M. A. Piestrup

Continuously tunable submillimeter wave source

A method for spanning the 100−1000−μ portion of the spectrum with continuously tunable coherent radiation is described. The approach is based upon laser light scattering from the long−wavelength side of the A1−symmetry soft mode in LiNbO3. In contrast with other techniques, this method uses a single fixed−frequency pump source, requires no magnetic field, provides continuous rather than discrete tuning, can cover most of the 100−1000−μ range, operates at room temperature, and is simple to tune. The experimental data show that tuning was obtained from approximately 150 to 700 μ.

Momentum modulation of a free electron beam with a laser

A Nd : YAG laser has been used to modulate the energy of a free electron beam. With a single pass of the 2−MW laser light through the 100−MeV electrons we have measured a 37−keV increase in the width of the energy spectrum for the particles. The interaction occurred in helium at standard temperature and pressure over a distance corresponding to 105 optical wavelengths, and a phase synchronism condition was maintained by means of the inverse Cerenkov effect. That is, along the direction of motion of the electrons the phase velocity of the electromagnetic wave equaled the electron velocity. Therefore, the electrons remained in an electric field of constant phase, resulting in significant energy exchange.

Transition radiation as a source of x rays

The generation of transition radiation resulting from the passage of relativistic charged particles through a periodic structure can produce intense and highly directional x rays. An optimization analysis accounting for material properties, multiple scattering, and statistical random errors in foil thicknesses is used to design an x‐ray emitter. The results demonstrate that transition radiation sources can be comparable in brightness to synchrotron emitters.

Two-dimensional x-ray focusing from compound lenses made of plastic

We have measured the intensity profile and transmission of x rays focused by a series of either spherical or parabolic lenses fabricated using Mylar® (C5H4O2) or Kapton® (polyimide). The use of plastics can extend the range of operation of compound refractive lenses, improving transmission and aperture size and reducing focal length. The number of unit lenses range from 193 to 600 for each compound refractive lens. Two-dimensional focusing was obtained for photon energies 8–14 keV with imaging distances of less than 1 m. For example, full-width-half-maximum linewidths down to 16 μm at a distance of only 47 cm from the lens were achieved at 9 keV. The effective apertures of the refractive lenses were measured between 250 and 364 μm with peak transmissions between 10% and 33%.

Observation of bright monochromatic x rays generated by relativistic electrons passing through a multilayer mirror

We have observed the emission of 15 keV x rays produced by 500 MeV electrons passing through a x-ray multilayer mirror. The mirror consisted of 300 pairs of W and B4C layers with layer spacing of 12.36 A and supported by a 100 μm Si substrate. The x rays were emitted at the Bragg angle θγ=33.15 mrad with respect to the mirror surface and at the angle θD=66.3 mrad with respect to the electron-beam direction. The spatial distribution and the spectral angular dependence of the x rays were measured and shown to be larger than the parametric x rays emitted from the Si substrate. The value of the differential photon efficiency was estimated to be about 0.22 photons/electron/str.

CYLINDRICAL COMPOUND REFRACTIVE X-RAY LENSES USING PLASTIC SUBSTRATES

We have measured the intensity profile of x rays focused by compound refractive lenses (CRLs) fabricated using acrylic (Lucite) and polyethylene plastics. A linear array of closely spaced holes acts as multiple cylindrical lenses. The important parameters for this type of focusing are the focal length and absorption, and, for wavelengths shorter than 3 A, low atomic number plastics are suitable. We have experimentally demonstrated that we can achieve one-dimensional focusing for photon energies between 9 and 19.5 keV with focal lengths between 20 and 100 cm. For example, using 12 keV x rays we have achieved focal full width at half maximum linewidths down to 21 μm at a distance of only 20 cm from the CRL. The x-ray source was a synchrotron emitter whose source size in the vertical dimension was 445 μm.

A design of mammography units using a quasimonochromatic x-ray source

In this article we present a mammography unit design using a parametric x-radiation (PXR) source. We show that PXR can provide a fanned quasimonochromatic x-ray beam that can be used to obtain mammography images of higher contrast and lower dose than those obtained from a conventional x-ray system. Changing the Bragg angle of the PXR crystal with respect to the electron beam changes the photon energy, improves image quality, and minimizes dose. Monte Carlo computer simulations are given that show that the PXR source with a 5% bandwidth gives a figure of merit close to that of the ideal monoenergetic source and significantly higher than that of the filtered-x-ray-tube sources. In order to simultaneously obtain adequate flux and achieve bandwidths below 5%, we utilized an electron-beam energy of 35 MeV and an average current of 300 μA to 1 mA for 3 s (depending upon breast thickness and density). Slits after the PX radiator are used to define both the spatial distribution and the spectral bandwidth of the x...

The prospects of an X-ray free electron laser using stimulated resonance transition radiation

This paper considers stimulated X-ray emission from a relativistic beam of electrons passing through a periodic, heterogeneous medium and interacting with a plane wave. Amplification of the wave occurs when there is bunching of the electrons due to this interaction. The gains for warm and cold electron beams are derived neglecting space charge and multiple scattering. High-current, ultrarelativistic electron beams appear to be the most likely to produce reasonable gain. An estimate of the multiple scattering for the examples cited show the net gain to be severely affected.

Characteristics of the thick, compound refractive lens

A compound refractive lens (CRL), consisting of a series of N closely spaced lens elements each of which contributes a small fraction of the total focusing, can be used to focus x rays or neutrons. The thickness of a CRL can be comparable to its focal length, whereupon a thick-lens analysis must be performed. In contrast with the conventional optical lens, where the ray inside the lens follows a straight line, the ray inside the CRL is continually changing direction because of the multiple refracting surfaces. Thus the matrix representation for the thick CRL is quite different from that for the thick optical lens. Principal planes can be defined such that the thick-lens matrix can be converted to that of a thin lens. For a thick lens the focal length is greater than for a thin lens with the same lens curvature, but this lengthening effect is less for the CRL than for the conventional optical lens.

The Effect of Unit Lens Alignment and Surface Roughness on X-ray Compound Lens Performance

The required alignment tolerances and surface roughness for unit lens elements in a compound refractive lens (CRL) for x rays are discussed. Contrary to what one might expect and what has been stated in the patent literature, alignment tolerances are large and for typical parameter values the effect of misalignment is minor. For a parabolic lens the focusing properties of the CRL are unaltered by misalignment and there is a small increase in absorption. For a lens with spherical aberration, there is a slight change in focal length, a minor translation of the image, and a small increase in absorption. This article also shows that lens gain is not appreciably reduced if the phase shift that is introduced by the roughness is limited to ±π/4 or if the transverse period of the roughness exceeds a specified value. The CRL can benefit from a managed misalignment of the elements to reduce the phase error introduced by surface imperfections of the lens.