Abstract:
A closed cycle cryocooler is thermally connected to an elongated, cup-shaped sample well and cools down the sample well. Gaseous helium at a relatively low pressure is introduced into the sample well so that, as the sample well is cooled by the cryocooler, the gas in the sample well is also cooled. A sample is attached to a sample stick assembly which is then lowered into the sample well where the sample is cooled by the cooled gas to carryout experiments at low temperature. The sample stick assembly is mechanically attached to the spectrometer magnets and a flexible rubber bellows connects the sample stick assembly to the sample well so that vibration generated by the cryocooler is not transferred to the sample.

Description:
BACKGROUND 
       [0001]    Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes that include a transition metal ion. According to quantum theory, an electron has a spin which can be understood as an angular momentum that produces a magnetic moment. If the electron is placed in a magnetic field the magnetic moment will tend to align with the magnetic field. However due to quantum effects, the electron can only have two states: one with the magnetic moment aligned parallel to the applied field and a second with the magnetic moment aligned anti-parallel to the field. Each of these two states has a different energy level. If electromagnetic radiation is applied at a frequency that corresponds to the separation between the two energy levels, energy is absorbed from the electromagnetic field and this absorption can be measured. An EPR spectrum can be produced by varying either the electromagnetic radiation frequency or the applied magnetic field strength and measuring the energy absorption. In practice, the latter is generally varied. 
         [0002]    Because most stable molecules have all their electrons paired, the EPR phenomenon is not generally observable in those molecules. Some molecules, known as paramagnetic molecules, have an odd number of electrons, which obviously cannot be paired. It is these molecules that are commonly studied via EPR techniques. This limitation to paramagnetic species also means that the EPR technique is one of great specificity, since ordinary chemical solvents and matrices do not give rise to EPR spectra. 
         [0003]    In many EPR experiments, it is either advantageous or necessary to measure the EPR sample at greatly reduced temperatures (4-10K). The advantages of operating at low temperature include an increase in signal levels from samples where relaxation times are very short at room temperature and the ability to study phase transitions. 
         [0004]    There are several methods for cooling a sample to the range of several degrees Kelvin. The most widely used method is to immerse the sample in a bath of liquid helium or to place the sample in a sample well where it is immersed in vapor flowing from evaporation of the liquid helium. However, this method has several drawbacks. Liquid helium itself is relatively expensive and, if the liquid helium must be shipped to the work site, there is inevitably some loss of liquid helium due to boil-off, making the liquid helium even more expensive. Further, as the helium evaporates, the gas is generally vented to the atmosphere from the top of the sample well and lost so that typical experiments use several liters of liquid helium each. Since helium boil-off is continuous, it is not economical to allow the EPR apparatus to remain at low temperature between experiments, thus experiments must be conducted as rapidly as possible and scheduled together to conserve helium. In any case, the helium must be replenished every few hours of operation and thus long term experiments are not possible. 
         [0005]    In order to overcome these difficulties, systems have been developed that do not use liquid helium. These systems generally use a closed-cycle refrigerator, such as a conventional Gifford-McMahon (GM) refrigerator or a pulse tube refrigerator to cool a metal “cold head” to the required temperature. The sample to be cooled is mounted on the cold head and cooled by direct conduction. These systems also have drawbacks. First, since the sample is mechanically connected to the cold head, any vibrations produced by the refrigeration mechanism are transferred to the sample. These vibrations are typically on the order of 1-2 hertz and typically do not cause problems with pulsed EPR experiments because the pulse time is much shorter than the vibration cycle time. However, the vibrations can cause problems with continuous EPR experiments. Second, in order to insulate the cold head and the sample, these latter elements are typically enclosed in a housing which is evacuated. Therefore, the cold head must be brought to a raised temperature and the housing must be vented prior to changing the sample. After the sample has been changed, the housing must be evacuated and the cold head brought down to the correct temperature, both of which are time-consuming operations. 
