US3281600A

US3281600A - Stimulated gamma ray emission

Info

Publication number: US3281600A
Authority: US
Publication date: 1966-10-25
Anticipated expiration: 1983-10-25

Description

Oct. 25, 1966 v, VAL] ETAL 3,281,600

STIMULATED GAMMA RAY EMISSION Filed D90. 26. 1963 24 v; OUTPUT FIG.2 T I I I 20 ID 2 L l CONTROL E3 F E ELEE L 4 O RAD'ATOR CRYSTAL CONDITION (E13. REAcTo ENERGY LEvEL E.G.

TEMPERATURE) l8- 1/; INPUT SOURCE OF SIGNAL TO BE AMPLIFIED 7a F|G.3

INVERTED 2 [POPULATION 053 SEC E LEVEL 5 =8 54 Kev l 4.6, SEC

ll35Kev E0 1 6g STABLE DEBYE TEMPERATURUQ), BELOW WHICH SUBSTANTIAL FRACTION(F) OF GAMMA RAYS EMIT WITHOUT RECOIL INVENTORS. VICTOR VALI WALTER VALI A TTORNE S United States Patent 3,281,600 STIMULATED GAMMA RAY EMISSION Victor Vali, Seattle, Wash., and Walter Vali, Palo Alto, Calif., assignors to The Boeing Company, Seattle, Wash., a corporation of Delaware Filed Dec. 26, 1963, Ser. No. 333,462 12 Claims. (Cl. 25084) from another aspect, the present invention provides a technique for accelerating the decay rate, i.e. reducing the half life, of certain metastable nuclei.

The acronym gaser can be used to categorize such gamma ray stimulated emission devices in that such essentially provide gamma ray amplification by stimulated emission of radiation, drawing an analogy from what are generally described as masers and lasers, i.e. devices for microwave or light amplification by stimulated emissionof radiation. It is to be emphasized, however, that the mode of stimulate-d emission characteristic of the present invention arises from controlled transition rates between nuclear energy levels, rather than between electron energy levels.

A selectively controllable gamma ray radiation device or gaser has utility in communications fields as an amplifi'er or oscillator functioning at extremely high frequency with sharp frequency selectivity and with high directivity.

The high intensity and highly directive nature of gaser produced energy transmission also raises the possibility of certain weaponry applications of the invention, with target destructive energy being beamed to the target as a short pulse or burst of high intensity energy. The utility of such energy release will be apparent, such as for line-of-sight antimissile purposes, for example.

A further field of application of the invention lies in the utilization of a gaser type energy source as an initiator or so-called trigger for nuclear fusion reactions, either uncontrolled or controlled. In the use of electromagnetic energy to supply the heat necessary to initiate and sustain a fusion reaction, the extent of energy conversion from electromagnetic to kinetic form (i.e. the interaction between t-he energy source and the high temperature plasma necessary for the fusion reaction) is in general proportional to the square of the frequency of the electromagnetic energy. Thus, high density gamma rays, such as available from a gaser according to the present invention, interact much more efficiently with the high temperature plasma than can rays from lower frequency electromagnetic energy sources such as lasers or masers. Moreover, in contrast to fission reaction type triggers which are necessarily self-destructive when developing suificient energy input to sustain nuclear fusion, a gaser type energy source has the capability of sustaining a fusion reaction without self-destruction.

Another field of application of the present invention is found in its capability of use for extremely small nuclear reactors involving controlled energy release by transitions between nuclear energy levels, i.e., nuclear transition as distinguished from nuclear fission or nuclear fusion, the energy released being essentially in the form of gamma ray radiation, thereby obviating the neutron shielding problems incident to conventional nuclear reactors. Characteristically, also, reactor source materials are not restricted to fissionable isotopes, and a self-sustaining reaction can be readily controlled, such as by means of relatively easily effected change in physical conditions.

