OPERATION OF THE WEB/JANIS MÖSSBAUER SPECTROMETER
Joseph W. Stucki
University of Illinois
Specifications
WEB/Janis Model SHI-850-5 closed-cycle refrigerated Mössbauer spectrometer
Operating temperature: 4.0 to 300 K
Temperature control at both the cold head and the sample positioner
Preliminary Checks
1. Helium gas turned on
2. Nitrogen gas turned on
3. Diffusion pump and roughing pump on
4. High vacuum in vacuum space of cryostat
5. Height of vibration-isolation table (VIT) set at 4 7/16 inches (from the bottom face of the top shelf to the center of the lowest joining flange of the cryostat (see mark on white wooden ruler)).
6. Pressure in the red tank stable at about 28 psi, which serves as the source of pressure to inflate the feet of the VIT. The valve on the red tank functions as follows: when the red ring is turned closest to the tank (fully clockwise), the tank’s contents are connected to the nitrogen gas supply line for pressurizing the tank; when turned away from the tank (fully counter-clockwise), the tank is connected to the feet.
7. Lead shielding in place
8. None of the power cords or vacuum or gas lines touching the VIT
9. The VIT is level in both horizontal axes. This means that the sample positioner insertion port (attached to top shelf of VIT) is exactly vertical to the cryostat entry neck (the rubber bellows are vertical, not angled, from the perspective of both horizontal axes). (link to hints for adjusting)
10. Electrical leads from temperature controller properly connected to cold stage (also to sample positioner if installed)
11. Sample space of cryostat is closed at the top with a rubber stopper
Starting the Closed Cycle Refrigerator (CCR)
1. Close the high-vacuum valve at the base of the cryostat.
2. Isolate the roughing pump from the diffusion pump (black valve handle pointing down)
3. Flush the sample space with helium gas:
Open the small valve toward the back of and above the roughing pump (roughing valve)
Slowly rotate the valve at the back, near the top, of the cryostat (cryostat valve) and slowly evacuate the sample space
Slowly rotate the cryostat valve toward the opposite or right direction (He gas position) to slowly backfill the sample space with He gas, up to atmospheric pressure
Repeat two times, and leave at atmospheric pressure with the valve in the vertical position
4. Turn on the Sumitomo CKW-21 Helium Compressor Unit
Check He pressure at the gauge on the front, should read 2.05 to 2.10 (if not, click here)
Monitor cold head temperature, should ultimately reach about 3.6 K
As cold head cools, the pressure inside the sample space will drop. This is ok as long as the fill gas is He.
5. Close the roughing valve, reconnect the roughing pump to the diffusion pump (black valve handle pointing toward the diffusion pump)
Powering Up the Mössbauer Spectrometer (Follow Steps In Order, Unless Noted)
1. Oscilloscope (order insensitive)
Power: on
CH1: x – connected to
CH1: y – connected to
Triggering:
Source: int
Mode: auto
Slope: in
Volts/div: 1 DC
CH1: selected
Time Base: 10 sec/div
2. WEB WTC102 Dual Temperature Controller
Power: on
CH1: connected to sample positioner
CH2: connected to cold stage
3. Ortec Model 541 Ratemeter (order insensitive)
Counts/sec: 100K
Std Dev: 10
BNC: connected to Bipolar connection on Canberra Model 241 Amplifier
4. NIM BIN Power CTI Model 1400
Power: on
5. WEB WPG300 Pulse & Waveform Generator
Power: on
6. WEB W200c Servo Amp
Power: on
Black Dial: Sensor
Velocity: 4 or 12 (usually)
Gain/BW: adjustment potentiometers to refine waveform, seldom adjusted
7. Canberra Model 241 Amplifier
Coarse Gain: 125
Fine Gain: 0.10
Other switches: positive
P/Z
Input: connector empty
Unipolar: connector empty
Bipolar: connector to ratemeter
8. Canberra Model 3102D HV Power Supply (goes to detector)
Polarity: positive
Voltage: 88
Power: on
Value: 1.76
9. Drive computer on and booted, execute MCS32 program
10. Temperature controller computer on and booted, execute WTC102 program
(If temperature controller and computer get out of synch, turn controller off and back on, then reboot computer and re-execute the WTC102 program)
Sample Positioning
1. The sample positioner is very delicate and can be easily damaged by bending (takes sample out of vertical alignment) or twisting actions (can break small wires leading to heater and diode). Handle with extreme care. Always lay flat with many supporting points, or hang exactly vertically.
2. If the vibration isolation table (VIT) is at the specified height of 4 7/16 in (see above), the vertical alignment of the sample can be finely adjusted if necessary by loosening the silver collar near the top and slightly loosening the knurled brass nut just below it. The shaft can then be moved gently up or down (do not rotate more than 10̊), depending on the need. The best position can be identified as the point where the ratemeter gives the highest count rate. The knurled nut should then be tightened and the silver collar tightened exactly above the brass nut.
3. Once a sample has been loaded into a sample holder (cup or other suitable container), the sample holder should be placed into the circular opening of the brass fitting at the end of the sample positioner. The sample itself should be placed as close to the vertical center axis of the positioner as possible, with the remaining portion of the sample holder (e.g., the cup in which the sample is placed) extending toward the detector.
Sample Loading
1. Load a suitable amount of sample into a plastic cup or other suitable holder in order to achieve between 5 and 10 mg Fe/cm2 of cross section intercepted by the beam. To make this calculation, the cross-sectional diameter of the sample holder and the total Fe content of the sample must be known.
2. Attach the sample holder such that the sample position is as close as possible to the center of the vertical axis of the positioner, then secure with kapton tape wrapped two times (kapton is transparent to gamma rays). Take care that no part of the sample, tape, or holder is outside the diameter circumscribed by the cylindrical brass base of the sample positioner (in order to prevent any part of the sample or its holder from touching the wall of the cryostat sample space).
