Vibration-free refrigeration transfer

Abstract

An apparatus for transferring refrigeration to an object to be cooled without mechanical contact of the refrigerator and object so that vibrational loads are not transferred to the object. The apparatus is characterized in that a fluid transfer medium is caused to circulate in a confined path adjacent the refrigeration source and the object to effect cooling of the object. An apparatus such as disclosed is ideally suited for cooling samples for Mossbauer Spectroscopy.

Description

BACKGROUND OF THE INVENTION

This invention pertains to the field of refrigeration and, in particular, transfer of refrigeration from a refrigeration source to an object to be cooled without transfer of vibrational forces from the refrigeration source to the object being cooled. Such vibrationless transfer of refrigeration is required for among other things, cooling samples to be examined by emission and absorption spectroscopy. One branch of spectroscopy for which this type of refrigeration transfer is absolutely critical is that which is known as Mossbauer Spectroscopy. In the late 1950's, one Rudolf Mossbauer discovered that nuclei that are embedded in solids can emit and absorb low-energy gamma rays which display the natural line width and possess the full transition energy. No recoil energy is transferred to the lattice vibration. This discovery is known as the Mossbauer effect. In those nuclei with low Debye temperatures, it becomes necessary for the gamma-ray emitter and absorber to be cooled to cryogenic temperatures on the order of 10.degree.K (Kelvin). In order to achieve these temperatures, it had been necessary to employ a source of liquid helium and/or liquid nitrogen. For studies following coulomb excitation or nuclear reactions, the time required for a single run can comprise several days. In order to achieve a single run, the cost of liquid helium and the labor involved in maintaining a level of liquid helium are significant.

More recently, it has been proposed to use a closed-cycle helium refrigerator to cool the sample for Mossbauer studies. This is set forth in an article entitled, "The Use of a Helium Refrigerator for Mossbauer Studies," by Y. W. Chow, E. S. Greenbaum, R. H. Howes, F. H. H. Hsu, P. H. Swerdlow, and C. S. Wu, which appeared in Nuclear Instruments and Methods 66 (1968) at Pages 177-180 published by the North-Holland Publishing Company. In this article the authors described a helium refrigerator mechanically coupled to the specimen holder to be cooled by a closed-cycle helium refrigerator. The article sets forth a bellows arrangement to minimize transfer of the vibrations associated with the closed-cycle helium refrigerator from being transferred to the sample holder.

The foregoing article and most of the prior art devices that are used commercially require a mechanical contact between the refrigerator and the sample being cooled, such contact is usually achieved by flexible braided copper straps to further minimize transfer of vibration from the refrigerator to the sample being cooled.

It is apparent that if the mechanical coupling has a large enough cross section to be a good heat conductor then it will also transmit vibration thus a compromise is always required between the desire to have a large cross section for good heat transfer and a small cross section to minimize transmitted vibration.

SUMMARY OF THE INVENTION

In order to overcome the above-described problems and to provide a more effective vibration-free transfer of refrigeration from a mechanical refrigeration source to a sample to be cooled, it has been discovered that if a quantity of a fluid transfer medium is circulated in a confined path adjacent the refrigeration source (e.g. cold end of the closed-cycle helium refrigerator) the circulating fluid transfer medium can be brought in contact with the sample holder, and the sample can thus be effectively cooled. In the foregoing apparatus, the sample holder can be mounted independently of the refrigerator, thus avoiding any mechanical contact between the source of refrigeration and the sample undergoing cooling.

Therefore, it is the primary object of this invention to provide an improved apparatus for transferring of refrigeration.

It is another object of this invention to provide an apparatus for transfer of refrigeration between a refrigeration source and a sample to be cooled without mechanical contact of the refrigerator and the sample.

It is still another object of this invention to provide an apparatus for transfer of refrigeration from a closed-cycle helium refrigerator to a sample holder.

It is a further object of this invention to provide a minimized vibration environment for cooling a sample holder in Mossbauer Spectroscopy.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an elevational view partially in section of an apparatus according to the present invention.

FIG. 2 is an enlarged section taken along line 2--2 of FIG. 1.

FIG. 3 is a fractional portion of the apparatus of FIG. 1 enlarged for clarity and illustrating another embodiment of the present invention.

