Doses, radiation and measurements
A beginner dosimetrist's guide

In the previous article we looked on overall radiological conditions in Chernobyl Zone. Today we will talk about measurements of radioactivity levels. Surely, you have seen pictures of people pointing their Geiger counters to spots across the Zone, but in fact it is not always that straightforward. So let's start our course of young dosimetrist.

Doses and units

When we deal with radioactivity, there are two main things that take the most attention — the power of radioactive emission and the dose. Let's first look to γ- and X-ray emission. In general, the dose of radiation is the amount of exposure to it. However, the problem is, that processes that happen after exposure are very diverse and variable, so it is not possible to just measure it in one and the same way. Therefore, in radiation protection science there are 4 types of doses and also different measurement units. All of this is applicable in different situations.

Exposition dose
It is an amount of ionization of dry air in the conditions of normal atmospheric pressure. According to SI system, its unit is Culon per kilogram (Cl/kg). There is non-system unit, called Roentgen — particularly it was the main unit of measurements in the Soviet Union, so you can widely meet in across literature and official reports of that time (normally marked as P, e.g. 100 P). One Cl/kg equals to 3867 Rouengens

Absorbed dose
It is an amount of energy, that radiation delivered to matter. The SI unit of absorbed dose is Gray (Gr), non-system is rad. 1 Gr = 100 rad.

Equivalent dose
This type of dose allows to estimate the effects of irradiation. The logic behind it is that the same absorbed dose of different types of radiation can have different effects on the tissue, as some particles can make higher ionization, hence higher damage. To get the equivalent dose, the absorbed one has to be multiplied to efficiency coefficient. The unit for this type of dose is Sievert (Sv) or, obsolete and non-system, Roentgen equivalent man (rem) (in Soviet sources marked as Biological Equivalent of Roentgen (BER)), where 1 R = 1 rem.

Effective dose
This type of dose being used for overall estimation of future effects of radioactive exposure to human body, organs and tissues. It estimated based on the fact that some organs are more sensitive to radiation than another. Therefore, to get an effective dose, an equivalent dose value has to be multiplied to a special correction coefficient, which will give show the overall effect of exposure to the organism. The units are the same — Sievert or BER.

The overall dose of radiation a human, that not involved in contacts with radioactive sources, should not exceed 1 mSv/year. For those, who deal with radioactivity professionaly, the maximum annual dose is 20 mSv.

The power of radiation

Given that different types of radioactivity mean different types of particles emitted with specific physical parameters, the way of estimation varies. When you have a γ-source, and take a radiometer, you get a power of the dose per period of time. Most of modern devices measuse it in Sv/h, or, as Sievert is very big amount, more common is uSv/h. On the picture below, MKS-U device registered 10.02 uSv/h, which means that in one hour on this place one would ger approximately 10 uSv of dose.

10 microsieverts

In case of α-, β- and neutron radiation, the point of measurement is density of particle flow, or activity per surface size. In the SI system the unit for activity is Becquerel, which means one radioactive decay per second. Typically, devices show the flow per 1 square cantimeter.

From the point of safety regulations, a level of γ-radiation that does not exceed 0.3 uSv/h is considered safe; for other types — activity should not be more than 20 particles per minute.

Radiometers, dosimeters and spectrometers

Despite in common talks everybody calls devices for measurements dosimeters, it is not fully correct. A dosimeter is a device for measuring dose, typically absorbed and equivalent. Depending on its tasks, it can be a piece of complex electronics, or a simple (quartz fiber, thermoluminiscent, film) assembly — like this one, based on special crystal pills:

Personal dosimeter

In another hand, a radiometer is one for measuring the power of the dose and/or activity. Most of the devices for home use are capable to register only photon emission (hard γ-radiation). For other types additional detectors required. Most of modern devices are two in one — dosimeters-radiometers.

There are few types of detectors that can be used in equipment:

Geiger–Müller tube
It used for the detection of γ-radiation, α-, β-particles and x-ray emission. Technically, it is a sealed tube filled with gas, equipped with electrodes with high voltage. The gas gets ionized once a particle flies through it, so the tube produces a recognizable electric implulse, that can be counted by device's logic board. Altough it is the cheapest and the most popular type of detector, it has certain limitations for measurements of high radiation levels, as well tend to slowly burn-out. The accuracy of measurement depends also on size of the tube used — bigger one will catch more.

Scintillation counter
It is a block of scintillator — a matherial (e.g. a crystal of Cesium Iodide) that produces photons when exposed to radiation, and a photomultiplier — a tube that registers light and produces electric impulses. On the headline picture is a recorder of aspiration system, that uses exactly this type of detectors.

Ion chamber
It is also a gas-filled detector, however, it does not use the avalanche effect of ionization. Typically they are used for dosimetry control outposts.

Depending on capabilities and types, not always a particular detector can catch all variety of radioactive emission — for example, due to low energy of some particles or weak flow, etc. So here comes the spectrometer — a device that capable to measure energy and identify isotopes.

Given that every radioactive element has a known unique emission spectrum, it is possible to compare the measured with etalon one, thus, identifying what exactly is this matter.

Practice

When you come to the Chernobyl Zone, these few hints may come very handy:

In general, to make a measurement, you need to place the detector close to the target object. Most of the devices have the mark on the enclosure that represents the location of detector. It is important not to place a device directly on surface, as it can cause its contamination, making results inaccurate.

If the device capable to show directly the power of the dose, give some time and get the readings. Normally, it is possible to understand the power of emission simply on how fast a device reacts. If the device works in counts per second, set it up for period of time and start recording, then divide result to mentioned period. To convert counts to other units, refer to device's documentation.

Remember, that measuring of individual radioactivity of some object can be less or more accurate when you do this in environment with normal background radiation or use isolation lead chamber. In the Zone, in most cases readings will be affected by it, making famous pictures with Geiger counter close to apples the illustations of quite useless activity.

Most of devices that capable to measure γ-radiation and have removable shield over detector are possible to use to check if the object emits harder particles, like β To do this, make a measurement in the same mode with shield on and off and check the readings. If the dosimeter capable to measure the density of β-flow, also do not forget to remove shield.

Use personal protection. Very strong α- and β-particles flow in some cases can make burns. So take the gloves on.

Write a comment