STABILIZATION OF THE MAGNETIC FIELD IN MAGNETIC SPECTROMETERS

An instrument is described which uses the phenomenon of nuclear proton resonance to stabilize the magnetic field in a beta spectrometer. The stable point of operation may be chosen anywhere in the range 140 to 1400 gauss. The degree of stabilization is approximately 2 x 10/sup -5/ for fields greater than 300 gauss and 4 x 10/sup -5/ for fields greater than 140 gauss. For fields up to 940 gauss, the coil is a toroid of volume 13 cm/sup 3/ and Q-value about 70. The circuit of the amplifier bridge, 1.f. amplifier, phase-detector, and d-c amplifier are given. The equipment was used over a period of four months for investigating the electron spectrum of RaTh in the range 140 to 2600 kev. The stabilization coefficient of the circuit against changes in the current in the main field coil is 100. The main field was supplied from accumulators and had a drift rate of 0.01%/sec in current. The dominant time constant in the feedback circuit was 5 sec. A note added in proof reports that the lower limit to the field which can be stabilized has recently been reduced to 12 gauss, while measurements may extend down to 5 gauss. (TCO)