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/H (or uR/H) of CS-137".

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

Another very useful and common probe is the 44-6, a GM probe with 2100 CPM per

mR/H. Considered by all to be the modern version of the CDV-700 hot dog probe with improvements.

Many meter units are scaled to read this probe in  mR/H 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.

.

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. One huge disadvantage is that to accomplish a

smooth response, all the probe can do is REDUCE ALL radiations down to the lowest level of any radiation. What

this does in practice is greatly reduce the reading given by low energy radiations. Mostly of the time, you would prefer

the larger readings, even though they may not be technically accurate. If for some reason you must absolutely have an

accurate quantification of a wide range of energies, then the energy compensated GM probe is the least expensive way to accomplish that.

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