FIG. 4 is an elevational view partially in section showing another embodiment of the present invention.

FIG. 5 is an elevational view partially in section showing a device according to the present invention suitable for cooling a superconducting magnet.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention in its broadest aspects will be described as applied to a sample holder for Mossbauer Spectroscopy. The sample holder portion of the Mossbauer Spectroscopy technique is shown in FIGS. 1 and 2 and includes a closed-cycle helium refrigerator shown generally as 10. The refrigerator 10 is of the two-stage type comprising a first stage 12 and a second stage 14. Such a refrigerator is capable of producing temperatures of 10.degree.K at the lower end of the second stage (cold end of the refrigerator) and extension stud 16. Stud 16 is generally of a high conductivity material such as copper. A refrigerator such as 10 is disclosed in detail in U.S. Pat. No. 3,620,029, which Patent specification is incorporated herein by reference. The present applicant is also the patentee of the aforementioned U.S. Pat. No. 3,620,029.

The refrigerator 10 is mounted in a support ring 18, which is in turn fixed to a rigid support such as the floor of the laboratory building by support member 20. The device also includes a sample holder designated generally as 22, which sample holder is held by support 24 which support 24 is in turn fixed to the optical bench by support members 26. The refrigerator 10 stages 12 and 14 are inserted inside sample support 22. In order to provide a closed system, a flexible vibration isolating sleeve 28, such as a very thin-walled, corrugated rubber sleeve, is affixed to the refrigerator 10 and sample holder 22 with hose clamps as shown in FIG. 1.

The sample holder 22 includes an outer vacuum shroud 30 with a suitable vacuum outlet 32 for evacuating the cavity defined by the vacuum shroud and the inner system components as will hereinafter be more fully described. Disposed within the vacuum shroud and surrounding the first stage 12 of the refrigerator is a stainless steel inner vacuum jacket sleeve 34. Affixed to the lower end of sleeve 34 is a radiation shield support adaptor 36. Disposed below the support adaptor 36 and affixed thereto as by a threaded connection is a second stage radiation shield 38. The radiation shield 38 can be a solid copper cylinder or can be with an aluminized plastic overwrap such as is commonly employed in cryogenic devices. Disposed within radiation shield 38 and affixed in fluidtight relation to sleeve 34 is an inner vacuum jacket extension 40 which surrounds the second stage 14 of the refrigerator 12. Affixed to the bottom of jacket 40 is a specimen holder adaptor 42, which in turn supports the sample holder 44. The specimen holder adaptor 42 includes a series of vertical passages 43, the function of which will be more fully explained hereinafter.

Disposed around the first stage 12 of the refrigerator is a first stage heat exchanger 46 and disposed around the copper stud 16 is a second stage heat exchanger 48. The heat exchangers are shown in cross section in FIG. 2 and can in one embodiment be manufactured from a sheet of copper, which is wrapped in cylindrical fashion with a plurality of vertical spacers 49, thereby defining a series of vertical flow passages through the heat exchangers 46 and 48. The heat exchangers 46 and 48, while different in size, are identical in structure. The heat exchangers provide for convective circulation of the fluid transfer medium through the vertical passages as will hereinafter be more fully explained.

The system is completed with an instrument feed through 50 for measuring temperature and the like. There is also provided a fitting 52 for introducing the fluid transfer medium into the area between the refrigerator stages 12, 14 and sleeves 34 and 40. Fitting 52 includes a pressure relief valve for maintaining one atmosphere pressure of fluid transfer medium in the nonevacuated spaces. There is also inlet conduit 54 and outlet conduit 56 for introducing and removing helium from the refrigerator 10. The inlet and outlet conduits are connected to a compressor (not shown) by flexible conduits (not shown) so that the compressor can be isolated from the refrigerator thereby minimizing any vibration forces produced by the compressor from being transferred to the refrigerator 10 and hence to sample holder 44.

