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March 6, 1962
-
P. M. MOSER
3,024,410
CONTINUOUS ‘READING FREE NUCLEAR PRECESSION
MAGNETOMETER AND METHOD
Filed June 25, 1957
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INVENTOR.
PAUL
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BY
M. MOSER
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/{?TORNEYISQ
March 6, 1962
CON TINUOUS READP.N M.
MAGNET
MOSER
E NUCLEAR
Filed June 25, 1957
‘
PRECESSION
3,024,410
-
AND METHOD
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FREQUENCY
M ETER
INVENTOR.
PAUL
BY
M. MOSER
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'TTORNEIY’
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Uited States Patet
??ce
,,,,,,,ed Mgfiflflig
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3 e24 410
ples other than free nuclear precession, e.g., saturable re
actor type, require that the sensing head be maintained
coNrrNUoUs guanine’ FREE NUCLEAR ran
CE§SION MAoNnToMETEn AND MErnon
Paul M. Maser, Eiliins Park, Pa., ass'ignor to the United 5
States of America as represented by the Secretary of .
the Navy
Filed June 25, 1957, Ser. No. 668,tl04
12 Claims. (Cl. 324-.5)
(Granted under Title 35, US. Code (1952), sec. 256)
10
With great accuracy .at some ?xed orientation with the
direction of the magnetic ?eld undergoing measurement.
This usually means, especially in cases of moving mag
netometer carriers, that elaborate and expensive apparatus
for maintaining the orientation must be provided, which
places additional limitations on the accuracy which is ob
tainable.
Hence, there is need for an accurate con
tinuously recording magnetometer which will function
The invention described herein may be manufactured
and used by or for the Government of the United States
of America for governmental purposes without the pay
under conditions of variable orientation with respect to
the ?eld undergoing measurements.
The present invention overcomes the disadvantages of
ment of any royalties thereon or therefor.
the prior free nuclear precession methods ‘and apparatus
The present invention relates to a method and apparatus 15 described abové as Well as other types by eliminating the
for measuring the intensities of magnetic ?elds and more
pulsed polarizing ?eld and its attendant drawbacks in
particularly to a method and apparatus utilizing principles
cluding the need for a heavy duty electric power source.
of nuclear precession for measuring in a continuous man
The invention in a preferred embodiment passes a ?uid
ner the intensities of magnetic ?elds, especially weak mag
sample containing atomic nuclei of large gyromagnetic
netic ?elds.
20 ratio successively through a strong polarizing magnetic
Present methods and apparatus relying on free nuclear
?eld such as one caused by a permanent magnet and a
precession for measuring the intensity of a magnetic ?eld
coil appropriately located for detecting the voltage at the
or otherwise correlating the magnetic properties of the
precession frequency. The voltage induced in the coil is
atomic nuclei portions of atoms with applied magnetic
ampli?ed and its frequency measured by standard meth
?elds initially subject a sample of substance possessing 25 ods. The ?uid sample may be circulated continuously
nuclear magnetic moments to a polarizing magnetic ?eld
at some angle to the direction of the magnetic ?eld to be
measured. The polarizing ?eld is then suddenly removed
and the nuclei thereby begin to precess about the vector of
the magnetic ?eld to be measured, hereafter referred to as 30
the precession ?eld. In returning to their equilibrium
state, the nuclei, acting as individual magnets with their
within a closed system.
An object of the present invention is the provision of a
method and apparatus for measuring the intensity of a
magnetic ?eld by the use of continuous free nuclear
precession frequency measurements.
