Geiger Counter Dose calibration

Dose measurement

Radiation dose measured by radiation detectors like iMetry is a sum of background (environmental) radiation and radiation from the sample. In case of iMetry, with some error of linearity, measured dose is a simple sum, too.

To measure sample's radiation accurately, you need Environmental radiation shielding with lead.

Dose calibration

  • For dose calibration, you need a sample radiation source of Cs137 and a calibrated dosimeter for reference.
  • Theoretically, in a fully shielded environment, radiation dose from point radiation source decreases inversely with the square of distance.
  • So, on a correctly calibrated dosimeter, when you measure 5cm and 10cm distance from a point radiation source and subtracted the background, value of 10cm distance equals to 1/4 of the value of 5cm, in theory.
  • In the real world, with factors like attenuation by air and effect of sensor shape, such cannot be observed. There is also error by linearity.
    To minimize the effect of linearity, measure at sufficiently low cpm, and calibrate at doses close to target.

  • Also, if there is a monitoring post in your neighborhood, measuring cpm around the monitoring post and calibrating with the displayed dose (μSv/h) might be a handy way of calibration. iMetry works with one dose calibration point, so this might be the easiest and most reliable DIY way of relcalibration.

Dose measurement with Cs137 sample radiation source

Prepare a paper pasted on a board, draw concentric circles in a regular radiuses like 5cm, 10cm, 15cm, to make a calibration board.
Example of a dose calibration board
Place a Cs137 sample radiation source (*1) at the center of concentric circles, and sensor position (often marked with +) of a calibrated dosimeter on each radiuses.
Dosimeter values may oscillate in a certain range, so read values multiple times (30 times in our case) with appropriate interval and average them. Take this value as dose in the distance.

  • *1 Cs137 sample radiation source with 0.25μCi (17 months from generation)
    • There is a Table for Ci (Curie) to Bq (Becquerel) conversion. With that, 0.25μCi=9.25kBq.
    • Survival rate of radioactive nuclide can be calculated at Half-life of radioisotope - high accuracy calculator site (in Japanese). At the site above, choose "セシウム137=Cesium 137" for "放射性元素=Radioisotope", give elapsed time in "Period=計算期間" in years (17/12=1.417 in case of 17 months) and push "計算=Calculate" button. Page will be reloaded and a chart is displayed, you can see the survival rate (%) of the radioisotope corresponding to the elapsed time at the very bottom of the page.
    • Theoritical dose value (Sv/h) in some certain distance from a point radiation source with a certain Bq can be calculated by multiplying the Becquerel with survival rate and give it to "放射性物質量=Amount of radiation source" field of Point radiation source Becquerel(Bq) to Sievert(Sv) conversion tool (in Japanese) and also give the distance.
    • Theoritical dose of a Cs137 point radiation source (Originally 0.25μCi, 17 months from generation) obtained this way is as follows.
Distance Theoritical dose from a Cs137 point radiation source (Originally 0.25μCi, 17 months from generation)
(μsv/h) Reduced to 70% (*2)
5cm 0.1953
10cm 0.049
15cm 0.0217
20cm 0.0119

  • *2 According to Simplified calibration parameter sheet comes with iMetry, claims the indicated value is reduced by 30% for compensation.

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.
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),
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 14:28:14