Magnetic field gradient measuring device

Disclosure is made of a magnetic field gradient measuring device comprising two sensors arranged at points of a magnetic field which are to be investigated, and connected with their outputs to inputs of a phase detector. Each of the two sensors comprises an absorption cell filled with atoms of a working matter. Arranged at the input of said absorption cell is a light source intended for optical pumping of atoms of the working matter. At the output of said absorption cell there is arranged a photocell intended to register the beam of light passing through the cell. The cell itself is located inside a radio frequency coil energized with current of a variable frequency close to the magnetic transition resonance frequency of atoms of the working matter, which is determined by the magnetic field intensity at the sensor location. The coils are energized by a means common for both sensors.

The present invention relates to devices for measuring parameters of 
magnetic field and, more particularly, to a magnetic field gradient 
measuring device. The invention is applicable to geophysics, where it can 
be used for detecting magnetic anomalies from board of aircraft. The 
invention can also be used for measuring the intensity of magnetization of 
rock and other substances, as well as for measuring the intensity of 
permanent and variable magnetic fields inherent in or generated by 
biological objects. 
There are known magnetic field gradient measuring devices (gradient meters) 
of the type that comprises two self-oscillating quantum sensors located at 
points of a magnetic field which are to be investigated, and connected 
with their outputs to inputs of a phase detector. Each of the sensors 
comprises an absorption cell arranged inside a radio frequency coil and 
filled with atoms of a working matter. Said atoms of said working matter 
are optically pumped by a light source which is a spectroscopic lamp 
arranged at the input of the absorption cell. Arranged at the output of 
said absorption cell is a photocell intended to register the beam of light 
passing through said cell. The output of the photocell is connected to the 
input of an amplifier. The radio frequency coil is energized with current 
of a variable frequency close to the magnetic transition resonance 
frequency of atoms of the working matter, which is determined by the 
intensity of the magnetic field at the location of the sensor. In order to 
supply variable-frequency current to the radio frequency coil, each of the 
sensors is provided with a feedback circuit which connects the output of 
the amplifier to the input of the radio frequency coil of that sensor. 
In the conventional devices, magnetic field gradients are measured either 
on the basis of a difference in oscillation frequencies of the sensors, 
which difference is determined by a difference in the parameters of the 
magnetic field at the sensors locations, or on the basis of direct current 
in the feedback circuit, proportional to the difference in the oscillation 
frequencies. Said direct current is applied to an additional feedback coil 
of one of the sensors from the output of the phase detector whose inputs 
are connected to the outputs of both sensors (cf. U.S. Pat. No. 3,252,081, 
Cl. 324-5). 
Gradient meters of the above type can measure great magnetic field 
gradients with a sufficient degree of accuracy at small base distances. 
However, measurements of small magnetic field gradients at small base 
distances involve difficulties due to the effects of one sensor upon the 
other. One of the sensors acquires the oscillation frequency of the other 
sensor, which is a serious obstacle to effective magnetic field gradient 
measurements. 
In case of a gradient meter which registers differences in the sensor's 
frequencies, the effectiveness of filtering the output signal depends upon 
the value of the magnetic field gradient, i.e. upon the difference in the 
frequencies. A decreasing magnetic field gradient makes it increasingly 
difficult to filter the output signal, which results in a narrowing 
frequency range of measurements. 
Conventional magnetic field gradient measuring devices of the type that 
incorporates a d.c. feedback circuit provide for a sufficiently high 
accuracy of measurements only in the absence of mechanical displacements 
of the feedback coil. At the same time gradient meters of this type are 
ineffective in measuring magnetic field gradients at small base distances 
due to the effects of scattering of the magnetic field of the feedback 
coil upon the reference sensor. 
Besides, conventional gradient meters lack means to eliminate noise 
generated by electric networks, which results in a reduced noise immunity 
in the course of measurements. 
It is an object of the present invention to provide a magnetic field 
gradient measuring device which would make it possible to measure small 
gradients. 
It is another object of the invention to provide a magnetic field gradient 
measuring device which would make it possible to measure magnetic field 
gradients at small base distances. 
It is still another object of the invention to provide a magnetic field 
gradient measuring device featuring a high noise immunity and a high 
vibration resistance. 