Another object is to provide a method and apparatus
for correlating in a continuous manner the magnetic prop
own magnetic ?elds, induce an alternating voltage in a
erties of atomic nuclei with applied magnetic ?elds.
coil which is placed conveniently for this purpose. The
A further object of the invention is a method and ap
frequency of this voltage is that of the precession fre 35 paratus for continuously measuring the intensity of the
quency of the nuclei which is directly related to the mag
netic ?eld intensity. A full discussion of the principles in
volved and an example of typical apparatus for carrying
out the procedure just described, appears in Reissue Patent
23,769, dated January 12, 1954, for “Method and Means
for Correlating Nuclear Properties of Atoms and Mag~
netic Fields,” issued in the name of R. H. Varian.
earth’s magnetic ?eld based on free nuclear precession and
not requiring constant orientation with the direction of
the ?eld undergoing measurement.
Still another object is to provide .a technique and ap
paratus relying on principles of free nuclear precession
for measuring the intensity of a magnetic ?eld relying on
simple, reliable equipment capable of accuracy heretofore
unobtainablc.
A technique of the variety just described has several
practical disadvantages. For one, the polarizing ?eld is
The exact nature of this invention, as well as other
applied in pulses, with the time between successive pulses 45 objects thereof, will be readily apparent from considera
devoted to the measurement of the output signal which
tion of the following speci?cation relating to the annexed
consists of a series of time-decaying pulses. Since the
drawings in which:
signal cannot be observed while the polarizing ?eld is
FIG. 1 illustrates schematically by a view along B-—-B
being pulsed, and since over half the time of making a
of FIG. 2 one embodiment of this invention.
determination is involved in pulsing action and waiting for 50
FIG. 2 is a View along A—A of FIG. 1.
unwanted transients to decay, the time interval in which
FIG. 3 shows the apparatus of FIG. 1 incorporated
frequency measurements may be made is quite small, and
into a typical system for initiating its operation and res
the accuracy of precession frequency measurement, de
cording its results.
pendent on the duration of observation, is severely limited.
There is shown in FIGS. 1 and 2 a magnetometer 10
Furthermore, because the decay time of spurious transi 55 comprising in this instance a closed- cycle ?uid ?ow sys
ents must be kept to a minimum, the intensity of the
tem constructed of tubing 12 made from non-magnetic
polarizing ?eld must be kept down to about 100 oersteds,
material such as glass for circulating therein in the direc
which limits the degree of polarization obtained. Also,
tion of the arrow a liquid 13 of the type hereinafter de
pulsing of a magnetic ?eld can be obtained only with an
scribed by a pump 14.
electromagnet requiring a heavy duty electrical power
source and switching devices, thereby limiting the port
ability of apparatus of this type. Another disadvantage of
the pulsed system is that it may fail to detect meaningful
Any liquid or ?uid can be used in tubing 12 for the
purpose of this invention provided the ?uid selected‘
?uctuations of short duration in a varying magnetic ?eld
‘
contains nuclei having magnetic moments, and prefera
bly a very large gyromagnetic ratio. Hydrogen is one
of the best materials for this purpose and it occurs abun-'
due to the fact that actual measurements are being taken 65 dantly in water, which is thereby very satisfactory for
during only a fraction of the monitoring time. Present
use in this invention. The water 13 may be modi?ed
continuously recording magnetometers based upon princi
so as to produce an intensi?ed output by adding a para
3,024,410
magnetic salt in solution in order to reduce the relaxa
tion time, as is understood in the art. Examples of
these salts are ferrous and manganous sulfate.
Tubing 12 at one point ?ares into a bulbous portion
comprising the polarizing chamber 16 preferably rec
tangular in cross-section as best shown in FIG. 2, for
best occupying the space provided. Polarizing chamber
16 is large enough to provide adequate dwell time for the
A
resonance, or the latter may be dispensed with entirely,
depending on circumstances and intended use of mag
netometer 10.
