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The concept of the graser (or gamma-ray laser) is discussed, and recent Russian and American proposals are surveyed. The difficulties in building a gamma-ray laser are outlined; and specific recommendations are made for delimiting the extent of NRL involvement in graser research in the near future.
This report is addressed to the problem of the state-of-the-art of gamma ray laser development. It is intended to identify the various disciplines and specific research areas which can best contribute to resolving the question of 'graser' feasibility. Topics discussed include the following: The basic mechanisms of laser action; Gamma ray emission from nuclear isomers; The resonance cross section; Nonresonant absorption; Photon kinetics; Inhomogeneous line breadths; Problems of pumping.
Recent approaches to the problem of the gamma-ray laser have focused upon upconversion techniques in which metastable nuclei are pumped with long wavelength radiation. At the nuclear level the storage of energy can approach tera-Joules (10 to the 12th power J) per liter for thousands of years. However, any plan to use such a resource for a gamma-ray laser poses problems of a broad interdisciplinary nature requiring the fusion of concepts taken from relatively unrelated fields of physics. Since 1978 we have pursued an approach for the upconversion of longer wavelength radiation incident upon isomeric nuclear populations that can avoid many of the difficulties encountered with traditional concepts of single photon pumping. Recent experiments have confirmed the general feasibility and have indicated that a gamma-ray laser is feasible if the right combination of energy levels and branching ratios exists in some real material. Resolution of the question of the feasibility of a gamma-ray laser now rests upon the determination of: 1) the identity of the best candidate, 2) the threshold level of laser output, and 3) the upconversion driver for that material.
Recent approaches to the problem of the gamma-ray laser have focused upon upconversion techniques in which metastable nuclei are pumped with long wavelength radiation. At the nuclear level the storage of energy can approach tera-Joules per liter for thousands of years. However, any plan to use such a resource for a gamma-ray laser poses problems of a broad interdisciplinary nature requiring the fusion of concepts taken from relatively unrelated field of physics. Our research group has described several means through which this energy might be coupled to the radiation fields with cross sections for stimulated emission that could reach 10 to the minus 17th power sq. cm. Such a stimulated release could lead to output powers as great as 3 X 10 to the 21st power Watts/liter. Since 1978 we have pursued an approach for the upconversion of longer wavelength radiation incident upon isomeric nuclear populations that can avoid many of the difficulties encountered with traditional concepts of single photon pumping. Recent experiments have confirmed the general theory and have indicated that a gamma-ray laser is feasible if the right combination of energy levels and branching ratios exists in some real material. Of the 1,886 distinguishable nuclear materials, the present state-of-the-art has been adequate to identify 29 first-class candidates, but further evaluation cannot proceed without remeasurements of nuclear properties with higher precision.
The most productive approaches to the problem of the gamma ray laser have focused upon upconversion techniques in which metastable nuclei are pumped with long wavelength radiation. At the nuclear level the storage of energy can approach tera-Joules (10(exp 12)J) per liter for thousands of years. However, any plan to use such a resource for a gamma ray laser poses problems of a broad interdisciplinary nature requiring the fusion of concepts taken from relatively unrelated fields of physics. Our research group has described several means through which this energy might be coupled to radiation field with cross sections for stimulated emission that could reach 10(exp -17) sq cm. Such a stimulated release could lead to output powers as great as 3 x 102 Watts/liter. Since 1978 we have pursued an approach for the upconversion of longer wavelength radiation incident upon isomeric nuclear populations that can avoid many of the difficulties encountered with traditional concepts 0 single-photon pumping. Experiments have confirmed the general theory and have indicated that a gamma-ray laser is feasible if the right combination of energy levels and branching ratios exists in some real material. Of the 1,886 distinguishable nuclear materials, the present state-of-the-art has been adequate to identify 29 first-class candidates, but further evaluation cannot proceed without remeasurements of nuclear. A laser-grade database of nuclear properties does not yet exist but the techniques for constructing one have been developed and utilize under this contract.
The possibility of extending the laser principle into the hard x-ray region above a few keV depends upon the ability of a pump to create the critical density of population inversion for which gain overcomes loss by absorption. Although this critical density decreases with the wavelength of the radiation to be stimulated, the power required to generate it depends upon the lifetime of the state being pumped. The lifetimes of inner-shell vacancies of atoms are very short. Nuclear states, on the other hand, have much longer lifetimes, ranging from fractions of picoseconds to millennia. Moreover, in the so-called recoilless or Moessbauer transitions of nuclear isomers, it was observed that the resonance cross section often exceeds the nonresonant absorption cross section by several orders of magnitude: just the condition for lasing in an inverted population. If, other things being equal, the absorber foil of a Moessbauer experiment contained an excess of excited states, then, instead of the absorption dip normally observed at resonance, there would be an increase of intensity, and amplification by stimulated emission would be achieved. The problem in making a gamma-ray laser is, therefore, simply that of obtaining an inverted population without inhibiting the Moessbauer effect. Research on this problem is reviewed.
This report summarizes the IDA research effort in FY 1985 in investigating the feasibility of developing a gamma-ray laser.
The present approach to a gamma-ray laser is based on the use of a long-lived nuclear state acting both as the storage and lasing level. The present approach requires that nuclear resonant emission occur from long-lived states. We have examined the issue both theoretically and experimentally. Theoretically, as discussed in a recent paper, there are reasons to believe that for long lived states the controlling width for nuclear resonance is not the natural linewidth but the homogeneously broadened relaxation width. This would make observation of the Mossbauer effect from long-lived states less stringent than previously believed. Experimentally, our efforts have continued to focus on observing the Mossbauer effect with the 88 keV level in Ag109 possessing a meanlife of 57.14 seconds. A facility dedicated to these experiments and based on a vibration-free He-closed cycle cryostat which can provide for T-variation and control at cryogenic temperatures was developed during the course of this grant. The first results on the resonant self absorption experiments on this facility are now available and are discussed in section II of the report.
This report summarizes the IDA research effort in FY 1988 in investigating the feasibility of developing a gamma-ray laser.