W edge and Strip Anode Array

The wedge and strip anode array (Siegmund et al. 1988, Siegmund et al. 1986, Lampton et al. 1986, Schwarz and Lapington 1985) operates by the division o f incident charge between several electrodes. It consists of three electrodes (wedge, strip and zigzag conductors) that share the anode plane. The wedge pattern provides a charge capture that varies linearly with position in the Y direction while the interleaved strips have widths that increase linearly with X (Fig. 9.3).

W edge

Strip

F ig u re 9.3. Schem atic o f a three elem ent w edge and strip anode (from S ieg m u n d et al. 1986).

Since three electrodes share the total charge from each photoevent, x and y may be simultaneously determined for each photon by the ratios o f the pulse heights. Each o f the three electrodes is connected to a low noise charge sensitive amplifier whose output is passed though a bipolar shaping circuit. The amplitude o f the signals is then sampled by performing a peak and hold operation, and the resultant dc levels are converted to digital signals (Q^, Q$, and Q^) which are then used to calculate the events’ positions in software. Provided that the incident charge cloud is larger than the repetition period o f the anode the centroid position is given by;

a

& + 6 v + Qz

a

(9.5)

(9.6)

The spatial resolution o f this detector is limited by the charge partition noise and the charge am plifier’s noise. Since the charge amplifier’s noise contribution is sensitive to the anode’s capacitance, spatial resolution is determined by the size of the anode array, unless the M CP gain is increased or a mosaic o f anodes are used to compensate. A relatively large gap between the rear M C P and the anode is required (-1 5 mm) so that the charge cloud can spread out enough to cover several elements at the anode. This large gap can make the detector susceptible to electromagnetic fields affecting geometric and spatial linearity.

9.1.4 Spiral Anode

The spiral anode (SPA N ) is an enhanced derivative o f the wedge and strip anode array. It consists o f a planar structure o f six electrically isolated electrodes. The ratio o f the charges collected by the electrodes determines a two-dimensional position. Although this design has double the number o f electrodes (three electrodes per axis) o f the wedge and strip anode array, it has a marked spatial resolution advantage since the spatial resolution is an order of magnitude higher than the charge measurement accuracy (Lapington et al. 1991). For each direction, the three electrodes have a form whereby their amplitudes and wavelengths decrease in a sinusoidal fashion so that each position on that axis has a unique electrode fractional area ratio (Fig. 9.4a). The locus o f the co-ordinate representing the variation in the electrode fractional areas along the axis describes a curve in space which lies on a plane (Fig. 9.4b), because the sum o f the electrode fractional areas is equal to the width or pitch which is constant. This curve is an Archimedean spiral {r=kQ) which gradually spirals in towards the centre with decreasing amplitude and wavelength. The form o f the spiral is further constrained by keeping the rate o f change of arc length with respect to position (bs/bx) constant along the length o f the spiral so that the resolution will not vary along the axis. For two-dimensional implementation the pitches are interleaved and angled at 45° to each other. In deciding the parameters governing the pattern (pitchwidth, amplitude, wavelength and its rate of change) the size and shape o f the charge cloud must be taken into consideration. The wavelength must be long enough and the pitch small enough that the circular charge cloud will encode its position with the correct fractional areas defined by the measured charge values.

(a)

(b)

F ig u r e 9.4. T he SPAN detector, a) Schem atic o f the im p lem entation o f a tw o-dim ensional spiral anode. T he shaded pitches encode the y-axis and the others the x-axis. T h e box ben eath rep resen ts one pitch o f the spiral anode dem onstrating the unique fractional area ratio o f the electrodes (A, B and C) at each position along the axis, b) V ariation o f the electrodes along the p attern axis plotted in the 3-D volum e defined by the fractional electrode areas. T he arc length, along the spiral, gives the position along the axis. From Lapington et al. 1991.

These anodes are made from 60 mm diameter, 2 mm thick quartz blanks which are coated

with a very thin layer of chrome (for adhesion) followed by 2-3 |Lim layer of aluminium. A laser

and an X-Y co-ordinate table (with accuracy of 1 p,m) are used to remove narrow (10-20 |Lim)

lines of aluminium to form the insulating gaps between the electrodes, the final pitch ( 6

electrodes) being about 680 |im. Each electrode has its own electronics consisting of a charge sensitive preamplifier, a shaping amplifier and an analogue to digital converter. The charge collected on each electrode is measured and then read into a computer for image decoding and processing.