Another field of application of the invention lies in the field of energy storage devices, such as for reaction energy propulsion usage. Such a device, employed as a controllable, nuclear reactor type propulsion reaction heat source, provides a theoretical capability of energy release on the order of several megaelect-ron volts (Mev.) per atom in contrast to the energy release capability of chemical reaction propulsion where the energy release is on the order of only a few electron volts (e.v.) per atom. The relatively small size of the nuclear reactor here involved is also a very important advantage in reaction propulsion applications, as compared with the inherent size require ments of conventional nuclear reactors. Yet a further advantage pertains in that only slight shielding is required for gamma ray type radiation. Still another advantage is in the inherent safety of a gamma ray nuclear reactor with appropriate physical condition control (e.g., change in temperature) in that the reaction, under certain conditions, can be termed self-controlling or self-damping so will not become explosive.

Incident to its generation of a given particular frequency or energy level of gamma ray radiation, gaser devices according to the present invention can also be employed as very accurate frequency standards in the gamma ray spectrum for research purposes.

In general, a gamma ray stimulated emitter device according to the present invention involves selection and preparation of a suitable isotope having an upper energy level of half life longer than a lower energy level in order to maintain what may be termed inverted population for a reasonable usable time. The equilibrium population distribution among the possible energy levels in a nucleus is governed by what is known as the Boltzmann ratio and, accordingly, the higher energy levels are normally less populated than lower energy levels. In isotopes having energy level differences corresponding to the gamma ray spectrum (i.e. a frequency corresponding to the energy differences between two particular energy levels), the equilibrium condition is with all nuclei at the ground state. However, if electromagnetic radiation of resonance frequency falls on the medium, then an upward redistribution of the populations of the energy levels occurs.

An ensemble of nuclei in which for a finite time an increased upper energy population density exists, can be described as being metastable and as having an inverted population condition. In this condition, a net emission of energy results at a frequency corresponding to the difference in energy levels, and for such time as such condition exists the ensemble radiates more energy than is absorbed. A general analogy to the inverted popula tion condition here involved can be drawn to the electronic inverted population'condition incident to maser operation, as discussed in Bloembergen, U.S. Patent No. 2,909,654, for example.

FIG. 1 is an energy level diagram;

FIG. 2 diagrammatically portrays the operation of a gaser;

FIG. 3 is the energy level diagram of the isotope Ge; and

FIG. 4 is a graph showing change in the fraction (1) with change in temperature (T).

In order to illustrate in principle appropriate energy level relations for practice of the present invention, FIG- URE 1 presents a hypothetical energy level diagram of a four energy level system, wherein E is the ground state or stable energy level, and E E and B are excited energy levels. Characteristic of a suitable isotope, the separation between energy levels E and E is of an order of magnitude falling in the gamma ray spectrum, and the half life T at energy level E is very much smaller Patented Get. 25, 1966 than the half life T at energy level E which is in turn larger than the half life T at energy level E to establish an inverted population level at energy level E when the isotope is radiated. The isotope is chosen with energy level relations which do not decay directly from the inverted population level E to stable or ground state energy level E since such decay relation has a high probability of nuclear absorption. It is important also that relatively rapid decay occur from energy level E to avoid absorption losses.

While a suitable isotope characterized by at least four energy levels, as above described, is preferred for practice of the invention because an inverted population condition is relatively easy to establish therein, it Will be seen that a suitable isotope characterized by a three energy level system can also be employed, in which event the highest, most unstable energy level becomes E the inverted population level becomes E and the lower energy level becomes E the half life T at energy level E being smaller than the half life T at energy level E, with the induced gamma rays being emitted at a frequency corresponding to the difference between energy levels E1 and E0.

This basic principle, applied to electron energy level states, provides stimulated emission of radiation by transition between electron energy levels in maser and laser devices. However, such stimulated emission of radiation in the gamma ray spectrum has not been possible heretofore because gamma rays are normally emitted with recoil and because the extremely high energy involved results in such a change in frequency of the radiation as a result of the recoil that the characteristic resonance condition necessary for induced emission cannot be achieved. As is known, the line width essentially determines the resonance condition, and normal nuclear recoil resulting from gamma ray emission causes a shift in frequency of the radiation which in general can be said to be many orders of magnitude, as compared with the line width.

