Accurate knowledge of the x-ray spectra used in medical treatment and radiography is important for dose calculations
and material decomposition analysis. Indirect measurements via transmission through materials are possible. However,
such spectra are challenging to measure directly due to the high photon fluxes. One method of direct measurement is via
a Compton spectrometer (CS) method. In this approach, the x-rays are converted to a much lower flux of electrons via
Compton scattering on a converter foil (typically beryllium or aluminum). The electrons are then momentum selected by
bending in a magnetic field. With tight angular acceptance of electrons into the magnet of ~ 1 deg, there is a linear
correlation between incident photon energy and electron position recorded on an image plate. Here we present
measurements of Bremsstrahlung spectrum from a medical therapy machine, a Scanditronix M22 Microtron. Spectra
with energy endpoints from 6 to 20 MeV are directly measured, using a CS with a wide energy range from 0.5 to 20
MeV. We discuss the sensitivity of the device and the effects of converter material and collimation on the accuracy of
the reconstructed spectra. Approaches toward improving the sensitivity, including the use of coded apertures, and
potential future applications to characterization of spectra are also discussed.
A Compton spectrometer has been re-commissioned for measurements of flash radiographic sources. The determination of the energy spectrum of these sources is difficult due to the high count rates and short nature of the pulses (~50 ns). The spectrometer is a 300 kg neodymium-iron magnet which measures spectra in the <1 MeV to 20 MeV energy range. Incoming x-rays are collimated into a narrow beam incident on a converter foil. The ejected Compton electrons are collimated so that the forward-directed electrons enter the magnetic field region of the spectrometer. The position of the electrons at the magnet’s focal plane is a function of their momentum, allowing the x-ray spectrum to be reconstructed. Recent measurements of flash sources are presented.