The device is assembled as shown with the flexible sleeve 28 fitted in fluid-tight relationship between the refrigerator 10 and the sample holder 22. A fluid transfer medium, preferably helium, is introduced through fitting 52 and fills the space between the refrigerator 10 (stages 12 and 14) and the sample holder internal sleeves 34, 40, and 42. The fluid transfer medium port 52 is connected to a source of fluid transfer medium that will maintain one atmosphere pressure and the refrigerator is energized, thereby producing refrigeration at the lower end of first stage 12 and second stage 14. The refrigeration causes the fluid transfer medium to circulate in a vertical manner through the heat exchangers 46 and 48 by convective phenomenon. At the lower end of second stage 14, the passages 43 is specimen holder 42 lengthen the circulation path of the fluid transfer medium, thereby achieving good exchange of refrigeration between the second stage 14 of refrigerator 10 and the specimen holder 44. Since there are no mechanical connections between the heat exchangers and the sample holder, there is no vibrational loading directly communicated between the refrigerator and the sample holder. Furthermore, the flexible vibration isolating sleeve 28 prevents transfer of vibration or other motion induced forces from other parts of the refrigerator to the sample holder 22. The vibration isolating sleeve 28 can be of a very thin flexible material as long as the system operates at one atmosphere, thereby having equalized pressure inside of the cavity filled by the fluid transfer medium and outside as determined by ambient pressure. As long as there is no differential pressure, the flexible isolating sleeve will isolate vibration and prevent transfer of vibration forces to the sample holder 44.

It is apparent that with the device, as illustrated in FIG. 1, the design criteria of maximum transfer of refrigeration per minimum volume of fluid transfer medium can be achieved by the convective circulation. The driving force for circulation is the difference in density between the warm and cold gas and the length of the confined circulation path as determined by the length of the heat exchanger 48 in relation to the extreme cold end of the refrigeration source. The circulation is somewhat enhanced by the passages 43 and specimen holder 42, however, there is shown in FIG. 3 a further means of enhancing the circulation. As shown in FIG. 3, a generally cylindrical chimney 100 is interposed between the heat exchanger 48 and the specimen holder 42, thereby defining a long flow path through the heat exchanger down to the sample holder 44 and around through passages 43 back up to the upper end of heat exchanger 48. Such a chimney is preferably made from a nonconductive material such as bakelite or other plastic material. The chimney does not mechanically link the refrigerator 10 to the sample holder 44 so no vibrational loads are transmitted to the sample holder 44, thus making the device ideally suited for Mossbauer effect studies.

It is also possible to achieve various temperature levels and temperature control by using a different transfer fluid in conjunction with the refrigerator 12 and the sample holder 22 as shown. For example, at high temperature levels (e.g. 20.degree.K) more effective refrigeration transfer can be achieved when the fluid transfer medium is hydrogen and is condensed to a liquid and then evaporated. A mixture of transfer fluids will achieve a different range of temperatures so that this condensing mode can be adequately effected.

There is shown in FIG. 4 yet another embodiment of the present invention wherein the overall length of the refrigeration system is shortened. This is achieved by, in essence, folding over the two stages of the circulating fluid transfer system to effect the shortened version of the system. In the embodiment of FIG. 4, there is shown a refrigerator 10' having a first and second stage 12' and 14' respectively as with the embodiment of FIG. 1.

In the embodiment of FIG. 4, the refrigerator is disposed inside of a fluid housing 70. The housing 70 is isolated from the refrigerator as in FIG. 1 by a thin, flexible sleeve 28'. The first stage 12' has two heat exchangers 58, 60 with a chimney device 62 therebetween. The chimney device is affixed to heat exchanger 58 but does not contact heat exchanger 60. The second stage 14' has heat exchangers 64, 66, and chimney 68 constructed in an identical manner. The second stage 14' heat exchangers 64, 66 are disposed within sleeve 72, which sleeve is made to be recessed inside sleeve 70 as shown. Disposed on the end of sleeve 72 is a cold end cap 74, which is cooled by the second stage 14' cold end of the refrigerator. As with the embodiment of FIG. 1, the fluid transfer medium, e.g. helium, is introduced through a suitable port 52 to fill the space between the refrigerator stages 12', 14' and corresponding shells 70, 72. The heat exchanger and chimney combinations, in co-operation with the cooling effect of the refrigerator, causes convective circulation in both stages of the fluid transfer medium, thus affecting refrigeration without the mechanical coupling of the refrigerator to the cold end plate 74.

