Fluke Biomedical 943-25 - Manual (Page 11)

Theory of Operation
Scintillator and Photomultiplier Tube
3
3-1
Section 3
Theory of Operation
3.1 Scintillator and Photomultiplier Tube
When beta particles enter the scintillation disc, pulses of light are emitted. The light pulses striking the
photocathode of the photomultiplier tube excite the electrons in the cathode to a high-energy state
causing them to escape from the surface of the cathode. The freed electrons are attracted by a voltage
potential to the first dynode of the photomultiplier tube. This starts a cascading effect where a charge is
passed from dynode to dynode, increasing in size at each stage until a shower of electrons is passed on
to the preamplifier.
3.2 Preamplifier
The schematic diagram for the preamplifier is located in Appendix A. The preamplifier provides
amplification
of the output from the photomultiplier tube and cable driving capabilities. Negative pulses
derived from the photomultiplier tube are applied to the input of the preamplifier. Operational amplifier Z1
is configured as a non-inverting amplifier with a gain of approximately six (ratio of R6 + R7/R6). The
amplified pulses are coupled to transistors Q1 and Q2.
Resistor R11 allows impedance matching of the preamplifier to the 50-ohm transmission line connected to
the readout. The non-inverting input of Z1 (pin 3) is biased at +10 VDC potential. This enables the
operational amplifier, in conjunction with transistor Q1 and Q2, to drive 1500 feet of cable and produce a
6 V pulse at the readout
.