Magnetic silence – a cabin of mu-metal

Cabin under construction The measuring chamber for biomagnetic investigations which is the heart of the new Laboratory for Biomagnetic Measurements in Berlin-Charlottenburg is the "magnetically most silent place in the world". Shielded as in a space capsule, the patient lies in the cabin whose thick mu-metal walls ward off any external magnetic field. This metal, which consists of 78 % nickel, 16 % iron and 4 % cobalt, gathers the magnetic field and pilots it around the inside of the cabin. With the overall dimensions of 12 m x 12 m and a height of 12 m, this measuring chamber is the greatest chamber of this kind worldwide. 24 t of mu-metal have been used alone to fabricate the over-thick floor and walls which enclose the relatively small interior of 3 m x 3 m x 3 m. Altogether, with a weight of 100 t, the chamber makes head against any shocks and vibrations – assisted by a base 200 t in weight. As we see, with success: the measurable vibrations on the inner wall of the cabin are still below 3 micrometers. Ultimate safety is offered to the metrologists by an incorported active shielding which with three sensors on the outer walls does not allow any magnetic signal to pass. Via a control loop a demagnetizing field will be set up forthwith. View into the cabin for neurological examinations A second cabin which has also been constructed recently is suitable for neurological examinations in particular. The instrument is placed directly at the head of the bed and protects the head like a helmet so that the measurement covers the whole brain. The test person or patient places his or her head in the depression, and with the aid of the cooled SQUIDs all magnetic fields can be recorded, which are, for example, caused by acoustic impulses in the head. A special acoustic shield ensures that solely the sounds used in the examination can be heard. Other sounds would not only disturb the concentration of the test person but also initiate cerebral activities and, thus falsify the measurement result. An intercom system ensures the contact with the person in the cabin. But even behind walls of such a thickness the cerebral and nervous signals of the human body are barely detectable. Highly sensitive magnetic field sensors – SQUIDs – sense the signals.