There is shown in FIG. 5 yet another embodiment of a refrigeration system wherein the refrigeration system is applied to cooling a superconducting magnet, which magnet structure includes an outer vacuum shell 80, a first heat shield 82 which is referred to as a 75.degree.K shield, a second heat shield 84 referred to as a 20.degree.K shield, a liquid helium dewar 86 containing liquid helium, a magnet cavity 88, magnet 90, helium fill tube 92 and electrical feed through 94. The refrigerator 10' is identical to that shown in the embodiment of FIG. 4 having first stage 12' and second stage 14' with heat exchangers as shown. The first stage 12' serves to maintain heat shield 82 at temperature of about 75.degree.K and second stage 14' serves to maintain shield 84 at a temperature of about 20.degree.K thereby minimizing heat leaking into the liquid helium in dewar 86. With the above-described setup, it would be possible to provide a Joule-Thompson type cryostat in conjunction with the refrigerator to maintain the level of liquid helium in the dewar 86 by taking the vaporized helium in the space above the liquid level and recondensing it to liquid. Such cryostats are well known to a worker skilled in the art and their inclusion would be readily achieved by a skilled artisan.

While the heat exchangers have been described as preferably being constructed from a sheet copper with spaces brazed thereon, it is possible to use many types of heat exchangers including folded copper sheet, a plurality of tubes or the like that will provide the same elongated closed flow path for circulation of the liquid transfer medium at the various stages of the refrigerator.

It is also within the purview of this invention to use a single-stage refrigerator for providing a fixed cryogenic temperature at the cold end of the refrigerator in an identical manner using similar structure.

It is also within the purview of the invention to use other fluid transfer means such as nitrogen, argon, air, hydrogen, halocarbons, noble gases, methane, and mixtures thereof.

It is still further within the purview of the invention to provide small mechanical fans or a like forced circulating device at the extreme ends of the various stages of the refrigerator to further enhance fluid transfer medium circulation.

Having thus described my invention, the following is desired to be secured by Letters Patent of the United States.

Claims

I claim:

1. Apparatus for transferring refrigeration from a cold refrigeration source to an object to be cooled comprising in combination:

a source of cryogenic refrigeration;

means for confining and circulating a fluid transfer medium in contact with the source of refrigeration; said means including a structural member affixed to the source of refrigeration thereby producing an elongated confined flow path for the fluid transfer medium; and

a support means for holding an object to be cooled in contact with the circulating fluid transfer medium, the support means being mounted in spaced relation to the refrigeration source to prevent forces induced by motion of the refrigeration source from being transmitted directly to the object support means.

2. An apparatus according to claim 1 wherein the refrigeration source and object support means are disposed within a vacuum jacket and the fluid transfer medium is maintained at a pressure of one atmosphere.

3. An apparatus according to claim 1 wherein the means for confining and circulating the fluid transfer medium includes a first heat exchanger in contact with the refrigeration source, the heat exchanger having a plurality of elongated generally parallel fluid passages therein for inducing convective circulation of the fluid transfer medium.

4. An apparatus according to claim 3 including a second heat exchanger positioned coaxially to the first heat exchanger in contact with the refrigeration source thereby extending the length of the circulation path of the fluid transfer medium.

5. An apparatus according to claim 4 wherein a chimney is interposed between said first and second heat exchangers to further enhance convective circulation of the fluid transfer medium.

6. An apparatus according to claim 1 including a flexible bellows-shaped sleeve between the refrigeration source and the object support means operable as part of the means for confining the fluid transfer medium, thus preventing transfer of forces induced by motion of the refrigeration source to the object support means.

7. An apparatus according to claim 6 wherein the fluid transfer medium is maintained at a pressure of one atmosphere.

8. An apparatus according to claim 6 wherein the fluid transfer medium is helium.

9. A superconducting magnet cooling apparatus comprising in combination:

an elongated cryogenic refrigerator with a cold end disposed adjacent the superconducting magnet;

means for confining and circulating a fluid transfer medium in contact with the refrigerator cold end, said means including a heat exchanger affixed to the refrigerator cold end whereby the heat exchanger causes the fluid transfer medium to circulate in an elongated confined flow path; and

a superconducting magnet support, said support positioning the superconducting magnet in contact with the circulating fluid transfer medium, the support mounted to prevent forces induced by motion of the refrigerator from being transmitted directly to the superconducting magnet.