<P> An X-ray laser device was proposed as part of the Reagan Administration's Strategic Defense Initiative in the 1980s, but the only test of the device (a sort of laser "blaster" or death ray, powered by a thermonuclear explosion) gave inconclusive results . For technical and political reasons, the overall project (including the X-ray laser) was de-funded (though was later revived by the second Bush Administration as National Missile Defense using different technologies). </P> <P> Phase - contrast X-ray imaging refers to a variety of techniques that use phase information of a coherent x-ray beam to image soft tissues . It has become an important method for visualizing cellular and histological structures in a wide range of biological and medical studies . There are several technologies being used for x-ray phase - contrast imaging, all utilizing different principles to convert phase variations in the x-rays emerging from an object into intensity variations . These include propagation - based phase contrast, talbot interferometry, refraction - enhanced imaging, and x-ray interferometry . These methods provide higher contrast compared to normal absorption - contrast x-ray imaging, making it possible to see smaller details . A disadvantage is that these methods require more sophisticated equipment, such as synchrotron or microfocus x-ray sources, X-ray optics, and high resolution x-ray detectors . </P> <P> X-rays with high photon energies (above 5--10 keV, below 0.2--0.1 nm wavelength) are called hard X-rays, while those with lower energy are called soft X-rays . Due to their penetrating ability, hard X-rays are widely used to image the inside of objects, e.g., in medical radiography and airport security . The term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself . Since the wavelengths of hard X-rays are similar to the size of atoms they are also useful for determining crystal structures by X-ray crystallography . By contrast, soft X-rays are easily absorbed in air; the attenuation length of 600 eV (~ 2 nm) X-rays in water is less than 1 micrometer . </P> <P> There is no consensus for a definition distinguishing between X-rays and gamma rays . One common practice is to distinguish between the two types of radiation based on their source: X-rays are emitted by electrons, while gamma rays are emitted by the atomic nucleus . This definition has several problems: other processes also can generate these high - energy photons, or sometimes the method of generation is not known . One common alternative is to distinguish X - and gamma radiation on the basis of wavelength (or, equivalently, frequency or photon energy), with radiation shorter than some arbitrary wavelength, such as 10 m (0.1 Å), defined as gamma radiation . This criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known . (Some measurement techniques do not distinguish between detected wavelengths .) However, these two definitions often coincide since the electromagnetic radiation emitted by X-ray tubes generally has a longer wavelength and lower photon energy than the radiation emitted by radioactive nuclei . Occasionally, one term or the other is used in specific contexts due to historical precedent, based on measurement (detection) technique, or based on their intended use rather than their wavelength or source . Thus, gamma - rays generated for medical and industrial uses, for example radiotherapy, in the ranges of 6--20 MeV, can in this context also be referred to as X-rays . </P>

Range of energy used in diagnostic x rays