** MANDATORY REQUIREMENTS **

Ø       Operate a liquid He detector only with the following items installed and operating.

 

- liquid He fill port safety fixture with relief valve on top and either a check valve or 2 ft. min. length draped hose on the side port.

- Overpressure relief valves for:
            a) bolometer detector (guard) vacuum space.
            b) liquid N₂ reservoir fill port.

Ø       Read these procedures, receive training from and show proficiency to an infrared beamline staff member.  

Ø       Ensure that a copy of the manufacturer’s instruction manual and these procedures are at the beamline.  Note that these procedures supercede their equivalents in the manual.

Ø       Take the web-based BNL Cryogen Safety Training (OSH-025): http://training.bnl.gov/demo/courses/courselist.htm

** CAUTION: Wear proper eye and hand protection when handling cryogens.

** CAUTION: Keep clear of the space above the detector's top surface during operation with liquid He.

 

Detector Preparation Procedures:

Note: See attached figure on last page for additional guidance.

1.      Evacuate the detector's guard vacuum space by connecting the side pump-out valve (guard vacuum valve) to an appropriate high-vacuum pump system (e.g., turbo pump).  Pump until the pressure reaches <1x10^-4 Torr as read at the pump.  Then close the guard vacuum valve, turn off the pump and disconnect the pump from the detector.

2.      Check the Teflon washer on the liquid He fill port (top of detector).  If it is dirty, clean it with alcohol.  If it shows excessive wear or no longer fits in the groove properly, replace it. 

3.      Pre-cool the detector interior with liquid N₂.  Fill the liquid nitrogen reservoir (outer port) first.  When the vapor stream from the fill port subsides (indicating that the reservoir has reached temperatures near 77K), fill the detector liquid He reservoir (top center port) also with liquid N₂ so that it will be pre-cooled to 77K.  Avoid overfilling either reservoir, causing liquid N₂ to spill excessively over the detector's top flange.  This can cause the vacuum O-rings to leak, causing partial vacuum loss.  Symptoms of this problem are the entire detector container becoming quite cold with condensation appearing on its exterior surface.  If this happens, allow the top flange to warm up and re-evacuate the detector guard vacuum space (step 1).

4.      Allow time (> 20 minutes) for the detector's liquid He reservoir and associated cold plate components to reach 77K.  The vapor stream (due to liquid N₂ boil off) from the liquid He reservoir will subside when this is achieved.  Shining a flashlight down the fill port should reveal a calm liquid (Wear eye protection!).

5.      Drain the liquid N₂ from the liquid He reservoir.  This can be accomplished either by inverting the detector and draining the liquid N₂ from both reservoirs into a proper cryogen storage vessel, or by using an liquid N₂ extraction tube to remove the liquid N₂ from the liquid He reservoir.  Be sure to use either dry nitrogen or helium gas and keep the pressure below 1 psi.  A liquid N₂ hand dewar can be used to recover the liquid.  With either method, check that the liquid He reservoir is completely free of liquid N₂ (if you're not sure, a flashlight light can be used to look down the central fill tube - Wear eye protection!).  If the inverted detector method is used, both the liquid He and liquid N₂ reservoirs will be empty.  Be sure to refill the liquid N₂ reservoir with liquid N₂ afterwards.  In either case, cover the liquid N₂ fill port with its combination cap and 0.5 psi relief valve (Note:  if you need to transport the bolometer at any time, cover the liquid N₂ fill port with aluminum foil during transport.  Once at the beamline, then screw on the cap and relief valve). During cold operation, check the detector's liquid N₂ reservoir level periodically (~8 hrs) and fill as needed.

Prepare liquid He Transfer Line

6.      Purge the liquid He transfer line with He gas.  Check that you have a compression fitting to form a seal around the transfer line when it is inserted into the liquid He storage dewar.  Vent the liquid He storage dewar through its vent valve and close the valve leading to the storage dewar's low-pressure relief fitting.

7.      Close the storage dewar vent valve, open its top valve, and carefully insert the liquid He transfer line.  The compression fitting screws onto the threaded top fitting of the storage dewar.  It should be tight enough to hold the pressure necessary for transferring liquid.  Insert the transfer line slowly to avoid excessive (more than 3 psi) pressure build-up in the storage dewar.  You should open the vent valve if the pressure exceeds 5 psi and close it once the pressure has dropped to 3 psi or less.