The operation of magnetometer 10 shown in FIGS. 1
and 2 is as follows: Magnetometer 10 or just detection
chamber 20 is immersed in the ambient or precession
?eld. Pump 14 maintains the circulation of water 13
continuously throughout tube 12 in the direction of the
arrow while the polarizing ?eld in chamber 16 produced
terminates in a length of tubing 18 necked down con 10 by permanent magnet 22 orients or points the magnetic
moments of the hydrogen nuclei in the water into align
siderably in order to provide ?ow fast enough to pro
ment with the polarizing ?eld as is understood in the
vide comparatively fast transfer of the nuclei to detec
art. Water 13 then passes quickly from chamber 16
tion chamber 20 which must be placedv far enough away
through tubing 18 into detection chamber 20 where the
from polarizing chamber 16 to avoid being subject to
hydrogen nuclei are subject to the precession ?eld. Coil
any signi?cant effects of the polarizing ?eld. Detection
l1 adjacent chamber 20 picks up a voltage at the preces
chamber 20 which is subject to the precession ?eld is
sion frequency due to the processing of the magnetic
made suf?ciently large to provide adequate dwell of the
?elds of the nuclei returning to orientation with the am
water 13 therein equal to the relaxation time of the
bient ?eld. The signal voltage in coil I1 is ampli?ed and
nuclei, approximately two seconds for water, and termi
recorded as hereinbefore described.
nates in the feedwater tubing for pump 14.
20
It will be evident, as already noted, from the descrip
The polarizing ?eld for chamber 16 is provided by
tion of magnetometer 10 and its operation that the flow
a permanent magnet 22 designed so that its poles termi
of water 13 must be controlled in such a way that the
nate adjacent opposite sides of chamber‘ 16 and wide
nuclei dwell in the polarizing ?eld for a time of the order
enough to subject substantially the whole chamber 16
to the ?eld between the poles, for maximum effect. The 25 of magnitude of the relaxation time of the nuclei, which
is about two seconds for protons in water, for the purpose
uniformity of the polarizing ?eld permeating chamber
of insuring complete orientation. Also, the nuclei once
16 is not critical and hence a simple magnet of the type
polarized must be transferred to detection chamber 20
illustrated is adequate for the purposes of this invention.
during a time interval which is small compared to the re
When the intensity of the precession or ambient mag
laxation time, so that most of the relaxation time will be
netic ?eld is of the order of magnitude of the polarizing
available for measurement. Then, the nuclei must re
?eld, or when the former ?eld is comparatively weak
main in detection chamber 20 for a time interval of the
when compared to the latter as would be the case when
order of magnitude of the relaxation time in order to per
measuring the earth’s magnetic ?eld and it is not possible
mit their giving up a considerable portion of the energy
or practical to isolate detection chamber 20 from the
in?uence of the polarizing (permanent magnet) ?eld, a 35 to I1; and permanent magnet 22 must be sufficiently dis
tant from the coil I1 so as not to disturb the homogeneity
magnetic shield to enclose chamber 16 may be provided.
of the ?eld within chamber 20 nor cause any time de
For this purpose, a pair of coaxial cylindrical shield cans
pendent variation therein. That is, for the device to give
24 and 26 separated, and the latter supported, by foam
?eld measurements which are independent of the orienta
rubber 29, and extensions 28 and 30 fully enclose polar
izing chamber 16, permanent magnet 22, and adjacent 40 tion of the instrument, the ?eld intensity at coil I1 due to
permanent magnet 22 must be less than the smallest varia
tubing including the necked-down portion 18. Magnet
tions in the intensity of the ?eld to be measured. With
22 is supported by a plurality of struts 31 of non-mag
these design considerations in mind, the relative sizes of
netic material such as wood or plastic. Water 13 passes
the tubing and various chambers, and proper rate of ?uid
from the polarizing ?eld to the precession ?eld to be
flow, can easily be calculated.
measured rather quickly, and well within the relaxation
It is seen that the method and apparatus described
period for the liquid 13 being used. The usual mag
above will accomplish the result of producing a con
netic shielding material, such as Permalloy or soft iron,
tinuous reading of the precession frequency for deter
is chosen for constructing the magnetic shield cans 24 and
mining the intensity of a magnetic ?eld, such as the earth’s
26, and extensions 28 and 30.
