WO1981000455A1

WO1981000455A1 - Nuclear magnetic resonance gyro

Info

Publication number: WO1981000455A1
Application number: PCT/US1980/000972
Authority: WO
Inventors: H Wang
Original assignee: Hughes Aircraft Co
Priority date: 1979-08-01
Filing date: 1980-07-31
Publication date: 1981-02-19
Also published as: EP0033340A1

Abstract

Nuclear magnetic resonance gyro including an enclosure (20) having a first region (23), a second region (24) and a connecting region (25), the enclosure (20) confining a quantity of an inert gas with two odd mass numbered isotopes capable of undergoing nuclear magnetic resonance. A magnetic field (32) is applied to the enclosure (20), and the gas in the first region (23) is pumped to an excited state to provide both polarized ground state atoms and discharge products therein, the atoms then being diffused through the connecting region (25), acting as a filter with respect to undesired discharge products, to the second region (24) where only polarized ground state atoms, which have retained their polarization, are able to propagate. Nuclear magnetic resonance transitions occur in the second region (24), and the frequency changes and the precession of the polarized atoms about the applied magnetic field (32) due to the rotational motion of the enclosure (20) are sensed by sensing circuitry (26, 21, 34).

Description

NUCLEAR MAGNETIC RESONANCE GYRO The present invention relates generally to gyros and more particularly to gyros of the nuclear magnetic resonance type.

BACKGROUND OF THE INVENTION Nuclear magnetic resonance gyros are presently known in the art, and existing designs have incorporated a single chamber, or gas cell, in which both optical pumping and observation of nucelar magnetic resonance take place. During the optical pumping cycle, atoms of a confined gas, or gases, are generally excited by an intense resonant light source and/or an RF source. The optical pumping creates nuclear polarization of the ground state atoms, and in addition, certain ions, electrons, metastable atoms, and other discharge products are also created. Generally, only the polarized ground state atoms are of interest in the observation region while the discharge products only serve to lower system performance and frequency resolution. Additionally, inter-atomic collisions between the discharge products and the polarized ground state atoms create unwanted frequency shifts and linewidth degradation (broadening of the resonance line) which lead to errors in the gyro readout.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a nuclear magnetic resonance gyro which reduces linewidth degradation and systematic nuclear magnetic resonance frequency shifts due to interactions between polarized ground state atoms and the unwanted discharge products.

It is a further object of the present invention to provide a nuclear magnetic resonance gyro which has improved frequency resolution.

Another object of the present invention is to provide a nuclear magnetic resonance gyro which is substantially independent of magnetic field fluctuations. In accordance with these and other objects of the present invention, there is provided a nuclear magnetic resonance gyro comprising an optically transmissive enclosure confining a mixture of two odd mass numbered isotopes of inert gas capable of undergoing nuclear magnetic resonance. Pumping means for creating a population inversion in the inert gas is disposed adjacen to the enclosure and includes a light source for providing circularly polarized resonant light and an RF discharge source for providing RF energy. The pumping means creates both polarized ground state atoms and discharge products during the pumping process. Means for applying a magnetic field is disposed adjacent to the enclosure. Sensing means for sensing the precession frequency of atoms undergoing nuclear magnetic resonance within the enclosure is disposed adjacent thereto.