In order for stimulated emission of gamma ray radiation to occur, an appreciable fraction of the gamma rays have to be emitted without recoil, and the recent work of Mossbauer and others has shown that under certain conditions recoilless emission of gamma ray energy can result. Representative literature discussions of the Mossbauer effect are found at Naturwiss, vol. 45, p. 538 (1958); Z. P. Naturforschung, vol. 41a, p. 211 (1959); Phys. Rev. Letters, vol. 3, p. 554 (1959) and Scientific American, issue of April 1960, at p. 72.

To obtain controllable gamma ray radiation, the basic precept of the present invention involves isolating a metastable isomer characterized by a suitable inverted population nuclear condition, establishing such isomer in an ensemble of appropriate crystalline form and mass, initially maintaining the ensemble under conditions involving a natural decay rate, i.e. with emission being characterizedessentially by recoil induced change in frequency, and then establishing an appropriate change in physical condition of the ensemble to reach or at least approach the physical conditions necessary for recoilless emission, whereupon substantial induced emission at resonance frequency occurs. With external, i.e. exciting, radiation at the resonance frequency applied as an input, the induced emission appears at increased intensity, and variations (e.g. moduation) of the input radiation results in corresponding but amplified variation of the induced emission, hence a gaser.

Diagrammatically, as shown at FIGURE 2, a gaser type device according to the present invention can com prise an isotope containing crystal 10, with as high a fraction as possible of the nuclei at energy level E so that an inverted population condition exists. As will be understood, such excited or pumped up energy level condition of crystal can be obtained by suitable means, as by neutron bombardment from a radiator 12, such as a reactor, as diagrammatically indicated at 14. Also, the

4 excitation bombardment 14 can be accomplished by electromagnetic radiation at, or including, energy at frequency corresponding to the difference in energy levels E and E Such excitation bombardment 14 can be effected either as a preparatory step or simultaneously with gaser operation, to prolong operational life.

The size of crystal 10 is preferably selected to be just slightly subcritical to avoid the possibility of a chain reaction, and the initial physical conditions of the crystal 10 are established and maintained so that no substantial gamma ray induced emisison occurs, i.e. only a natural decay rate exists. Input signal source 16 at frequency v radiates crystal 10 as indicated at 18. The a input signal at 18 is at a frequency corresponding to the energy level difference between energy levels E and E The physical condition of crystal 10 is controlled by a physical change factor 20, as indicated at 22 (such as by lowering of the crystal temperature to about or less than the Debye temperature), to bring the physical condition of crystal 10 to the point where substantial recoilless emission from crystal 10 begins to occur. The gamma ray induced emission output 24 is at the v frequency but at considerably increased intensity because of the induced emission. It will be seen that change in level or modulation of signal input 18 results in corresponding change in level or modulation of output 24, and also that the amplification factor (output/input) of the gaser is affected by the extent of control generated by the physical change factor 20. Should crystal 10 be of critical size, and physical change factor 20 be also maintained critical to the generation of gamma ray induced emission exponential with time, then a chain reaction can occur so long as the inverted population condition exists in crystal 1t} and so long as crystal 10 is maintained intact for sufficient time, considering that the time for completion of such a chain reaction theoretically can be less than a microsecond.

Also to be noted with respect to the input signal 18 to crystal 10 in FIGURE 2, the corresponding induced gamma ray emission appearing at output 24 by known principles is inherently very directional and coherent.

For purpose of definition of the nature of the present invention, it may be said that the-fraction of gamma rays emitted without recoil as compared with the total gamma rays emitted becomes substantia at temperatures below the Debye temperature and such terminology as to a substantial fraction of the gamma rays being emitted without recoil can vary depending upon other factors as set forth in Equation 10 below, from as low as about 0.1% to 10% or more. Generally, however, the most satisfactory isotopes for-purposes of the present invention will be characterized by induced gamma ray emission when such substantial fraction is more than about 1%.

