Geiger Mueller detectors are used in the system along with a gamma filter
to select the desired energy spectrum. The filter limits the low energy
response and the detector design limits the upper energy, which can be
detected. The available energy at the filter is a function of the initial energy
of the gamma radiation from the source and the path length through the
material.
The count rate at the detectors is ratioed to a standard set of conditions
in order to eliminate drift of the system and the effect of aging of the
radioactive material over long periods of time.
This table lists the mass attenuation coefficients for suggested calibration
materials covering the possible range of photon energy. The values are
calculated from data included in "Gamma Cross Sections, Attenuation
Coefficients, and Energy Absorption Coefficients from 10 keV to 100 GeV"
published by NIST.
After using this data to correct the metallic materials, the experimental
count rates will give an equation, which is still not applicable for construction
materials. Assuming that most construction materials will have a composition
between limestone and granite, the metallic values of A and C can be used
to calculate a value of B which applies to these materials or other values of B
can be determined for any material.
Experimental data must be used and not the values from the above table.
The initial gamma energy is known to be 0.662 MeV but the average energy
for the interactions would be impossible to determine. Gamma filters are
used with the detectors to limit the lower energy in order to reduce the
errors due to chemical composition.
Using energy discriminating detectors, the lowest possible chemical error
for limestone and granite is ± 0.4%. With Geiger Mueller detectors and
mechanical filters, the practical limits are about 2% for backscatter modes
and 1.5% for direct transmission.
Material
0.10
0.15
0.20
0.30
0.40
0.50
0.60
Magnesium
0.1610
0.1360
0.1220
0.1060
0.0944
0.0861
0.0796
Magn./Alum
0.1620
0.1350
0.1210
0.1040
0.0931
0.0849
0.0784
Aluminum
0.1620
0.1340
0.1200
0.1030
0.0922
0.0841
0.0777
Limestone
0.1920
0.1460
0.1280
0.1080
0.0960
0.0874
0.0808
Granite
0.1640
0.1370
0.1240
0.1070
0.0950
0.0867
0.0802
Lime/Gran.
0.1780
0.1415
0.1260
0.1075
0.0955
0.0870
0.0805
Water
0.1680
0.1490
0.1360
0.1180
0.1060
0.0967
0.0895
Mass Attenuation Coefficients (cm²/g)
GAMMA ENERGY(MeV)
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