RSDetection
User’s Manual
Copyright
©
2016 General Electric Company. All Rights Reserved
S131-200-UM Rev A
Page 19
g
There are other characteristics that are important such as accuracy, stability, directional
response, and others which are not covered in this section. These parameters are functions of the
entire system including electrometer, analog signal conditioning, etc., and therefore are covered in
the system specifications.
4.1.1
Energy Response
Energy response is the signal measured as a function of the gamma ray photon energy.
Ideally this response should be flat, such that no energy generates more ions than it would
in air. The response is characterized by a curve whose X-axis is energy and whose Y-axis is
the normalized energy response. The normalization of the response means that the
nominal response for any energy would be 1.0 if the detector were ideal. For the HPIC the
response curve is shown in Figure 7.
Figure 7 HPIC Energy Response
The response is not flat across the measurable energy range, and the ability to measure
gamma’s falls off below 60KeV. The result of this is that some isotopes over-respond and
some under-respond as compared with the air equivalent standard. Overall, this difference
is not great and in most applications it is not considered. One reason for this is that to
apply a correction requires one to know the energy of the incident photons. If this is
known, a correction factor can be applied. This factor, referred to as the “F” factor, is the
correction to be applied to compensate for the non-ideal nature of the response. Since
each isotope has a different response, the calibration of the detector must be referenced
to a known isotope. For this sensor, the reference is Ra226.
4.1.2
Determining the “F” Factors
The first task is to calculate the normalized response for the isotopes. If the isotope emits
one energy unit, this would be a matter of finding the response value on the Y-axis for the
energy of the photons. Most isotopes, however, have more than one energy line. Each
energy line and the percentage of that energy to the total must be considered. The factors
for the most common elements are shown in Table 3.
0.1
1
10
10
100
1000
10000
R
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la
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s
ponse
Gamma Energy (keV)