The present invention essentially consists in providing a magnetic field 
gradient measuring device (gradient meter) comprising two sensors to be 
arranged at points of a magnetic field which are to be investigated, and 
connected with their outputs to inputs of a phase detector, each of the 
two sensors comprising an absorption cell filled with atoms of a working 
matter, at whose input there is arranged a light source intended for 
optical pumping of atoms of the working matter, whereas arranged at the 
output of the absorption cell is a photocell intended to register the beam 
of light passing through the cell arranged inside a radio frequency coil 
energized with current of a variable frequency close to the magnetic 
transition resonance frequency of atoms of the working matter, determined 
by the intensity of the magnetic field at the sensor locations, which 
device includes, according to the invention, a means for supplying 
variable-frequency current to the radio frequency coils of both sensors. 
It is expedient that the function of the means for supplying 
variable-frequency current to the radio frequency coils should be used by 
a variable-frequency oscillator. 
The means for supplying variable-frequency current to the radio frequency 
coils may be a feedback circuit provided in one of the sensors, the 
parameters of the feedback circuit being selected so as to ensure a phase 
balance and a resonance frequency amplitude balance in the feedback 
circuit, which circuit is to be electrically connected to the input of the 
radio frequency coil of the other sensor. 
It is desirable that the function of the means for supplying 
variable-frequency current to the radio frequency coils should be 
performed by an additional sensor indentical with the first two sensors 
and provided with a feedback circuit whose parameters are selected so as 
to ensure a phase balance and a resonance frequency amplitude balance, 
which circuit is electrically connected to the inputs of the radio 
frequency coils of the first two sensors. 
In the latter case it is expedient that the device should include a 
frequency (phase) detector connected to the output of the additional 
sensor, and a filter tuned to the frequency of noise to be compensated and 
connected with its input to the output of the frequency (phase) detector, 
whereas the output of the filter is connected to the input of the radio 
frequency coil of the additional sensor and, via a signal level regulator, 
to the inputs of the radio frequency coils of the first two sensors. 
The gradient meter of the present invention makes it possible to measure 
small (in the order of 10.sup.-8 gausses) magnetic field gradients. 
The effects of one sensor upon the other at small base distances are ruled 
out due to the fact that the proposed device operates at a single 
frequency coherent for all the sensors, the effectiveness of filtering the 
output signal of the gradient meter is not affected by the value of the 
magnetic field gradient, which value may be close to half the width of the 
magnetic resonance line. 
The proposed gradient meter features a high immunity to noise generated by 
electric networks, and a high vibration resistance.

Referring to the attached drawings, the proposed magnetic field gradient 
measuring device (gradient meter) comprises two identical sensors 1 and 2 
(FIG. 1). Each of the sensors 1 and 2 comprises an absorption cell 3 
filled with saturated vapour of an alkali metal, for example, cesium, and 
provided with means to prevent relaxation of atoms of the working matter. 
The cell 3 is arranged in a radio frequency coil 4. The beam of light 
emitted by a spectroscopic lamp 5 performing the function of a light 
source travels through a light guide 6, a lens 7 and an interference 
polarization plate 8 to the input of the cell 3. Arranged at the output of 
the cell 3, downstream of the beam of light, are a lens 9 and a photocell 
10 connected with its output to the input of an amplifier 11. The 
spectroscopic lamp 5 is powered by a generator 12. 
The outputs of the amplifiers 11 are the outputs of the sensors 1 and 2 and 
are connected to inputs of respective frequency multipliers. The output of 
the frequency multiplier 13 connected to the output of the sensor 2 is 
coupled via a phase shifter 14 to an input of a phase detector 15. The 
output of the frequency multiplier 13 connected to the output of the 
sensor 1 is directly connected to another input of the phase detector 15. 
The frequency multipliers 13 are conventional devices built around, for 
example, an automatic phase frequency control circuit. The known frequency 
multipliers may be complemented with units intended to stabilize the 
output signal phase with respect to the input signal phase. 
The radio frequency coils 4 of both sensors 1 and 2 are supplied with 
variable-frequency current by a common means connected with its output to 
the inputs of the radio frequency coils 4. In the embodiment of FIG. 1, 
said means is a variable-frequency oscillator 16. The frequency of the 
oscillator 16 is selected so as to be close to the magnetic transition 
resonance frequency of atoms of the working matter determined by the 
intensity of the magnetic field at the points the transmitters 1 and 2 are 
located. 
