Operating potentials for the IPD electrodes

The tubes are operated with the anode near ground potential and the photocathode at between three and five thousand volts negative, depending on the number of MCPs. When discussing inter-electrode potentials, those which accelerate electrons towards the anode are deemed to be positive and those which retard the electrons are considered negative.

6.4.1 Voltages for the single stack device

The values of the resistor chain are selected to give the required electrode voltages. A "standard" set of resistor values has evolved which is appropriate for the majority of tubes, but some tubes require variations on this. The fine tuning of the microchannel plate voltage V^cp is achieved by adjusting the overall voltage once the correct resistor ratios have been determined. This makes only a small and relatively unimportant difference to the other electrode voltages. The other potentials required are: , between the photocathode and the first microchannel plate, and between the last microchannel plate

and the anode. The voltage requirements for operation of the tube are outlined below.

It is recalled from paragraph 2.5 that this should be as high as possible to ensure maximum penetration of the ion barrier and minimum lateral motion of the photoelectron. However, as discussed in paragraph 3.8.4, field emission spots can become a problem and it is sometimes necessary to reduce the voltage in order to minimise this effect. Too high a voltage can also cause internal breakdown inside the tube, resulting in damage to the photocathode and microchannel plates. Internal breakdown has been a persistent problem with ITL tubes and is, therefore, limited to 500 volts for these devices.

^mcp* This is chosen to give a good Gaussian pulse height distribution at the optimum amplitude for the processing electronics. The channel plates should be run at the highest possible gain consistent with stable operation and a well formed pulse height distribution. As this gain is similar within each class of detector it is possible to use a standard charge/shaping amplifier and to adjust the channel plate voltage to give the correct range of pulse heights as measured on a pulse height analyser. If a pulse height analyser is not available, for example in the case of repairs in the field, it is possible to set the voltage approximately by illuminating the detector and adjusting the voltage for maximum count rate from the processing electronics. Sometimes the amplifier gain has to be modified to suit individual tubes. This can happen when a tube exhibits instability at a lower than normal gain, or when the tube has to be run at a lower gain in order to minimise a hot spot or other blemish on the image caused by a microchannel plate defect.

The MCP voltage can be applied across the entire stack so that it is shared equally between all the plates, provided that they all have a similar resistance. However, the first channel plate in the stack is filmed, as explained in paragraph 6.2.3. Because these plates are manufactured separately it is not always economic to select plates which have good resistive matching to the remainder of the stack. When

matching is not achieved, an intermediate connection is fitted to the junction of the first MCP and the remainder of the stack. In this configuration the filmed plate is normally driven with one third of the total stack voltage, but this can sometimes be varied to obtain an improved pulse height distribution.

This is not a critical parameter as long as the voltage is not so low that the charge cloud spreads over the edge of the anode. A potential of 100 volts is satisfactory.

6.4.2 Voltages for the double stack device

These devices require two separate channel plate potentials, and ^ztncp the V and Z MCP stacks. The potential of the gap between the two stacks is These voltages need much more careful fine tuning than with the single stack detectors. The optimum operating voltages are assessed by ITT prior to shipment and are detailed in the data sheet supplied with each tube.

: As with the single stack device, this should be as high as possible. The ITT devices are particularly tolerant of high potentials and 700 volts is the usual value used for these tubes. Although higher potentials could probably be used this is the maximum value permitted under the conditions of the tube warranty.

V ; This should be as large as possible consistent with safe

v m c p

operation of the device, and is usually about 1800 volts.

There are two modes of operation for the gap, which initially appear to be contradictory. The first is to operate with a positive potential to accelerate the electron cloud so that it covers a relatively small area on the face of the Z-stack. The second is to apply a negative potential which repels the lower energy electrons, allowing only the higher energy ones to reach the Z-stack. The rejected electrons will be those produced towards the end of the Vmcp channels which have low energy. Those emitted further back along the

V-stack channels are able to gain enough energy to overcome the gap potential before they leave the channel. They are, therefore, highly collimated and have very little lateral spread. The negative mode does, obviously, introduce a significant gain reduction, typically a factor of ten, so the Z-stack has to be run at a higher gain to compensate.

Floryan and Johnson (1989) have demonstrated that the best combination of resolution, pulse height distribution and counting efficiency is obtained with a negative usually between -50 and -200 volts.

Again, this is not critical and is normally in the region of 100 volts.