Non-shielded relative Geiger counter

What's discussed here is a general idea about the measurement limit. It does not apply at all for that if you measure with a sufficient margin.
Assume a given set of circumstances, room temperature, normal humidity, constant environment, and iMetry sensor with sensitivity of 6000cpm/μSv/h, showed 0.06μSv / h after one hour measurement of the background radiation.
iMetry shows 360cpm, that is 21600counts for an hour. If you assume a Poisson distribution, sd becomes 147counts, if you adopt 3sd for significance, it will be about 2% of the entire count value.

Therefore, in a non-shielded environment, where background radiation is 0.06μSv / h, measurable decision possible radiation dose, assume 3sd for significance, no temperature or over time drift, etc., one hour measurement, would be 2% of 0.06μSv / h background, about 0.0012μSv / h or more, relative MDA in this one case. Here, it is assumed that self-noise count of iMetry, can be ignored.
Depends on the sensitivity of iMetry and radiation background levels, MDA is slightly different due to individual differences.

When the background is 0.06μSv / h, the measurement limit amount of a small relative radiation, as determined in 3sd, would be about 0.0012μSv / h with iMetry sensor sensitivity of 6000cpm/μSv/h is considered.

When the background is about 0.06μSv / h, there is a problem that iMetry does not yet support the energy correction, in relative measurement, in comparison with the control, about 0.0012μSv / h in a non-shielded environment extent is theoretically possible with multiple measurements and comparisons, but in practice, in order to avoid a erroneous judgment, it is desirable for such a low level of radiation measurement to be performed in a lead-shielded environment, with the presence or absence of the spectrum peak.

Further, prior to the measurement in a non-shielded environment, environmental radiation should be checked whether being stable in about 0.0012μSv / h range, and the system including iMetry should be verified stable enough. However, to discuss the MDA of non-shielded environment in this case, as the actual background is not measured yet, you will need to measure the diurnal variation of total iMetry system and the background.

Needless to say about environmental radiation levels of cesium, what makes a very small amount of radiation measurement in a non-shielded environment difficult as well as drift of equipment, is a short period variation of radon dose in the room. Prior to measurement, you must also observe the variation of the background dose and equipment in that environment continuously and comprehensively.

You also have to set iPad to airplane mode to perform such subtle measurement.
線量安定度.png
As an example, in an air conditioned concrete room, operated iMetry for a long time and observed the time variation. In an hour out of total 6 hours of measurement, top to bottom 0.00135μSv/h of time variation was observed.
Possible causes of the variation are
・Captured the short time variation of radiation from radon
・Ccaptured the variation of K40 of human body origin in the vicinity, due to the movement of people
・Temperature drift, or any iMetry drift
But as out of our poor equipment, the most stable is iMetry, so we have no idea what was captured.
This is just an example, but anyway, the radiation measurement in a non-shielded environment, under the normal dose, to compare the magnitude of the dose at 2% of the background at one time measurement may be a problem, in terms of the diurnal variation.

Before jumping to conclusions, we may pair-calibrate two units of iMetry in the background, and measure in an environment where people are far enough away. From the experiment, depending on the results, we might make somewhat progress if its due to temperature variation or not or something.

It is one example, but in the present case, taking enough safety factor, of 10% of the background, the measurable magnitude of the radiation in one time measurement in non-shielded environment may be about 0.006μSv / h.
For the diurnal variation, we would like to further measure in environment without people.

Background-spectrum.PNG
In the long-time measurement, environmental or nearby human body origin spectrum peak of K40 and peak around 600keV, possively Bi214 origin has been observed.

As shown above, radiation measurement in a non-shielded environment can not be focused to target radiation, due to

Variation of the background dose itself
Variation of the background dose, as a Poisson distribution, expressed with SQRT(COUNT)/COUNT
Background spectrum

Upon very small amount of radiation measuring, by these circumstances, sufficient shielding by lead is required.



  • 最終更新:2013-05-21 14:27:19

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