Fluke Biomedical 943-27 Detector - Manual (Page 9)

Theory of Operation
Functional Description
2
2-1
Section 2
Theory of Operation
2.1 Functional Description
The detector is comprised of a thin calcium fluoride scintillation disc located behind an ultra-thin (0.0002
in) Titanium end window. The end window permits the penetration of Beta rays as low as 80 Kev. The
calcium fluoride disc is optically coupled to the active cathode of a one-inch diameter photomultiplier tube
(PMT). A schematic representation of the detector is shown on drawing 943-27-32, located in Appendix
A. As shown in the schematic, the cathode of the PMT is tied to a negative high voltage through a ten
(10) megohm current limiting resistor, R1. The PMT dynodes are biased by the divider string formed by
resistors R2 through R13. The output signal is taken directly from the PMT anode, which is held at a
virtual ground potential by the input of the associated integrating electrometer.
2.2 Theory of Operation
Beta particles that have enough energy to penetrate the end window and impinge upon the disc will
produce light pulses, proportional to the energy deposited in the disc. The disc is thin so that gamma rays
will not have a high incidence of interaction with the disc. This yields a high rejection of gamma rays while
still possessing good sensitivity to beta particles.
A photomultiplier tube, optically coupled to the scintillation disc, detects visible light emitted from the disc
and converts this light to electrical energy that is proportional to the energy deposited by the beta particle.
The electrical charge is sent to an electrometer in an external preamplifier that converts the charge into a
digital value for display and processing.