Patent Publication Number: US-6334314-B1

Title: Cryostat nozzle a method of using a cryostat

Description:
TECHNICAL FIELD 
     The present invention relates to the use of a cryostat for maintaining a specimen at a low temperature so as to allow scientific examination or experiment to be accurately performed. More particularly, the present invention is concerned with a nozzle for use in such a cryostat. 
     BACKGROUND ART 
     X-ray crystallography is well-known as one particular scientific method in which a specimen may be subjected to a beam of x-rays with the x-ray diffraction patterns obtained being indicative of the crystalline structure of the specimen. Another known method is neutron crystallography. In general, biological samples subjected to such methods are more robust if they are frozen, and uniform results are more likely to be obtained if their temperature is kept stable. Thus, chilling of samples, for example, by the use of a cryostat, and particularly by open stream cooling, is well known. Very cold temperatures, for example those at less than the 77.4 degrees Kelvin boiling point of liquid nitrogen, are highly desirable, since they allow crystal phase changes to be studied. Such very cold temperatures are obtainable by the use of a ‘cryogas’, a term which in this specification is defined as meaning helium, neon or hydrogen in a gaseous or liquid phase. 
     U.S. Pat. No. 5,653,113 discloses a cooling system comprising a nozzle pipe for jetting low-temperature gaseous nitrogen from a tip end opening thereof, the nozzle pipe being disposed inside a cooling chamber supplied with ordinary temperature nitrogen gas. However, the system has the disadvantage of not being adapted to cool samples below the boiling point of nitrogen to temperatures at which x-ray crystallography is particularly fast and accurate. 
     U.S. Pat. No. 4,295,339 discloses a cryostatic system utilising a liquefied gas in which an object to be cooled in a thermostatic chamber is cooled initially by a liquid coolant at a cryogenic temperature and then by a gaseous coolant. The coolant is typically nitrogen or carbon dioxide, thus the system suffers the same disadvantages as that disclosed by U.S. Pat. No. 5,653,113, when the coolant is in its gaseous form. 
     It is an object of the present invention to provide an improved method of using a cryogas cryostat and an improved nozzle for use in such a cryogas cryostat. 
     DISCLOSURE OF INVENTION 
     In accordance with the present invention, a method of using a cryogas cryostat comprises supplying a stream of cryogas over a specimen, and surrounding said stream in the vicinity of the specimen by a dry annular flow of cryogas at ambient temperature, characterised in that the stream of cryogas is helium at a temperature of between 4.2 and 77.4 degrees Kelvin and the dry annular flow of cryogas is the same cryogas. 
     Preferably in the vicinity of the specimen there is no solid barrier between said stream and said annular flow, and the speed of flow of said stream is substantially the same as the speed of said annular flow. 
     Preferably further in the vicinity of the specimen there is no barrier at all between said stream and said annular flow, whereby said stream is supplied adjacent the surrounding annular flow. 
     Preferably further in the vicinity of the specimen there is supplied an outer annular dry ambient airflow surrounding the annular flow of ambient temperature cryogas. 
     Preferably further the speed of flow of said stream in the vicinity of the specimen is at least 25 cms/second. 
     Preferably further the speed of flow of said stream in the vicinity of the specimen is less than 10 meters/second. 
     Preferably further the speed of flow of said stream in the vicinity of the specimen is between about 50 cms and 1 meter/second. 
     Preferably the method includes positioning the cryostat in such a disposition that said stream is supplied generally downwardly. 
     The present invention further consists in a nozzle for a cryogas cryostat comprising a central feed tube for supplying a stream of cryogas over a specimen, and an annular feed opening for supplying a dry annular flow of cryogas at ambient temperature surrounding said stream in the vicinity of the specimen, characterised in that the stream of cryogas is a stream of helium, that the dry annular flow of cryogas is also helium, and that said cryogas cryostat also comprises means for supplying said stream of helium at a temperature of between 4.2 and 77.4 degrees Kelvin to the central feed tube and means for supplying helium at ambient temperature to the annular feed opening. 
     Preferably in the vicinity of the specimen there is no solid barrier between said stream and said annular flow. 
     Preferably also the annular feed opening is surrounded by a shield extending beyond a mounting point for the specimen. 
     Preferably further the shield is beryllium having a high thermal conductivity and good thermal capacity to remain above freezing point in use. 
     Alternatively the shield is optically transparent and the nozzle has means to supply dry ambient temperature air along the outside of the shield in the vicinity of the specimen. 
     Preferably the annular feed opening is connected from a corresponding annular feed chamber. 
     Preferably further the annular feed chamber and annular feed opening connected to it comprise a radially inner wall which is of insulating material. 
     Preferably further the interior of the nozzle is a vacuum chamber. 