The improvement comprises an enclosure having a first region, a second region, and a connecting region therebetween, the first region providing a volume in which the pumping of the isotopes occurs so as to create the population inversion therein. The connector region provides filtering means wherein the pumped atoms diffused therethrough in a manner such that the polarized ground state atoms substantially diffuse into the second region having retained substantial polarization, while the discharge products decay by the process of interatomic collision with other atoms and with the walls of the connecting region. The second region provides a volume thus containing a substantial quantity of polarized ground state atoms which undergo nuclear magnetic resonance, the second region further providing a volume in which nuclear magnetic resonance transitions occur which are sensed by the sensing means. Additionally, the pumping means are disposed adjacent to the first region while the sensing means is disposed adjacent to the second region. In operation, the pumping means creates a population inversion in the atoms of the confined gas. This process creates both polarized ground state atoms and discharge products. The pumped gases diffuse through the•connecting region and into the second region wherein the observation of the nuclear magnetic resonance takes place. ^'The- length of the connecting region is such that only the polarized ground state atoms can diffuse through the connecting region into the second region while retaining their polarization. This is due to the relatively long relax- ation time of the ground state nuclear polarization. The discharge products are effectively prevented from reaching the second region due to their sensitivity to wall collision relaxation into a thermal equilibrium state in the connecting region. Accordingly, the connecting region acts as a filter with respect to the unwanted discharge products. In addition, the use of two isotopes of the inert gas compensates for variations in the applied magnetic field. In essence, two nuclear magnetic resonance observations are made in the second region and the effective variations in the magnetic field may be substantially eliminated by comparing the two sets of data obtained.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects and features of the present invention may be more readily understood

OMPI with reference to the following detailed description taken in conjunction with the accompanying drawing in which like reference numerals designate like structural elements, and in which: The single FIGURE of the drawing is an illustration of a nuclear magnetic resonance gyro made in accordance with the principles of the present invention.

DETAILED DESCRIPTION Referring now to the drawing, the gyro generally comprises a resonant light source 21, which may be a^' Xenon discharge lamp, or the like, which provides circu¬ larly polarized resonant light, and an RF discharge source 22, for providing RF energy. The enclosure 20, which includes a first region 23, a second region 24, and a connecting region 25 therebetween, is disposed such that the first region 23 is adjacent to the resonant light source 21 and the RF source 22. The enclosure 20 is generally comprised of an optically transmissive material, such as Pyrex, glass, or the like. Generally, the first and second regions 23, 24 have an inner diameter on the order of two to three centimeters, while the connecting region 25 has an inner diameter of about three millimeters and a length of about two centimeters. Means is provided adjacent to the enclosure 20, such- as a magnetic source 33, for applying a magnetic field (designated by arrow 32) to the enclosure 20. The orientation of the magnetic field 32 is not critical, but is generally parallel to the major axis of the connecting region 25, and may have a field strength on the order of about 1 Gauss.

The enclosure 20 confines a mixture of two odd mass numbered isotopes of an inert gas, such as Xenon, or the like, which are capable of undergoing nuclear magnetic resonance. Typical of these isotopes is Xe and 131Xe. The enclosure 20 is generally sealed and evacuated prior to the introduction of the two isotopes, so as to lessen the amount of contamination therein.

Sensing means is provided for sensing the precession frequency of atoms undergoing nuclear magnetic resonance within the enclosure 20. The sensing means generally comprises two substantially identical sensing circuits

26, 34. The first sensing circuit 26 generally includes a resonant tank circuit comprising an induction coil 27, a capacitor 28, and an amplifier 30 assembled to form a nuclear magnetic resonance controlled oscillator, such as a marginal oscillator, or the like. The first sensing circuit 26 is tuned to respond to the precession frequency of the first isotope, while the second sensing circuit 34 is tuned to respond to the precession frequency of the second isotope.

Signal processor means 31, such as a microcomputer, or the like, is coupled to the two sensing circuits 26, 34 for processing signals received therefrom in accordance with data processing methods well known in the nuclear magnetic resonance gyro art.

In operation, the resonance light source 21 applied circularly polarized resonant light to the first region 23 of the enclosure 20. Simultaneously, the RF discharge source 22 applied RF energy, by means of weak RF discharge, to the first region 23. The two isotopes of Xenon confined within the enclosure 20 are initially in an equilibrium ground state condition. The RF discharge pumps these isotopes into an excited state, thus creating a mixture of ground state atoms, metastable atoms, electrons and ions within the first region 23. Applying the circularly polarized resonant light to the confined isotopes, commonly known as optical pumping, allows for the transfer of angular momentum stored in the circularly polarized light to the metastable atoms of these isotopes. By means of atomic collision processes,

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_^Q-ΛPI the metastable atoms subsequently transfer angular momentum to the ground state atoms confined within the first region 23 which causes a population inversion therein. In the presence of the applied magnetic field 32, the ground state atoms are put in a polarization state wherein the nuclear spins of the atoms are oriented in a direction substantially parallel to the magnetic field 32.

