Objective
- The objective of the present research is to develop a large area, fast, high resolution solid photocathode gaseous photomultiplier for photon imaging over a broad spectral range from visible to VUV;
- Main applications foreseen are :
- UV and visible photon imaging for fast readout of solid scintillators in medical applications such as PET (positron emission tomography) and gamma cameras;
- applications in industrial radiography for non-destructive evaluation;
- applications in high energy particle physics, atomic physics and astrophysics.
Expected Outcome
Following performance from the device :
- High quantum efficiency in the spectral range of interest;
- Stability with time in a relatively unpurified gaseous environment;
- Sum-mm two-dimensional localisation accuracy over large surfaces, without parallax error;
- Sub-nanosecond timing;
- Efficient detection of photons due to the high single electron detection efficiency of the multistage electron multiplier;
- High rate capability due to fast ion removal at low-pressures, the low charge density in the avalanche and the possibility of subdividing the readout electrodes into arrays of individual pixels;
- Low ageing of the photocathode due to the multistage operation and the possibility to incorporate an electric gate, to stop back-drifting ions;
- Possibility to use the device in a triggered mode with built-in-electronic delay (drift in gas), further increasing its high rate capability and immunity to background.
Besides the immediate application to PET and the large spectrum of possible applications of these devices, in basic and applied research, the study may provide new valuable physical data about the properties of new photosensitive and scintillation materials.
Results
- We performed a complete study of CsI as a photo-emitter. We realised that the discrepancies in the measured quantum efficiencies around the world were due to incorrectly referenced standards supplied by some manufacturers, or to the effect of the substrate. We have now agreed, in the framework of the RD26 collaboration, to a common data.
- We have studied the effect of aging by radiation, by light and by current. We have determined that the gas used for amplification has no effect on the quantum efficiency, when the electric field is large enough.
- We have measured the effect of the incident angle, which is of high interest for application as Cerenkov detectors. The quantum efficiency of amorphous silicon was measured, with different doping percentages. We found that the highest value is obtained for p- doped photocathodes. About twelve organics or organometallic compounds were studied.
- We conclude that only the derivatives of ferrocenes exhibit potentially useful quantum efficiencies. Similar organometallic compounds, containing different metallic elements, are much worse than those containing iron. Our best choice is decamethylferrocene, that has some unique characteristic : it is solid, its quantum efficiency is high at 220nm and it is not air-sensitive.
- Study of the characteristics of photocathodes materials such as the determination of the quantum efficiency and its dependence on wavelength and temperature;
- Photocathode compatibility with gaseous amplification media under various operating conditions;
- Matching of a chosen scintillator crystal with a suitable photocathode;
- Design and construction of a gaseous imaging photomultiplier;
- Various systematic tests of the photomultiplier and the scintillator-photomultiplier assembly;
- Possible incorporation in a PET scintillator system;
- Design and construction of a sealed photomultiplier and tests.
The first material studied is the well-known inorganic photo-emitter Cs1. Other materials currently investigated are the organometallic compounds having a low ionisation threshold.
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Coordinator
91128 Palaiseau
France
The total costs incurred by this organisation to participate in the project, including direct and indirect costs. This amount is a subset of the overall project budget.