Detector tubes do not detect Gamma rays directly very well.

 

First the Gamma Ray must interact with an atom by one of several methods outlined in another

article ( Compton scattering, Photoelectric, Pair Production).

The secondary charged particles created in those reactions can easily be detected by the GM tube and this is the vast majority

of the detection methodology.

Since this critical interaction takes place in the wall material of the tube, it is obvious that the secondary charged particle must be created close

to the inner layer of atoms so that it may escape into the gas volume for detection. Charged particles created deeper inside the wall

will simply be absorbed.

 

 Keeping that in mind, we now consider energy response.

Different tubes will
respond to varying energy levels according mainly to the construction
materials used, and volume of fill gas ( size of tube). In general,
low energy Gammas must be of sufficient strength ( meaning energy
level, not number of disintegrations) to penetrate the housing
material. The Z number is used
to indicate density of any material, based on atomic makeup.Once
inside the tube, lower energy Gammas are much more likely to cause
an interaction at the surface layer of the wall, and therefore be counted. At some point as the
energy level increases, the ray will simply pass into the wall, and eventually out of the tube and
not be counted. These facts account for the whipsaw shape of the
energy response curves of all GM tubes. External filtering may be
applied to compensate for this non linear effect, resulting in a
probe that is called" energy compensated". Be aware that this
procedure knocks all the response down to the lowest level, and that
although now nearly perfectly linear, such a probe will give lower
reading than you may be used to from the more common "energy
dependant" probes.

Unfortunately most all GM and scintillation detectors are "energy
dependant". Basically this means the number of pulses the probe will give
off depends on the energy level of the radiation causing the pulse. To add
to the confusion, the curve is not a straight line, far from it.

Ludlum probes are usually specified in " counts per minute per m/R ( or uR)
of CS-137".

For example the 44-2, probably the most practical Gamma scintillator for the
home lab enthusiasts have an output of 175 CPM per 1 uR/H.

Another very useful and common probe is the 44-6 and it's a GM probe with
2100 CPM per mR/H. Many meter units are scaled to read this probe in mR
directly.

With that known, you can go around accurately measuring Cs-137 all day long,
but other isotopes will read out differently. Fortunately the manufacturers
are making the energy response charts available, and I've posted a few as
PDF files in the FILES section. Armed with the Cs-137 figures, and one of
these charts, you can go about figuring what levels are actually present, as
long as the isotope is known.

If all this seems overly complicated, I agree, but it's part of
understanding what we are doing.

A more direct approach would be to only use "energy compensated" probes,
like the Ludlum 44-38 or Eberline HP-270. These units straighten out the
curve so the probe responds equally to all energy levels. There are of
course limitations and restrictions imposed by such probes, so most of us
will keep using pancakes and hot-dogs for the majority of our work.

The main thing, as always, is to be aware of the quirks, shortcoming as
peculiarities of our instruments.

Lastly I want to add once again that the probe does all the work. Our meter
units no matter how complicated, or simple they may be, merely take the
pulses from the probes and display them for out interpretation.*

*Update on this last paragraph, modern meters are doing wonders with processing

probe data, using software to analyze the speed at which the counts come in and making calculations to

surmise facts not obvious to the casual observer. Techniques like "Time to Count", pulsing the High Voltage,

Deadtime Correction, Clock Time/ Real Time analysis etc. are making it into hand held equipment now, a direct result of 9/11.


Have Fun

Geo