DE1213912B - Method and device for measuring weak magnetic fields using nuclear induction - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

ALAJk

GERMAN JMWSsSSl PATENT OFFICE Int. CL:

GOIr

EDITORIAL

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German class: 2Ie-12

1213 912
C20380IXd / 21e
December 16, 1959
April 7, 1966

The invention relates to a method and a device for measuring weak magnetic Fields using nuclear induction and a liquid containing protons, in which one dissolves a paramagnetic impurity in the liquid, the electronic one Has resonance line with hyperfine structure, this resonance line with the help of an auxiliary magnetic field high frequency that is perpendicular to the field to be measured and the core frequency in that too measuring magnetic field determined.

The measuring technique making use of core induction was first used by Bloch (Physical Review, 70, p. 460, 1946) stated, it consists essentially in the fact that one the frequency of the free Measures the precession of a set of nuclei that have been placed in a permanent magnetic field and have a kinetic moment, the spin, as well as a magnetic moment, both of which are non-zero. To get a signal with a noticeable amplitude even with weak magnetic fields, one is forced, on the one hand, hydrogen nuclei, d. H. Protons, in a relatively large volume of liquid or water of 11 or more and, on the other hand, to use a trick called by Packard and Varian (Physical Review, 93, S. 941, 1954) and has the purpose of increasing the polarization of the nuclei. Man can also cite Wat er s and Francis (Journal of Scientific Instruments, 35, p. 88, 1958).

This method of measuring a weak DC magnetic field h, as stated by Packard and Varian, is based on the exploitation of a phenomenon which can be explained as follows:

Protons contained in water, for example, have or in the absence of a magnetic field if only a weak magnetic field is applied, all spins are in the same direction, and it shows It turns out that statistically 50% of these spins have one sense and the other 50% the opposite sense to have.

The action of a magnetic field of large amplitude changes this distribution slightly in favor of one of the spin populations, that is, it "polarizes" the protons in the sense that when the thermal equilibrium between the spins and the environment is reached, a small magnetization μ occurs, which directed parallel to the field η and by the resulting magnetic moment method and device for measuring
weak magnetic fields using core induction

Applicant:

Commissariat a l'ßnergie Atomique, Paris
Representative:

Dipl.-Ing. R. Beetz, patent attorney,
Munich 22, Steinsdorfstr. 10

Named as inventor:

Georges Bonnet, Grenoble;

Antoine Salvi, Saint Egreve, Isere (France)

Claimed priority:

France of December 23, 1958 (782472) -

is given, where X is the susceptibility of the measuring medium.

If a container with a liquid containing such cores is brought into the small magnetic field η to be measured, for example the earth's field, and is also exposed to a permanent field S which is perpendicular to the first field and significantly greater than this field, the liquid mass takes after a For a certain period of time, which depends on the "relaxation time", a macroscopic magnetization occurs in the equilibrium state, which is practically parallel to Jp and is given by the formula (1).

If the field h 'is suddenly made to disappear, so that only the field η to be measured remains, the vector of the resulting magnetization μ leads, due to the gyromagnetic properties of the nuclei to which this is due, a precession movement around the direction of the Field fi until the new equilibrium orientation is reached.

The rotating field with the precession of the resulting magnetization vector μ induces in

a coil surrounding the measuring material, which is arranged in a suitable manner, an alternating EMF, the frequency / with the size or amplitude H of the field η by the formula of Larmor

= XH

is linked, in which γ is a characteristic physical constant of the nucleus in question. the

609 557/131

3 4

Measurement of a magnetic field using which is increased. The magnetic moment of the core-core induction So it is based on a frequency measurement that spins in the fluid through the electronic returned. Spins are relaxed, can then under exactly defined

The use of the auxiliary field ~ H ' corresponds to ned conditions a considerable value

a dual purpose; a magnification is achieved that is 5 in very weak fields (smaller than

tion of the resulting magnetization of the substance about 10 Gauss) regardless of the size of the

and can therefore produce a sufficiently strong signal on the magnetic field. For example, in the

take and continue a precession of this earth field will approximate the polarization of the protons

Magnetization brought about. 4000 times stronger than without this effect. Then leads

The relatively Schacher magnetic fields for measuring the phenomenon of the use of a strong polarization-capturing nuclear resonance, which is required by the measurement, by means of different vertical fields W of the size from 100 to 200 Oersted driving measures.

