Capintec CRC-25R, Руководство пользователя

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CAPINTEC, INC CRC
®
-25R
The figure depicts the sensitivity of the ionization chamber as a function of photon energy up
to 1.9MeV. Above a photon energy of 200keV, the ionization in the chamber is mainly due to
electrons resulting from Compton scattering of photons by the filling gas (argon) and the
chamber walls (aluminum).
The peak in the low-energy region of the sensitivity curve is due to the rapid increase in
photoelectric effect as photon energy decreases and to the attenuation of low energy
photons by the sample holder, the chamber liner and the chamber walls, as well as the
absorption of photons in the sample material and its container.
Although a significant fraction of photons with energies below 50keV are stopped in the
chamber wall, some photons could enter the sensitive volume of the chamber and could,
therefore, contribute to the activity measurement. All photons with energies below about
13keV are stopped before they reach the sensitive volume of the chamber and, therefore,
these photons do not contribute to the activity measurement.
Calibration Setting Numbers
The relationship between the response of the detector and the gain setting (relative to that
for Co60, in order for the instrument to give a direct reading of the activity) is given by:
A
R
1
G
A
≡
(4)
The calibration setting number is linearly related to the chamber response.
All the calibrators are calibrated with certified Cobalt 60 and Cobalt 57 standard source.
A calibration setting number of 990 was assigned to Co60 and 112 was chosen for Co57.
The calibration setting number of CRC
®
Calibrator for radioisotope A, N
A
, is given by:
( )
( )
( )
( )
57 60
57 60
60
57 60
57 60
A A
- 1 R N
Co Co
Co Co
Co
Co Co
Co Co
R R
N N
N
N N
R R
−
−
∗
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
∗
−
−
− =
(5)
Entering numerical values:
(6)
( )
0.080 R 1076 N : obtains one
2% 0.189 R 1.000 R
112 N 990 N
A A
Co57 Co60
Co57 Co60
− =
± = =
= =
The accuracy of the sensitivity curve and the calibration number determination was tested by
calculating calibration numbers for all the radioisotope standards used for the studies of the
sensitivity. The agreement between the calculated and the observed responses were all
within
±
3%.
A2 - 2 APPENDIX II June 09