Patent GB-1161501-A (GB)
Title: Measuring Weak Magnetic Fields by Compensation to Zero
IPC: G01R33/26
Filed: 1966-08-16
Abstract: 1,161,501. Measuring magnetic holds. CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE. Aug. 16, 1966 [Aug. 16, 1965], No. 36541/66. Heading G1N. To compensate an ambient magnetic field to zero and thereby measure it, its effect on the orientation of the macioscopic magnetic moment of the atoms or molecules in an absorption cell subject to optical pumping is detected optically along three athogonel axes (Ox, Oy, Oz) and currents derived therefrom are applied to three pairs of coils symmetrically arranged along these axes which produce fields equal and opposite to the corresponding ambient field components thereby reducing them to zero, the magnitudes of the currents being an indication of the corresponding field components. The magnetic field is measured using an optical pumping system in which the resonant gas is excited by circularly polarized radiation applied along, e.g. the Oz axis. Due to the polarization of the gas atoms, it is found that the components of the magnetic moment of the absorption cell atoms or molecules along the Ox and Oy axes proportional to the ambient field components Hy, Hx respectively and also that the component along the pumping beam axis itself is proportional to the field component Hz. Alternately, by applying a further weak A. C. field along, e. g. the Ox axis, it is found that the magnetic moment components along the axes are modulated, the direct components of the moment along the Ox and Oy axes, being proportional to the field components Hy, Hz respectively and the modulation components along, e.g. the Ox, Oz axes having an amplitude proportional to the field components Hz, Hx respectively. The magnetic moment component along any axis may be measured by detecting the change in interiaty of a circularly polarized detection beam, the rotation of the polarization plant of a linearly polarized beam or the polarization of a fluorescent light beam in each case directed along the axis and through the absorption cell. In the embodiments, the resonance cell 11, Fig. 4, consists of a bulb of glass or quartz containing, e.g. <SP>87</SP>R6. The bulb is coated on the inside with paraffin or the gas is mixed with a buffer-gas to improve its relaxation time. In carrying out the first method outlined above, measurement takes place in two stages. In the first stage the bulb is irradiated along the Oz axis with light from a lamp 21, also containing <SP>87</SP>R6, to excite the gas, the beam having first been reflected from a mirror 23 and transmitted through a polarizing unit 22 to produce a circularly polarized optical pumping beam. Two circular polarized detection beams, produced from a lamp 31 by reflection at mirrors 33, 233 and transmission through polarizing units 32, 232, are transmitted through the excited gas and detected by photomultipliers or photo-electric cells 41 y , 41 x . The signals from cells 41 y , 41 x  are amplified by amplifiers 141 y , 141 x  and transmitted, through switches 144 y , 144 x , to control circuit 45 x , 45 y  whose outputs are fed to coils 46 x , 46 x <SP>1</SP> and 46 y , 46 y <SP>1</SP> to produce the required compensating fields along the Ox, Oy axes. In the second stage, to determine the required compensating field along the Oz axis, the mirror 23 is retracted so that it is no longer operative and the mirror 233 is rotated so that the optical pumping beam from the lamp 21 is reflected by it through the bulb 11 along the Ox axis. Simultaneously the positions of switches 144 x , 144 y  are changed to connect the output of amplifier 142 y  to control circuit 45 z  for coils 46 z , 46 z <SP>1</SP>, which produce the required compensating field along the Oz axis. A common switching device is provided to establish the two states of the apparatus alternatively, the duration of each state being of the order of one half of a second. Due to the frequency of alternance of the two states, the three components of the ambient field can be said to be permanently counterbalanced. The control circuits 45 x , 45 y , 45 z  also incorporate indicators enabling a reading of the currents applied to the pairs of coils to be made, from which the magnitude and orientation of the ambient field can be determined. In the modification in which a weak A. C. field is superposed on the ambient field along the Ox or Oy axis, Fig. 5 (not shown), the gas is again excited by a circular polarized beam of radiation from a lamp (21) along the Oz axis. In this embodiment, the absorption of energy from the pumping beam is measured by a photo-multiplier (41 z ), whose output signal is transmitted through a selective amplifier (242 z ), which amplifies the alternating component of the signal, and a phase detector (243 z ), in which the signal is compared in phase with the A. C. field signal. Similarly the absorption of energy of a circularly polarized beam directed along the Ox axis, from a lamp (31), is measured by photo-multiplier (41 x ), and transmitted through amplifier (242 x ) and phase detector (243 x ). To provide the weak A.C. field, an A.C. generator (51) is provided, connected, via a switch (52), to either a pair of coils (53 x ), (53 x ') arranged coaxially with the Ox axis or a pair of coils (53 y ), (53 y ') arranged coaxially with the Oy axis. When the A.C. field is directed along the Ox axis, the output from detectors (243 x ) and (243 z ) are connected, via switches (244 x ), (244 z ), to control circuits (45 z ) and (45 x ). When the A. C. field is directed along the Oy axis, the detector (243 z ) is connected to the control circuit (45 y ). A switching device is provided to alternate the positions of switches (52), (244 x ) and (244 z ) simultaneously. In a modification of this circuit, Fig. 6 (not shown), a weak A. C. field is provided by coils (53 x ), (53 x '), connected to the generator (51) only along the Ox axis. Again, the modulation component of the signal detected by the photo-multiplier (41 z ) is applied to the control circuit (45 x ). While the modulation component of the signal from the photo-multiplier (41 x ) is applied to the control circuit (45 z ) and the direct component of the signal from the photo-multiplier (41 x ) is amplified at (142 x ) and applied to the control circuit (45 y ).