8.      Gently insert the transfer line until it is reaches the bottom of the liquid He storage dewar, and then back it off slightly to avoid picking up ice that may be at the bottom.  If a plug develops (indicated by a lack of gas or liquid coming through the line), then vent the storage dewar, remove the transfer line and purge it with He gas or allow it to warm up completely before trying again (return to step 6). 

9.      Allow helium to flow and cool the transfer line interior before inserting the line into the detector liquid He fill port.  A milky plume indicates that liquid is reaching the far end.  If the pressure in the liquid He storage dewar falls to near zero, attach a helium gas line to the liquid He storage dewar vent line and open the vent valve.  Use helium gas from a regulated gas cylinder set to deliver about 2 psi. 

Fill Detector liquid He Reservoir

10.  Insert the transfer line into the detector liquid He reservoir fill port.  On a bolometer, be sure to turn on the pre-amplifier electronics if not on already.  This prevents the first stage JFET from getting too cold to operate.  Adjust the detector cryostat height so that the transfer line is near the liquid He reservoir bottom (while the transfer line's other end remains near the bottom of the liquid He storage dewar). 

11.  Fill the detector's liquid He reservoir.  Initially, a large plume of gas will evolve from the detector's liquid He fill port.  Once the detector's internal components reach liquid He temperature, liquid will begin to collect and the plume will drop dramatically while becoming more densely white.  Continue transferring liquid until the detector liquid He reservoir is full.  This is indicated by a change in the plume - it will appear very dense and tend to spill across the top of the detector.

12.  Close the liquid He storage dewar vent valve and disconnect the helium gas line from the storage dewar vent line connection.  Turn off the flow of helium gas at the gas cylinder. 

13.  Vent the liquid He storage dewar completely through the vent line.  Loosen the transfer line compression fitting and remove the transfer line from both the storage dewar and detector liquid He reservoir.  Close the storage dewar vent and top valves.

14.  Open the valve leading to the low-pressure relief fitting on the liquid He storage dewar.   This is a good time to visually inspect the other storage dewar's medium and high-pressure relief valves.

Prepare the Detector for Operation

15.  Insert the liquid He safety fixture into the detector liquid He fill port, and screw the fitting down firmly (hand tight) onto the liquid He fill port fitting.  This safety fixture has a small stainless steel tube that reaches past the liquid He reservoir's narrow neck and into the top of the helium reservoir, thus providing 2 separate vent paths from the helium reservoir.  An overpressure relief valve is attached to the top of the safety fixture.  Inserting this fixture slowly into the liquid He fill port helps to conserve helium.

16.  With the preamplifier power already "on", turn on the detector bias and then the input switch.  The detector is now ready for use.

See below for ice plugs and possible overpressure dangers.

Refilling a Cold Detector

17.  Prepare the liquid He storage dewar and transfer line following steps 6 through 9 above. 

18.  Remove the liquid He safety fixture from the detector's liquid He fill port.  The long central tube and copper radiation baffles on the safety fixture should be clean and free from ice.  If not, use a flashlight (or similar) to visually inspect the fill port tube that extends down to the liquid He reservoir (Wear eye protection!).  Any ice collected on the sides can be scraped off using a length of stainless tubing.  The ice will fall to the bottom of the liquid He reservoir where it is not a problem.  Ice in the detector liquid He reservoir fill tube indicates either an improperly configured safety fixture (not screwed on tight, leaking relief valve, or hose not draped downward) or failure to install the safety fixture promptly after filling with liquid He. 

19.  Perform the liquid He transfer following steps 10 through 15.

 See below for ice plugs and possible overpressure dangers.

 

Warm Up

20.  When finished using the detector, turn off the detector input switch, bias switch, and preamplifier power switch.  Leave the safety fixture installed (i.e., attached to the detector liquid He fill port) with hose or check valve.  The detector can be simply allowed to warm up to room temperature.  The time for this varies, but typically takes about 24 hours after the detector's liquid He reservoir is empty.