magnetic ?eld, permitting frequency measurements to be
Surrounding detection chamber 20 or otherwise ap
propriately placed for picking up the voltages induced
taken over as long a period as is required, thereby result
ing in higher precision than heretofore possible. Further
by the precessing hydrogen nuclei in the process of re
more, since no pulsing takes place, there are no spurious
orientation in the ?eld to be measured is a coil 11. A
variable condenser C1 may be placed across coil I1 for
transients to interfere with the results, and because the
tuning the combination to resonance at the precession 55 invention involves the use of a permanent magnet the
frequency for obtaining better results. As best shown
electrical power requirements are reduced substantially,
in FIG. 3, the output of this tuned circuit may be fed
although, of course, a D.C. electromagnet may be used
into an ampli?er 32 and subsequently a frequency meas
in particular circumstances. In application of the inven
uring circuit 34 which may include the usual recording
tion, it is not necessary to maintain a particular angular
apparatus, as understood in the art. For tickling the
relationship between the polarizing ?eld and the ?eld to
resonant circuit C1, 1,, at the outset, an oscillator 33 of
be measured, although, to obtain a reading at any par
conventional design operating at the approximate, ex
ticular instant, some angle, usually substantially greater
pected precession frequency may be connected through
than zero for desirable signal to noise ratio is required.
a small capacitor C2 and a switch 8-1 to one end of
coil I1. Switch S-l would be temporarily closed during 65 Where the magnetometer is being moved in the precession
?eld and thus the relative orientation is being continu
starting for the purpose only of stimulating the nuclei
ously changed, proper design will insure that periodic
to precess coherently. Once in operation, magnetometer
passage
of this angle through or near zero will not pre
10 will continuously provide a signal at a frequency equal
vent substantially continuous readings.
to the precession frequency, which is proportional to or
It should be understood of course that the foregoing
otherwise directly related of the intensity of the ?eld 70
disclosure relates to only a preferred embodiment of the
undergoing measurement. Capacitor C1 is initially ad
invention and that numerous modi?cations or alterations
justed for resonance, and would normally not require
thereof may be made therein without departing from the
further adjustment unless the precession ?eld changes con
spirit and scope of the invention as set forth in the
siderably. Of course, automatic means, not illustrated,
appended claims. For example, in cases where the pre
may be provided for continuously readjusting C; for
hydrogen nuclei in the polarizing magnetic ?eld, and
5
3,024,410
cession ?eld is weak when compared to the polarizing
?eld, it is not necessary to shield polarizing chamber 16.
Instead, if necessary, a shield between chambers 16 and
20 may be resorted to. Further, this invention may be
applied without the use of a closed cycle system provided
there is an ample supply of a suitable ?uid. It also may
be feasible to use a gas or solid material, the latter, for
6
subjected therethrough to a ?rst unidirectional magnetic
?eld whereby said nuclei therein are predominantly point
ed in a ?rst direction, a permanent magnet ?xedly dis
posed about a second segment of said path for produc
ing therethrough a second unidirectional magnetic ?eld
angularly displaced from the ?rst ?eld whereby said
nuclei therein are predominantly pointed in a second di
rection, a shield enclosing said magnet for mutually ex
cluding the ?rst and second ?elds in the segments, and a
example, being moved physically over the wheels of a
pulley, or under particular circumstances it may be de
sirable to rotate or oscillate the polarizing ?eld rather 10 detector coil ?xedly disposed about the ?rst segment for
discerning free precession as said nuclei circulating
than the sensitive material.
through the second segment into the ?rst segment change
What is claimed is:
1. Apparatus for continuously producing nuclear gyro
orientation from the second direction to the ?rst direc
tion.
magnetic moment free precession, comprising: gyromag
6. Apparatus as set forth in claim 5 further comprising
netic atom portions, means for circulating said atom por
means for decreasing the velocity of said circulating ?uid
tions in a continuous path within said apparatus, a ?rst
segment of said path adapted to be subjected therethrough @ at the segments, and for increasing the velocity of said
circulating ?uid in the direction of said ?rst segment
to a ?rst unidirectional magnetic ?eld whereby said atom
from said second segment.
