elements are damaged at high temperatures. This degeneration in the vacuum eventually leads to the situation in which a cooling element is no longer able (at least in an efficient manner or sufficiently fast) to cool the detector element to the desired temperature for efficient detection of the radiation. Thus, the detector lifetime is curtailed, especially as only limited cooling power is available to cool the detector. Furthermore, the outgassing into the vacuum space provides a significant thermal transfer path between the cold fmger and the outside of the detector, when the pressure exceeds 1 x 10 ^"3 Torr. In mechanical cooling systems, the thermal capacity drops as the temperature differential increases, so that with an insufficient initial vacuum, necessary operating temperatures may never be reached.

The cooling system is generally not provided with an extraordinarily large cooling capacity because this may introduce increased vibration, leads to inefficiency and increased size, and may be difficult to control. In operation, the desired vacuum condition in the housing reduces the heat transfer from outside the vacuum space, thus limiting the amount of heat which must be removed through the cold fmger. As the vacuum deteriorates, heat transfer from outside the space increases, imposing a greater heat load on the cryogenic system. Thus, the detector lifetime may be curtailed.

In U.S. Pat No. 4,474,036, a getter of a zeolite or synthetic zeolite material is disclosed. The getter may also be a molecular sorbent material formed in situ, minimizing the need for adhesives, such as silica gel. This material is formed into a shape which conforms to a cooled portion of the vacuum space, such as around the cold finger. The getter is generally provided as a formed element in substantial thermal contact with a cooled portion of the Dewar, such as by a low-outgassing epoxy. The epoxy may be filled with a thermally conductive material, such as silver powder.

Typically, the synthetic zeolite body of U.S. Pat. No. 4,474,036 is composed of particles having a width of at most a few micrometers and with somewhat

irregular inter-particle voids also in the body. The pores of the porous zeolite particles forming the body have a width comparable to molecule sizes (up to approximately 0.5 nm) of gases in the vacuum space and were formed by driving off the water of crystallization of the zeolite material before molding the zeolite particles together in an annular shape to form the body; the heating required to effect this dehydration is thus performed before mounting the getter in the Dewar envelope. The resulting molecular-size pores permeate the zeolite particles to give an extremely large internal surface area, as a result of which the cooled body can absorb a large volume of gas by adsorption on the inner surfaces of the pores.

Since the cooling element of U.S. Patent 4,464,036 is provided to cool to only a moderate cryogenic temperature, the good large-area thermal contact between the inner major surface of the getter body and the surface of the Dewar is particularly important in efficiently cooling the molecular-sieve body, i.e., the molecular sorbent getter, to obtain a high sorption efficiency. An annular configuration for both the getter body and the cooled surface also minimizes the amount of epoxy adhesive necessary to secure the getter body to the surface; this is important since a large amount of epoxy can increase out-gassing into the vacuum space. In a particular example the epoxy film may be typically 100 micrometers thick.

SUMMARY OF THE INVENTION

When employing a lithium drifted silicon X-ray sensor, a temperature of about 100 K to 110 K at the detector crystal is desired. According to the present design, an internal mass damper has an operating temperature of about 82 K, with a gradient along the cold finger and detector holder. Other types of detectors may have different optimum temperatures. For example, germanium detectors, which may detect X-rays and/or gamma rays, generally require a lower temperature, e.g., about 95 K, for effective and efficient operation. In this case, a closed circuit cryocooler system is provided which produces lower temperatures, thus allowing reduced temperatures at the detector. Improved insulation techniques to reduce parasitic heat load may also reduce temperatures at the detector. The cryocooler may be operated with a refrigerant composition effective for cooling to temperatures lower than those achieved through the use of liquid nitrogen, which exists in a liquid state at standard pressure at 77 K.

The cryocooler is subject to vibration from a number of sources. First, the refrigerant compressor produces a pulsatile pressure wave in the refrigerant streams, and

also transmits vibration through the refrigerant supply and retum lines. At the expansion chamber, the fluid refrigerant expands supersonically and turbulently, creating a noise and vibration. Further, the refrigerant vaporizes in the counterflow heat exchanger, creating the further possibility of vibration. The uncompensated detector is vibration sensitive, resulting in an apparent loss of resolution.