Said sensors 1 and 2 are mounted on a common rigid base (not shown in FIG. 
1) and spaced at a distance of a few centimeters to several meters, 
depending on the purpose of magnetic field gradient measurements. 
FIG. 2 is a block diagram of another embodiment of the proposed device. 
Unlike the device of FIG. 1, the means for supplying variable-frequency 
current to the radio frequency coils 4 is a feedback circuit 17 provided 
in the sensor 2. The parameters of the feedback circuit 17 are selected so 
as to ensure a phase balance and an amplitude balance at the magnetic 
transition frequency of atoms of the working matter. The feedback circuit 
17 is electrically connected to the input of the radio frequency coil 4 of 
the sensor 1. 
Unlike the device of FIG. 1, the means for supplying variable-frequency 
current to the radio frequency coils 4 of the embodiment of FIG. 3 is a 
third sensor 18 which is identical to the sensors 1 and 2. The sensor 18 
is provided with the feedback circuit 17 whose parameters are selected so 
as to ensure a phase balance and an amplitude balance at the magnetic 
transition frequency of atoms of the working matter. The feedback circuit 
17 is electrically coupled to the inputs of the radio frequency coils 4 of 
the sensors 1 and 2. 
Unlike the device of FIG. 3, the embodiment of FIG. 4 includes a frequency 
(phase) detector 19 for noise signal separation, connected to the output 
of the sensor 18. 
The output of the frequency (phase) detector 19 is connected via a filter 
20, tuned to the noise frequency range to the radio frequency coil 4 of 
the sensor 18 and via a signal level regulator 21 to the radio frequency 
coils 4 of the sensors 1 and 2. The output of the sensor 18 is also 
connected to all of the radio frequency coils 4 via isolation circuits 22, 
for example, active resistors. 
The proposed magnetic field gradient measuring device operates as follows. 
As the device is switched on, the high-frequency generator 12 produces a 
discharge in the spectroscopic lamp 5 (FIG. 1). 
The working matter of the spectroscopic lamp 5 is the same as in the 
absorption cell 3, so in the course of the discharge the spectroscopic 
lamp 5 emits a beam of light which is in resonance with the atoms 
contained in the absorption cell 3. 
In order to carry out optical pumping of atoms of the working matter in the 
cell 3, the beam of light of the spectral lamp 5 is directed through the 
light guides 6, lenses 7 and interference polarization plates 8 which 
separate a desired resonance radiation line for optical pumping and effect 
its circular polarization with respect to the absorption cells 3. 
The cells 3 containing saturated vapour of an alkali metal, for example, 
cesium, are filled with a buffer gas to prevent relaxation of atoms of the 
working matter on the walls of the cells 3 in the course of optical 
pumping. Another way of preventing relaxation is to coat the walls of the 
cells 3 with a thin layer of paraffin. 
As the radio frequency coils 4 are energized by the variable-frequency 
oscillator 16, a variable magnetic field is produced in the cells 3. 
If the frequency of the oscillator 16 is close to the frequency of 
transition between the magnetic sublevels of atoms of the working matter, 
between which the optical pumping produces an inverse population, and if 
the angle between the direction of the magnetic field H.sub.o and the axis 
of the beam of light in the cells 3 is close to 45.degree., the beam of 
light that passes through the cells 3 is amplitude-modulated. The maximum 
of modulation corresponds to the equality between the frequency of the 
variable-frequency oscillator 16 and that of magnetic transition. 
Upon passing through the cells 3, the amplitude-modulated beam of light is 
detected by the photocells 10 and amplified. 
In the absence of a magnetic field gradient, the phase shift of signals at 
the outputs of the identical sensors 1 and 2 is zero. The presence of a 
gradient .DELTA.H accounts for a phase shift .DELTA..phi. related to the 
magnetic field gradient as follows: 
##EQU1## 
where A is an atom constant, whereby the magnetic transition frequency is 
related to the magnetic field intensity; and .DELTA.f is the width of the 
magnetic transition line, which is dependent upon the transverse 
relaxation time of atoms in the cells 3 and the intensity of the pumping 
light. 
The phase shift .DELTA..phi. is measured by the phase detector 15. In order 
to improve the resolution of the device, signals from the sensors 1 and 2 
are applied to said phase detector 15 via the identical frequency 
multipliers 13. 