While in the equilibrium state, the confined atoms are randomly oriented as defined by the Boltzman distri¬ bution, and no nuclear magnetic resonant signal is observable. By optically pumping the gas, the population inversion is created, and in the presence of the applied magnetic field 32, allows for an observable nuclear magnetic resonance signal. For a better understanding of optical pumping and nuclear magnetic resonance in general, see "Optical Pumping", by W. Harper, Review of Modern Physics, 1972 and "Nuclear Magnetism", a text by A. Abragam, Oxford University Press. The pumping process in inert gases generally requires the creation of metastable atoms by means of the weak RF discharge, which also creates discharge products such as the electrons, ions and excited atoms. The polarized ground state atoms created by this process are the only atoms of interest in terms of measured resonance phenomena

In the present invention, two isotopes of Xenon are used, namely 129Xe and 131Xe. These isotopes of Xenon are particularly useful because they have long nuclear magnetic relaxation time in that the nuclear polarization in the ground state atoms is retained for a relatively long period of time. For example, the ground state nuclear polarization of the Xenon isotopes continues to exist for periods up to 24 minutes.

Accordingly, once the gas atoms have been optically pumped, they diffuse from the first region 23 through the connected region 25 and into the second region 24, which

^ acts as an observation region for the nuclear magnetic resonance. However, only the polarized ground state atoms which have a long nuclear magnetic relaxation time are able to diffuse through the entire length of the connecting region 25 and into the observation region while retaining their polarization. The unwanted discharge products, • on collision with the walls of the connecting region 25, decay into a ground equilibrium state while diffusing therethrough. As such, the connecting region 25 acts as a filter which allows only the polarized ground state atoms to diffuse into the second region 24.

During this process, the magnetic field 32 is applied to the enclosure 20. The applied magnetic field 32 is essential to the nuclear magnetic resonance process in that it defines the orientation of the polarized nuclear spins in the two isotopes.

Once the polarized ground state atoms have diffused into the second^' region 24, they are stimulated to make a radiative transition in response to the radio frequency field provided by the two sensing circuits 26, 34.

Each sensing circuits 26, 34 responds to the nuclear magnetic resonance signal of one of the two isotopes, in that the two sensing circuits are tuned to resonate at the magnetic resonance frequency of the respective isotopes. The radiative transition leads to an observable signal which is detected by the sensing circuits 26, 34. The sensing circuits 26, 34 are nuclear magnetic resonant controlled oscillators whose oscillation frequen¬ cies are the Larmor precession frequencies of the nuclear spins of the polarized ground state atoms of the two isotopes in the applied magnetic field 32. The general equation for this relationship is given by ω = γH - ω , where ω is the measured nuclear magnetic resonance fre¬ quency, γH is the Larmor precession frequency of atoms in a stationary system, γ is a known physical constant. H is the magnitude of the applied magnetic field 32, and ω is the angular rotation rate of the enclosure 20 about an axis parallel to the applied magnetic field 32.

Generally, the inertial rotation rate of practical systems are relatively small, being on the order of one part in 100 million of the nuclear Larmor precession frequency in a 1 Gauss field. However, frequency measure¬ ments of one part in ten to the fourteenth are easily detected by the present state of the art devices.

The frequencies of the nuclear magnetic resonance are detected by the sensing circuits 26, 34, and are applied to the signal processor means 31, such as a micro-computer, or the like. The signal processor means 31 may, for example, integrate the changes in frequency ^* over time to attain an angular rate of change which is indicative of the rotational motion of the gyro. This manner integration is well known to those skilled in the nuclear magnetic resonance gyro art.

