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
||Dr C N Booth
As a result, a much greater density of free carriers is produced in a semiconductor.
much greater density than for a gas
reduced ionisation energy (of the order of 1 eV to produce an electron-hole
pair, compared with 5 eV to ionise a solid insulator and 30 eV to ionise
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
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.
up to 40 by 9 mm in large detectors (useful for ionisation
measurements for particle identification, but providing limited position
resolution). Detectors with very small pads are sometimes known as
more commonly, many narrow strips - e.g. 50 microns across, are used in
providing very good position resolution. (Such detectors are frequently
used close to the interaction point of an experiment to act as vertex
detectors, able to identify the decay of short-lived particles.)
Two other potential problems with semiconductor detectors are susceptibility
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
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
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.
Advantages of Silicon Microstrips
Small energy per pair (~1 eV c.f. 30 eV in gas) and higher density
result in a large signal
No need for avalanche multiplication
Easy to produce very narrow strips (< 50 microns) leading to very
precise position information
Disadvantages of Silicon Microstrips
Silicon wafer size, and need for a connection per strip to a pre-amplifier,
mean that only small detectors are possible (a few cm2 compared
with m2 for gaseous detectors)
The presence of more material results in scattering and interaction of
the particles to be detected
Silicon suffers from radiation damage in high intensity environments
Detectors may need cooling to reduce noise
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