The cryocooler is physically connected to an intemal mass damper with a plurality prestressed flexed copper members. These prestressed copper members provide a thermal communication path as well as mechanical support. The cryocooler is mounted to the housing. The internal mass damper is thus cooled by the cryocooler, and has a significant thermal inertia. Therefore, it is desired to have an internal mass damper with a high mass-to- thermal capacity ratio for faster responsiveness. The intemal mass damper is further thermally linked to the cold finger by a flexed high-flexibility copper strap system having a number of straps of varying placement and orientation.

In order to maintain a high level of vacuum inside the detecting unit after sealing, a sufficient molecular sorption getter material is attached to a cold surface of the device to maintain molecular flow of gas molecules, e.g., a pressure of less than about 1 mTorr. Molecular sorption getter materials have the characteristic that, as cooled, they increasingly trap gasses from the cryostat, thus acting as an internal vacuum pump for the detecting unit. The total amount of gas that can be trapped by the getter material increases as the material gets colder and decreases as the ambient pressure drops. In such a system, the amount of clean getter material, its temperature and the amount of residual gas in the cryostat determine the operating pressure of the detecting unit.

The molecular sorption getter is preferably formed to occupy void space adjacent to cooled surfaces inside the evacuated space. However, the getter may also be provided as a plurality small portions mechanically retained against the cooled surfaces.

Activated carbon is a preferred getter material, and may be generated from preformed organic material which is converted to activated carbon while retaining its form. Thus, an easily formed organic material such as a polymer or biological material is pyrolyzed or processed into activated carbon which less easily shaped. For germanium detectors, various interfering factors, such as vibration and electrical interference, act to increase the FWHM of the output signal from the preamplifier FET from less than or equal to about 127 eV at 5.9 keV from ⁵⁵ Fe at 1000 cps with a 40μS time constant to a greater value, e.g., greater than about 130 eV. The interfering factors convolve with the signal produced by an X-ray to increase the FWHM.

See, Goldstein, J.L, et al., Scanning Electron Microscopy and X-Rav Microanalysis (2d Ed.). Plenum Press, New York (1992), pp. 310-313. As the peak width increases, the ability to distinguish closely spaced peaks diminishes.

As shown in FIG. 1, three X-ray peaks 61, 62, 63 are emitted from a sample. Due to characteristics of the detector system, vibration, electrical interference, and other sources of noise, the energies as received are dispersed to a received signal having a greater FWHM 64, 65, 66. The detector, because of the closeness of these peaks, receives a signal which appears like signal 67, and has difficulty resolving the individual peaks 64, 65, 66. Further, as the peaks are spread, the intensity is decreased, and the signal-to-noise ratio decreased. The X-ray peaks 61, 62, 63 emitted from a sample have a FWHM of about 2 to 3 eV. The detector and associated electromc circuit, however, spreads the energy over a wider range. For example, a liquid nitrogen cooled Dewar system having a lithium drifted silicon crystal operating at about 100-110 K has a peak width of about 137 eV using a standard measurement technique, measurement of a 5.9 keV X-ray from an ⁵⁵ Fe source, 1000 cps with a 40 xS time constant (Si(Li) crystal detector). As stated above, this peak is broadened by vibration, high temperatures, and other factors known in the art.

Vibrational energy which remains, may be damped by providing a large mass damper firmly linked to the refrigerant tubing. Vibrations passing the linkage are attenuated. The turbulent vibration is generated in the cryocooler proper, and therefore may not be easily filtered in the refrigerant lines. In order to attenuate vibrations generated in the cryocooler, an active or passive mechanical filter is provided at the cryocooler or between the cryocooler and the detector crystal. Passive filters include damping elements, which convert vibrations to other forms of energy. Active filters determine a force of a vibrational wave and apply an opposing force which sums with the vibrational wave, resulting in an attenuated waveform, at least in a particular region of interest, i.e., the detector. Since a complex mechanical structure bridges the cryocooler to the X-ray detector crystal, care may be taken to reduce undesired resonances and other detrimental acoustic or vibrational properties of the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features in accordance with the invention are illustrated, by way of example, in specific embodiments of the invention now to be described with reference to the accompanying drawings, in which:

FIG. 1 is a graph showing the influence of peak spreading on the detector performance;

FIG. 2 is a block diagram of a closed circuit cooler employed for achieving cryogenic temperatures; FIG. 3 is a perspective view of a closed circuit cryogenic cooler according to the present invention having an extemal damping mass;

FIG. 4 is a cross-sectional view of an X-ray radiation detector in accordance with a first embodiment according to the present invention;

FIG. 5 is a cross-sectional view of an X-ray radiation detector in accordance with a second embodiment of the present invention, having added getter material;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

EXAMPLE 1 In FIG. 2, cryogenic cooling system gas supply having a compressor

101, a heat exchanger 103 for cooling the compressed gas, also known as a condenser are provided. The compressor is electrically powered, preferably from line current through a line cord 47, shown in FIG. 3. The heat exchanger 103 is provided with a high surface area radiator portion which has a high heat transfer coefficient between an internal volume of compressed gas and a stream of air 48 forced over the portion by blowers 104.

