PHOTO-EMMISSIVE TECHNOLOGY
The photograph shows the ICCD-80X
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PHOTO-EMMISSIVE TECHNOLOGY
Photek manufactures a wide range of image intensifiers using single, double and triple channel plates in 25, 40 and 75mm active diameters. These can be coupled to a suitable scintillator to match customers' requirements, and to provide imaging covering the electromagnetic spectrum from UV to high energy gamma rays and particles. Coupling to film backs, CCD cameras and photon counting systems with PC interface and full 16 bit image processing software can also be undertaken. All Photek camera systems are manufactured to individual customer specification and requirement, and are optimised for that purpose.
Input image formats of greater than 75mm can be made by using a fibre optic taper between the scintillator and the image intensifier. The largest fibre taper generally available is 115mm diameter, though larger sizes have been made.
Fibre optic tapers have an optical transmission proportional to the area reduction factor, and are quite inefficient for large reduction factors. Photek are therefore developing a range of electrostatic focus demagnifying image intensifiers, which will allow much more efficient coupling between large scintillators and read-out cameras.
Photons in this energy range are strongly attenuated by air, but can be imaged in vacuum by phosphor screens, or micro channel plates. The detection efficiency of a microchannel plate for photons is a function of the angle of incidence and photon energy. Typical efficiency is 10% falling away at both low energy and high energy.
The efficiency can be icnreased to around 20% by coating such as CsI, but the thickness of this should be optimised for the desired photon energy.
Figure 1 - Thickness of Gd2O2S Required for 90% Absorption.
Inorganic powder scintillators provide 20-30 visible photons per absorbed kV and can be efficiently coupled with a fibre optic block to an image intensifier or photon counting tube. With a quantum efficiency of typically 10% at the image tube photocathode, it can be seen that every X-ray photon will generate several photoelectrons, ensuring an almost completely efficient detection probability.
At high photon energies the thickness of the scintillaor required for good absorption efficiency, increases as shown in Figure 1.
The spatial resolution is approximately equal to twice the thickness of the powder applied, so that, for example at 18 kV the limiting resolution wil be 100 microns for a scintillator capable of absorbing 90% of available photons.
For photons above this energy, it is necessary to choose between a scintillator optimised either for resolution or for counting efficiency.
Terbium glass fibre optic scintillators offer an easy solution for X-ray and gamma-ray detectors at higher energy. The conversion efficiency is lower at around 10 photons per keV, but since all the light is channelled down the 15 micron fibres, the thickness can be made whatever is necessary to achieve efficient absorption. Resolution is typcially 50 microns or better, and is largely independent of hte photon energy.
Other new glasses are becoming avaiable that are more efficient than Terbium doped material (Reference 1). Figure 3 shows the photon enrgy bands covered by the three detection systems discussed above.
Real-time radiographs are easily made with micro focus X-ray sources (40 kV, 100uA anode current) using a single stage intensifier coupled to a CCD camera. The energy required for a single frame with reasonable definition is of the order of 1 milli Rad.
Output is proportional to the input flux density for about 3 orders of magnitude from about 10-6 Rad/cm2. Lower flux density signals can be detected by increasing the integration time (slower frame rate on the camera) or by integrating several images into a computer buffer memory. CCD readout noise is not reduced by either technique, and the maximum light/dark ratio that can be reliably achieved with a video readout is approximately 256:1 (8 bits digitisation).
The resolution at the scintillator is approximately 50 microns. This is the primary limit for 25 and 40mm systems. With 75 and 115mm systems the resolution will be limited by readout electronics. For example a 75mm sensor readout with 768 x 585 CCD results in a pixel size of over 100 microns at the scintillator.
Photon counting systems are 10,000 times more sensitive than video systems, since they are able to detect signal levels down to 1 photon/pixel/hour. Resolution can also be greater. Photek can supply systems with a resolution of up to 3080 x 2304 pixels by using software interpolation methods developed by ESA and the astronomical community. These cameras can operate in both an analogue (high count-rate) and digital (photon counting) mode, giving very high dynamic range and also feature electronic zoom. Photon counting sytems are suitable for applications with low flux rates, and at present they are limited to a maximum count rate of 100,000 photons/sec (very approximately 10 micro Rad/sec). Nearly all the electronic and tube related noise is removed by the photon counting system, and the accuracy of the data is largely dependent upon the time and patience of the experimentalist. Images to 16 bits deep are stored in the image processor, and manipulation of this data is accomplished using 8/16/32 bit imaging software.
Thermal neutrons are easily imaged with our cameras using commercially available neutron scintillators such as NE426. This is made from ZnS doped with Lithium 6, and has a resolution of about 0.1mm (Reference 2).
All the scintillators described for X-ray detection can be used for Beta particles with similar conversion efficiency expressed in visible photons/KeV. The absorption of Beta particles is much stronger than X-rays, enabling much thinner scintillators to be used. Photon counting cameras are therefore much more sensitive than photographic film and give immediate quantitative data without the need for plate calibration and scanning. Both pathological analysis and living tissues can be studied using radioactive tracers such as Carbon, Phosphorus, Tritium etc. (Reference 3).
Low energy particles such as electrons, ions, atoms and molecular fragments are conveniently detected in vacuum by a microchannel plate (References 4 & 5).
At higher energy above 100 KeV in SEMs etc. conventional phosphors are the obvious choice. For nuclear physics, our detectors are conveniently attached to arrays of scintillating plastic optical fibre. Photek image intensifiers and photon counting tubes are used in CERN and are chosen for the ACE explorer satellite system for reading out arrays of scintillating plastic fibre.
Figure 4 - Mechanical Dimensions
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