Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 4. SOLID STATE DETECTOR THEORY 19
charged particle track
E
+
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+
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+
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+
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+
D
Figure 4.3: A charged particle traversing the detector generates electron-hole pairs along its
track.
N C and N V
are the weights of conduction and valence bands, E g is the band gap, k the
Boltzmann constant, T the absolute temperature, h is the Planck constant, m e and m h
are the eective masses of electrons and holes, respectively. The intrinsic carrier density
strongly depends on the band gap and the temperature. Materials with a low band gap,
implying a large number of intrinsic carriers, need either cooling down to temperatures
where the carriers are no longer excited or a reverse-biased pn-junction, which results in
a space charge zone free of carriers.
Initially, all free carriers inside the bulk are drained by the applied electric eld. There
is no charge movement in the bulk, except for thermally excited electron-hole pairs, which
immediately drift to the electrodes.
When a charged particle traverses the detector, electron-hole pairs are created along
the particle track. In the case of a MIP perpendicularly traversing a detector of thickness
D, the number of generated pairs is Q p = q p D. The electrons move towards the positive
electrode, while the holes drift in the opposite direction. As these carriers move, a charge
is induced at the electrodes, which can be observed by acharge-sensitive amplier, or, in
the case of high particle rates, measured as a DC current in the bias line. It is irrelevant
whether the generated charges nally reach the electrodes or not, only the length of their
path contributes to the (integral) signal. Especially when trapping or recombination
occurs (as in CVD diamond), many charges do not reach the electrodes.
Seen from the point ofa subsequent amplier, the detector is electrically represented
by a (pulse) current source in parallel to a capacitance (g. 4.4).
i(t)
C
Figure 4.4: The electric representation of a detector, a current source in parallel to a capacitance.