portions therein are predominantly oriented in a ?rst
7. Apparatus ‘for continuously detecting nuclear gyro
direction, means ?xedly disposed adjacent a second seg '20
magnetic moment free precession, comprising: means for
ment of said path for producing therethrough a second
circulating the atom portions in a continuous path, a
magnetic ?eld angularly displaced from the ?rst ?eld
?rst segment of said path adapted to be subjected there
whereby said atom portions therein are predominantly
through to a ?rst unidirectional magnetic ?eld whereby
oriented in a second direction, means for slowing the cir
culation rate of said atom portions through said second 25 the atom portions therein are predominantly oriented in
a ?rst direction, means ?xedly disposed adjacent a second
segment, and means for mutually excluding the ?rst and
segment of said path for producing therethrough a sec
second ?elds in the segments whereby said atom portions
ond unidirectional magnetic ?eld angularly displaced
circulating through the second segment into the ?rst seg
from the ?rst ?eld whereby the atom portions therein are
ment precess freely from the second direction to the ?rst
30 predominantly oriented in a second direction, means for
direction.
mutually excluding the ?rst and second ?elds in the seg
2. Apparatus for continuously producing nuclear gyro
ments, and means ?xedly disposed adjacent the ?rst seg
magnetic moment free precession, comprising: a continu
ous conduit ?lled with a ?uid containing gyromagnetic
nuclei, pump means for circulating said ?uid in said con
duit, a ?rst segment of said conduit adapted to be sub
jected therethrough to a ?rst unidirectional magnetic
?eld whereby said nuclei therein are predominantly
pointed in a ?rst direction, a permanent magnet ?xedly
disposed about a second segment of said conduit for pro
ducing therethrough a second unidirectional magnetic
ment for detecting the voltages induced as said atom por
tions, circulating through the second segment into the
?rst segment precess freely from the second direction to
the ?rst direction.
8. Apparatus for continuously detecting nuclear gyro
magnetic moment free precession, comprising: a con
tinuous conduit ?lled with ?uid containing the nuclei,
pump means for circulating said ?uid in said conduit, a
?eld angularly displaced from the ?rst ?eld whereby said
?rst segment of said conduit adapted to be subjected
nuclei therein are predominantly pointed in a second
therethrough to a ?rst unidirectional magnetic ?eld where
by the nuclei therein are predominantly pointed in a ?rst
direction, means for slowing the circulation rate of said
direction, a permanent magnet ?xedly vdisposed about a
?uid through said second segment, and a shield enclosing
said magnet for mutually excluding the ?rst and second 45 second segment of said conduit for producing a second
unidirectional magnetic ?eld angularly displaced \from the
?elds in the segments whereby said nuclei circulating
?rst ?eld whereby the nuclei therein are predominantly
through the second segment into the ?rst segment precess
pointed in a second direction, a shield enclosing said mag
freely from the second direction to the ?rst direction.
net for mutually excluding the ?rst and second ?elds in
3. Apparatus as set forth in claim 2 further comprising
means for decreasing the velocity of said circulating ?uid 50 the segments, and a detector coil ?xedly disposed about
the ?rst segment for detecting the voltages induced with
at the segments and for increasing the velocity of said
in said ?rst segment as said nuclei circulating through the
circulating ?uid in the direction of said ?rst segment ‘from
second segment into the ?rst segment precess freely from
said second segment.
the second direction to the ?rst direction.