       SUMMARY 
       [0006]    In accordance with the principles of the invention, a closed cycle cryocooler is thermally connected to an elongated, cup-shaped sample well and cools down the sample well. Gaseous helium at a relatively low pressure is introduced into the sample well so that, as the sample well is cooled by the cryocooler, the gas in the sample well is also cooled. A sample is attached to a sample stick assembly which is then lowered into the sample well where the sample is cooled by the cooled gas to carryout experiments at low temperature. The sample stick assembly is mechanically attached to the spectrometer magnets and a flexible rubber bellows connects the sample stick assembly to the sample well so that vibration generated by the cryocooler is not transferred to the sample. 
         [0007]    In one embodiment, the sample well is contained in a tubular vacuum shroud that extends perpendicularly to the cold head of the cryocooler, thereby allowing the sample well to operate with a narrow spacing between the poles of a conventional electron paramagnetic resonance spectrometer. 
         [0008]    In another embodiment, a thermal radiation shield is integrated with the sample well assembly and extends between the magnet poles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  is an end view of a cryostat assembly constructed in accordance with the principles of the invention. 
           [0010]      FIG. 1B  is a side view of the cryostat assembly shown in  FIG. 1A . 
           [0011]      FIG. 2  is a cross-sectional diagram of the vertical portion of the cryostat. 
           [0012]      FIG. 3  is a perspective diagram of an EPR magnet assembly showing an EPR probe mounted on a bracket attached to the magnet frame. 
           [0013]      FIG. 4  is an enlarged view of a probe with a mounting assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIGS. 1A and 1B  show end and side views of a cryostat constructed in accordance with the principles of the invention. The cryostat  100  is cooled by a closed cycle cryocooler  102 , which has a first stage  104  and a second stage or cold head  106 . The cryocooler  102  is a conventional closed-cycle refrigerator, such as a conventional Gifford-McMahon (GM) refrigerator or a pulse tube refrigerator that cools cold head  106  to a temperature of approximately 10K. A cryocooler suitable for use with the invention is a model RDK 408D2 manufactured and sold by Sumitomo Cryogenics of America, Inc. 1833 Vultee Street Allentown, Pa. 18103-4783. 
         [0015]    The first stage  104  and the second stage  106  of the cryocooler are enclosed in a vacuum shroud  108  which reduces convective heat transfer. Shroud  108  is closed by an end plate  110  which is bolted to the shroud  108 . Next to the inner surface of shroud  108  is a conventional cylindrical thermal radiation shield (not shown in  FIGS. 1A and 1B  for clarity) which is mounted on, and thermally connected to, cryocooler first stage  104 . The radiation shield extends close to end plate  110  without physically touching it, where the radiation shield is closed by a circular end plate. This radiation shield reduces heat transfer due to thermal radiation. The entire cryostat structure rests on adjustable support structures  112  and  114  which can be adjusted to vertically position the cryostat between the EPR magnets, as discussed below. 
         [0016]    In accordance with the principles of the invention, the vacuum shroud  108  has a vertical extension  116 , which houses a sample well and sample as discussed further in detail below and is shown in more detail in the cross-sectional diagram shown in  FIG. 2 . Extension  116  may have either a hollow rectangular or cylindrical cross-section and is bolted to the shroud  108  via flanges  118  and  120 . The vacuum shroud extension  116  encloses a sample well  122 , which is constructed from a conductive material in the shape of an elongated cup. The sample well  122  is thermally connected to the cryocooler second stage  106  via a thermally-conductive sample well extension  124  which extends through a hole  128  in radiation shield  126 . The sample well  122  is itself surrounded by a sample well radiation shield  130  (not shown in  FIGS. 1A and 1   b  for clarity) which also extends through hole  128 . The lower end  132  of the radiation shield  130  is attached to, and thermally anchored with, the first stage  104  of the cryocooler  102  using either a solid link connection  134  or a flexible link that has high thermal conductivity. Radiation shield link  134  has a flexible joint  136  to accommodate thermal contraction and expansion and mechanically supports the radiation shield  130 . The other end of the radiation shield  130  extends to the upper part of the sample well  122  as shown in  FIG. 2 . The radiation shield  130  intercepts the heat load that is conducted from the upper warmer part of the sample well  122  to the colder part of the sample well  122 . 
         [0017]    The sample well  122 , the sample well radiation shield  130  and the vacuum shroud extension  116  all have a window  134  that allows a laser beam to be applied to the sample. 
