Handling the MCPs for GIS

The MCPs for GIS came from Philips as sets of three with matched resistances because they were cut from the same boule. This meant that they did not need to have interme­ diate contacts to an external voltage network to maintain the same voltage across each plate, although, in fact, contacts were chosen to act as spacers as well as to protect against any change in resistance.

MCPs are delivered in little foil sachets surrounded by foam padding in plastic boxes. In the air flow bench (better than class 10) in the clean room, using powder-free, pure, natural, latex, clean-room gloves, the packaging is removed and the MCPs are ex­ amined for any travel damage and placed into watch glasses, supported on o-rings, in a locked desiccator cabinet. Thus, the MCPs are only supported by the extreme comers and are kept dry and protected. The o-rings and cabinet are kept scrupulously clean, using IPA and, for the watch glasses, an ultrasonic bath in Arklone. A record is kept of the serial numbers of each MCP, so that the sets do not get confused with each other.

Before use, a set of MCPs is mounted into a specially made stainless steel ‘toast rack’ and baked in the bake-out chamber at low pressure (less than lO ^m bar) at 250°C for 48 hours. The bake-out chamber is protected by valves which isolate the chamber if the power fails, maintaining a vacuum until the temperature has dropped to normal.

The MCP stack (Figures 20 and 25) is built up from the back contact plate which defines the aperture with a chamfered edge. Polycarbonate spacers position the MCPs and a contact plate is held onto the top MCP face by copper leaf spring clamps. The front plate is in contact with the MCPs only along the long edges in order to prevent masking of grazing incidence radiation by stmcture along the short edges. Nickel shims, 25 pm and 12.5 pm thick, are placed between the MCPs (Figure 10). The inner aperture

153

of the back face contact plate is smaller than the apertures of the inter-MCP contact shims, which are themselves smaller than the front face contact plate, so that the edge of the aperture is well defined. This thereby avoids any edge noise which may occur due to the stack spring clamp on the top MCP or from the naked short edges of the MCPs. Once the stack is built, the order of the MCPs and their orientation is never changed, so that calibration may be maintained.

Cleanliness is vital for the proper functioning of the MCPs. They must have a sec­ ondary electron emission coefficient, 6, of at least 1.3 (Eberhardt, 1981) to obtain a de­ sirable gain. Heavy hydrocarbons such as vacuum silicon pump oils, solder resins or epoxy deposited on the MCPs can break down under electron bombardment leaving mainly carbon, which reduces Ô. Smaller hydrocarbon molecules, such as alcohol, trichlor and other similar solvents will not leave a carbon residue. Carbon has a very high work function and the process of deposition is irreversible and therefore has to be avoided. The degradation is proportional to the quantity of heavy hydrocarbons and also to the current through the MCP. Thus, the bottom plate is the most degraded. Particulate contamination must be kept to a minimum because of the risk of generating hot spots.

Evacuating an MCP to a low pressure allows it to desorb water vapour and other light molecules. When in use, electron multiplication and impact, together with the heating due to the current, continues the desorption process. At first application of high voltage, the gain may be unusually high because of the ionisation of the residual gas. Baking the MCP before use reduces the likelihood of this.

McComas et al. (1987) report a series of experiments on CEMs, exposing them to controlled quantities of contaminants at partial pressure including Teflon, eutectic 63/37 solder, Epotech H77 non-conductive epoxy and Epotech H21-D conductive epoxy. After taking 1-2x10^ i events, there was no degradation of the CEM gains. Repeating these experiments with Viton and Apiezon-L vacuum grease shows some degradation, but falling by only -10% after IxlQH counts. With the ability to increase the voltage to compensate for gain loss, they predict that a 10 year lifetime at 6.3 kHz counting rate is possible with gains greater than 1x10^. Although these experiments were performed on CEMs and not MCPs, the results strongly suggest that use of these substances should not be prohibited for testing or in flight, although the use of hydrocarbons should always be kept to a minimum.

The detectors are operated, at MSSL, in a ultra clean UHV stainless steel chamber in a clean room, continually pumped to a pressure of no higher than 2 x l0 “6mbar, provided by an oil free turbo-molecular pump, backed by a rotary pump. If it is neces­ sary to open the chamber, dry nitrogen is used to backfill the chamber to atmospheric

pressure. The first few monolayers of gas adsorbed by the MCPs are therefore dry nitrogen rather than water vapour.

A Penning gauge is used to measure the UHV pressure. It is baffled from the chamber by being mounted on an elbow on the side of the chamber. It can be used as an ion source for stimulating the detector at very high count rates. Fraser (1990) warns that after-imaging has been observed after using an ion gauge, even when it is only used with the detector HV switched off! During the experiments described here, this problem was never observed.

The detector body can be isolated from the chamber to simulate the actual isolation from the optics bench in the instrument.

A voltage of up to 5 kV is applied across the MCP stack and anode gap. Turning the HV on to the MCPs must be a gradual process, particularly for the first time, in case of bursts of out gassing leading to HV break down. The rise time of the power supply is limited to longer than 2.5 s and the on-board software ramps the voltage up over 5 s at each commanded voltage step. For the first switch-on after exposure to atmosphere the voltage is raised in 500 V steps until the MCPs start to emit electrons. Then the voltage is raised in steps of 100 V (the flight power supplies have 20 V steps) with plenty of time (up to an hour) at each stage for outgassing. In the flight HV power supplies, monitors report the current and voltage measured. If the current monitor exceeds a pre­ determined value (selectable) the on-board software turns the power supply off to that detector. The voltage may then be re-applied more slowly with smaller steps in the hope that whatever caused the current trip can be removed by electron bombardment. Alternatively, the detector may just need more time to outgas. It is possible to have pressures of 10“^mbar inside the channels (see below) when the gauge on the vacuum chamber is reading less than 10“6 mbar.

If a laboratory power supply is being used a current monitor is needed in series with it. Whenever the MCPs are active, the chamber pressure (the ion gauge is turned off before voltage is applied), voltage, current and count rate are noted. Where possible, gain and PHD FWHM are measured.

From Holland, Steckelmacher and Yarwood (1974), the pressure inside a channel can be calculated approximately as follows:

Virtual le a k (0 = pressure(P) x pumping speed(t/) (67)

C/= 116 X aperture x W (68)

where W= l/ocxln(a), a being the length to diameter ratio of the channel. The channel aperture = 7cr2= 1 . 3 x 1 U = 6xlO~^^vn} s~^.

155

Q = outgassing rate of glass x surface area of channel (69) = 1x10“^x6x10”8 Pa m3 s“ ^

/. P = 1x10-4x6x10-8/6x10-10 = 0.01 Pa= 1x10-4mbar. (70) Thus, the pressure in the channels depends on the outgassing rate of the glass, which depends on the glass material and conditioning (baking and scrubbing), not on the chamber pressure.