Our team at Los Alamos National Laboratory has successfully employed Compton spectrometers to measure the X-ray spectra of both continuous and flash radiographic sources. In this method, a collimated beam of X-rays incident on a converter foil ejects Compton electrons. A collimator may be inserted into the entrance of the spectrometer to narrow the angular acceptance of the forward-scattered electrons, which then enter the magnetic field region of the spectrometer. The position of the electrons at the magnet’s focal plane is proportional to the square root of their momentum, allowing the X-ray spectrum to be reconstructed. A new samarium-cobalt spectrometer with an energy range of 50 keV to 4 MeV has been fielded at two facilities. The X-ray generating machines produced intense photon beams (> 4 rad at 1 m) with spectral endpoints below 3 MeV. Recent experimental results will be presented.
During the previous three years, a Compton spectrometer has successfully measured the x-ray spectra of both continuous
and flash radiographic sources. In this method, a collimated beam of x-rays incident on a convertor foil ejects Compton
electrons. A collimator in the entrance to the spectrometer selects the forward-scattered electrons, which enter the
magnetic field region of the spectrometer. The position of the electrons at the magnet’s focal plane is proportional to the
square root of their momentum, allowing the x-ray spectrum to be reconstructed. The spectrometer is a neodymium-iron
magnet which measures spectra in the <1 MeV to 20 MeV energy range. The energy resolution of the spectrometer was
experimentally tested with the 44 MeV Short-Pulse Electron LINAC at the Idaho Accelerator Center. The measured
values are mostly consistent with the design specification and historical values of the greater of 1% or 0.1 MeV.
Experimental results from this study are presented in these proceedings.
Nuclear Quadrupole Resonance (NQR) has been demonstrated for the detection of 14-N in explosive compounds. Application of a material specific radio-frequency (RF) pulse excites a response typically detected with a wire- wound antenna. NQR is non-contact and material specific, however fields produced by NQR are typically very weak, making demonstration of practical utility challenging. For certain materials, the NQR signal can be increased by transferring polarization from hydrogen nuclei to nitrogen nuclei using external magnetic fields. This polarization enhancement (PE) can enhance the NQR signal by an order of magnitude or more. Atomic magnetometers (AM) have been shown to improve detection sensitivity beyond a conventional antenna by a similar amount. AM sensors are immune to piezo-electric effects that hamper conventional NQR, and can be combined to form a gradiometer for effective RF noise cancellation. In principle, combining polarization enhancement with atomic magnetometer detection should yield improvement in signal-to-noise ratio that is the product of the two methods, 100-fold or more over conventional NQR. However both methods are even more exotic than traditional NQR, and have never been combined due to challenges in operating a large magnetic field and ultra-sensitive magnetic field sensor in proximity. Here we present NQR with and without PE with an atomic magnetometer, demonstrating signal enhancement greater than 20-fold for ammonium nitrate. We also demonstrate PE for PETN using a traditional coil for detection with an enhancement factor of 10. Experimental methods and future applications are discussed.
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.
Flash radiography is a diagnostic with many physics applications, and the characterization of the energy spectra of such sources is of interest. A Compton spectrometer has been proposed to conduct these measurements. Our Compton spectrometer is a 300 kg neodymium-iron magnet constructed by Morgan et al1, and it is designed to measure spectra in the <1 MeV to 20 MeV range. In this device, the x-rays from a radiographic source are collimated into a narrow beam directed on a converter foil. The forward-selected Compton electrons that are ejected from the foil enter the magnetic field region of the spectrometer. The electrons are imaged on a focal plane, with their position determined as a function of their energy. The x-ray spectrum is then reconstructed. Challenges in obtaining these measurements include limited dose of x-rays and the short pulse duration (about 50 ns) for time-resolved measurements. Here we present energy calibration measurements of the spectrometer using a negative ion source. The resolution of the spectrometer was measured in previous calibration experiments to be the greater of 1% or 0.1 MeV/c1. The reconstruction of spectra from a bremsstrahlung source and Co-60 source are also presented.