     The present invention also consists in a cryostat comprising a cryostat nozzle according to any one of the preceding statements of invention. 
     Preferably further the cryostat is provided with a source of cryogas to be connected to the central feed tube and operable at less than 77.4 degrees Kelvin and is provided with a dry source of the same cryogas at ambient temperature to be connected to the annular feed chamber. 
     Other preferred features of the invention will be apparent from the following description and from the subsidiary claims of the specification. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will now be further described, merely by way of example and relating to the use of gaseous helium as the cryogas, by reference to the accompanying drawings, in which: 
     FIG. 1 is a sectioned elevation of a cryostat nozzle according to a first typical example of the invention, and taken along the line  1 — 1  of FIG. 2, 
     FIG. 2 is an end view taken in the direction of the arrow  2  shown on FIG.  1 . 
     FIG. 3 is a sectioned elevation of a cryostat nozzle according to a second typical example of the invention, and taken along the line  3 — 3  of FIG. 4, 
     FIG. 4 is an end view taken in the direction of the arrow  4  shown on FIG.  3 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring to FIGS. 1 and 2 of the drawings, a cryostat nozzle  10  of the first typical example comprises a generally frustoconical ring  11  having an outer annular land  12  screwed at  13  onto the circular end of a body  14  of a cryostat and sealed thereto at  15 . The ring  11  has an inner annular land  16  screwed at  17  and sealed at  18  to an outer hoop  19  of a complex collar  20 . 
     The upper surface of the hoop  19  of the collar  20  extends inwardly in a flange  21  axially spaced from the land  16  to a downwardly extending tubular section  22 . The land  16  radially inward of the hoop  19 , the hoop  19  itself, the flange  21  and the tubular section  22  define an annular chamber  23  to be fed with dry helium gas at ambient temperatures, e.g. about 25 to 30 degrees Celsius, through a pipe  24  in the body  14 . 
     In the lower radially inward region of the annular chamber  23  there is provided an annular passage  25  leading to an annular clearance  26  between the remainder of the tubular section  22 , a conical lower end  27  to the collar  20  and the adjacent inner surfaces of the ring  11 . Ambient temperature helium gas in the annular chamber  23  passes through the annular passage  25  and along the annular clearance  26 . 
     The collar  20  also comprises a tubular body  28  upstanding from the conical lower end  27  and surrounding but spaced from a central feed tube  29  for supplying a stream of cold gaseous helium, usually at a temperature of less than 77.4 degrees Kelvin, from within the body  14  to an axial cylindrical hole  30  through the conical lower end  27  on which the adjacent end of the central feed tube  29  is mounted. 
     It is to be noted that all of the parts of the complex collar  20  which are shown cross-hatched in FIG. 1 are made of a good thermal insulating material such as expanded polystyrene foam, and in use the interior of the body  14  is evacuated, so as to minimise heat transfer from the body  14  or from the ambient temperature helium gas to the very cold helium gas in the feed tube  29  and the axial cylindrical hole  30 . The tubular inner face of the tubular body  28  is provided with a sealing tube  31  of a poor thermal conductor, such as stainless steel and the tubular inner face of the tubular section  22  is provided with a sealing tube  32  of a poor thermal conductor, such as stainless steel to maintain the integrity of the vacuum in the body  14 . 
     The tip  33  of the ring  11  opens to an axially-aligned circular tubular shield  34  of a material transparent to x-rays, e.g. beryllium. The shield  34  has an internal cross-sectional area which is approximately the same as the sum of the cross-sectional areas of the axial cylindrical hole  30  and the adjacent mouth of the annular clearance  26 . 
     Specimen mounting means are not shown, but in this first typical example, the length of the shield  34  is about the same as its internal diameter, which is sufficient to extend beyond a specimen mounting point  35  and to allow rotation of the specimen around a range of axes within a wide cone of maneuverability defined by the specimen mounting point  35  and the circular bottom edge of the shield  34 . Alternatively the shield  34  may be longer if the specimen mounting means permits. The specimen mounting point  35  is sufficiently far away from the tip  33  of the ring for the x-ray diffraction pattern emanating from the specimen to be unimpeded by the cryostat nozzle  10 , passing freely through the wall of the beryllium shield  34 . 
     The bottom edge of the axial cylindrical hole  30  is provided with an extension tube  36  of a poor thermal conductor, such as nylon. The tube  36  passes across the mouth of the annular clearance  26  to align the helium gas flows, but ends short of the specimen mounting point  35  in order not to impede the x-rays. The width of the gap between the tube  36  and the shield  34  is appropriate to maintain the integrity of the annular gas flow through it to the far end of the shield  34 , i.e. between about 1 and 3 mm. 
