Page 215 - Radiochemistry and nuclear chemistry

Detection amt Measurement Techniques 199

Because of the short duration of the absorption process for a single particle (10 -4 - 10 -9
s) the short current, i, is referred to as a pulse of charge AQ

i= AQIAt (8.3)

If this current passes through a resistor R it will produce a voltage pulse (of. (8.2))

AV = R AQIAt (8.4)

The pulse is usually referred to as the signal (for the preamplifier). While the signal from
semiconductor detectors is used as a charge pulse, most other detectors immediately convert
the current to a voltage drop over a resistor. In either case, the output from the preamplifier
is usually a voltage pulse. Preamplifiers are often integrated with the detector.
Pulse counting per se does not distinguish between different nuclear particles (c~,/3, "y,
etc.) or between particles of different energy. Such distinction is obtained by choosing
detectors of unique (or exceptionally high) sensitivity to the particles of interest. Energy
analysis, if desired, is achieved by the accompanying electronic circuitry because the pulse
charge or voltage (AQ or A V) is proportional to the energy of the absorbed particle.

8.2.1. Pulse generation

We shall use Figure 8.3 to describe the formation of a voltage pulse. A detector is
connected between points A and B. The detector has an internal resistance (because of the
limited charge carrier mobility in the detector) and capacitance (because of mechanical
construction), indicated by R i and C i. Figure 8.3(a) does not show the physical design of
the detector, but only its electrical equivalents; this will make it easier to understand its
function. When a particle enters the detector it produces charge carriers (this is symbolized
by the closing of switch S), and the collection of these at the electrodes gives a current
which, together with a small current from the bias supply through R e, flows through R i to
ground. R e is the resistance between the detector anode and the positive terminal of the bias
voltage supply (voltage + V0); the other terminal is grounded. We shall concentrate our
interest on the potential Vp at point P which is connected via the comparatively large
capacitance C e to the output. In general R e > R i (under conducting conditions), and C e ~,
Ci; for illustrative purposes we will assume R e = 50 kf/, R i = 10 kfl and C e + C i = 100
pF.
When S is open (no ionization in the detector), the potential at point P must be Vp = V 0,
i.e. the potential of the bias voltage. At time t = 0, S is closed (production of charge
carriers has occurred in the detector because of a nuclear particle), and the charge of C i +
C e flows through R i together with a small current from the bias supply through R e. The
potential in P decreases according to

Vp /V 0 = a (1 -e -bt) + e -bt (8.5a)

where a = R i I(R i +Re), b = (a R e C) -1 , and C = C i + C e ~. C e. For R e = 50 kfl, R i = 10
kO and C = 100 pF, a = 1/6 and b = 12 000 s -1. l/b is referred to as the time constant