In a gaser device used as an amplifier, the induced emission is kept subcritical because one or more physical conditions, such as crystal size, is maintained subcritical. This does not preclude, however, change in another physical condition, such as temperature, to vary or modulate the amplification factor of a gaser. Change in physical condition can involve one or more changes effected simultaneously, such as by a lowering of temperature, by increase in crystal size, by a change in sonic vibration, or by increase in pressure. A recently published work of Dr. Russell V. Hanks of Boeing Scientific Research Laboratory, Seattle, Washington, (appearing in Physical Review, vol. 124, No. 5, at pages 1319 and 1320) indicates that recoilless radiation (the Mossbauer effect) is to an extent directly dependent upon pressure.

For reactor applications, the exciting radiation can be the spontaneous radiation resulting from the natural decay of the isomer, and the criticality of the energy release conditions can be suitably physically controlled by one or more changes in physical condition from subcritical to critical.

With respect to use of temperature change as a controlling or contributing physical factor to either maintain gaser or reactor operating conditions, it is known that the fraction (f) of gamma ray emission without recoil as compared with the total gamma ray emission is related to the Debye temperature of a crystal lattice. FIGURE 4 graphically illustrates a typical correlation of such fraction (f) with the Debye temperature (0). Other conditions being suitable, the Debye temperature of a particular isotope in crystalline form then provides a convenient indicator as to approximately What temperature is necessary for substantial recoilless emission and occurrence of net energy emission, i.e. amplification. The Debye temperature for Ge" is about 370 K. and the Debye temperature is known per se or readily determinable by known principles for any selected isotope ensemble. As also known per se, various chemical forms of a given isotope ensemble have different'Debye temperatures, providing a means for adapting a given isotope to desired operating conditions by choice of an isotope compound having a Debye temperature most closely corresponding to the desired environmental temperature.

By rough analogy, a gaser arrangement as shown in FIGURE 2 can be compared to a tetrode type thermionic amplifier where the inverted population condition of the isomeric crystal is the equivalent of the tetrode plate voltage, the exciting energy is equivalent to the tetrode control grid voltage and the physical conditions determining the fraction of recoilless emission and consequent- -ly the amplification factor are equivalent to the tetrode screen grid voltage. As has been indicated, it is a characteristic of gaser amplifier applications that at least one of the physical conditions is at all times maintained subcritical. In oscillator and reactor applications of this principle, spontaneous emission is used to initiate and maintain the energy release and the physical conditions controlled to be subcritical until self excitation or triggering is desired, at which time such self excitation is accomplished by establishing all physical conditions critical. More specific considerations and relationships in these respects are developed in the following discussion of certain of the principles involved.

The possibility of controllably inducing stimulated gamma ray emission arises from the fact that at resonance the cross section (a) of gamma ray absorption or induced emission is much larger than the electronic scattering cross section for energies under consideration here.

where 1' and r are the reciprocal lifetimes of the upper and the lower energy states respectively, and A is the wavelength of the emitted radiation divided by 21r.

In order to obtain induced gamma ray emission the following requirements exist:

(1) An appreciable fraction of gamma rays have to be emitted without recoil, i.e. according to the Mossbauer elfect, in order to keep the radiation precisely at resonance. As generally known per se, this fraction can be improved for a selected isotope by the proper choice of chemical compound, in that the chemical form of a crystal affects the lattice structure.

(2) The internal conversion coefficient of the resonant gamma radiation preferably but not necessarily should be small.

(3) In order to have time to pump a considerable fraction of the nuclei into a high isomeric level the half life has to be of substantial duration.

(4) The line widening due to spin-spin interactions in the crystal lattice and other perturbations should be reasonably small (not many orders of magnitude bigger than the natural line width) 6 The radioactive decay rate is a'N/dt=-W where N is the number of excited nuclei and A is the decay constant. If some induced emission occurs, Equation 2 has to the first approximation, the following form:

where p is the probability for a gamma ray to stimulate an emission.

where N is the number density of excited nuclei, 0' the stimulated emission cross section and where N, is the number density of lower (or ground state) nuclei, a the resonant absorption cross section, N the total number density of atoms in the crystal and a the average electronic scattering cross section per atom.