However, the application of this method makes the corresponding to the French patent specification

is in principle easy, difficult. The pola- 1174136 becomes the amount of fluid which the

rising field must namely contain in an extremely short 15 paramagnetic impurities and the

Time span - in less than 30 με with the magnetic field H to be measured and one for this

generation of a magnetometer for measuring an alternating field H _RF perpendicular to the earth of the electronic re-

field - to be made to disappear, because the resonance frequency is subject to pollution,

the spectrum of the decrease in the magnetic field when surrounded by a coil, whose axis to the coupling

its elimination must be a spectral energy density 20 with the nuclear frequency field H _n in one direction.

at the Larmorian core frequency, which are arranged right to the two above-mentioned fields.

is big enough to achieve transitions. In net is.

Considering the relatively large amount of energy in the this coil forms the inductance of the resonance field the coil is stored when a relatively large circle of a pound spectrometer. This contains Field is generated, this condition 35 results in an oscillator that generates an alternating current, a shutdown problem that is very difficult to solve. its frequency roughly equal to the core frequency

In addition, the power required for generation in the field to be measured is. The one from above

of the polarizing field is necessary, a field generated by a coil does not have exactly that

considerable expenditure on equipment for the implementation frequency of the nuclear resonance. To determine this,

tion of this procedure. 30 is superimposed on the weak magnetic constant

Eventually the phenomenon of the field H becomes a very weak magnetic field h _a of about

free precession strong due to inhomogeneity of 50 Hz; the total modulated magnetic field H + h _a

measuring magnetic field influences what the main one also assumes the value of a nuclear resonance

represents the reason for measurement accuracy if one corresponds to the nance frequency and equals the frequency of the

takes into account the difficulty that is in the field generated by the coil. This response

right to maintain a high degree of homogeneity in a so then from the phenomenon of energy absorption

large volume of liquid lie, as determined in the vor- tion that occurs in the process. One then grasps

lying trap must be used. the instantaneous or instantaneous value of the

It would in itself be possible to reduce the consumption of elec- Period of the auxiliary alternating field in which the energy

tric energy and thus absorption arises, and from this determines the exact

would simplify the shutdown by taking nuclear magnetic resonance by applying the formula of

uses a less strong polarizing field and Larmo r.

significantly reduces the amount of liquid; Both of these procedures require a considerable amount of effort Measures are, however, disadvantageous for the accuracy of the expenditure on sensitive and complicated straight gears Measurement, as it is definitely the size of the 45th, in particular an oscillator, an oscillator for the generation of the precession frequency essential graph, a generator for the Weehsel field low resulting magnetic moment of lower frequency and the necessary electronic set and thus the relationship between the devices to be measured for determining the phase of the resosend Reduce the signal and the interference level. nance phenomenon. As a result, the implementation

Another method for measuring magnetic 5 ° this method difficult.

fields using the core induction, which is according to French patent 1177112 allows measurement of weak fields, is surrounded by the amount of liquid in a coil, Abragam, Solomon and Combrisson used part of an oscillating circuit with as much as possible (Comptes rendues de l'Academie des high surge coefficient (Q ^ 1000) bil-Sciences, 8. 7.1957, Tome 245, p. 157 to 160, as well as 55 det, whose resonance frequency is close to the core French patents 1174136 and 1177112). resonance frequency of the protons of the liquid

According to this procedure, which is the Overhauser set. The disadvantage of this measure is therein Effect as a trick to increase the nuclear polar- to see that under these conditions the measurement in the magnetic field to be measured uses its resonance frequency through the system the weak DC magnetic field is "falsified" because of the two Amount of liquid in which a paramagnetic coupled system with adjacent re-contamination is dissolved, which is an electronic sonanzfrequenzen a frequency shift be-resonance line with hyperfine structure, and is ding.

saturates this resonance line with a high frequency The present invention allows the after-field Hgp. The energy supplied to the electrons is partly to avoid measuring errors caused by the load described above, partly by relaxation on the protons.

transferred in which the proportion of spins of one The present invention relates to an improved

Sense in relation to the spins of the other sense 'procedure and a device for measuring