4. Apparatus for continuously detecting nuclear gyro
9. Apparatus as set forth in claim 8 further compris
magnetic moment free precession, comprising: gyromag 55
ing means for decreasing the velocity of said circulating
netic atom portions, means for circulating said atom por
?uid at the segments, and for increasing the velocity of
tions in a continuous path, a ?rst segment of said path
said circulating ?uid in the direction of said ?rst segment
adapted to be subjected therethrough to a ?rst unidirec
from said second segment.
tional magnetic ?eld whereby said atom portions therein
10. Apparatus for continuously measuring a magnetic
are predominantly oriented in a ?rst direction, means 60
?xedly disposed adjacent a second segment of said path
?eld, comprising: atom portions of known gyromagnetic
properties, means for circulating said atom portions in a
for producing therethrough a second unidirectional mag
continuous path, a ?rst segment of said path adapted to
netic ?eld angularly displaced from the ?rst ?eld whereby
be subjected therethrough to a ?rst unidirectional mag
said atom portions therein are predominantly oriented in
a second direction, means for mutually excluding the ?rst 65 netic ?eld whereby said atom portions therein are pre
dominantly oriented in a ?rst direction, means ?xedly
and second ?elds in the segments, and detector means
disposed adjacent a second segment of said path for pro
?xedly disposed adjacent the ?rst segment for discerning
ducing therethrough a second undirectional magnetic ?eld
free precession as said atom portions circulating through
angularly displaced from the ?rst ?eld whereby said atom
the second segment into the ?rst segment change orienta
70 portions therein are predominantly oriented in a second
tion from the second direction to the ?rst direction.
direction, means for mutually excluding the ?rst and
5. Apparatus for continuously detecting nuclear gyro
second ?elds in the segments, and ‘detector means ?xedly
magnetic moment free precession, comprising: a con—
disposed adjacent the ?rst segment for producing a signal
tinuous conduit ?lled with a ?uid containing gyromag
indicative of the strength of the ?rst ?eld as said nuclei
netic nuclei, pump means for circulating said ?uid in said
conduit, a ?rst segment of said conduit adapted to be 75 circulating through the second segment into the ?rst
3,024,410
segment prccess freely from the second direction to the
?rst direction.
11. Apparatus for continuously measuring a magnetic
?eld, comprising: a continuous conduit ?lled with a ?uid
containing nuclei of known gyromagnetic properties, pump
8
?uid at the segments, and for increasing the velocity of
said circulating ?uid in the direction of said ?rst segment
from said second segment.
References Cited in the ?le of this patent
UNITED STATES PATENTS
means for circulating said ?uid in said conduit, a ?rst
segment of said conduit adapted to be subjected there
through to a ?rst unidirectional magnetic ?eld whereby
said nuclei therein are predominantly pointed in a ?rst
direction, a permanent magnet ?xedly disposed about a
Re. 23,769
Carian ______________ __ Jan. 12, 1954
Re, 23,950
2,721,970
10 2,728,217
Bloch et al ___________ __ Feb. 22, 1955
Levinthal ____________ __ Oct. 25, 1955
Richardson __________ __ Dec. 27, 1955
746,114
Great Britain __________ __ Mar. 7, 1956
second segment of said path for producing therethrough
FOREIGN PATENTS
a second unidirectional magnetic ?eld angularly displaced
from the ?rst ?eld whereby said nuclei therein are pre
dominantly pointed in a second direction, a shield en
closing said magnet for mutually excluding the ?rst and
second ?elds in the segments, and a detector coil ?xedly
OTHER REFERENCES
Hunter: Oil and Gas Journal, vol. 54, No. 66, Aug. 6,
1956, pp. 144 and 145.
disposed about the ?rst segment for producing a signal
Sherman: Physical Review, vol. 93,
indicative of the strength of the ?rst ?eld as said nuclei
1954, pages 1429 and 1430.
circulating through the second segment into the ?rst seg
Suryan: Proceedings of the Indian
ment precess freely from the second direction to the ?rst 20
ences,
vol. 33, January-June 1951 (pp.
direction.
Hopkins: The Review of Scienti?c
12. Apparatus as set forth in claim 11 further compris
ing means for decreasing the velocity of said circulating
20, No. 6, June 1949, pp. 401 and 402.
No. 6, Mar. 15,
Academy of Sci
107-111).
Instruments, vol.
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