Normal operation of the phase detector 15 requires a certain phase shift 
between signals applied to its inputs. This shift is determined by the 
phase shifter 14. 
The gradient meter of FIG. 1 makes it possible to measure small gradients 
with small distances between the sensors. However, high effectiveness 
requires the presence of a magnetic field which would be stable with time, 
and a low level of variable magnetic noise which causes amplitude noise 
modulation of sensor signals. 
In order to measure the gradient of a magnetic field that varies with time, 
it is more convenient to use the gradient meter of FIG. 2, wherein the 
means for supplying variable-frequency current to the radio frequency 
coils 4 is the feedback circuit 17 provided in the sensor 2 and connected 
to the input of the radio frequency coil 4 of the sensor 1. The parameters 
of the feedback circuit 17 are selected so as to ensure a phase balance 
and a resonance frequency amplitude balance; as a result, the sensor 2 
operates in the self-oscillation mode, its oscillation frequency following 
variations in the external magnetic field, whereby normal functioning of 
the sensor 1 is ensured. 
The gradient meter of FIG. 2 makes it possible to measure gradients of 
magnetic fields which vary with time; however, this gradient meter is 
sensitive to variable magnetic noise that causes amplitude noise 
modulation of signals of the transmitters 1 and 2. 
In order to measure magnetic field gradients in the presence of magnetic 
noise, it is preferable that use should be made of the gradient meter 
shown in FIG. 3. 
In the embodiment of FIG. 3, the function of the means for supplying 
variable-frequency current to the radio frequency coils 4 is performed by 
the sensor 18 provided with the feedback circuit 17 which connects the 
output of said sensor 18 to the radio frequency coils 4 of all the three 
sensors 1, 2 and 18. The parameters of the feedback circuit 17 of the 
sensor 18 are selected so as to ensure a phase balance and a resonance 
frequency amplitude balance; as a result, the sensor 18 operates in the 
self-oscillation mode, its oscillation frequency following variations in 
the intensity of the external magnetic field, which automatically ensures 
normal functioning of the sensors 1 and 2. 
The permissible gradient of the magnetic field between the three sensors 1, 
2 and 18 must be of a value at which the self-oscillation frequency of the 
sensor 18 should be found within the interval between the resonance 
frequencies of the sensors 1 and 2. 
The gradient meter of FIG. 3 makes it possible to measure gradients of 
magnetic fields which vary with time at variable magnetic noise levels 
that are commensurable, in terms of magnetic units, with the width of the 
magnetic transition line. 
If the magnetic noise level is higher, for example, because of the presence 
of electric networks, the best accuracy of measurements is provided by the 
gradient meter of FIG. 4. 
The device of FIG. 4 includes a frequency (phase) detector 19 intended for 
noise separation and connected to the output of the sensor 18. The 
frequency detector 19 may have different circuitries; one of the possible 
versions is an automatic phase frequency control circuit operating as a 
narrow-band follow filter. An error signal is applied from the output of a 
phase detector, incorporated in the above-mentioned automatic phase 
frequency control circuit, to the filter 20 tuned to the noise frequency. 
The output signal of the filter 20 is applied to the radio frequency coil 
4, whereby in the cell 3 of the sensor 18 there is produced a magnetic 
field which is in anti-phase which the magnetic field of the noise; thus 
the noise is compensated. 
In the sensor 18, the degree of noise suppression is determined by the 
transfer factor K of the feedback circuit from the frequency detector 19 
to the radio frequency coil 4. The same noise, acting upon the sensors 1 
and 2, is compensated by the magnetic field at the noise frequency by 
connecting the radio frequency coils 4 to the output of the filter 20 via 
the signal level regulator 21. If the intensity of the magnetic 
compensation field is equal to half the sum total of the intensities of 
the noise fields acting upon the sensors 1 and 2, any change in the events 
can be detected at the output of the device. 
The noise suppression circuit of the sensor 18 reduces the noise signal 
(K+1)-fold. If similar circuits are introduced in the sensors 1 and 2, the 
noise gradient is reduced accordingly, which makes it unnecessary to 
adjust the noise compensation level. 
The gradient meter of FIG. 4 makes it possible to measure gradients of 
magnetic fields varying with time at levels of variable magnetic noise 
which are in excess, in terms of magnetic units, of the width of the 
magnetic transition line.