Choosing an isotope where the half life of the lower state is considerably shorter than the half life of the upper state, then N m N r (6) p e e) The induced emission across section can be written as Substituting Equation 8 into Equation 7 and As a suitable example of an isotope capable of use for practice of the present invention, specific consideration will be given to Ge. As shown in the simplified diagram presented at FIGURE 3, this isotope can be obtained by neutron bombardment of Ge or by 5+ decay of As, for example. The isotope Ge' has an upper isomeric state 54 kev. above the lower energy state. The half lives are 0.53 sec. and 4.6 microseconds for the upper and lower energy states. The lower energy state decays mostly through another gamma emission to the ground (stable) state of Ge' For this transition (1E8, f-=-O.5, and a =10- cm. Assuming availability of a crystal where an appreciable fraction of the nuclei are in the uppermost energy state of Ge, a condition of criticali-ty or near-criticality can be obtained, as desired. For the 54 kev. transition, the lifetime of the lower energy state is 4.6 microseconds; hence m approaches unity.

Further examples of suitable isotopes meeting the appropriate energy level conditions as discussed in connection with FIGURE 1 are C1 S0 Se Tc Rh RhIOB ra? A 109 na 127 129 131 and H0160 for example. As further and more detailed investigation of energy level systems of different nuclei evolve, it is to be expected that other appropriate isotopes suitable for the practice of the present invention will 'be determined.

For a large crystal the condition that it will become critical (or oscillating) is that the losses have to be less than the gains through induced gamma ray emission.

. 7 Using Equations 5, 6, and 8 this condition can be written as e) z+ 2 1)( e f The principles governing critical size for an induced emission of gamma rays increasing exponentially with time are essentially the same as those known with respect to chain reactions involving neutron emission. Quantitatively, however, the comparative order of critical size for an induced gamma ray emission chain reaction proves exceedingly small. As will also be understood, various factors will detract from the ideal condition and thereby increase finite size for criticality as to development of an induced gamma ray emission chain reaction, such as the presence of impurities, either in the form of other nuclei or of decay products. From this latter consideration it follows that an induced gamma ray emission chain reaction is completed in a very short time in that the chain reaction of itself exhausts the available excited nuclei at the inverted population level.

For single crystals the conditions (9) and (10) can be relaxed considerably. It is well known from the studies of transmission of X-rays that in a dislocation-free crystal the energy that flows along the diffracting planes is attenuated two or more orders of magnitude less than normal absorption (iii. the Botrrmann effect, noting Jrnl. Appl. Phys., vol. 30, p. 874). Hence w in Equations 9 and 10 decreases correspondingly. The direction half width of the Borrmann effect is a few seconds of are, giving under these conditions a very narrow beam of induced radiation. (There is a very strong angular correlation between the incident and. the induced gamma ray). The effect is quite similar to Kessel lines in X-ray spectroscopy.

What is claimed is:

1. A gamma ray emitter comprising a crystalline body containing a metastable isotope in an excited nuclear state characterized by an inverted population condition, and means for establishing the physical conditions of said body at those necessary for induced gamma ray emission from said isotope.

2. The emitter of claim 1, wherein the metastable isot'ope is selected from the group consisting of C1 8e G673 S681 95 R-hlfll Rhl03 A ltl'l ree 112 127 129 131 and H0160.

3. A gamma ray beam transmitter comprising a crystalline body containing a metastable isotope in an excited nuclear state characterized by an inverted population condition, means radiating said body at an energy corresponding to the energy difference between the inverted population level and a lower metastable energy level, and means establish-ing the physical conditions of said body at those necessary for induced emission of gamma rays from said isotope, the induced emission from said body emitting therefrom as a coherent, highly dire tional beam.

4. A gaser device comprising a crystalline body containing a metastable isotope in an excited nuclear state characterized by an inverted population energy level, the physical conditions of said body being subcritical but approaching the physical conditions necessary for induced gamma ray emission, energy input means radiating said body at a frequency corresponding to the energy difference between the inverted population energy level and a lower energy level, and means varying the intensity of such energy input whereby the induced gamma ray emission from said body reflects corresponding change in output intensity.

