Improving spatial resolution of high stopping power X- and gamma-ray cameras:: fibers or slat-structured detectors?

Abstract

For medical imaging applications, the earliness of the detection is an essential factor to increase chances of recovery; in the field of industrial imaging, nondestructive testing with lower detectivity threshold to ensure quality and safe conduct. Accordingly, in all areas using the up-to-date compact (much less-expensive facilities) high-energy pulsed electron accelerators (HF or induction linac, Marx generator) to produce energetic photons (bremsstrahlung), such as industrial and medical numerical imaging, flash radiography, radiotherapy positioning, computed tomography, detection of small- or low-contrasted details require two-dimensional (2D) detectors with an even more improved combination of sensitivity (which implies high stopping power), spatial resolution (millimetric or sub-millimetric) and speed, working in integrating mode (i.e. dose measurement) because bremsstrahlung X-ray sources provide short pulses. The purpose of this paper is to highlight some of the issues involved in the development of high-performance position-sensitive X- and gamma-ray cameras for high-energy flash imaging. The basic idea is that, examining in detail the energy deposition and its statistics (quantum noise), we shall be able to determine in real detectors the following features, such as detectors composition and pixel size, which can simultaneously lead to good detection efficiency and good spatial resolution. In general, conclusions can be transposed to other particle imaging detectors as neutron imagers (changing “dense” metal by “high energy transfer” material). There are, of course, challenges to get such detectors, although new technologies have already provided some prototypes offering more than 30% stopping power and less than 2mm spatial resolution (blur) for 50ns long 5MeV X-ray pulses. There are various detector-segmentation methods that can be applied in order to improve the stopping power (macroscopic cross-section) and reduce the effect of the lateral energy spread on the resolution. Technologies such as scintillator arrays (bundles), photoconductor plate piling, multi-hole- and multi-slat-solid detectors into gas chamber are employed. The first experimental results obtained at CEA are encouraging for applications such as high-energy low-dose fast imaging, flash radiography, radiotherapy positioning, nuclear waste or fuel survey, and, may be, high angular resolution imaging in the field of high-energy astrophysics. The relevance of “future” solutions is then examined, exhibiting theoretical and experimental frontiers: the most efficient structure for a future high-performance 2D integrating imager shall be a piling of thin metal layers, able to boost the quantum efficiency, sandwiched between a few hundreds of micrometers sensitive detectors as low-dark-current photoconductor (micro-strips) or parallel high-performance scintillating fiber sheets.