Page 240 - Radiochemistry and nuclear chemistry

224 Radiochemistry and Nuclear Chemistry

is removed through a resistor -capacitor network, Rf and Cf in Figure 8.18(b), which is
part of the amplifier f~xlback loop. This arrangement is called a resistor feedback
preamplifier. The feeAback resistor adds its inherent Johnson noise to system noise;
typically 300 eV in FWHM with a resistor at room temperature. The contribution to the
total noise level is, at least for a cooled resistor and FET, negligible in normal "y-
spectrometry. However, when measuring low energy X-rays with a Si(Li) detector it may
be important to reduce the noise further.
In the pulsed opticalfeedbackpreamplifier, Figure 8.18(c), the resistor is omitted and the
input FET is permitted to charge-up in steps by each new pulse. The output signal is in the
form of a staircase function and this is transformed to voltage pulses by differentiating
circuits in the main amplifier. When the accumulated charge becomes too large (i.e. the
output voltage is near its highest possible value) the sensor circuit triggers and the FET is
irradiated with a short light pulse from the built-in light emitting diode thereby causing a
short circuit in the FET, removing the accumulated charge. The reset operation produces
a large voltage swing at the preamplifier output which may easily overload the main
amplifier and make it distort pulses rapidly following the reset pulse. In order to avoid
making measurements during and immediately after the big reset pulse a blocking signal is
often available from the preamplifier during the reset pulse. A typical pulsed optical
feedback preamplifier can, with a Si(Li) X-ray detector, give a FWHM of < 150 eV at 5.9
keV. The good resolution is important in low energy applications, e.g. X-ray
measurements. A minor drawback of pulsed optical feedback is the extra dead time
introduced by blocking measurements during each reset operation.
The output from a voltage sensitive or resistor feedback preamplifier is a tail pulse with
a rather long decay time. Hence, some pulse pile-up is unavoidable, except at very low
count rates. Pile-up will cause the average level of this signal to increase with pulse rate,
which may approach the limit of linear operation of the preamplifier.
8.7.2. Amplifiers

The purpose of an amplifier to amplify a voltage pulse in a linear fashion and to shape
the pulse so that the event can be analyzed easily and correctly in a short time. A linear
amplifier accepts tail pulses as input, usually of either polarity, and produces a shaped and
amplified pulse with standard polarity and amplitude span (NIM standard is positive polarity
and 0 - 10 V amplitude). On most commercial linear amplifiers, the time constants for the
various pulse shaping circuits are adjustable to fit various detector and count-rate
requirements.
Biased amplifiers shifts the zero of the amplified pulse down while still producing only
positive output pulses. This effect is important in e.g. a-spectrometry as the lower energy
range is normally of no interest and it is better to use the available output amplitude span
to enlarge the interesting energy region than to cover the whole energy range.
Amplifiers with a logarithmic response have use in liquid scintillation counting for
compression of the higher energy part of the B-spectrum.