5 6

rather magnetic fields using the suppression or elimination of the magnetic

Core induction, in which the disadvantages or measurement errors are used in the auxiliary field.

the known methods are avoided; The company working according to the new process

Improvement takes advantage of the increase in the nuclear polarization gnetometer, which is now to be described as is

from that carried out on the Overhauser-Abragam effect be 5 portable measuring device. The source of energy is

rests. therefore a storage battery of 12 V (whether

The method according to the invention for measuring could probably also suffice 6 V),

a weak magnetic field utilizing the In the F i g. 1 is a high frequency oscillator 1 for

Nuclear induction and use of a liquid, the supply of to saturate the electronic

contains the protons, in which one is in the liquid i ° resonance lines which are in the measuring liquid 2

the ability to dissolve a paramagnetic impurity, finding paramagnetic impurity requires

which provided an electronic resonance line with hyperfine energy. The resonance frequency

structure, using this line of resonance depends on the nature of the paramagnetic radical

of an auxiliary magnetic field of high frequency saturates that and the selected resonance line from and from that

is perpendicular to the field to be measured, and the 15 value of the auxiliary magnetic field that determines the polarization

Core frequency in the magnetic field to be measured takes effect.

averages, is essentially characterized in that, in the event that the paramagnetic liquid used is exposed to an additional constant magnetic field h _c and the auxiliary field 3 Gauss - for example, the nitrosodisulphonate of potassium for the duration of some impurity, this resonance is the essentially perpendicular to 20 frequency near 60 MHz. A coaxial magnetic field H to be measured and the magnetic cable 3 of suitable length and a matching field of high frequency H ^ _F and its order of magnitude 4 connect the oscillator 1 with an order hardly greater than that of the field to be measured against the liquid A suitable dielectric H is that then the intensity of this auxiliary field cumulatively isolated high-frequency coil 5, which is arranged inside the container 2 to the value zero in a very short time. The container 2 has brought and after it has completely disappeared a capacity of about 100 cm ³ and entFeldes the frequency of the field of the core frequency held water in which the contamination is dissolved. will measure. As can be seen from Fig. 2, sets

Thus, according to the invention, the polarization in the high frequency coil 5 is one or two

the measuring liquid by applying the over 3 ° turns of an insulated wire 6 together, the

Hauser-Abragam effect, the auxiliary field has been aligned in such a way that the

only the task of orienting the resulting magnetic field perpendicular to the magnetic field η

changing magnetization so that it lies, which one wants to measure and its direction

falls in a direction approximately perpendicular to that illustrated by arrow 7.

of the field to be measured. In this way 35 another coil 8, the axis of which is perpendicular to

a ratio on the order of 5 that of the first coil 5 as well as perpendicular to it is sufficient

between these two fields. It is in the direction of the magnetic field S is immediate

This results in a very low amount of electrical equipment being wound around the vessel 2. This second

shear energy and a considerable simplification of the coil 8 has a double task:

Problem of the rapid interruption of the magnetic 4 ° On the one hand, it should create the auxiliary magnetic field.

sizing stream; this interruption can be caused by changing the alignment of the spins and

simple electronic means carried out due to their later precession, and on the other hand

will. also record the signal induced by the precession

Referring to the schematic fig.

3 to 3 of the drawing, an amplifier 10 is only intended to be used as an explanation. The frequency of the protons in the measured field details the process execution and which is attuned; the amplified signal is then order of the device, which is fed in connection with a frequency meter 11, which - in this example, are described as a component - is frequency meter of the decade counter type.
The high-frequency oscillator 1, the amplifier 10, however, it should be clear that all equivalent means and the frequency meter 11 are devices known in the art can be used in the same way without the type and are consequently not in their one-frame of the invention to exceed. details described.

