Biomedical Engineering Reference
In-Depth Information
fluctuations from pure Poisson statistics. It is defined as the ratio of the observed
variance and the variance predicted by the latter:
Observed variance
Poisson variance
F
=
.
(11.90)
Reported values of Fano factors for gas proportional counters are in the range from
about 0.1 to 0.2 and, for semiconductors, from 0.06 to 0.15. For scintillation detec-
tors,
F
is near unity, indicating a Poisson-limited resolution.
Dead Time
An instrument that responds sequentially to individual events requires a certain
minimum time to recover from one event before it is ready to respond to the next.
This recovery interval, called the
dead time
, can be due to physical processes in the
detector and to instrument electronics. When a radioactive sample is counted, there
is a possibility that two interactions in the detector will occur too close together in
time to be registered as separate events.
Two models have been proposed to approximate the dead-time behavior of coun-
ters. Following a count, a
paralyzable detector
is unable to provide a second response
until a certain dead time
τ
has passed without another event occurring. Another
event during
τ
causes the insensitive period to be restarted. A
nonparalyzable detec-
tor
, on the other hand, simply ignores other events if they occur during
τ
. Differ-
ences in the two models are illustrated in Fig. 11.7.
The top line represents seven events as they occur along the horizontal time axis,
and the two axes below show the responses of the two types of detector. Events 1
Fig. 11.7
Illustration of counts registered by paralyzable and
nonparalyzable models with dead time
τ
. See text.
