Patent GB-1056565-A (GB)
Abstract: 1,056,565. Stimulated emission of gamma rays. GENERAL ELECTRIC CO. Jan. 30, 1964 [Feb. 6, 1963], No. 3988/64. Heading H1C. [Also in Division G6] A device for generating gamma rays by stimulated emission is produced by irradiating a target isotope with nuclear radiation to form substances including a desired isotope in an upper isomeric state, isolating the isotope and growing an elongated crystal therefrom. The crystal is maintained in a non-homogeneous magnetic field, produced by a tapered coil 4 (Fig. 5, not shown), and/or in a non-homogeneous gravitational field produced by mounting the crystal vertically, and/or at a temperature above the minimum attainable by an associated cooling system, so that premature triggering is avoided. The crystal is also placed inside a magnetic shield. To initiate stimulation the current through coil 4 is terminated, the crystal is placed horizontal and the temperature is lowered. Emission may be triggered by a spontaneous event or by a beam of electrons or X-rays. Gravitational effects may be eliminated by mounting the device in a satellite or a freely-falling body. The crystal 3 may be of Zn<SP>69</SP>, which generates radiation at 435 key., and may have dimensions of one millimetre in length and one micron in diameter. Other suitable materials include Cd<SP>113</SP>, Xe<SP>131</SP>, Xe<SP>133</SP>, Ba<SP>135</SP>, and Ce<SP>137</SP>. To produce the excited isomer of Zn<SP>69</SP> a covalent compound of Zn<SP>68</SP> is irradiated with neutrons, formation of the desired isotope being accompanied by rupture of the covalent bond and permitting chemical separation by the Szilard-Chalmers method. A suitable covalent compound is Zn(C 2 H 5 ) 2 . Theory Emission of a gamma ray photon from a nucleus. generally produces a recoil which reduces the energy of the emitted photon below the energy separation of the energy levels involved. In a crystal lattice the recoil energy is converted into lattice vibrations and the Doppler effect causes a shift of the absorption line. A gravitational field gradient and Zeeman splitting of the nuclear levels cause differences in the photon energy from different nuclei, so that to sustain a photon chain reaction the environmental condition of all the nuclei have to be kept the same. It is shown mathematically that, to sustain a chain reaction, a nuclear isotope must (a) have a crosssection for stimulated emission which is greater than that for non-resonant adsorption, (b) have a high fraction of nuclei in the upper energy state, (c) have a high probability for a transition of the crystal lattice from the ith to the jth vibrational state, and (d) be as free as possible from nuclei other than the desired isomer. In crystals exhibiting the Mossbauer effect emission takes place without recoil and a population inversion of energy states is shown to be necessary, whereas in non-Mossbauer crystals inversion is not necessary. When population inversion is required it is shown that this may exist when the desired isotope is removed from the reactor in which it is produced, or may occur at some later time depending particularly upon the relative lifetimes of the isomeric states, the branching ratio (i.e. the fraction of decays which occur by emission of beta rays after first making the isomeric transition from the upper to the lower state), and the relative cross-sections for formation of the upper and lower isomeric states.