Geiger Counters
Do you
need advice on how to select the best Geiger tube for your application?
How do I select a Geiger counter for a particular application?
The type of Geiger counter to be purchased will depend on the
radiation to be detected. The application will involve the detection
of either Alpha, beta, gamma or X radiation.
For the detection of Alpha particles a thin end window Geiger
tube is required like the ZP1401. This type of radiation is stopped
very easily so the window should be no more than 1.5mg/cm2in
thickness, other wise the detection efficiency will be too low.
Beta particles also need an end window tube which can be 2/3mg/cm2.
The exception is high energy betas above 2.5MeV which can be
detected by thin side wall tubes. However the thickness of the
metal wall of the tube should ideally be no more than 30/40mg/cm2.
Gamma rays are high energy rays so the tube thickness is not
important.
All tubes will detect gamma radiation. The two main gamma ray
emission
energies are 633KeV (Cs137) and 1.33MeV (Co60).
X-Rays are low energy gamma rays and again end window tubes are
normally used as the typical X-Ray emission energy is around
20-30keV. You can also use an X-Ray proportional counter like
the ZP1600 series.
How does the radiation dose level affect my selection?
As a rule of thumb the higher the dose rate level the less volume
you need inside the Geiger counter tube. The larger the volume
of gas in the tube the more sensitive it will be and hence as
the radiation increases the quicker the Geiger will go into saturation.
Once the Geiger saturates it becomes non linear and is no longer
useful as a measuring device, so it is always important to consider
very carefully the dose rate range over which you need to use
the device to always ensure it is in the unsaturated state no
matter what the dose rate. Very high dose rates will require
a small volume tube like the ZP1300 for example.
If I
am trying to detect the radiation from various isotopes how can
I compensate for the different energy levels?
This can be done by a method called "energy compensation"
of the Geiger.
A shield normally made of plastic and metal can be placed around
the tube to flatten the response of the Geiger tube over a wide
energy band. Energy compensation is a technique used on many
small tubes used in hand held pocket dosimeters for example so
irrespective of the isotope being used the reading can be correct
over a guaranteed energy range to within a specified tolerance
( +/- 15% is typical).
How
can I power my Geiger tube?
Most current day Geiger tubes have an halogen quenching agent
for the gas and this means the typical operating voltage of the
Geiger will be around 400 -600 volts. Because of the low current
consumption the tubes actually require very little power.
Typical power supplies are small high voltage blocks although
they can also be easily driven from fairly basic laboratory power
supplies.
Are there any special requirements in mounting Geiger tubes?
Most Geiger tubes have two connections only one for cathode and
one for the anode so the electrical connection is very simple.
However when mounting the Geiger it is important not to let the
Geiger connection become too hot ( i.e.: don't linger with the
soldering iron for too long) as many of the tubes have fragile
glass to metal seals which can crack if heat is applied for any
length of time. It is often a good idea to sit the tube on a
soft material particular if they are used in portable equipment
as this will help to absorb any shock and vibration in transit
and also take stress off the connections.
Why is the glass components of all Geiger tubes painted black?
The reason is to prevent the Geiger tubes from being light sensitive.
The sun is an excellent radiation emitter and as such if a Geiger
is used externally as a probe for example with no case protection
the Geiger would be light sensitive if the glass was not paint
black. This is not an important issue if the Geiger is fully
enclosed in a light tight box. In these applications unpainted
Geiger's can be used.
What
are the typical applications for Geiger tubes?
The main application areas are as follows:-
Radiation detection equipment
for nuclear power installations or military or civil use.
Industrial non contact gauging
applications. (Liquid level and density gauges).
Educational aids for the teaching
of radiation and its detection.
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This article
has been kindly supplied by: ALRAD
