Time Resolved X-Ray Spot Diagnostic

A diagnostic was developed for the determination of temporal history of an X-ray spot. A pair of thin (0.5 mm) slits image the x-ray spot to a fast scintillator which is coupled to a fast detector, thus sampling a slice of the X-ray spot. Two other scintillators/detectors are used to determine the position of the spot and total forward dose. The slit signal is normalized to the dose and the resulting signal is analyzed to get the spot size. The position information is used to compensate for small changes due to spot motion and misalignment. The time resolution of the diagnostic is about 5 ns and measures spots from 0.5 mm to over 3 mm. The theory and equations used to calculate spot size and position are presented, as well as data. The calculations assume a symmetric, Gaussian spot. The spot data is generated by the ETA II accelerator, a 2kA, 5.2 MeV, 60 ns electron beam focused on a tantalum target. The spot generated is typically about 1 mm FWHM. Comparisons are made to an X-ray pinhole camera which images the X-ray spot (in 2D) at four time slices.

DIAGNOSTIC DESCRIPTION

This diagnostic is used to determine the time history of an X-ray spot on the ETA accelerator. A slice of the X-ray spot is projected on to a detector using 2 thin (0.5 mm) horizontal slits in 8 inch thick lead bricks. The first slit is nominally about 5 m from the converter target, and the second slit is about 5 m behind the first. These distances change as dictated by other hardware and available space. Additional shielding of 8 inches (or greater) of lead is around the detectors so that < 1 % of the signal is noise. The slits are aligned to be in line with the center of the beam spot.