The Wedge and Strip anode

Anodes of this type use a completely different technique in order to determine position by charge division. The charge cloud from the microchannel plates is spread over a pattern of interleaving electrodes. The pattern is such that the position of the charge centroid can be computed from the ratios of the pulses measured on each electrode.

The Wedge and Strip is the most well known configuration of this class of anode, but other geometries are possible. The anode consists of three electrodes termed wedge, strip and Z. The first consists of a row of identical triangular wedges aligned with the Y-axis of the anode. A row of strips is interleaved between the wedges. Unlike the wedges, the strips decrease in width across the X-axis of the anode. The Z electrode simply fills in the space between the wedges and strips.

Prototype detectors have been made in collaboration with Instrument Technology Ltd, The theory of the anode, originally designed by H.O. Anger, is described by Martin et al (1981). It is an ingenious device, and is illustrated in figure 4,2.

The positional algorithms are given by: 2S X = [4,12] W+S+Z 2W Y = --- W+S+Z

These are realised in exactly the same way as the resistive anode algorithms, and the same Signal Processing Unit is used, with slight modifications to realise the new algorithm.

The advantages of the Wedge and Strip anode are generally quoted as: 1. It lacks the large inherent current noise of the resistive anode, 2. It can easily be made in large sizes,

3. It is inherently linear.

It is clear from figure 4,2 that to ensure accurate charge division the electron cloud from the MCP stack must be spread out over a sufficiently wide area so that it covers at least one full cycle of the pattern. In order to do this an electron drift region of about 15mm is provided between the MCP stack and the anode. The potential

gradient across this gap must be uniform. This is achieved by providing a series of conducting rings around the perimeter of the gap, held at intermediate potentials. Although the anode is claimed to be distortion free, problems in the electron drift area can introduce distortions, as demonstrated by Siegmund et al (1983).

The dominant sources of noise are amplifier voltage noise and partition noise. The amplifier noise is dependent on input capacitance, which is much greater than with the resistive anode. This is largely caused by inter-electrode capacitance and can be very high as the Z electrode weaves its way through the wedges and strips. The length of the Z electrode means that it will also have a finite resistance and therefore an additional noise contribution.

The amplifier noise is given by: 3kt

+ 4KTR.

^FET [4.13]

where g is the transconductance of the input FET of the amplifier. and are the capacitances due to the anode and the FET. is the stray input capacitance and R is the resistance of the Z electrode. It was seen in paragraph 4.8 that image perturbation due to the noise generated by a resistive anode is independent of position, and so the resolution is the same over the full image area. This is because the dominant noise components are anti-correlated in the denominator of the algorithm, and therefore cancel out. However, this is not the case for the Wedge and Strip anode and it has been shown (Martin et al, 1981) that at the edge of the image the perturbation due to noise is twice that in the centre.

electrons between the electrodes, and is given by Allington-Smith and Schwartz (1984) as: R = 2.36L p ( f - f . J 7 Q [4.14]

where is the FWHM resolution component due to the partition noise and L is the length of the sides of the anode. Q is the total charge per event and f is the fraction of charge collected by the electrode, which lies between the limits f , and f

m i n m a x

The devices made for UCL were not particularly successful and the imaging performance was very poor. This was attributed to manufacturing problems in the anode. It is clear from the literature that excellent performance can be obtained once the necessary expertise has been accrued. However, a demountable vacuum system is required for such experimentation and the lack of success with the two sealed devices made by ITL meant that further investigation of this technique was liable to be prohibitively expensive. These devices were, therefore, forsaken in favour of the resistive anode.

Resistive anodes have been harshly dismissed by some in favour of Wedge and Strip anodes. (Allington-Smith and Schwartz, 1984; Schwartz and Lapington, 1985). However, their comparisons of the imaging performance were made with the outmoded rise-time method of resistive anode decoding, which has several limitations. In particular, the repeated references to the temperature dependence of the resistive anode simply do not apply when the charge ratio method is employed. Also, the resistive anode charge noise is much less significant when used with state of the art high-gain microchannel plate stacks.

Resistive anodes have been found to be both reliable and repeatable, and for many practical purposes just as effective as Wedge and Strip anodes in terms of image quality, speed and linearity.

Figure 4.1

The C irc ul a r Arc T e r m i n a t e d Anode

B order re sisto r Ri_ / u n i t le n g th A Resistivity R, D Radius

a

Figure 4.2

The Wedge and Strip Anode