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Evaluation of Cooling Technologies for HPGe Detectors Joel Forrester 1 , James Colaresi 2 , Herbert Gohla 3 , Ethan Hull 4 , Michael Pickering 5 , George Rybicki 6 , George Shipman 5 , Robert Werzi 3 , Michael Yocum 2 1 Pacific Northwest National Laboratory, 2 Canberra Industries, 3 Provisional Technical Secretariat, CTBTO, 4 PHDs Co., 5 General Dynamics Advanced Information Systems, 6 Air Force Technical Applications Center The views expressed here do not necessarily reflect the opinion of the United States Government, the United States Department of Energy, or the Pacific Northwest National Laboratory OVERVIEW The radionuclide (RN) monitoring stations of the International Monitoring System (IMS) employ mechanical coolers to maintain the low temperature needed for operation and excellent sensitivity of HPGe crystals. Failures related to the mechanical coolers comprise a significant portion of missing IMS data availability for aerosol systems. This work is a preliminary gathering of data to support investigations leading to adoption of new cooler technology. Past, current and R&D systems are considered. US-based General Dynamics Advanced Information Systems (GDAIS), in concert with the IMS, has accumulated >87 instrument years (~ 4 calendar years) of operating experience with X-Cooler models. A significant number of spontaneous temperature excursions were observed among the IMS RASA stations. The state-of-health (SOH) data from the RN stations were examined with particular emphasis on the crystal temperature. Some examples of spontaneous temperature excursions are presented. A significant excursion is defined as a temperature increase of greater than 30 °C above the normal baseline temperature (typically -185 °C) that is not due to loss of cooler power. Of the 11 spontaneous temperature excursions recorded, none were related to power outage, vacuum problems, or other apparent problems and the effect to mission capability was no more than a few days. The GDAIS-maintained RN IMS stations , specifically RN66 and RN70 - RN80, have recorded more temperature excursions than among the other IMS RN stations. This is possibly due to the following: 1. The use of switching UPS units rather than true on-line UPS 2. Difficulty in maintaining climate control and adequate protection from dust and other environmental factors -- the addition of a building airlock might provide more environmental isolation One general note is that many of the temperature excursions occur soon after shipping or movement. Future Activities Several activities are being discussed in a growing collaboration interested in solving the cooler issue: • Collecting and mining SOH data including past cooler failures • Merging failure rate data among users • Performing an accelerated aging study which stresses weak points in the coolers • Selecting a limited set of next generation or R&D technology for a field test involving multiple copies of each selected system in a challenging environment The X-Cooler II, manufactured by MMR Technologies and ORTEC, is the current cryogenic cooler installed on all IMS particulate samplers (except RN75 & RN76). (Photo obtained http://www.ortec-online.com) The PHDs Co. RASA 1 prototype holds, cools, and instruments a detector as large as 10-cm diameter and 10- cm long. Pictured here it holds a 70-mm diameter x 70-mm long coaxial detector. This detector system cools a large coaxial detector to 50-60 K using a powerful cooler. The gettering and sealing in the vacuum-system design provide an extremely stable vacuum that should never require maintenance. (Photo courtesy PHDs Co.) The PHDs Co. RASA 3 prototype detector system holds the detector in a configuration orthogonal to the direction of the cooler. Like RASA1, this detector system cools a large coaxial detector to 50-60 K using a powerful cooler. The gettering and sealing in the vacuum-system design provide an extremely stable vacuum that should never require maintenance. (Photo courtesy PHDs Co.) The Canberra Cryo-Pulse 5 HPGe detector system incorporates a 4-watt split pulse-tube cooler manufactured by Thales Cryogenics, Eindhoven, Netherlands. The pulse tube cooler utilizes the same helium compressor technology as many split Stirling coolers, however, unlike Stirling cycle coolers, there are no moving parts in the cold tip of a pulse tube. Fewer moving parts results in less wear and contamination failures and leads to longer operating lifetime. The lack of moving parts in the cold tip also reduces the vibration transmitted to the detector chamber giving lower detector noise and better spectroscopic performance. (Photo courtesy Canberra Industries) The Canberra Cryo-Cycle HPGe detector cooler is a closed-system nitrogen (N 2 ) reliquifier incorporating a 15-watt free-piston linear Stirling cooler. The Cryo-Cycle continuously condenses the LN 2 boil-off inside a closed Dewar to maintain a 22-liter reservoir of LN 2 . Once charged, the system does not require additional N 2 except in the event of a long-duration power outage or to replace gas loss due to seal leakage. An N 2 gas generator can be integrated with the standard Cryo-Cycle to allow unattended operation in the RASA. (Photo courtesy Canberra Industries) This example is quite typical of many spontaneous temperature excursions — several days of slowly rising temperature are followed by a sudden and dramatic increase in temperature. In some cases, the detector spontaneously begins to cool, in others the temperature rises to ambient and remains at that level requiring operator intervention. Temperature (°C) A gradual temperature rise followed by a dramatic spontaneous excursion. Note that the base temperature following the temperature excursion is lower than before the excursion. Temperature (°C) Temperature (°C) Multiple small excursions within a short period of time is a fairly common behavior. When it became clear that a temperature excursion was in progress, the cooler was turned off and the system allowed to undergo a temperature cycle. This is the only spontaneous temperature cycle this system has undergone. Hardware and performance specifications of recently retired technology, current generation of technology, and near-future technology. Mean Time Between Failure (MTBF) rates are from the GDAIS study of IMS RASA systems. Performance and MTBF data from other sources are being compiled. For more information, please contact: Joel Forrester Pacific Northwest National Laboratory P.O. Box 999, MS K1-90 Richland, WA 99352 USA +1-509-375-4484 [email protected] Temperature (°C) Power turned off to allow temp cycle Recently retired technology Manufacturer Technology Power Size (compressor) Cooler Mass MTBF (Manufacturer) Service Life (Maint Interval) Heat Lift @ ~80K Capacity Electricool (III) ORTEC J-T ~ 520 W 45 x 31 x 37 cm 32 kg 2.8 yr (5 yr) N/A --- CryoTiger Polycold J-T 500 W 45 x 31 x 38 cm 32 kg N/A (5 yr) 3 W @ 77K --- Current generation of technology LN Generator (LNP20 - 20 L/day unit) Cryomech GM/Pulse Tube ~ 4 kW 48 x 56 x 56 cm 80 kg N/A N/A --- 100 L LN Generator (LNP10 - 10 L/day unit) Cryomech GM/Pulse Tube ~ 2 kW 48 x 56 x 56 cm 65 kg N/A N/A --- 35 L LN Generator (StirLIN-1 - 10 L/hr unit) Stirling Cryogenics Stirling 16 kW N/A 1800 kg 4.4 yr (6 000 hr) --- 500 L X Cooler II MMR/ORTEC J-T < 400 W 32 x 32 x 28 cm 17 kg N/A N/A N/A --- CryoPulse 5 Canberra Pulse Tube 325 W 14 x 28 x 31 cm 27 kg (>50 000 hr) N/A 4 W @ 80K --- CryoCycle Canberra Stirling (Hybrid) 300 W 43cm dia x 61cm tall 30 kg (empty) (>50 000 hr) >5 yr N/A 22 L Near future technology Prototype PHDs Co. Stirling N/A 570 cm 3 3 kg N/A >5 yr ~11 W @ 77K --- Prototype Canberra Stirling (Hybrid) N/A N/A >25 kg (>50 000 hr) N/A N/A 22 L PNNL-SA-66757