The cryogenic cooling system has a supply mbe 1, from a coupling 111, which feeds a stream of compressed refrigerant at approximately ambient temperamre. A return tube 117 is provided to recycle the gas through the system from the cryocooler 120. A pressure relief valve 121 is provided in the supply line to prevent overpressure conditions. A supply line gas filter 109 and a return line gas filter 122 is are provided to reduce particulate contamination. A pressure gauge 118 is provided to indicate line equalization, and a pressure relief valve 116 is also provided in the return line proximate to the cryocooler.

The compressor employs an oil lubricant, and thus includes an oil separator 106, and oil return line 107.

As shown diagrammatically in Fig. 2, the cryocooler operates on a standard Joule-Thomson principal, with a counterflow heat exchanger for precooling the compressed refrigerant in the supply path 112 with the expanded refrigerant in the return path 120. The compressed refrigerant is present in a mostly liquified state at the end of the

7 counterflow heat exchanger. A throttle valve 113 is provided to limit refrigerant flow into the expansion chamber 114, where the pressure drop in the gas flowing through the orifice occurs supersonically and isenthalpically so that the refrigerant experiences a temperature reduction, in accordance with the Joule-Thomson principle. The refrigerant flow in the expansion chamber is turbulent.

An extemal mass damper 46, formed of a steel block having a weight sufficient to damp compressor induced vibrations, e.g., greater than about 20 lbs., is provided firmly connected to the supply 43 and return 44 lines and resting on the floor to reduce any pulsatile variation in the refrigerant mbes. The closed circuit cryogenic cooler, without damping, produces an unacceptable level of vibration, e.g., about 7.4 mg RMS, over a frequency range of 0-500 Hz, along the long axis of the cold fmger. When two 27.75 lb steel blocks are placed on the refrigerant mbes between the compressor and the cryocooler, the level is reduced to about 6.5 mg RMS. When a 13 lb lead ring is placed around the housing of the cryocooler, the vibration levels are reduced to about 4.5 mg RMS. When both the steel block and the lead ring are employed, the vibration levels are reduced to about 3.2 mg RMS.

As constructed into a cryogenic radiation detector according to a preferred embodiment of the invention, with external mass damping of the refrigerant supply and return lines and internal inertial mass damping, the system produces 3.5 mg RMS from 0-2 kHz along the X axis (long axis), 1 mg RMS along the Y axis and 1.3 mg RMS along the Z axis. A comparable 10 liter liquid mtrogen cooled Dewar system produces 0.4 mg RMS from 0-2 kHz along the X axis (long axis), and around 0.5 mg RMS along the Y and Z axes. With this level of vibration damping, energy resolutions of less than or equal to 137 eV at 5.9 keV from an ^5S Fe source, 1000 cps with a 40μS time constant can be obtained with a lithium drifted silicon crystal detector according to the present invention. Without such damping, the energy resolution would be at minimum about 140 eV, and likely greater than about 150 eV under the same conditions, which results in an inferior instrument.

The output of the supply line from the external mass damper 1 leads to the cryogenic refrigerator of the radiation detector device. As stated above, the cryogenic refrigerator includes a counterflow heat exchanger 3 which precools the compressed supply refrigerant to an almost liquid state with the returning expanded refrigerant. The expanded refrigerant vaporizes in the counterflow heat exchanger 3. The precooled pressurized supply refrigerant passes through a throttle valve 113, which selectively restricts the flow of refrigerant into an expansion chamber 114. The throttle 113 valve includes a needle, having

a portion which extends into the expansion chamber, with an expanded neck portion which is proximate to a conical valve seat (not shown). As the expansion chamber 114 cools, the needle contracts, causing the expanded neck to restrict refrigerant flow by partially seating in the conical valve seat. In general, the throttle valve 113 is more open during cool down, where the cryogenic cooling system has a greater cooling capacity, but reaches a higher minimum temperamre because of the increased back pressure in the expansion chamber. After the system reaches a target temperamre, the throttle valve 113 is less open, so that the flow rate is reduced and the pressure differential between the compressed refrigerant supply 112 and the expansion chamber 114 is increased. This decreases the theoretical minimum temperamre while having the effect of reducing the heat removal capacity of the system. This type of control helps to achieve cool down rapidly, while providing integral temperamre regulation.

