Closed-cycle cryocooled SQUID system with superconductive shield for biomagnetism

Abstract

We developed a cryocooled SQUID system with which human magnetocardiogram (MCG) and possibly magnetoenceparogram (MEG) can be measured. To reduce cyclic magnetic noises originating from the regenerator of the cold heads of the cryocooler, a superconductive shield (99.5% Pb) was used to protect the SQUID sensors, and a ferromagnetic shield (78% Ni alloy) was used to screen the cold head. In addition, the SQUID sensors' chamber was placed at a distance of 1.8 m from the cold head chamber to install the cold-head chamber outside the magnetically shielded room (MSR) for future development. The loss in cooling power due to the increased distance was compensated by increasing the number of thermal rods, and thus the SQUID sensor and superconductive shield could be refrigerated to 4.8 K and 5 K, respectively. The superconductive shield successfully rejected thermal noise emitted from metallic blocks used to improve thermal conduction. The noise of the SQUID system was 3 fT/Hz 1 2 , and the cyclic magnetic noise could be reduced to 1.7 pT. We could obtain a clear MCG signal while the entire cryogenics was in operation without any special digital processing.

Compact, ultra-low vibration, closed-cycle helium recycler for uninterrupted operation of MEG with SQUID magnetometers

“…Figure 1 shows the design of the recycler. The high-pressure helium gas from the compressor is delivered to the rotary valve-motor assembly (4) via a pair of long flexible tubes (3). The motor assembly is detached from the cold head (5) to reduce vibration of the recycler assembly (7).…”

“…Using a template matching noise rejection algorithm, the noise of the recycler (2 Hz with harmonics) could be reduced to below the system noise level (white noise level 12-18 fT/Hz) to measure evoked averaged MEG signals from human volunteers. Yu et al [3] have used a similar design with the cold-head of the cryocooler inside the MEG cryostat. A superconducting plate was successfully used 25 mm above the MEG magnetometer array to reduce the noise from the cryocooler to as little as 1.7 pT/Hz at the center of the superconductor plate.…”

Low-noise magnetoencephalography system cooled by a continuously operating reliquefier

“…Therefore, an MEG system not using LHe at all or not requiring periodic LHe refills is desirable. There have been several attempts to cool the SQUID array using a cryocooler of either Gifford-McMahon (GM), GM with Joule-Thompson, or pulse-tube (PT) type, where the SQUID arrays are cooled by thermal conduction from the cold head of the cryocooler [4][5][6]. The thermal conductor material can be copper rods or flexible braids of copper.…”

Operation of a high-TCSQUID gradiometer with a two-stage MEMS-based Joule–Thomson micro-cooler

“…Commercially available mechanical cryo-coolers (Stirling, Gifford-McMahon or pulse tubes) are hampered by their dependence upon moving metallic parts and their generation of vibrations [13]. It has been shown that magnetic noise from the moving displacers can be reduced by introducing superconducting and mu-metal shields between the cold head and the SQUID [14]. In order to isolate mechanical vibrations, SQUIDs must be separated from valves and other parts that cause disturbances [15].…”