VIBRATION CHARACTERIZATION AND REDUCTION OF A JOULE-THOMSON CRYOCOOLER FOR A SQUID-BASED METAL DETECTION SYSTEM
G. J. J. B. de Groot, M. A. Santin, T. Thijssen, John G Weisend, John Barclay, Susan R. Breon, Jonathan A. Demko, Michael DiPirro, J. Patrick Kelley, Peter Kittel, Arkadiy Klebaner, Al Zeller, Mark Zagarola, Steven Van Sciver, Andrew M. Rowe, John M. Pfotenhauer, Tom Peterson, Jennifer Lock · AIP conference proceedings · 2008
Very sensitive high-temperature SQUID (Superconducting Quantum Interference Device) magnetometers are used for the detection of very small metal contaminants in food and other products. When the SQUIDs are cooled with liquid nitrogen, a detection sensitivity of 500 nA-m2 in a 150×150 mm2 orifice is obtained. However, when a commercial Joule-Thompson cryocooler was used, the sensitivity was reduced by a factor of 5 due to spurious magnetic signals.In this study the emphasis is on understanding noise that may be created due to mechanical vibrations in the cryocooler cold head. The natural frequencies of the cryocooler cold head are modelled and two mechanisms of noise generation are considered and analysed. Calculations show that the noise created by vibrating dipoles in the cold head may create noise of 1 pT up to 20 pT in the frequency range of 10 to 50 Hz.The vibration characteristics of several parts of the cryocooler cold head assembly were determined with a laser vibrometer, a fluxgate magnetometer and through finite element simulations. Two natural frequencies of approximately 21 and 49 Hz are particularly noticeable and are also observed in the SQUID noise.Anti-vibration measures are introduced to reduce external vibrations.