In the drawing only those for understanding are 55 It should be noted, however, that

nis of the invention necessary elements shown; the amplifier and the frequency meter very easily

Corresponding elements of the figures can be equipped with transistors; in

same reference numerals. It shows that the power supply arrangement needs

1 shows a schematic representation of a Manungl2, which in the schematic representation by gnetometer, which symbolizes a rectangle using the core inductance 60 and from the battery 13

tion according to the present invention works, is fed, only the operation of the high-

Fi g. 2 shows a section through the high voltage required in the flow frequency oscillator

speed in which the nuclear spins deliver for the measurement, d. H. about an output of IS W, if the

be used; This section also shows the arrangement coil 5 a high frequency power of approximately 5 W tion of the coils for the high and low frequencies 65 must be supplied.

quenz, When operating or using the new

Fig. 3 is a schematic representation of the Um- device are two chronologically successive pre-

Switching arrangement, which is particularly to distinguish between fast gears:

7 8

a) A) is sent via the relay circuit 9. The resistor 15 is used for this purpose, the ohmic value of which in direct current in the low-frequency coil 8 is around 500,000 Ω; the value Q resulting magnetization of the protons is then close to 10, and the oscillation has to be oriented at an angle to the field to be measured η . a duration of a few milliseconds with a maximum voltage of the order of magnitude of the type of paramagnetic contamination 10 V. There is practically no switch-off or its concentration as well as the type of solution switching problem.

by means of. If as a paramagnetic impurity, even if you do under these conditions

nitrosodisulphonate of potassium in millimoles- without further ado, use a mechanical relay

If the concentration is dissolved in water, it could be, but the use of an electronic one

This procedural step only requires a time 'relay to be preferred because the interruption by

of about 2 seconds. The orienting magnet- such a relay takes place very quickly and also

field is of the order of 3 Gauss. While not deviating from the state of metallic contacts,

the amplifier 10 is short-circuited during this period. As a result, the measured values are

sen and to repeat or reproduce the counter of the frequency meter 11 to zero 15 res,

set. Since the internal resistance R of an electrical

b) With the help of a simple program circuit, the ironic relay is not zero, then a time constant is then triggered via the relay circuit 9 in the circuit of the loan interruption of the auxiliary field and the field-generating coil 8 is introduced. In the ge. the low-frequency coil 8 to the input terminals ao selected embodiment, the ratio LJR of the amplifier 10 is applied in order to determine the precession frequency of the order of 500 μβ, and the alignment frequency. tion or orientation of the nuclear spins takes place in the

The duration of the frequency count is in the same magnetic auxiliary field without any precession, of the order of magnitude as that of the previous one. The relay 16 receives an actuation signal S,

Process step. 25 his closing contact 17 conducts via the

It should be pointed out that without any further resistance 18 of approximately 500 Ω a sufficient the high frequency oscillator 1 while counting high polarization voltage that grids the tube period can stop, whereby the mean current 19 (Type 12 AU 7) makes positive. Simultaneously load, consumption is reduced. The closing of the contact 20 of the relay 16 acts through the over- The polarization induced by the Hauser effect runs like the excitation of the relay 21. The closing of the Konauch the free precession - after one with the clock 22 this relay connects the battery, the Time decreasing law, which is due to the relaxation with its negative pole at terminal 23, time is given. via the tube 19, the internal resistance of which is through

The switching device in the relay switch- the potentiometer 24 (for setting the »auxiliary time«) tion9 is contained, was regulated in more detail 35 in FIG. 3, with the coil 8. The relay 21 is connected. det through its second contact 25 the cable 26,

To a disturbance of the field at the location of the measurement that leads to the input of the amplifier 10, with the to avoid this arrangement is spaced mass.

of 1 or 2 m from the low-frequency coil 8 at the end of the "orientation period" is created and with this coil via a coaxial cable 40 excitation of the relay 16 is interrupted. The contact 17 14 connected, the capacity of which C _{0 is} not negligible, opens, as a result of the action of the additional battery, can be detected. 27 (of about 10V) become the grids of the tube

There is therefore a parasitic capacitance C ₀ of 19 strongly negative, whereby the tube 19 is blocked 100 or 20OpF at the terminals of the inductance. The duration of the interruption of the electronic L _{0 of} the low frequency coil 8. 45 see is through the resistor 28 and the relay

If you suddenly determine the magnetizing current capacitance between grid and ground. With interrupts that flows through L ₀ , a resistance 28 of 100,000 Ω occurs, this is a balancing process in the L ₀ -C ₀ circuit, duration in the order of magnitude of a microsecond.
which can be periodic or attenuated. Opening the contact 20 removes the excitement

