Diffraction Radiation Diagnostics for Moderate to High Energy Charged Particle Beams

Diffraction radiation (DR) is produced when a charged particle passes through an aperture or near a discontinuity in the media in which it is traveling. DR is closely related to transition radiation (TR), which is produced when a charged particle traverses the boundary between media with different dielectric constants. In contrast to TR, which is now extensively used for beam diagnostic purposes, the potential of DR as a non-interceptive, multi-parameter beam diagnostic remains largely undeveloped. For diagnostic measurements it is useful to observe backward reflected DR from an circular aperture or slit inclined with respect to the beam velocity. However, up to now, well founded equations for the spectral-angular intensities of backward DR from such apertures have not been available. We present a new derivation of the spectral angular intensity of backward DR produced from an inclined slit for two orientations of the slit axis, i.e. perpendicular and parallel to the plane of incidence. Our mathematical approach is generally applicable to any geometry and simpler than the Wiener Hofp method previously used to calculate DR from single knife edges. Our results for the slit are applied to the measurement of orthogonal beam size and divergence components. We discuss the problem of separating the simultaneous effects of these beam parameters on the angular distribution of DR and provide solutions to this difficulty. These incude using the horizontal and vertical polarization components of the radiation from a single slit and interferences from two inclined slits. Examples of DR diagnostics for a 500 MeV beam are presented and the current limitations to the technique are discussed.Comment: 32 pages,including 14 figures; submitted to NIM

Higher order parametric X-ray spectra in mosaic graphite and single silicon crystals

We have observed up to eight orders (n) in the spectra of parametric x-radiation, in the range 5-40 keV, produced by the interaction of a 90 Mev electron beam with mosaic graphite and 90 and 35 Mev beams with single silicon crystals. The measured yields and intensity ratios, I(2)/I(n= I), in graphite are not in agreement with the theory of PXR for mosaic crystals. In comparison, the yield and ratios of intensities in silicon are close to the predictions of PXR theory for perfect crystals. The bandwidths of spectral lines measured in both silicon and graphite are in good agreement with theoretical predictions, and are determined by the angular field of view of the detector.U.S. Department of EnergyDNANaval Postgraduate SchoolContract No. DE-FG03-91ER8109

Initial Search for 9-keV XTR from a 28-GeV Beam at SPPS

The potential to use x-ray transition radiation (XTR) as a beam diagnostic and coherent XTR (CXTR) as a gain diagnostic in an x-ray FEL was proposed previously. At that time we noted that the unique configuration of the SLAC Sub-picosecond Photon Source (SPPS) with its known x-ray wiggler source, a special three-element x-ray monochromator, x-ray transport line, and experimental end station with x-ray detectors made it an ideal location for an XTR feasibility experiment. Estimates of the XTR compared to the SPPS source strength were done, and initial experiments were performed in September 2005. Complementary measurements on optical transition radiation (OTR) far-field images from a 7-GeV beam are also discussed

Production of x-rays by the interaction of charged particle beams with periodic structures and crystalline materials

We describe our recent experimental study of the production of x-rays by an electron beam interacting with a crystal lattice, i.e. parametric x-ray (PX) generation. In this radiation process the virtual photon field associated with a relativistic electron traveling in a crystal is diffracted by the crystal lattice in the same way that real x-rays are diffracted by crystals. The radiation produced satisfies the Bragg condition associated with the diffraction of the virtual photons which are nearly parallel to the velocity of the electrons. This phenomenon is associated with a more general class of radiation production mechanisms which include transition radiation (TR), diffraction radiation (DR), and Smith-Purcell radiation. In each case, radiation is produced when the particle's fields are altered by interacting with a material whose dielectric constant varies along or near the particle's trajectory. The usual acceleration mechanism for the production of radiation is not involved in these phenomena. In the case of a crystal, the periodic electric susceptibility interacting with the particle's field produces parametric x-rays. We will also present a theoretical overview of this phenomenon which can be used to generate monochromatic, linearly polarized, directional x-rays. Accelerators with energies ranging from a few MeV to hundreds of MeV may be used as drivers for novel parametric x-ray generators for various applications requiring the unique properties of these sources

Polarized angular distributions of parametric x radiation and vacuum-ultraviolet transition radiation from relativistic electrons

We present quantifiable images of the angular distribution (AO's) of parametric X radiation (PXR) and vacuum-ultraviolet transition radiation (vuv TR) from 230 MeV electrons interacting with a silicon crystal. Both AD's are highly polarized. The vuv TR and optical TR data provide measurements of the beam energy and effective divergence angle. Using these quantities and separately known values of the electronic susceptibility |Xo|, we show that the measured PXR AD is in good agreement with the predictions of single crystal theory. Our analysis suggests a method to measure |Xo| using PXR AD's.Ths work was sponsored in part by DOE SBIR Grant No. DE-FG03-91er80199; NCI SBIR Grant no. 1-R43-CA60207-01 and the Canadian Natural Science and Engineering Research Counci

A compact tunable x-ray source based on parametric x-ray generation by moderate energy linacs

Parametric x-radiation can be described as the diffraction of virtual photons associated with the electric field of a relativistic charged particle passing through a crystal. In analogy with Bragg reflection of x-rays, these diffracted photons appear as real photons, with an energy which satisfies Bragg's law for the reflecting crystal planes. We describe the results of experiments performed on the Naval Postgraduate School linac which were designed to explore the basic properties of PXR in order to assess its potential application as a compact tunable x-ray source. Experiments using a mosaic graphite radiator show that this radiator produced multiple order, narrow bandwidth reflections from 5 - 45 keV. The measured production efficiency is found to exceed that predicted for spectral orders n > 1. We demonstrated the tunability of PXR by rotating the crystal in order to change the Bragg angle relative to the incident 90 MeV electron beam.This work was partially supported by the Defense Nuclear Agency, the Naval Postgraduate School and USDOE SBIR Contract (No. DE-FG03-9lER81099

A method for measuring dark current electron beams in an RF linac

X-ray fluorescence from thin foils inserted into the Naval Postgraduate School linac has been used to measure the integrated electron beam intensity when the accelerator is operating with dark current. The measured x-ray flux, the known inner shell ionization cross sections and radiative transition probabilities are used to obtain measurements of dark currents of the order of 10-14 Amperes. The same arrangement allows continuous, in-situ energy calibration of our SiLi detector in the electromagnetic noise environment of the linac. This technique was originally developed to perform absolute production efficiency measurements of parametric x-ray generation in the 5 - 50 keV range.This work was partially supported by the Defense Nuclear Agency, the Naval Postgraduate School and USDOE SBIR Contract (No. DE-FGO3- 9 1ER8 1099)

Diffraction radiation produced by a charged particle passing near or through a dielectric sphere

We have derived expressions for the spectral distribution of diffraction radiation produced when a charged particle of constant velocity passes near or through a dielectric sphere of radius a. Our expressions, which are valid in the long wavelength limit ka .. O and the impact parameter is large. Certain limiting cases of our expressions are found to be significantly different from the corresponding results previously published by other workers. 4 refs., 7 figs