Selection of a specific design of strip detector and associated readout electronics

The requirements of an ‘ideal’ silicon scatter detector for a Compton camera application were summarised in section 2.4.3, where it was stated that a similar performance would be achieved by either combining a pixel size of 1 mm with 2 keV FWHM energy resolution Or a pixel size of 300 pm with 6 keV FWHM energy resolution. Such general results are now applied to the specific case of a strip detector design.

In silicon strip detectors, the energy resolution is dominated by the electronic noise, which can be expressed as a function of various components, as will be discussed in more detail in chapter four. The most important components of the noise are associated with the detector leakage current (typically in the order of 5-10 nA per strip) and the VLSI readout electronics. The noise of an integrated preamplifier may be expressed as the standard deviation (cr) of the noise spectrum in keV and as a function of the input capacitance (see chapter four, section 4.4). Assuming 10 pF of input capacitance, examples of noise (cr values) of integrated preamplifier channels are 0.8 keV (Viking chip [Toker, 1994]), 2.3 keV (APV6 chip [Raymond, 1997]) and 4.2 keV (MX3 chip [Schwarz, 1994]). One may therefore assume that a FWHM energy resolution value between 5 and 7 keV (at 100 keV energy deposition) could be achieved by selecting good quality silicon wafers and by making use of a low-noise chip such as the APV6, which was employed in this project. As the APV6 had been designed for use with AC coupled detectors, this type of coupling became an additional requirement on the detector characteristics for this project.

In a strip detector, the requirements on ‘pixel size’ have to be translated in terms of

the strip pitch, which is the characteristic distance that separates adjacent implants. If

one assumes that the hit position is given by the strip with the highest signal, the FWHM spatial resolution of a strip detector has an inherent geometrical limit, which can be expressed in terms of the pitch p as follows [Weilhammer, 1994]:

Chapter 2 Design and Selection o f a Compton Scatter D etector fo r Positron Emitter Imaging

^FWHM

However, if the strip pitch is comparable to or smaller than the width of the charge packet created by the recoil electron at interaction, better precision can be obtained by applying centre of gravity (COG) algorithms (the limit on the achievable resolution is in this case dependent on the signal to noise ratio). The current limitation on the readout pitch of the existing front-end electronics is 50 pm. If sampling o f narrow distributions is required, the detector may be designed with a strip pitch that is smaller than the readout pitch (i.e. the separation between the strips that are actually wire-bonded to the electronics). In such applications, intermediate strips are left floating, with the pre-amplifier channels connected every «-th strip; the principle of capacitive charge division is employed for the detector read-out [England et al.,

1981]. Typical values of spatial precision obtained in HEP experiments are in the order of 5 -10 pm, considering detectors with a strip pitch < 50 pm and a readout pitch ^ 1 5 0 pm [Damerell, 1995].

A Monte Carlo study was carried out in order to determine the profile o f the recoil electron range following Compton scatter of 511 keV photons in silicon (see Fig. 2.26). It was shown that the FWHM value of the spatial distribution is approximately 280 pm. Although this estimate does not account for diffusion effects and for the presence of an electric field, it suggests the need for a relatively large strip pitch (in the order of a few hundred pm) so as to prevent the produced charge being shared by a large number of strips, resulting in a poor signal/noise ratio. On the other hand, charge sharing may improve spatial resolution by applying the COG position finding method to two or three strips. It was concluded that the optimum strip pitch was approximately 500 pm. According to equation (2.13), such a value would provide a FWHM spatial resolution of 340 pm or better, thus fulfilling the position sensitivity requirements established for the Compton scatter detector.

-o_0> D Û0 ■ 0.8-1 □ 0.6-0.8 □ 0.4-0.6 ■ 0.2-0.4 □ 0-0.2 X (rnm) y (mm)

Fig. 2.26: Monte Carlo simulation o f the spatial distribution o f the energy deposited in silicon by the recoil electrons, following Compton scatter o f 511 keV photons.

In section 2.4.3 it was also stated that the desirable thickness of the scatter detector was 1 mm, in order to maximise single Compton efficiency. Satisfying such a requirement posed some difficulties; the standard silicon wafers used in detector manufacture are only 300 pm thick, since they are usually designed for the detection of charged particles or low-energy X-rays. A compromise solution was found in the choice of the IPS 60x60-500 NX-NY128 double-sided silicon microstrip detector, which is manufactured by the French company Eurisys^^. The detector is AC coupled, punch-through biased, with a sensitive area of 6 x 6 cm^, a thickness of 500 pm and a strip pitch of 470 pm in both planes (128 channels/side). A detailed description of the strip configuration and the technical specifications of the detector is given in the next chapter. The detector area, strip pitch and type of coupling of this detector design were a perfect match of the required characteristics, which were selected on the basis of the computer model. However, the detector thickness was half the value stated as ‘ideal’, causing the absolute efficiency of single Compton scatter at 511 keV to

Chapter 2 Design and Selection o f a Compton Scatter Detector fo r Positron Emitter Imaging