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R. Forty and O. Ullaland
the CsI photo cathode can either be deposited as a reflective, Fig. 7.18b, or as
a semi-transparent layer [54]. The latter would, in the case of Fig. 7.18b lay-out,
be a layer on the quartz window. The maximum quantum efficiency for semitransparent CsI is for a thickness of about 11 nm in the wavelength range from
210 to 170 nm. The thickness does not matter for a reflective photo cathode and is
normally in the range of 150–200 nm. A semi-transparent CsI photo cathode will
have a quantum efficiency of about 0.7 compared to a reflective one. It should
be noted that the photon conversion efficiency is strongly depending on the bulk
structure and morphology of the CsI layer; that is, the roughness of the substrate
and the connectivity of the layer. Particularly thin layers can become a collection of
unconnected islands. Post-production heat treatment has proven advantageous.
As for the photosensitive vapours, a CsI photo cathode will be sensitive to
the photon feed-back from the gas amplification process, see Sect. 7.4.4.1. A
stable operation of the chamber is therefore a compromise between single photon
efficiency, electronics sensitivity, signal shaping and gas amplification.
As few, if any other photon detector, can beat a gas based detector in cost
efficiency and geometrical acceptance, a number of similar, but not identical,
detector set-ups are proposed and investigated. The main emphasis is on limitation
of photon feed-back, on better and more stable photo cathodes and on time
resolution. This work is also partially driven by very large Cherenkov detectors
for astrophysics. A very promising research and development is in gaseous micro
pattern detectors with Bialkali photo cathodes. We will not discuss these here,
but refer the reader to [55]. An overview of the current status and perspectives of
gaseous photon detectors can be found in [56].
7.4.4.3 Vacuum Based Photon Detectors
The working principles of vacuum based photon detectors like photo multiplier
tubes are discussed in Chap. 3. Although small diameter PM tubes, diameter 10 mm
upwards, have been used for a long time in Cherenkov detectors, cost, balanced with
space resolution and material budget, made them less attractive. The introduction of
multi anode and pixilated silicon anode detectors, together with fast and sensitive
electronics changed this. The first generation of multi anode photo tubes required a
lens system [57] in order to give good geometrical acceptance. See Fig. 7.19a.
The schematic of a Hybrid Photon Detector [60, 61], HPD, is shown in Fig. 7.19b.
In these detectors the encapsulated pixilated silicon detector is bump-bonded onto
the read-out electronics. The capacitance is thereby small and the associated noise
low. It also requires only a few vacuum feed-throughs. The photo cathode is
normally a S20. 16 Under the influence of the electric field, the photo-electron
is accelerated onto the silicon detector. In the example given in Fig. 7.19b, the
20 kV potential between anode and cathode gives a cross-focusing field with a
16 S20 is a tri-alkaline (Sb-Na-K-Cs) semi-transparent photo cathode.
R. Forty and O. Ullaland
the CsI photo cathode can either be deposited as a reflective, Fig. 7.18b, or as
a semi-transparent layer [54]. The latter would, in the case of Fig. 7.18b lay-out,
be a layer on the quartz window. The maximum quantum efficiency for semitransparent CsI is for a thickness of about 11 nm in the wavelength range from
210 to 170 nm. The thickness does not matter for a reflective photo cathode and is
normally in the range of 150–200 nm. A semi-transparent CsI photo cathode will
have a quantum efficiency of about 0.7 compared to a reflective one. It should
be noted that the photon conversion efficiency is strongly depending on the bulk
structure and morphology of the CsI layer; that is, the roughness of the substrate
and the connectivity of the layer. Particularly thin layers can become a collection of
unconnected islands. Post-production heat treatment has proven advantageous.
As for the photosensitive vapours, a CsI photo cathode will be sensitive to
the photon feed-back from the gas amplification process, see Sect. 7.4.4.1. A
stable operation of the chamber is therefore a compromise between single photon
efficiency, electronics sensitivity, signal shaping and gas amplification.
As few, if any other photon detector, can beat a gas based detector in cost
efficiency and geometrical acceptance, a number of similar, but not identical,
detector set-ups are proposed and investigated. The main emphasis is on limitation
of photon feed-back, on better and more stable photo cathodes and on time
resolution. This work is also partially driven by very large Cherenkov detectors
for astrophysics. A very promising research and development is in gaseous micro
pattern detectors with Bialkali photo cathodes. We will not discuss these here,
but refer the reader to [55]. An overview of the current status and perspectives of
gaseous photon detectors can be found in [56].
7.4.4.3 Vacuum Based Photon Detectors
The working principles of vacuum based photon detectors like photo multiplier
tubes are discussed in Chap. 3. Although small diameter PM tubes, diameter 10 mm
upwards, have been used for a long time in Cherenkov detectors, cost, balanced with
space resolution and material budget, made them less attractive. The introduction of
multi anode and pixilated silicon anode detectors, together with fast and sensitive
electronics changed this. The first generation of multi anode photo tubes required a
lens system [57] in order to give good geometrical acceptance. See Fig. 7.19a.
The schematic of a Hybrid Photon Detector [60, 61], HPD, is shown in Fig. 7.19b.
In these detectors the encapsulated pixilated silicon detector is bump-bonded onto
the read-out electronics. The capacitance is thereby small and the associated noise
low. It also requires only a few vacuum feed-throughs. The photo cathode is
normally a S20. 16 Under the influence of the electric field, the photo-electron
is accelerated onto the silicon detector. In the example given in Fig. 7.19b, the
20 kV potential between anode and cathode gives a cross-focusing field with a
16 S20 is a tri-alkaline (Sb-Na-K-Cs) semi-transparent photo cathode.