PHY6040 Particle Detectors Dr C N Booth

Semiconductor Particle Detectors

We saw that in gaseous detectors, the passage of a charged particle left a trail of free electrons and positive ions.  A similar effect occurs when a charged particle passes through a sample of semiconductor, except that in this case it is electron-hole pairs which result.  Two advantages of semiconductor as a detecting medium are As a result, a much greater density of free carriers is produced in a semiconductor.

Silicon Hodoscope

A reverse biased p-n junction, with sufficient bias voltage to cause complete depletion of the carriers, forms a semiconductor ionisation chamber.  For example, if a p-type silicon layer is formed on top of an n-type wafer substrate, narrow insulating tracks of silicon dioxide can be used to separate regions of the p-type layer into pads, each pad behaving as an independent detector.  The passage of a charged particle creates about 20,000 e-hole pairs, and the holes drift to the negative p-type pad, giving an observable signal.

Various pad geometries have been exploited

The fact that each strip in a strip detector needs an individual connection to a preamplifier and readout electronics, together with the maximum size of semiconductor wafers, limit the size of silicon strip detectors.

Two other potential problems with semiconductor detectors are susceptibility to radiation damage in a high intensity environment and the need, under some circumstances, to cool the detector to reduce thermal noise.

Charge-Coupled Devices (CCDs)

Charge-Coupled devices are more commonly used as optical detectors (in solid state cameras and video recorders) but can also be used as particle detectors.  They typically consist of a rectangular array of some 30,000 to 200,000 potential wells (pixels) about 30 microns square covering the surface of the semiconductor.  The potential wells are maintained by a set of phased electrodes insulated from the bulk of the semiconductor.  When a charged particle passes through the detector, the charge is trapped in one of the wells.  Subsequently, if the electrode potentials are "clocked" in an appropriate manner, the entire image of charges is moved coherently across the detector.  One side of the detector is adapted to form a shift-register which can be clocked to move the charge in a perpendicular direction.  The readout sequence thus consists of moving the image across by one column, then reading a column out pixel by pixel from the shift-register port, before clocking the image across another column, etc.  In this way, the 100,000 or so channels of the whole CCD can be read out through a single preamplifier and readout channel.  The price paid for this is that the readout is very slow (of the order of milliseconds), with the device integrating the signal over a long period.  Typically, the particle source must be vetoed during this period, and the detector cooled with liquid nitrogen to reduce thermal noise.  CCDs have very good spatial resolution, typically of the order of 5 microns in each direction.

Click here to view an animations of charge transfer in a CCD.


Silicon Drift Chamber

In analogy with gaseous drift chambers, a semiconductor device can also be made where the position resolution in one dimension is provided by the drift time of the ionisation.  The silicon drift chamber is a form of strip detector, with readout at one end of each strip.  A "gutter" potential causes electrons (or holes) to drift parallel to the strip, to the readout point (a distance of 1 to 2 cm).  The drift time then provides the co-ordinate in the direction parallel to the strip direction.  This technique also provides a resolution of about 5 microns.

Comparison of Silicon Microstrip Detectors and Multiwire Proportional Chambers

Advantages of Silicon Microstrips

Disadvantages of Silicon Microstrips

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