Preliminary investigation fo the non-linear response of image intensifiers used for gamma-ray imaging

Abstract

Image intensifiers combined with columnar scintillators have found application in x-ray and gamma-ray, biomedical imaging and other fields. In scintillator imaging, hundreds or thousands of optical photons can illuminate the faceplate of the image intensifier in a small area, essentially simultaneously. This is a situation not found in the typical design application for an image intensifier, night vision or low-light-level imaging. Microchannel plates (MCPs) are known to exhibit gain saturation that could result in non-linear signal response in scintillator imaging, limiting quantitative measurement capabilities. A calibrated LED photon source was developed that can provide a known average number of photons per unit area in a small spot size, similar to that seen due to a gamma-ray interaction in a BazookaSPECT imager. A BazookaSPECT imager is composed of a columnar scintillator and an image intensifier, with output light optically imaged onto a CCD camera. The calibrated source was used to investigate gain-saturation effects for two Proxivision, GmbH image intensifiers, a single-stage BV 2583 EZ and a two stage BV 2583 QZ-V 100N in a BazookaSPECT imaging configuration. No gain saturation was found for the single-stage image intensifier up to more than 100 optical photons per microchannel, but significant gain-saturation non-linearities were measured in the two-stage image intensifier at high gains for >12 optical photons per microchannel. Implications for scintillator imaging using such systems are discussed.

Analogue saturation limit of single and double 10 mm microchannel plate photomultiplier tubes

“…This analysis confirms the assumption made in previous studies of MCP saturation in a low rate, multi-photon environment (as opposed to a high-rate, single photon regime) that looked at saturation in an MCP-based image intensifier from Proxivision. 7 The assumption was that any saturation would result in a signal depression independent of the pulse width, provided it is significantly shorter than the recharge time of the MCP, which is in the order of milliseconds, and is also suggested by a modelling analysis. 8 Both of these studies indicate a continued increase in signal output up to a level considerably beyond the linear region.…”

Imaging properties of pixellated scintillators with deep pixels

“…The high gain noise in the image intensifier [10] is likely the most limiting factor, followed by the summed readout noise from the large number of CCD pixels corresponding to each scintillator pixel. Microchannel-saturation effects [11] might also make a minor contribution to the limited energy resolution. One phenomenon that probably does not limit energy resolution in this case, is the variation in light transmission with interaction depth along the long, thin pixels as we shall see in the next section.…”