Our standard photomultipliers comprise of 9124B RbCs Bialkali tubes (supplied by Electron Tubes Ltd.) fitted into our own proprietary housings. The tubes are specially selected to provide good response in the blue / green region with rapidly decreasing sensitivity to red and infrared light. For experiments using long wavelength visible dyes (e.g. SNARF) the S20 version of the tube with enhanced red response is usually supplied. If the tube type is not indicated then it will be the default 9124B.
The photomultiplier housing has two input and one output BNC connectors. From left to right these are labelled HV (high voltage), GATE, and SIG(nal). The high voltage connector should be coupled to the PMT SUPPLY module in the main electronics housing via the specially rated cable provided (identified by its large stainless steel plugs). The signal output is in the form of a negative current and should be connected to the input socket of the INPUT amplifier or PMT AMP via one of the standard moulded BNC leads provided. If more than one photomultiplier are fitted then each one should be connected to a different power supply and amplifier. The GATE input allows the photomultiplier to be effectively "switched off" rapidly (by turning off the first multiplication stage in the tube) without the destabilising effect of cutting the high voltage supply. Gating is achieved by applying a +5V TTL signal to the BNC socket. This should normally be left disconnected during setting up and trouble shooting, but during operation it can be optionally connected as follows:
- If a SWITCHER module is fitted in the system box then the OUTPUT from this can be connected to the GATE input of the photomultiplier. This has the effect of gating the photomultiplier whenever the shutter is closed. This helps to prevent accidental over exposure of the photomultiplier whilst recordings are not in progress.
N.B. The SWITCHER can be configured to give an output signal of either polarity - please refer to the relevant section to ensure that this is correct. - If using the Spectrophotometer in conjunction with our Flash Photolysis system it is important to gate the photomultiplier during flashes. This is achieved by connecting the OUTPUT of the Flash Photolysis unit to the GATE input of the photomultiplier.
- The photomultiplier gating may be controlled by one of the trigger outputs of our computer interface module or from any third party device capable of producing digital control signals.
