Energy Dependence and Directional Dependence
The following radiation sources were used to obtain the data presented in this section:
Co-60 (mean energy 1250 keV),
filtered X radiation according to the N series (»Narrow spectrum«) from ISO 4037-1.
The diagrams show typical curves, in practice slight deviations are normal and cannot be avoided.
All energy curves are normalized to the indication at Cs-137. Directional dependencies of H
normalized to the indication in preferential direction at the same energy. Directional dependencies of
H*(10) models are normalized to the indication in preferential direction at Cs-137. The reason for the
different normalization of directional dependencies is that the change from H
accompanied by a change of official requirements:
models, the energy dependence in preferential direction was required not to deviate by more
than ±30% referred to Cs-137. The directional dependence within a cone of ±45° around the
preferential direction was required not to deviate by more than ±20% referred to the same energy in
preferential direction. This means that requirements for energy dependence and directional
dependence were completely independent of each other. There was no consideration for the fact that
errors may compensate or add. For example, if an instrument had an error of -30% at some energy,
and at that energy additionally an error of -20% at some direction, it meets the requirements,
although the errors add to a total error of -44% (0.7
0.8 = 0.56 = 1.00 - 44%). On the other hand, an
instrument with an energy error of -30% and a directional error of +40% does not meet the
requirements, although both errors almost compensate leaving an error of only -2% (0.7
For H*(10) models, there is no separate but a combined requirement for energy and directional
dependence. It says that, for all energies
all directions within a cone of ±45° around the
preferential direction, the error must not exceed ±40% referred to indication at Cs-137 in preferential
direction. The new requirement has little to do with the new quantity H*(10) itself; it was introduced
together with H*(10) merely because it was regarded to be more realistic.
The range meeting the requirements is called »nominal energy range«. When using the instrument you
have to observe that energy is within that range. In other words, you need to know something about the
radiation to be measured:
If radiation energy is considerably lower than the beginning of the nominal energy range, the
instrument’s response will decrease considerably even down to »blindness«. You will then
underestimate the radiation field. This may particularly happen with low energy X radiation.
It depends on the type of instrument how it responds to energies above the end of the nominal energy
range. A common question is how an instrument will respond to the 6 MeV radiation of N-16 that
occurs in a nuclear facility. It is well-known that energy compensated GM tubes as used in the
6150AD will always overrespond at such high energies (up to three times the true value). Therefore,
although the 6150AD is not suited to measure such a high energy radiation field correctly, it will
overestimate the radiation and thus the radiation risk. In terms of radiation protection, it will be more
strict than it has to be.
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