3. If the CCR is not already turned on and cooled down, go to that procedure at this point.
4. Isolate the roughing pump from the diffusion pump (black valve handle pointing down)
5. Flush the sample space with helium gas:
Open the small valve toward the back of and above the roughing pump (roughing valve)
Slowly rotate the valve at the back, near the top, of the cryostat (cryostat valve) to the left (vacuum position) and slowly evacuate the sample space
Slowly rotate the cryostat valve toward the opposite or right direction (He gas position) to slowly backfill the sample space with He gas, up to atmospheric pressure
Simultaneous remove the rubber stopper from the opening to the sample space and open the cryostat valve fully to the He gas position. Maintain maximum flushing of He while sample is being loaded (minimizes the entry of atmospheric gases that will freeze onto the cold head)
6. Carefully insert sample positioner into the sample space, lowering it until the nylon collar reaches the opening. With a slightly more forceful push, insert the collar into the opening until it contacts the o-ring under the metal flange, and quickly turn off the He gas supply by rotating the cryostat valve to the up (closed) position. The flange of the sample positioner should be seated onto the o-ring with the black position marks aligned. Insert the retaining screws and tighten gently with the long Allen wrench.
7. Reconnect temperature controller cable to the sample positioner B connector.
8. Slowly evacuate the sample space to about -20 on the gauge
9. Close cryostat and roughing pump valves
10. Reconnect roughing pump to diffusion pump (black valve handle pointing toward the diffusion pump)
11. Begin sample analysis routine
Sample Analysis Routine
1. Select the desired velocity range on the WEB W200c Servo Amp
2. Verify that a suitable velocity calibration standard has been obtained for the selected velocity setting.
3. Verify that ample counts are traversing the sample by checking the ratemeter. With a 20 mCi source and smectite loaded at about 10 mg/cm2, the count rate is about 0.5 x 100K/sec.
4. Set the desired temperature using only Channel 2 (cold head), and monitor the reported temperature in both Channels 1 and 2. Wait for sample to reach desired temperature.
5. Set the energy window for the detector as follows:
Select <Acquire>, <SCA Sweep> in the MCS32 program
After a few passes when the peaks are clearly visible and strong, select by <left-mouse-click, drag, and release> the 14.4 keV peak (third from left), selecting on each side of the peak just above the background, then select <Save SCA>. Record the upper and lower voltages, which should be in the vicinity of 1.5 to 2.2 V, respectively.
6. Start data acquisition
Select <Acquire>, <MCS–>buffer> in the MCS32 program
Select <Cear>, then <Sart>
For a good absorption effect, the counts should exceed the number of passes.
7. Generally, allow at least 1 M counts/channel
8. When finished, select <Stop>, then save data in two formats, i.e., <File>, <Save Data As>, and <File>, <Export ASCII>
9. Record all information in ProCite data base Mossbauer Log on computer jws177-105.
Changing Sample Temperature
Target 4 K
1. Follow procedure to start the CCR, making sure the sample space is filled with He gas and that vacuum is established in the insulation space of the CCR (using the diffusion pump and valve at the base of the CCR).
2. Set the target temperature in Channels 1 and 2 to 3.5 K.
3. Watch pressure inside the CCR sample space to be sure it drops below -20 inches.
4. Report the sample temperature as that displayed on Channel 1.
Target 77 K
1. Follow the same procedure as for a target of 4 K, except for the following:
2. Be sure the WTC102 temperature controller is turned on, both diodes connected, and the heater switches turned on.
3. Set the target temperature on Channel 2 to 77 K.
4. Enter 500 in the window beside Gpi for Channel 2, press <Enter>.
5. When the temperature on Channel 1 (the sample diode) reports within a few degrees of the target temperature on Channel 2, a target temperature of 77 K may be entered into Channel 1. Enter 100 in the window beside Gpi for Channel 1, press <Enter>. Heating the sample often is unnecessary, but may be used to fine-tune the sample temperature.
Target 298 K
1. With the CCR in operation, reaching 298 K places more strain on the system than is desirable.
2. Turn off the Sumitomo CKW-21 He compressor.
3. Place full vacuum on the CCR sample space.
4. Be sure the diffusion pump is operating at a vacuum of about 10-5 Torr or better.
5. Open the valves connecting the diffusion pump to the CCR insulating vacuum space.
6. Set the target temperature on Channel 2 to 298 K.
7. Enter 500 in the window beside Gpi for Channel 2, press <Enter>. If the temperature increase is too slow, this value may be increased to 1000, or more if needed.
8. When the temperature on Channel 1 (the sample diode) reports within a few degrees of the target temperature on Channel 2, a target temperature of 298 K may be entered into Channel 1. Enter 100 in the window beside Gpi for Channel 1, press <Enter>. Heating the sample often is unnecessary, but may be used to fine-tune the sample temperature.
Sample Change Procedure
1. Be sure sample data collection is completed and data saved properly.
2. Bring the atmosphere in the sample space to one atmosphere
Isolate the roughing pump from the diffusion pump (black valve handle pointing down)
Open the roughing valve
Slowly open the cryostat valve to the He gas position and slowly backfill the sample space with He gas, up to atmospheric pressure
3 Remove the retaining screws from the top flange of the sample positioner
4. Raise the sample positioner enough to clear the o-ring, then immediately open the cryostat valve fully to the He gas position. Maintain maximum flushing of He while sample is being removed.
5. Slowly and carefully raise the sample positioner vertically until it clears the sample space opening
6. Gently place positioner on the table in a fully supported horizontal position
7. Place the rubber stopper securely into the sample space opening and turn the cryostat valve to the off position.