Bibliography - School of Physics - University of Melbourne

136 CHAPTER 10. RUTHERFORD SCATTERING
10.4.2.1 Detector and electronics
The electronics of the detection system are given in the block diagram shown in figure 10.3.
Detector Pre−amp Amp SCA
Bias
Counter
Figure 10.3: Block diagram of the electronics. SCA = Single Channel Analyser. The scaler
and clock together form a counter, which is a single unit.
The detector is a solid-state Si detector and is, in essence, a type of miniature ionisation
chamber. The active volume is formed by the depletion region of a large area p-n junction.
It is shown schematically in figure 10.4. The detector is a p-n junction where the charged
particle enters the detector through the p-type layer. This layer is as thin as possible in order
to minimise the energy loss through the material from ionisation. A reverse bias voltage
applied as shown in figure 10.4 increases the volume of the depletion region.
A charged particle entering the depletion region creates electron-hole pairs (free charges)
which then move in the electric field induced by the positive and negative charges created
by the application of the bias voltage in the p- and n-type layers. The mean energy loss of a
charged particle required to produce one electron-hole pair in Si is 3.5 eV, and so 2.87 × 10 5
such pairs are produced on average per MeV energy deposited.
The effect of the total energy lost in the depletion layer is to produce a potential V dependent
on the total amount of charge collected in the capacitor: V = Q/C, where Q is the charge
collected. The capacitor recharges through a resistor after the discharge. A charge-sensitive
preamplifier produces a signal whose amplitude is proportional to the charge collected and
hence also to the energy of the detected charged particle.
p−type
n−type
incident
particle
+
+
+
+
+
+
+
+
+
+
−
−
−
−
−
−
−
−
−
−
EMF
depletion
layer
Figure 10.4: Detector and response as a function of time.