5. A gamma ray emitting nuclear reactor comprising a crystalline body containing a metastable isotope in an excited nuclear state characterize-d by an inverted population energy level, and means for establishing at near critical the physical conditions of said body necessary for induced gamma ray emission.

6. A gamma ray emitting energy source comprising a crystalline body containing an isotope being characterized by a ground state energy level E and at least two excited energy levels E and E the natural decay from energy level E being relatively rapid and the energy difference between levels E and E falling within the gamma ray spectrum, with the half life T at energy level E smaller than the half'life T at energy level E said isotope being in an energy state having an inverted population at energy level E and the physical conditions of the body being essentially those required to induce gamma ray emission by accelerated change in energy level population from level E to level E 7. A gaser device comprising a crystalline body containing an isotope being characterized by a ground state energy level E and at least two excited energy levels E and E the natural decay from energy level E being' relatively rapid and the energy difference between levels E and E falling within the gamma ray spectrum, with the half life T at energy level E being smaller than the half life T at energy level E said isotope being in an energy state having an inverted population at energy level E the physical conditions of said body being subcritical but approaching the physical conditions necessary for substantial induced gamma ray emission, energy input means radiating said body at a frequency corresponding to the energy difference between energy levels E and E and means varying the intensity of such input frequency whereby the induced gamma ray emission from said body reflects corresponding change in intensity of output at the same frequency.

8. A gamma ray emitting nuclear reactor comprising a crystalline body containing an isotope being characterized by a ground state energy level E and at least two excited energy levels E and E the natural decay from energy level E being relatively rapid and the energy difference between levels E and E falling within the gamma ray spectrum, with the half life T at energy level E being smaller than the half life T at energy level E said isotope being in an energy state having an inverted population at energy level E the physical conditions of said body being near critical as to those necessary for substantial induced emission of gamma rays at a frequency corresponding to the difference in energy levels E and E 9. The method of generating coherent gamma ray radiation, comprising establishing a crystalline body containing a metastable isotope in an excited nuclear condition characterized by an inverted population energy level, and selectively producing stimulated gamma ray emission from said body by establishing a change in physical condition of the body to approach the physical conditions necessary for induced gamma ray emission.

10. The method of inducing gamma ray emission, comprising providing a crystalline body containing a metastable isotope having an upper energy level with a half life considerably longer than the half life of a lower energy level and with an energy difference between the levels which falls within the gamma ray spectrum, radiating-said body to establish an inverted population condition in said upper energy level of said isotope, maintaining such body in a physical condition where gamma ray emission occurs at the natural decay rate, then inducing gamma ray emission by subjecting said body to change in at least one physical condition to increase the fraction of gamma rays emitting without recoil to the point where the gamma ray cross section for induced emission exceeds the electronic scattering cross section.

11. The method of claim 10, comprising changing the physical condition of said body to bring about stimulated emission by effecting at least one of the following physical changes; (1) lowering temperature, (2) increasing body size, (3) changing sonic vibration, and (4) increasing pressure.

12. The method of claim 10, comprising employing a metastable isotope selected from the group consisting of 19 C1 8e Ge, Se T0 Rh Rh Ag Ag Lustig, The Mossbauer Effect, American Journal of 111112, T6127, T6129, T6131, and Physics, vol. 29, No. 1, January 1961, pp. 1 to 18.

References Cited by the Examiner Schiflfzr et7a1P.,hRe coi111;ss Eesolnance Algsorpltison1 gamma UNITED STATES PATENTS 51216 eS ysrca evrew etters, ec. 9, pp. 2,909,654 10/1959 Bloemberger 31394 3,130,315 4/1964 Hamermesh 250 106 The NJossbauer Effect. A Tool of Sc1ence by Werthe1m,

Nucleonlcs, v01. 19, No. 1, January 1961, pp. 5257. OTHER REFERENCES Josephson, Temperature Dependent Shift of Gamma 10 RALPH G. NILSON, Primary Examiner.

E't db 'd,PhlR' Ltt 1.4, 5 2 gin 253 341 3 6 GT8 v0 J. W. LAWRENCE, Assistant Examiner.