PARAMETRIC X-RAYS (PXR) PRODUCTION

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Click here to see the FIRST images taken with a PXR beam

Click here to see some experimental data

What is PXR?

Parametric X-ray radiation belongs to a group of several radiation types that are emitted when relativistic charged particles interact with a crystal.

When relativistic electrons enter a crystal they are transported in the repeated structure of the crystal. As a result of the interaction of the electrometric wave associated with the electrons and the repeated structure polarized photons are emitted and diffracted in the crystal itself. The emitted X-rays have several interesting and useful features:

• The photons are quasi-coherent.
• The emitted X-ray energy depends on the angle between the crystal and the electron beam, which makes the source energy tunable.
• The natural line width (energy distribution) is very small (less than 2 eV) and thus the practical line width depends on the experimental geometry.
• The radiation can be emitted at large angles relative to the electron beam, which helps reduce background from bremsstrahlung radiation.
• The X-rays are emitted with a narrow angular distribution and are simultaneously reflected to several angles around the crystal. This effect can be used in order to run several experiment simultaneously.
• The X-rays are linearly polarized.
• The efficiency of converting electrons to photons is high when compared to other mechanisms that produce monochromatic X-rays.

The geometry for PXR production is illustrated in figure 1.

Figure 1 - Electron beam incident on a crystal and creates Parametric X-rays.

The electromagnetic field associated with the electrons in the crystal can be treated as virtual photons. The energy of the emitted X-rays depends on the angle f between the crystal planes and the electron beam and is given by

:                              (1)

where t is the magnitude of the reciprocal lattice vectorn which is a function of the crystal lattice spacing d and the reflecting plane Miller indexes (hkl).

                   (2)

The Bragg diffraction occurs in twice this angle (f=q_b, q_d=2f) where the spatial distribution of the X-rays is given by:

         ₍₃₎

This distribution is shown in figure 2

Figure 2 - Distribution of PXR parallel to the interaction.

A calculation showing the intensity and energy of the PXR emitted form an Si crystal are shown inf Figure 3. The plot is a cut along qy=0. The solid angle of the detector (shown on the plot above the left peak) can be used to estimate the expected energy distribution (resolution) measured by the detector.

Figure 3 - Angular and energy distribution of PXR. The small line above the left peak represents the detector solid angle.

Our objective in this research is to use the PXR mechanism to cerate an intense X-ray source for the RPI LINAC. In order to construct such a source several aspects of PXR are studied.

• Find the most suitable crystal material (graphite, Silicone, Germanium, Metalic Crystals).
• Optimize the crystal thickness for a give X-ray energy.
• Investigate the effect of the electron beam divergence.
• Calculate the effect of electron straggling in the Crystal using Monte Carlo Methods
• Investigate PXR production in the Laue and Bragg geometries.
• Design a crystal target for high electron beam currents address heat and charge transfer problems.

The properties of the source are suitable to many applications that require an intense monochromatic polarized X-rays. Some of the applications we are currently investigating

• Phase imaging for none destructive and medical applications.
• Real-time imaging (pulsed X-ray source)..
• Explosive detection by diffraction
• Fissile material detection by edge absorption and fluorescent.
• Measurment of photon interaction cross sections.

1. Y. Danon, B.A.Sones, R.C.Block, Novel X-Ray Source at the RPI Linac, American Nuclear Society Transactions, 2002 summer meeting Hollywood, Florida, volume 86, June 9-13,2002.
2. B.A.Sones,Y. Danon, R.C.Block, Optimization of Parametric X-Ray Production, American Nuclear Society Transactions, 2002 summer meeting Hollywood, Florida, volume 86, June 9-13,2002.
3. B. Sones, Y. Danon, and R. Block, LiF Crystal Advantages in Parametric X-Ray (PXR) Production, Transactions of the ANS/ENS, New Orleans, Louisiana, Volume 89, 2003.
4. B. Sones, Y. Danon, and R. Block, Novel X-Ray Imaging Opportunities for the RPI Linear Accelerator's Tunable, Quasi-monochromatic X-ray Source, 2004 ANS Annual meting, Pittsburgh, PA, June 13 - 17, 2004
5. Yaron Danon, Bryndol Sones and R. C. Block, Dead time and pileup in pulsed Parametric X-rays spectroscopy, Nuclear Instruments and Methods A, 524. p. 287 –294, 2004.
6. B. Sones, Y. Danon, and R.C. Block, Lithium Fluoride (LiF) Crystal for Parametric X-Ray (PXR) Production, NIM B, 227, p. 22-31, 2005.
7. Report to DOE, Development of a Novel Tunable X-Ray Source for the RPI-LINAC, Department of Energy's Information Bridge.