Answer to Question #10416 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

I have a high purity germanium detector operated on liquid nitrogen. After a week it was observed that detector warmed up. What is the procedure to cool it down?

The use of high purity germanium (HPGE) type detectors have the great advantage that they can be allowed to warm up and not suffer damage. This is unlike the older lithium drifted detectors that were not tolerant of any significant warm up because of the associated migration of the lithium ions, resulting in an improperly functioning detector. You do not specify the conditions under which your detector warmed up; in particular, whether the high voltage was applied during the time that the detector warmed up? Most systems are interlocked to prevent this, but if the voltage had been applied the system may have sustained some damage because of the high leakage current that would have resulted and would most likely have burned out the field-effect transistor (FET) used in the preamplifier. Repairing this constitutes a major job since this part is normally within the vacuum cryostat.

The precise procedure for recooling the detector with liquid nitrogen depends, in good measure, on the particular configuration of the detector/cryostat and the dewar vessel, but some fairly common considerations apply to most.

Assuming that no voltage was on while the detector was warming, if the detector has not already warmed completely to room temperature, allow it to do so. The system may require a couple of days to reach room temperature and thermally equilibrate (it sounds as if yours already has warmed). Apply no voltage and remove electronic connections.

With the cryostat/dipstick assembly mounted in the dewar vessel, make sure that the cryostat vent tube and fill tube are open and free of any obstructions. It is often desirable to attach an extension tube to the vent to carry cold vented nitrogen at least several feet away from the cryostat. If you are filling the cryostat by gravity it is usually preferable to connect a metal funnel to the inlet with a short piece of tubing (plastic tubing may be used but it is subject to cracking, and a flexible metal tube is better). The intention is to fill the cryostat without getting liquid nitrogen or cold vented nitrogen in direct contact with the outside of the cryostat or with detector and associated electronics to avoid damage to these components that can result from dramatic changes in temperature; sudden contractions or expansions can lead to failures in vacuum flanges, which require major repair efforts to reestablish the required vacuum. If you are using a pressurized reservoir of liquid nitrogen to fill the cryostat directly, check with the manufacturer to ensure that you are using the correct pressure. It is common practice to position the dewar on a flat platform scale so that the weight of added nitrogen can be determined, and the supply can be terminated when the desired capacity has been reached. If this is not used it is generally adequate to fill carefully until some slight overflow is observed in the vent pipe.

Once the dewar has been filled, you must wait long enough for the detector to completely cool before applying any voltage. This waiting time may vary with cryostat/detector and dewar configuration, but a minimum time of about eight hours is common. I assume you have experience handling liquid nitrogen and are familiar with safety precautions, and I will not say more about that.

There are variations on recommendations for recooling detectors that have warmed, depending on manufacturer and the types of cryostats and detector configurations being used. Here is a link to a useful document from PGT, one of the major providers of HPGE detection systems. Download the document and refer to the Table of Contents. Section 2 discusses filling of cryostats with liquid nitrogen, and some other sections, particularly Section 3, may be helpful.

Good luck!

George Chabot, PhD

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