Patent Number: 040428273
Section: summary

Soon after the demonstration of laser action had made the potentialities of laser devices apparent, speculation arose concerning extending amplification by stimulated emission of radiation beyond the visible light frequencies through ultraviolet light frequencies into the X-ray region of the electromagnetic spectrum. However, prior proposals for X-ray lasers have not yielded promising approaches to the realization of X-ray laser action from a practical point of view. In addition to difficulties similar to those in achieving laser action at longer wavelengths, the X-ray laser utilizing a population inversion for stimulated emission of radiation requires an inversion between levels widely separated by an energy on the order of hundreds of electron volts. Even assuming that one would first endeavor to achieve short pulses of directional X-rays generated by stimulated emission, the conditions for amplification of X-radiation by stimulated emission must never-the-less prevail for a time long enough to produce a sufficient, detectable number of stimulated X-ray photons. The present invention provides a practicable system for overcoming the inherent difficulties in the problem of directional X-ray generation by stimulated emission. In the system of the invention, there is a complementary combination of a system producing the requisite pumping energy in a directable form such as a beam, and a geometry of the amplifying region of the generator such that the active region can be scanned along the X-ray generation path at light (and X-ray) propagation velocities. As later described, the invention in preferred embodiments utilizes inverted populations in the X-ray region produced from selective inner-shell vacancies such as are obtained in intermediate energy ion-atom collisions. A beam swept at nearly the speed of light along the length of an extended interactant material provides an active region in synchronization with a resonant X-ray pulse traveling along the material. While the first system considered here is one which involves a collision between a high velocity ion and a neutral atom, there are further systems which involve the collision of one ion with another ion. It should be understood, therefore, that either a collision between an ion and an atom or a collision between an ion and another ion may equally well be the basis of a system according to the present invention. For simplicity, such collisions are referred to as ion-atom collisions, whether the second collision partner be a neutral atom or another ion. In a specific embodiment the large cross sections for selective production of atomic inner-sheel vacancies by heavy-ion bombardment at intermediate energies (.about.1 keV/amu) of metal targets are utilized to advantage. The physical process is thought to be an electron promotion mechanism that occurs at level crossings in the quasimolecule formed during the collision, and/or rotational excitation. The vacancies produced may occur in the ion or the atom or both. Thus for ions passing through a thin foil, on the downstream side of the foil a significant number of ions will contain inner-shell vacancies. By choice of collision partners as well as beam energy and foil thickness, population inversions desired for particular applications will be achieved,. Moreover, by sweeping the ion beam along the length of an extended target, such population inversions may be obtained in synchronism with a traveling wave front due to radiative decay of the states produced. Such a system will amplify this wave front advancing in the direction of sweep of the beam by stimulated emission. Systems of this nature would be useful X-ray sources when operated in a single-pass mode with no cavity. Such systems would of course also have great value when placed in a suitable X-ray laser resonant cavity. An X-ray resonant cavity in accordance with presently developed technology would likely be both cumbersome and expensive. However, advances are to be expected from proposals such as are now being investigated or as may be available at the time of instrumentation of such a cavity-including system. For ion-atom combinations such that inner-shell energy levels of interest match, ion to atom, cross sections for the selective production of vacancies in these shells are given approximately by taking for the level-crossing radius a value equal to the sum of the radii of the two electronic shells involved. As an example, the match in energy between the carbon K shell and the argon L shell allows selective vacancies to be produced in the L shell of argon. In particular, measurements show that for bombarding energies below .about. 80 KeV effects of double L-shell excitation are not observed. Furthermore, it was found that at an ion energy of .about. 50 KeV, the resulting X-ray spectra indicated the strong predominance of the 224-eV line due to a 3s-2p transition. The radiative lifetime of this transition may be taken as 2.8 .times. 10.sup.-.sup.11 sec and a fluorescence yield of 1.67 .times. 10.sup.-.sup.3 is attainable. It will be understood that, for understanding the invention in terms of an amplification phenomenon, it is expedient to consider the amplification that would be imparted to an input X-ray wave; this is probably not the most immediately useful application of the invention. Rather, it is expected that the invention will have as a first principal utility the generation of a directional X-ray beam. A highly directional beam of X-radiation (compared with previously known X-ray sources) will be generated by virtue of the sweeping of an active region for amplification by stimulated emission of radiation along an extended path at a velocity approximately equal to the velocity of light. In such case, the X-radiation amplified will be X-radiation emitted within the "input end" of the active region, by spontaneous emission for example. Reference is made to additional explanatory material in articles by R. A. McCorkle and J. M. Joyce, "Practical X-ray Amplifier," by R. A. McCorkle, Physical Review Letters, Vol. 29, page 982, October, 1972; and "Threshold Conditions for Amplified Spontaneous Emission of X Radiation," Physical Review A, Vol. 10, page 903, September, 1974; and also the the numerous references thereto appended. From the foregoing description it will be understood that the present invention provides a heretofore unavailable practicable system for producing amplification of X-radiation through the phenomenon of stimulated emission of radiation and which will provide, among other useful effects, the effect of generating X-radiation which is substantially more directional than that obtainable with previously known X-radiation sources; the generated radiation will generally also have substantial monochromaticity. Apparatus according to the invention would find many uses, for example the more precise application of X-radiation to the human body for medical purposes. In addition to providing advantages and utilities described above, it is an object of the present invention to provide apparatus in which there are produced an ensemble of excited particles in a region of predetermined geometry, such that X-radiation is amplified by stimulated emission in the region to produce a non-isotropic X-radiation emission. It is another object of the present invention to produce an ensemble of highly excited particles in a region by the scanning of an interactant material with an interactant beam at approximately the velocity of light. It is a further object of the present invention to produce an ensemble of excited particles in the manner described above, in which such particles form a medium with a population inversion, and in which the situs of the inverted population travels at approximately the speed of light so as to be in synchronism with an X-radiation pulse to be amplified by stimulated emmission. It is a still further object of the present invention to provide apparatus in which a high-current ion beam is swept at nearly the speed of light along the length of an extended foil, whereby particles with inner-shell vacancies are produced in intermediate energy ion-atom collisions in an active region traveling in synchronization with a resonant X-ray pulse traveling parallel to the foil surface.