     The ring  11  is provided with an electrical heater  37  wrapped around its exterior just below the inner annular land  16 . The heater  37  can be powered when necessary to ensure that the ring  11 , and (by thermal conduction) the shield  34  are warm enough to avoid atmospheric water condensation, e.g. about 25 to 30 degrees Celsius. 
     When the cryostat is used with the specimen mounted at the mounting point  35 , the feed tube  29 , the axial cylindrical hole  30  and the extension tube  36  supply a stream of cold gaseous helium usually at a temperature of less than 77.4 degrees Kelvin over the specimen. The speed of flow of the cold helium stream is important, preferably being at between 25 cms and 10 meters per second in the vicinity of the specimen, more preferably between about 50 cms and 1 meter per second, to provide uniform flow of helium gas of low density towards and against the higher density air beyond the shield  34 , with reasonable economy of helium usage. The density of helium gas is higher at lower temperatures, which can be as low as its boiling point of 4.2 degrees Kelvin, thereby reducing the difference of densities between gaseous helium and room temperature air and allowing the speed of flow of the cold helium gas to be reduced. 
     At the same time, the stream of cold gaseous helium is surrounded in the vicinity of the specimen by the annular flow of dry helium gas at ambient temperature, e.g. about 25 to 30 degrees Celsius, from the annular clearance  26  and around the outside of the extension tube  36 . The speed of the annular flow is preferably the same as the speed of the cold helium stream in order to match laminar flows and hence minimise mixing between the cold helium stream and the ambient helium annular flow, which mixing would begin to heat the stream of cold helium. 
     The ambient temperature helium annular flow alongside the inside of the shield  34  stops the latter from being chilled, and hence prevents ice formation from the surrounding environment on the shield  34  and the cryostat nozzle  10 . This effect of the ambient temperature helium is supplemented by thermal conduction from the cryostat body  14 , and can be boosted by the heater  37  if required. Thus not only ice but even water condensation on the shield  34  can be avoided. 
     Preferably the cryostat is disposed to supply the stream of cold helium generally downwardly, but perhaps at an angle of some 10 to 45 degrees to the vertical for a stable gas flow pattern, to keep good control of the helium flow and to keep air out of the shield  34 . 
     If the specimen is to be examined by x-ray crystallography, the x-ray source is to one side of the shield  34  which does not significantly affect the x-rays incident on the specimen, or the x-rays diffracted by the specimen. 
     In the second typical example of the invention shown in FIGS. 3 and 4, to which reference is now made, a set of co-axial optically transparent shields  40 , 41  replaces the beryllium shield  34  of the first typical example. This enables a beam of light, for example from a laser, to be trained on the specimen cooled by the cryostat, and the resulting reflected, refracted or diffracted images studied. A polyamide such as “KAPTON” (Trade Mark) is preferably used, or a polyester such as “MYLAR” (Trade Mark). These plastics can be very thin, such as 0.5 mm, which means that x-ray crystallography can still be carried out with this second example without too much degradation of the x-ray diffraction patterns. 
     In FIGS. 3 and 4, the same reference numerals are used for the same parts as those in FIGS. 1 and 2 of the first typical example, but the beryllium shield  34  of the first typical example is changed to a “KAPTON” inner shield  40  of the same configuration, and a “KAPTON” cylindrical outer shield  41  is provided around and coaxial with the inner shield  40 . The outer shield  41  is mounted on frustoconical air jacket  42  that surrounds the ring  11  below the heater  37  to feed dry ambient temperature air from a supply  43  as an outer annular dry ambient air flow between the inner shield  40  and the outer shield  41  in the vicinity of the specimen at approximately the same speed as the annular flow of ambient temperature helium beside it but on the inside of the inner shield  40 . 
     The reason for adding the air jacket  42  and the outer shield  41  in the second typical example is that the plastics materials used for the inner shield  40  have poor thermal conductivity and low thermal capacity. 
     Consequently, any stray inter-mixing between the helium flows which even temporarily chills a portion of the plastic inner shield  40  would cause condensation or freezing if moisture-laden ambient air were present, whereas the high thermal conductivity and relatively good thermal capacity of much thicker beryllium prevents this from happening in the first typical example. 
     The uniform speed of flow for all of the gas streams reduces turbulence and promotes laminar flow when their separation by the shields ceases, keeping the nature and integrity of each of the gas streams over and for as far as possible away from the specimen mounting point  35 . 
     Gaseous helium has been used as the cryogas in the descriptions of the first and second typical examples above, but at about 4.2 degrees Kelvin helium may be used in mixed phases with droplets of liquid helium dispersed in gaseous helium. Helium may also be used as a liquid as long as the specimen is adequately mounted.