The response of most detectors, ionisation chambers included, shows a
dependence on
the energy as well as the intensity of the radiation
they are measuring. The amount of radiation absorbed in tissue is also energy
dependent. Fig. 
shows the number of gamma photons of different energy which have to strike a
area of tissue each second to produce
a dose rate of 
.
When an ionisation chamber is used in an instrument to make dose or dose rate measurements its response to different energies of radiation will not be the same as the response of tissue. This is hardly surprising since ionisation chambers are made of metal, gas, plastics and other materials.
Ionisation chambers can be partially compensated
for this by suitable shielding around the tube and appropriate tube wall
design. The corrected detector will be reasonably tissue equivalent
in a restricted energy range. Fig.
shows the typical response of the corrected detector.
For example, a practical gamma survey meter
will read within 
25%
of the true dose, for gamma energies from about 50 keV to 3 MeV. Below 50 keV
the shielding effect of the tube walls eliminates the response to low-energy
photons. Above 3 MeV the reading will be high and therefore over-estimate the
risk.
Figure: Energy dependence of
detectors .