For reasons of power consumption it is better to open 5 ° of the relay 21, but the capacitor 30 to work with a periodic equalizing process.This relay delays its switch-off, provided that the natural frequency remains much higher as a result of this also during the duration des is than the Larmor frequency at the core precession. Switch-off compensation process in the coil 8 of the The low-frequency coil 8 has switched on damping resistor 15 in the example and the 4000 turns, which are divided into four disks; 55 amplifiers short-circuited. As soon as the relay 21 the wire has a diameter of ⁴ Ao mm and switched off, the resistor 15 is switched off and is insulated with silk. The natural frequency of this on and the coil 8 is connected via the contact 31 with the order is with a line capacitance Co of 150 pF amplifiers, so that now a measurement is approximately equal to F ₀ = 15 kHz. To an auxiliary field of the precession frequency can be done. To create in Anbe-3 Gauss, one comes with a current 60 endeavoring the small voltage increase at that of 10 mA, which corresponds to an ohmic voltage equalization process, the tube 19 is possible to drop V from 1V. to replace with a transistor. However, this must

It is known that a high-frequency transistor can be used in an overvoltage circuit so that it can _{carry out a ratio of the circuit L 0} -C _{0 of} the oscillating fast shutdown or separation. it must be able to dissipate about 100 mW (when supplied with amplitude C- V and its envelope curve with a 6V) as a loss and an anti-time constant of QInF ₀ decreases. A voltage in the direction of the emitter-base of 20 V is interested in reducing this overvoltage. allow.

It can be seen that during the period of the precession frequency measurement the relays are not fed in order not to disturb the magnetic field which is being measured at that moment. For the same Basically, small relays (miniature relays) are used for remote control if necessary, their volume of ferromagnetic material is very low.

It has already been said above that the amount of liquid to be measured should be at most 100 cm ³ , which simplifies the requirement for homogeneity of the field. Although the device just described is portable and has a low power consumption, the accuracy of the measurement is of the same order of magnitude as that of the much larger known devices which perform a measurement of free precession. If you are satisfied with an accuracy of ± 10 ~ ⁵ Oersted, you can reduce the liquid volume to 10 cm ³ and only use 1W high frequency power.

Claims (3)

Patent claims: 1. A method for measuring a weak magnetic field using nuclear induction and using a liquid containing protons, in which a paramagnetic impurity is dissolved in the liquid, which has an electronic resonance line with a hyperfine structure, this resonance line is saturated with the aid of an auxiliary magnetic field of high frequency, which is perpendicular to the field to be measured, and determines the core frequency in the magnetic field to be measured, characterized in that the liquid - for a period of a few seconds - is exposed to an additional DC magnetic field (h _c ) which is essentially perpendicular to the measuring magnetic field (H) and the high frequency magnetic field (H _RF ) and its order of magnitude is hardly larger than that of the field to be measured (H) , that the intensity of this auxiliary field is then brought back to zero in a very short time and after complete Disappearance of this field the frequency of the field of the core frequency ge will measure. 2. Device for performing the method according to claim 1, in which the saturation of the electronic resonance line with hyperfine structure is carried out by a coil whose axis is perpendicular to the field to be measured, characterized in that the device has a second coil in a manner known per se (8), the axis of which is perpendicular to the field to be measured (H) or arrow (7) and to the axis of the first coil (5), which, however, serves to generate a DC magnetic field (h _c ) , the magnitude of which is hardly higher than that of the field to be measured; that devices (relay circuit 9) are provided in order to bring this weak direct magnetic field to the value zero in a very short time, and that switching means (9, 10) are also present, which allow the connection of a frequency meter (11) to the second coil during the Allow the duration of the measurement during which the weak DC magnetic field has disappeared. 3. Apparatus according to claim 2, characterized in that for the rapid "interruption of the direct magnetic field (h _c ) of the second coil (8) by negative polarization of two control grids of a double triode (19 in Fig. 3), the anode current of the current in the coil controls, and that this coil can only be connected to a frequency meter (11) after the electrical oscillations, which arise due to the disappearance of the DC magnetic field, have decayed. Considered publications:
Revue Generale de l'Electricite, 67 (1958), pp. 365 to 384. For this purpose, 1 sheet of drawings 609 557/131 3.66 © Bundesdruckerei Berlin