The applied magnetic field 32 generally fluctuates with time, and in accordance with the present invention, the use of two isotopes of Xenon compensates therefore. Accordingly, two nuclear magnetic resonances are measured which provides a means for eliminating any fluctuations in the magnetic field 32. In essence, and considering the equation above, the use of two isotopes of Xenon provides for two equations and two unknowns thus elimina¬ ting the magnetic field from the equations, and as such the rotational frequency of the enclosure 20 may be easily calculated by the signal processor means 31.

The present invention has incorporated the use of two isotopes of Xenon so as to circumvent the practical problem of a fluctuating magnetic field 32. Clearly, the invention is in principle operational when using only one isotope. Additionally, other elements may be used, such as mercury or rubidium. However, these elements have polarization relaxation times on the order of one second or so, which would require a very short connecting region and a consequent reduction in signal to noise ratio due to losses in nuclear polarization. Also, with such short relaxation times, the nuclear magnetic resonance frequency resolution is reduced, thus providing a less effective gyro. As a matter of note, certain inert gases cannot be used, since they have no odd mass number isotopes. Such a gas is argon. Also, neon may be used, but it has only one odd mass number isotope. Thus, the magnetic field fluctuations could become a problem in gyros incorporating neon.

Thus, there has been described an improved .gyro which has improved frequency resolution due to the elimimination of unwanted metastables from the observation region, and which eliminates error due to changes in applied magnetic field.

It is to be understood that the above-described embodiment is merely illustrative of but a small number of the many possible specific emobid ents which can represent applications of the principles of the present invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A nuclear magnetic resonance gyro comprisin an enclosure (20) confining a quantity of an odd mass numbered isotope of an inert gas capable of undergoing nuclear magnetic resonance, pump structures (21, 22) for pumping the inert gas to an excited state, magnetic field generating circuitry (33) for applying a magnetic field (32) to the enclosure (20), and sensing circuitry (26, 31, 34) for sensing the nuclear magnetic resonance and for providing an indication of the rotational motion of the enclosure (20) about an axis parallel to the magnetic field (32) , characterized in that: said enclosure (20) includes a first region (23) , a second region (24) , and a connecting region (25) connecting said first region (23) and said second region (24) , said first region (23) being disposed adjacent to said pump structure (21, 22) and having an optically transmissive portion thereof, said second region (24) being disposed adjacent to said sensing circuitry (26, 34) , said first region (23) providing a region in which said pumping creates polarized ground state atoms and discharge products, said connecting region (25) acting as a filtering circuit wherein said polarized ground state atoms and said discharge products diffused therethrough in a manner such that said polarized ground state atoms diffuse into said second region (24) having retained sub¬ stantial polarization while said discharge products diffuse thereto having decayed in a ground thermal equilibrium state, said second region (24) providing a volume containing a substantial quantity of polarized ground state atoms which undergo said nuclear magnetic resonance that is sensed by said sensing circuitry (26, 31, 34) .

2. The nuclear magnetic resonance gyro according to claim 1, characterized in that said pump structure (21, 22) includes a resonant light source (21) providing circularly polarized resonant light, and includes an RF discharge source (22) providing RF energy in the form of a weak RF discharge in said first region (23) thereby pumping said isotopes into an excited state and creating a mixture of ground state atoms, metastable atoms, electrons and ions within said first region (23) , while the angular momentum stored in said circularly polarized light is transferred to said metastable atoms of said isotopes.

3. The nuclear magnetic resonance gyro according to claims 1 or 2, characterized in that said sensing circuitry (26, 34) includes first and second sensing circuits (26 and 34) each having a resonant tank circuit (27, 28) coupled to an amplifier (30) to form separate nuclear magnetic resonance controlled oscillators, said first sensing circuit (26) being tuned to respond to the precession frequency of a first isotope, while said second sensing circuit (34) being tuned to respond to the precession frequency of a second isotope.

4. The nuclear magnetic resonance gyro according to claim 3, characterized by a signal processor (31) coupled to said first and second sensing circuits

(26 and 34) for processing signals received therefrom.

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