Background Geiger Counters/Spectrometry

Background dose measurement

Next, measure background (Environmental radiation of measurement point). Without sample radiation source or other radiation sources, use calibrated dosimeter as above, read values multiple times (30 times in our case) with appropriate interval and average them.

Or, you may use values with a sample radiation source. As the radiation dose varies in reverse proportion to the square of distance, and observed dose is a sum of the background and the radiation from the source, you can obtain background dose with 2 measurement points of the sample source.
Example) If a dose in 10cm distance from a source is 0.14μSv/h, while 20cm distance show dose of 0.08μSv/h, define A, B, and X as follows,
A: Radiation dose from the sample source in 10cm distance
B: Radiation dose from the sample source in 20cm distance
X: Background radiation dose

A + X = 0.14、B + X = 0.08
B = A / 4
So X can be derived as
X = (0.08 * 4 - 0.14) / 3 = 0.06μSv/h

But in the real world, with many factors like attenuation by the air, radiation dose from a sample does not always meet the inverse proportion low.

How to perform calibration

Deduct the background dose from each dose values obtained above and write into the calibration board.
(example)
Distance Dose(μsv/h)×70%(*3)
5cm BG+0.1953
10cm BG+0.049
15cm BG+0.0217
20cm BG+0.0119

This is because the background may change due to various factors with time, while the radiation from sample source with long half-life such as 30 years in CS137 won't change that much.
So, to perform calibration, everytime you have to measure background as shown above, and add it to the dose from the standard radiation source.
  • *3 According to Simplified calibration parameter sheet comes with iMetry, claims the indicated value is reduced by 30% for compensation.

Set a calibrated dosimeter and iMetry at the same distance from the sample radiation source (See the picture below),
radicalib.jpg
Set the CPM value read and the sum of BG and radiation from the sample radiation source in μSv/h to the calibration parameter of iMetry App.
  • Linearity of iMetry is limited, becomes non-linear in high dose area. Example:60000cpm or higher
  • Regarding calibration with point radiation source, please be noted that at 5cm distance, iMetry tend to show higher values, due to background radiation, error caused by placing the radiation source too close, increasing covered open angle without linearity, and iMetry's CsI crystal being 2cmx1cmx1cm cuboid.



To operate iMetry in low dose areas, 3 point calibration as follows is recommended.
  • With Full fledged radiation shielding box, calibration to the already calibrated dosimeter
  • Background around 0.05μSv/h, with the already calibrated dosimeter
  • With a sample radiation source, set a certain distance where iMetry shows around 600cpm, with the already calibrated dosimeter

  • 最終更新:2013-06-14 14:31:10

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