As shown in FIG. 4, The cryogenic refrigerator and the cooled components of the detector are contained within an evacuated chamber 15, 20, 21, 25. The vacuum reduces the thermal conduction through a surrounding gas to the outside, which is, e.g., at ambient temperature. The evacuated chamber 15, 20, 21, 25 is initially brought, during manufacmre, to a vacuum pressure of about 1 x IO ^*7 Torr. This allows an amount of degradation of the vacuum, to about 1 x 10 ^*3 Torr (operating), before the thermal transfer through the gas in the envelope becomes limiting, and the gas tends to be less molecular and more viscous in its characteristics. This degradation of the vacuum may occur due to, e.g., outgassing of the detector or diffusion through seals. Since the evacuated chamber is sealed, degradation of the vacuum limits the operating life of the detector before repair or replacement. The housing is sealed using a single hard seal, available from Helicoflex (not shown). The expansion chamber 114 of the cryogenic refrigerator is linked in thermal communication to an internal mass damper 8, provided in order to reduce vibration transmitted to the radiation detector 34. This intemal mass damper 8 reduces vibration transmitted through the refrigerant supply 1 and retum 2 lines , as well as any vibration from turbulence, resonance or other vibration from the expansion chamber 4. The intemal mass damper 8 may also reduce mechamcal coupling from extemal vibration in the supply 1 and return 2 lines to the detector system. The intemal mass damper 8 is a copper cylinder weighing about 2 lbs. Vibrations reduce the resolution of the detector 34 and may create microphonics in the output signal.

The cryocooler 3 is mechanically and thermally linked to the internal

mass damper 8 by a plurality prestressed flexible copper straps 6, shown in FIG. 6, which are flexed in position. This connection serves to allow the internal mass damper 8 to attenuate vibrations from the cryocooler 3, while acting as a thermal conduit. These preflexed flexible copper straps 6 also serve to mechanically support the intemal mass damper 8.

The intemal mass damper 8 is linked to a flexible thermal coupling 10 to the cold finger 32. This flexible thermal coupling 10 consists of a plurality of flexible braided copper webs, which are flexed to provide maximum compliance along the axis of the cold finger 32. The cold finger 32 is thus supported separately from the cryocooler 3, as the flexible thermal coupling 10 between the internal inertial mass damper 8 and the cold finger 32 does not provide stiff mechanical support. The flexible thermal coupling 10, however, cannot be formed with sufficient flexibility to provide sufficient vibration isolation, and therefore it alone is insufficient to vibration isolate the detector 34.

The cold finger 32, which is preferably formed of copper, is concentrically secured within an extension of the envelope 35 from the cryocooler 3 and internal mass damper 8 by a thermally insulating disk 17. The thermally insulating disk 17 preferably is formed of G-10 fiberglass reinforced epoxy. Near the detector, a further spacing support is provided as a strip of Velcro ^® fastener hook portion 26 about 0.25 inch wide, wrapped circumferentially around the detector holder 23, which centers the detector holder 23 in the extension of the envelope 35.

The cold finger 32, which may be formed of G-10 fiberglass reinforced epoxy material, is a circular cylindrical copper cylinder which is preferably about 0.250 inches in outside diameter and is at a temperamre of less than about 100 K with a temperamre gradient from end to end. The cold finger 32 has a polished external surface that has a specular finish to provide a relatively low emissivity factor, for example, around 0.1. This low emissivity minimizes radiation coupling of the cold finger 32 to the housing members 35, 21, 15, which radiation coupling is the major source of heat transfer. The cold finger 32 is then wrapped in aluminized Mylar ^® sheet.

A radiation detector holder 23 assembly is thermally conductively secured to the cold fmger 32. This radiation detector holder 23 is preferably formed of aluminum. The extension of the envelope 35 has a thin, X-ray radiation transparent window 25 formed at an end thereof. This window 25 is sealed, so that the vacuum within the housing is maintained.

