Which devices can you use to take geolocalised measurements?
In Japan, new equipment appeared necessary to meet the expectations of the population after the Fukushima accident, to allow individuals to take measurements independently using easy-to-use low-cost devices. On this basis, various dosimetric applications and systems have been developed, benefitting from the context of internet and new information technologies, able to simplify connections between citizens and sharing skills. On this basis, new systems offer new options for recording geolocated measurements (location, date, value) and can be used to share measurements via platforms.
Dosimeters designed for the "general public", which can be used for geolocated measurements, exist in three main sub-groups:
- Dosimeters connected to a smartphone, which use a Geiger counter or a semi-conductor as a sensor, e.g. OpenRadiation,
- Autonomous dosimeters (measurements and geolocation functions), such as the bGeigie nano by Safecast (using a Geiger meter),
- Applications using the CMOS sensor of the smartphone's camera.
On the basis of which criteria should the device be selected?
The main criteria and settings to be taken into consideration when selecting a dosimeter are as follows:
- Price: applications using the smartphone's CMOS camera cost just a few euros, connected dosimeters cost a few hundred euros, or over €500 for "professional" equipment. Applications are generally available for Apple and Android for dosimeters connected to a smartphone.
- Life and power: rechargeable or other type of battery.
- Type of radiation detected: photons (X, gamma, beta, etc.). A photon dosimeterdosimeter is sufficient in most cases.
- Operating range: a dosimeterdosimeter can be used for a given range in terms of dose rate and the energy of the radiation measured. These ranges are generally compatible with "classical" environmental measurements. Precautions must be taken in some cases: photons with a low or very low energy level, or a high dose rate, etc.
- Sensitivity: this aspect must be sufficient for measuring the natural ambient background noise level (approx. 0.1 µSv/h), i.e. the most frequent situation.
How can accurate measurements be taken in the field?
Data recorded in the environment, outside of housing or buildings, is worthwhile for all, although it can be useful or relevant to measure the ambient dose rate in a home. It can be worth measuring levels near to an old radium alarm clock or a radioactive stone, however this data is not relevant for the environmental data base. In addition, these dosimeters are not appropriate for measuring the radioactivity of food or water.
Depending on the selected device, the implementation procedure may differ, it is important to refer to the seller's recommendations.
In order to obtain measurements which accurately reflect ambient radioactivity levels, with an acceptable level of quality and which can be compared, a few recommendations are necessary, i.e.:
- Take the measurement outside, in as clear a space as possible to minimise disturbance (nearby wall or building, etc.) and obtain a measurement which is as representative as possible of the site
- Stay away from any known source (people having recently been subjected to a nuclear medicine examination, system generating ionising radiation: X-ray generator, etc.)
- Wait until the measurement has stabilised; in fact, the lower the radiation level, the longer the measurement must be to obtain an adequate measurement statistic (measurements generally last a minute or more for background noise)
- Place the dosimeter approximately 1 metre above the ground: this height is generally accepted as a representative means of assessing the average dose received by an individual.
The units used to quantify the radioactivity and associated exposure
The sensors proposed to citizens generally indicate a number of radiation counts detected per minute (CPM), which is translated into microsievert using an internal algorithm.
Radioactivity can be quantified with specific units. The main three are as follows:
- Becquerel (Bq): the number of becquerels corresponds to the number of disintegrations per second for a radioactive source. The greater the number of becquerels, the greater the activity of the source. The curie is replaced with the equivalence 1Ci = 3.7 10E10 Bq or 1 Bq = 2.7 10E-11 Ci
- Gray (Gy): grays are used to quantify the dose absorbed, i.e. the quantity of energy released by the ionising radiation in the target material. One gray represents 1 joule per kilogram of material. It replaces the rad with an equivalence of 1 rad = 0.01 Gy or 1 Gy = 100 rad
- Sievert (Sv): sieverts are used to quantify the risk for humans. The effects of the different types of radiation vary depending on the organs or tissues affected, with some more sensitive than others. The effective dose in sieverts is obtained from the dose absorbed by each organ, weighted with one factor, which depends on the type of radiation, and another factor depending on the sensitivity of the organ. 1 Sv = 1000 mSv = 1 000 000 µSv.
For information, weighting factors for organs are as follows: 0.12 for bone marrow, the colon, lungs and breasts; 0.08 for gonads; 0.04 for the bladder, liver, esophagus and the thyroid; 0.01 for the skin, bone surface, brain and saliva glands; 0.12 for all other tissues.