Digital Caesium Vapour Magnetometer

Description

The final product of John Stanley’s PhD research was the development of this “Digital, Alkali Vapour magnetometer using Integrated Circuits”. The Larmor Frequency signal generated by the self-oscillating, optically pumped sensor shown in the foreground (3.498 Hz per nT or typically about 180 kHz in a field of 50,000 nT), was digitally counted using TTL logic in the form of integrated circuits. Given that the Larmor Frequency varied by 3.49869 Hz per nT, a Frequency Counter that used a timing gate the reciprocal of this value (0.2858 s) would yield a count numerically equal to the magnetic field in nT. A timing gate 10 times wider (2.858 s) would deliver a count in units of 0.1 nT. This measurement was displayed in nT (gammas) using six, state-of-the-art incandescent digits. This magnetometer also included a Period Counter which delivered a faster count rate, but in this case the count interval changed according to the field value. By using the Larmor signal to gate a high frequency clock, 0.01 nT resolution could be delivered in approximately a 2.2 s period, 0.1 nT in 220 ms or 1 nT in 22 ms. In the absence of digital recording technology at this time, an internal digital to analogue converter following the Period Counter provided an output that could feed a time-based chart recorder.

Absolute Total Field measurement.

Resolution: 0.01 nT @ 0.5 Hz. 1 nT @ 45 Hz,

Serial Number: 1

Year of manufacture: 1973

Origin: University of New England. Armidale, Australia