Deadtime in Geiger Mueller Tubes >Tutorial by
Geo
Inside any discharge tube is a fill
gas that ionizes when it encounters a charge that exceeds its breakover
potential.
Some examples of discharge tubes are neon lamps, mercury vapor
lamps, microwave T-R switches, Voltage regulators, and Geiger Tubes. In all
cases,
the "on" resistance* is very low compared to the "off" resistance*,
and external means are provided to limit the total current drawn by the
device,
as it is essentially a short circuit when the gas is ionized.
For
most of the glow tube applications, discharge, once established, is maintained
as a part of the tube's function. To extinguish the discharge,
the supply
Voltage is reduced significantly, or removed.
Consider now the a tube
which is charged to a steady DC Voltage somewhat below it's ionization
potential. Any extra energy added to the gas will cause it to ionize totally.
This energy can be in the form of RF, light, electricity, x-ray, or a charged
subatomic particle. Indeed, RF "sniffers" have been made
that rely on this
principle.
Since the extinguishing Voltage level is much lower than the
ionizing Voltage level, a tube once ionized will remain so until the steady
state
Voltage is reduced or removed.
When a GM (Geiger-Muller) tube is
used in a Geiger Counter, a series of pulses is desired instead of a single
pulse. Removing the Voltage and
reapplying it would be difficult and
cumbersome**, so another gas, called a quench gas is introduced into the fill
gas. The purpose of this addition is
to chemically extinguish the ionization
after a short period of time, rendering the tube ready to respond to the next
charged particle. In
practice this approach works well, but takes a finite
amount of time to complete each cycle.
During the time in which the tube is
ionized by the last radiation particle, it is of course not available to
register any subsequent particles that may
arrive, until the quench cycle is
complete.
Quench cycle time delay is short but measurable and very
meaningful, as it gives rise to two important parameters that deleteriously
affect all GM tubes, namely "SATURATION" and "DEADTIME". Both are
easy to understand in theory but somehow are just as easily overlooked in
practice.
All GM (Geiger-Mueller) tubes need a
certain time to refresh themselves after a pulse, before they can be ready for
another pulse. The period of time after a pulse but before the tube can create
another pulse is called DEADTIME.
Even though this time is short, on
the order of 20 microseconds for the pancake tube, longer for metal tubes, when
the pulses come fast and furious there reaches a point where the tube simply
can't respond any more. Modern instruments recognize this situation and respond
with an alert of some kind. Some sound an alarm; most send the meter to beyond
full scale. Without this protective action, a meter can actually read zero in a
very high radiation field. Such a condition is called SATURATION.
When in saturation, a tube cannot
indicate radiation any more and presents a short circuit to the
electronics. CDV-700's have no such protection circuit so will go dead in
a high radiation field.
Way before saturation, a GM tube
will start to suffer effects of deadtime. Simply stated, doubling the
actual radiation field will not yield twice the count rate, but less than twice.
The higher the count rate, the less will be the increase, up to a point where
there will be no further increase in a higher field.
First let's examine
SATURATION. It should be obvious if a heavy stream if radiation is
encountered, the particles will arrive faster than the tube has
a chance to
recycle itself. Such a condition results in continuous discharge, and in essence
the tube goes dead. A saturated GM tube will seem
to be reporting low or even
zero radiation, even though the actual radiation is dangerously high. A serious
and possibly life threatening situation for
workers in areas where such high
fields might be encountered, as in nuclear power generation and nuclear medicine
applications. In order to at least
recognize the onset of saturation, most
modern Geiger Counters are equipped with a "saturation alarm" which is an
electronic system that recognizes the
effects of over saturation and alerts
the operator that reading are inaccurate at that point. Some manufactures
include an audible alarm, but
almost all will cause the meter reading to go
full-off-scale. Photo Multiplier Tubes (PMT) used in scintillators have no such
limitations, and can report many times more pulses than a GM tube, depending
upon the crystal in use. Plastic scintillation material, although only about 15%
as effective as NaI, does
have a much faster (narrower) pulse than NaI (Tl),
into the nanoseconds. When a scintillator is used with a Geiger Counter that
incorporates a
saturation alarm, the alarm must be disabled. Some makers
include a switch for the purpose, in some models there is no way to easily
disable the
circuit, so essentially hey are made for GM use only. We have
overcome this deficiency in some units, and published some easy mods on the
Web.
Next
we'll investigate DEADTIME.
The action of recycling a GM tube takes a
certain amount of time as we have discovered. In a typical pancake tube that
time is 20 microseconds, some
end-window tubes run 90-150 microseconds, and
metal hot-dog tubes are typically 100 microseconds.
Using the pancake’s
figure of 20 uS, you might expect that the tube could produce 1,000,000/20=
50,000 CPS or
50,000 CPS x 60S= 3,000,000 CPM.
In practice I have found the
max pulse rate to run far short of the calculated, by experimenting with 4
similar Alpha sources, each capable of
giving 150,000 CPM (on a pancake). Two
such sources used together yield 300,000 CPM as expected. Increasing the number
of sources to three only
increases the count to 400,000, not the 450,000
expected. Adding the fourth source does not increase the count beyond 400,000
CPM. This indicated to me
that deadtime has come into play. Even with 4
sources, we still have not saturated the tube however.
