Production and application of a novel energy-tunable X-ray source at the RPI LINAC

Abstract

The 60-MeV electron linear accelerator at Rensselaer Polytechnic Institute (RPI) is used to produce parametric X-rays (PXR). PXR is an intense, quasi-monochromatic, energy-tunable, and polarized X-ray source derived from the interaction of relativistic electrons with the periodic structure of crystal materials. Experiments were performed using highly oriented pyrolytic graphite (HOPG), LiF, Si, Ge, Cu, and W target crystal radiators. Smooth X-ray energy tunability is achieved by rotating the crystal with respect to the electron beam direction. Measured energy linewidths consistently agreed with predicted values except in cases using lower quality HOPG. When the predicted energy linewidth was narrower than our Si X-ray detector resolution (350eV at 17.5KeV), a near-absorption edge transmission technique that takes advantage of the PXR energy tunabilty was used to measure the PXR energy linewidth for example, Si(400) FWHM of 134eV at 9.0keV (2%). Per electron, the photon production efficiency of PXR is comparable to synchrotron radiation sources. A theoretical model that considers electron multiple scattering, electron divergence, and crystal mosaicity was used to broaden the PXR photon distribution in order to calculate the predicted PXR photon yield. Comparing measurements and calculations resulted in a typical relative error below 50%. In some cases with LiF, the differences between predicted and measured values were as low as 2% for LiF(400). Finally, this work reports for the first-time PXR imaging. This was achieved using LiF(220) interacting with 56MeV electrons with electron beam currents up to 6μA. The LiF and graphite PXR target crystals were compared for use in soft tissue imaging, e.g. mammography using energies 17–20keV. Low Z materials like graphite and LiF were most suitable for PXR production because of their low Bremsstrahlung production, electron scattering, and photon absorption. Graphite was more efficient at producing PXR photons while the LiF energy line width was narrower.