         [0018]    The sample well assembly  122  uses a double bellows setup. A thin metal bellows  138  connects the sample well  122  to a vacuum housing interface flange  140  that is at room temperature and forms part of a vacuum housing extension  142  that bolts to the vacuum housing  116  to form a vacuum chamber enclosing the sample well  122 . The bellows  138  allows the assembly flexibility for alignment and helps to reduce the conductive heat load from the interface flange  140  to the sample well  122 . 
         [0019]    Another soft flexible rubber bellows  144  is mounted between the interface flange  140  and a flange assembly  146  that is clamped to the sample stick (not shown in  FIG. 2 ). During the installation and transportation of the cryostat, four support posts, of which two  148  are shown, are removably attached between the sample stick interface flange  146  and the vacuum housing flange  140 . The support posts  148  keep the sample stick interface flange  146  stable and are removed during operation. 
         [0020]    A helium inlet fitting  150  is connected via a tee fitting  156  to a helium inlet tube  152  that communicates with the interior of the sample stick interface flange  146  and, in turn, with the interior of the sample well  122 . The helium inlet tube  152  allows the sample well  122  to be filled with gaseous helium. During experiments, the pressure of the helium in the sample well  122  is maintained at 3.44 kPa to 6.89 kPa (0.5 psi to 1 psi) by a relief valve  154  which is attached to tee fitting  156 . The sample well  122  is cooled by the cryocooler  102  and cools the gaseous helium, which, in turn, cools the sample (not shown in  FIG. 2 ). 
         [0021]      FIGS. 3 and 4  show the EPR magnet and sample stick assembly in more detail. A typical EPR magnet assembly  300  is shown in  FIG. 3  and consists of a pair of electromagnets  302  and  304  with pole caps  306  and  308 . Magnets  302  and  304  are, in turn, supported by magnet frame  310  which rests on stand  312 . 
         [0022]    The sample stick assembly  400  is schematically shown in  FIG. 4 . Conventional parts of the assembly have been omitted for clarity. A mounting plate  402  is used, as described below to mount the sample stick assembly  400  on the magnet frame  310 . The sample stick assembly  400  has a sample holder  403  which accepts a sample that is typically placed in a long quartz tube (not shown). The tube is inserted into a hole  404  in the sample holder  403  and extends to near the end  432  of the sample stick assembly. 
         [0023]    The mounting plate  402  has four arms  406 - 412 , each of which has a slot  414 - 420 , respectively. The slots  414 - 420  fit onto the arms of a bracket  316  shown in  FIG. 3 . Once in proper position, the sample stick assembly can be locked in place by means of four knobs  422 - 428  that tighten screws against the bracket arms. The sample stick assembly also includes a sample stick flange  430  which mates with the sample stick interface flange  146  on the cryostat. 
         [0024]    During experiments, the cryostat assembly shown in  FIGS. 1A ,  1 B and  2  is positioned on the floor beneath the magnets  302  and  304  with the sample well  122  extending upwards between the pole caps  306  and  308 . The sample stick assembly  400  is then inserted into the sample well  122  and is supported on, and affixed to, the bracket  316 , which is, in turn, attached to the frame  310 . Once the sample stick assembly is properly aligned, the flanges  146  and  430  are clamped together and the four support posts  148  are removed leaving the cryostat connected to the sample stick assembly only by the flexible rubber bellows  144 . The bellows  144  isolate the sample stick assembly  400  from any vibration induced by the closed cycle cryocooler  102 . Ultra low vibration in the nanometer range can be achieved using this method. 
         [0025]    The inventive design employs several unique features. The vacuum housing  116  is sized to fit between magnet pole caps  306  and  308  that are set 55 to 57 mm apart. The size also allows a hall sensor to be mounted on the pole caps  306  and  308  without any physical interference. In one embodiment, the inner diameter of the sample well  122  is 40 to 43 mm diameter and is designed to accept existing sample stick sizes. The physical dimensions of the cryostat provide for the highest magnetic field and sensitivity during experiments and can operate with narrow EPR magnet pole spacings of 55 mm. This design allows researchers to run long-term experiments, to simplify the logistics of using liquid helium dewars in the laboratory, to simplify and automate operation of the system and to reduce longer term operating costs. 
         [0026]    While the invention has been shown and described with reference to a number of embodiments thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.