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Note: Magnetic noise from the inner wall of a magnetically shielded room
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http://aip.metastore.ingenta.com/content/aip/journal/rsi/84/5/10.1063/1.4802845
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Image of FIG. 1.

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FIG. 1.

Looking down on the inner layer of the MSR, with inside dimensions of 3.0 m (in x), 4.0 m (in y), and 2.4 m (in z). This layer consists of four sheets of 1 mm moly-permalloy, mounted on a high-conductivity aluminum layer of 10 mm thickness. The MSR contains the MEG helmet with 102 magnetometers (small squares), of which 46 on each side are summed separately, to make two large magnetometers. These measure the magnetic noise in the x-direction, generated thermally in the left and right walls. The nose of an imagined MEG subject is seen at the left of the helmet. The inset is a perspective view of the same helmet.

Image of FIG. 2.

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FIG. 2.

Each magnetometer is assigned a weight from 0 to 1, depending on its angle to the direction. Taking all weights into account, the total effective area on each side is 28 times the area of each small square. Extreme left-right magnetometers have a weight of 1.0 and they are about 22 cm apart, resulting in incomplete cancellation of wall noise due to its x-gradient.

Image of FIG. 3.

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FIG. 3.

Frequency analysis of a wall-noise measurement, for 30 min recording. The MEG bandwidth was 0.1 to 200 Hz, the sampling rate was 600 Hz, at 24 bits. Three sets of data are shown: (1) A single channel chosen which directly faces -ward. (2) The two large magnetometers, added together. (3) The same but in the subtracted mode. Only two “clear” sections of the 1-curve are shown, to avoid crowding. The complete lower two curves are dominated by usual MSR artifacts: wall vibrations below 50 Hz, and 60 Hz and its harmonics. However, two regions are indicated which are almost clear of artifacts, hence due only to broad-band noise. Two vibration peaks are labeled “” and “.”

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/content/aip/journal/rsi/84/5/10.1063/1.4802845
2013-05-01
2014-04-23

Abstract

We measured the thermal magnetic noise generated by the inner high-permeability wall of a magnetically shielded room. This room houses a magnetoencephalogram (MEG), which contains 102 “small” identical magnetometers. For the measurement, we created two large magnetometers by summing the outputs of 46 magnetometers equally on the helmet's left and right side, to look at the summed noise of the right and left vertical walls. From these summed outputs, we calculated the rms noise amplitude due to all six walls at the MEG location to be ∼0.5 f T/ at 100 Hz, only slowly rising with lower frequency. This is well below the system noise of each small MEG magnetometer, hence is negligible for the MEG.

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Scitation: Note: Magnetic noise from the inner wall of a magnetically shielded room
http://aip.metastore.ingenta.com/content/aip/journal/rsi/84/5/10.1063/1.4802845
10.1063/1.4802845
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