7.
Some more theory
In nuclear physics radioactive radiation sources are called RADIONUCLIDES. The radiation
energy is measured in mega-electronvolt (MeV) or kilo-electronvolt (keV):
MEGA = 1,000,000 = 10
6
or KILO = 1,000 = 10
3
Where this radiation arrives it is measured in sievert (Sv) or rem, where
100 rem = 1 Sv or 1 rem = 0.01 Sv
0.1 rem = 1 mSv or 0.1 mrem = 1
µ
Sv
Normal background radiation:
120 mrem/a = 1.2 mSv/a = 0.015 mrem/h = 0.15
µ
Sv/h
In principle it can be said that the counting e
ffi
ciency of a measurement rises with the
sensitivity of a Geiger-Müller counter tube. But that always applies only to one specific
RADIONUCLIDE or its radiation energy. The penetration capacity (range) of radiation can be
derived from the radiation energy. Whether the radiation can be detected by a Geiger counter
and can therefore be measured depends on the radiation energy of the RADIONUCLIDE and the
transparency/sensitivity of the counter.
The radiation energy of a NUCLIDE has nothing to do with its activity (decay per second), which
is measured in becquerel (Bq). This also applies to the detection limit (DL) which refers to the
minimum activity (Bq) of the radiation source required to allow its measurement. The radiation
energy (keV) and its activity (Bq) are two di
ff
erent factors which, together with the type of
radiation (ALPHA, BETA and GAMMA radiation) cause the radiation exposure.
Dosimeters (energy dose) are designed for measuring GAMMA radiation. These show the
radiation in sievert (Sv) or rem. Contamination measuring instruments must be much more
sensitive. They must allow measurement of BETA and possibly also ALPHA radiation.
The specifications of radiation detection tubes also always state the radiation energy required
so the tube can detect the radiation (quality characteristic). The end window tubes A and G can
detect
e.g.
ALPHA radiation from 1.9 MeV
BETA radiation from 0.09 MeV
and
GAMMA radiation from 0.01 MeV
Immersion probes B and FSZ can detect no ALPHA radiation and BETA radiation from 0.2 MeV
and GAMMA radiation from 0.02 MeV.
The immersion probes can compensate for this drawback by the geometry factor. In
immersion the surface of the probe receiving radiation is larger than in surface
measurements. In surface measurements the counter tube absorbs the radiation from
one side only and even the smallest distance results in scatter losses.
OPERATING INSTRUCTIONS
for Counter Tube Type G
JS Monday 10 January 2011 Page 7 of 9