The radiation detector holder 23 assembly holds an X-ray detector crystal

34, which is preferably a germanium or lithium drifted silicon X-ray detector, as known in the art. The preferred size is 10 mm ² . This crystal detector 34 operates with an externally generated bias voltage, to sweep the induced charge to the gate of a FET 28. A wire conductor couples the sleeve to a source of bias voltage (not shown) for biasing detector. The FET is electrically and mechanically coupled to the detector crystal by a beryllium-copper spring 36, which resiliently holds the detector crystal 34 in place. The electrical output from the FET amplifier circuit is a thin wire which exits from the envelope for connection to other electronic circuits (not shown).

The radiation detector holder 23 assembly also supports other portions of a field effect transistor (FET) electrical amplification circuit (not shown). Other elements of the electronic circuit include a resistive heater, which dissipates about 0.25 W, provided to heat the FET by about 40 K, several diodes, a light emitting diode to blank the FET, and other elements. If other electrically dissipative elements are provided, they may advantageously be mounted near the FET to help provide the necessary heat. Other types of detectors are known, and may be employed, including germanium crystals, used as X-ray or gamma ray detectors, and which generally require lower temperamres for high performance than silicon (lithium drifted) crystal detectors.

EXAMPLE 2 The apparams generally according to Example 1 is provided with a large amount molecular soφtion getter 5, 16, provided as portions of activated carbon material, as shown in FIGs. 5 and 6. According to this embodiment, the void volume 22 in the evacuated chamber is minimized. This is in contrast to the design according to Example 1 , wherein the void volume 22 is not particularly minimized, except to reduce the internal surface area. In fact, under normal circumstances, the void volume in the apparams of Example 1 is not minimized, so that the adverse effects of outgassing are reduced or diluted in a larger volume.

The molecular sorption getter 5, 16 therefore may occupy a sigmficant portion of the void volume 22 in the evacuated space. For example, the molecular sorption getter 5, 16 may be placed around the expansion chamber 4 and adjacent to the counter flow heat exchanger 3. The molecular sorption getter 5 preferably does not link the internal mass damper 8 and the cryocooler 3, as these should be free for effective damping of vibration. An additional molecular sorption getter 16 may also be provided on the internal mass damper 3, e.g., adjacent to the flexible copper thermally conductive strap 10.

The molecular sorption getter 5 is provided in firm thermal contact with cooled components of the device, by, e.g., a thermally conductive epoxy. Such a thermally conductive epoxy may be a silver powder filled two-part epoxy material. Additional molecular sorption getter 16 material is provided in a thermally conductive pouch mounted with epoxy to the intemal inertial mass damper 8, e.g., thin wall aluminum foil having gas permeable apertures.

Altematively, the molecular sorption getter may be provided as an organic polymer shaped in the desired configuration which is pyrolyzed to produce a shaped getter. The molecular sorption getter may also be a zeolite or synthetic zeolite material, as known in the art.

EXAMPLE 3

In order to provide improved electrical performance, it is desired to electrically isolate the compressor system from the detector. The compressor 45 is electrically operated, and thus has a magnetic fields which may induce small currents.

Further, the compressor 45 may be distant from the detector, and therefore the possibility of significant ground loops is present. Finally, by electrically isolating the detector 34 from the compressor 45, the electronic outputs of the detector may be more easily integrated with the electronics of the X-ray device without interference. Normal supply 1 and return 2 mbing for compressed refrigerant gas is formed of malleable copper mbing. This mbing 1, 2 is conductive. Generally available flexible non-conductive mbing may be unsuitable for this purpose, as it may leak, introduce contaminants or kink.

The present invention employs standard conductive malleable metal mbing with an electrical isolation device 50, as shown in FIG. 8, in line with the mbing. The isolation device 50 must contain the high pressure of the supply line, without introducing contaminants.

The electrical isolation device includes a glass or ceramic length of tubing 52, preferably Macor ^® , which is about preferably about 5-10 cm in length, although any length sufficient to provide electrical insulation may be used. Each end of the mbe 52 is counterbored with a recessed groove 56. The mbing 52 is brazed to stainless steel fittings 51, 53 which conform to the counterbored recessed grooves 56 to form a gas-tight and mechanically strong seal. The brazed fittings 51, 53 are further coupled to stainless steel mechanical fittings 54, 55, for connection to the supply 1 and return 2 line mbing.

In a preferred embodiment, on at least one side of the isolation device 50, the stainless steel fitting is a standard-type self sealing, quick release fitting, available from Aeroquip, allowing the detector to be easily separated from the compressor.

It should be understood that the preferred embodiments and examples described herein are for illustrative purposes only and are not to be construed as limiting the scope of the present invention, which is properly delineated only in the appended claims.