Measuring Radioactivity
Radioactivity is not a real concern for most potters in their studios, but if you either use or collect pieces of ceramics (or glass) with Uranium then you may want to know about any adverse affects from its radioactivity. To do this, you need to know how radioactivity is measured, and how emissions from your plates compare against other sources. Here we try and explain the many ways in which radioactivity may be measured.
There are many ways in which radiation can be measured, and this is confusing when you first look in to the subject. I find the best way to view it is as a heirarchy of measurements, starting with the basic physics of the rate of radioactive emissions and the half life of the emitting material, and then adding in the energy in those emissions. From there, we go onto the radiological aspects, looking at the different methods of assessing the effect of radiation on the body. These are presented in sequence below. The units used have also changed with time, so as well as giving the SI units some of the other units that have not yet disappeared from use are also given.
Types of Radiation
𝛼 radiation consists of a helium nucleus - two protons and two neutrons. This is the radiation that converts matter from one element to another. They have a high mass and low speed. This, coupled with their charge (+2), means that they readily interact with matter, and lose their energy quickly. They are very short range, less than 10cm (4"), and easily stopped by a sheet of paper or 0.1mm of skin, so unless an alpha emitter is ingested these will not do you any harm. However if ingested they do cause damage to their immediate surroundings, as although short range the radiation energy is high, as shown by the high WR value.
𝛽 particles are electrons emitted by the atom. These are lighter, faster particles that can penetrate up to 2cm of the body, or a few millimeters of aluminium sheet, depending on their energy levels. They are less damaging to health than other types of radiation, and are perfectly safe if kept within suitable containers, though as with 𝛼 emitters they are more harmful if inside the body.
𝛾 rays are photons, and as such are energy as opposed to particles, equivalent to the man-made X-Rays. The rays have greater penetrating power, being able to pass through the human body. They are best shielded by dense materials such as lead sheeting. This radiation is the most damaging to health, in part because of the difficulty in containing it. However there is not a huge difference between emitters inside and outside of the body.
Uranium Decay Paths
Rate of Emissions
Radioactive substances are not emitting radiation continuously, but rather atoms randomly decay, emitting a particle or energy at the time of their decay. The Becquerel (Bq) (1Bq = 1 decay/second = 10-6 rutherford (Rd) = 2.703-11 curie (Ci)) is the measure of the average rate of emission of radioactivity from a source (it may also be given as the measure of energy, rather than the rate, in electronvolts (eV) or Joules (J).
| Material | Radioactivity (Bq) | |
| 1 banana | 15 | |
| 1kg of clay | 63-2400 | |
| 1kg of Brazil nuts | 400 | |
| 1kg of rutile | 650-3400 | |
| 1kg of ilmenite | 650-4850 | |
| 1kg of coffee | 1000 | |
| 1kg of granite | 1000 | |
| Air in a 100sq.m. house (low Radon) | 3000 | |
| 1 adult human | 4500 | |
| 1kg of zircon | 4900-9900 | |
| Air in a 100sq.m. house (high Radon) | 30000 | |
| 1kg of depleted Uranium | 15000000 | |
| 1kg of Uranium | 25000000 | |
Half Life and Decay Constant
Related to the rate of decay is the half life, t½. The half life is the time for half of the atoms in a sample to decay. So if we have 1000 atoms of a substance with a half life of 10 minutes, then at the end of 10 minutes 500 will have decayed, and 500 not. Then, in another 10 minutes, half of the undecayed atoms will decay, i.e. 250, giving a total of 750 decayed and 250 not. And so it goes on, with the number of atoms decaying reducing in each time period.
This can also be expressed as the decay constant, 𝜆. The relationship is: t1/2 = 0.693/𝜆.
Radiation Exposure
Radiation exposure is a measure of the level of ionizing radiation (i.e. 𝛽 and 𝛾 radiation) in a given mass of air. Thus it combines both the rate of radiation and the amount of energy emitted in the radiation. Units: 1 Coulomb/kg (C/kg) = 3876 röntgen (R). As a measure of radiation damage, it has largely been replaced by the absorbed dose, which takes into account the absorption characteristics of the material.
Absorbed Dose
The absorbed dose measures the energy of radiation absorbed per unit mass of absorber, measured as the amount of energy absorbed per kilo of absorber matter. The unit is the gray (Gy). 1 Gy = 1 J/kg = 100 rad = 1x104 erg/g = 6.24x1012MeV/kg.
This represents the radiation dose that causes short term adverse medical effects from high level doses - radiation sickness or, more formally, acute radiation syndrome. Typically, an exposure of at least 0.75gray is needed for adverse medical effects. In comparison, a chest X-Ray gives an exposure of 0.001 gray, whereas radiotherapy may give an exposure of 50 grays (over a very small area).
Equivalent Dose
So far, this is all from the physicist's point of view, but now we need to look at how different types of radiation have greater or lesser effects on the body to determine the equivalent dose. For this, we multiply the absorbed dose by the radiation weighting factor WR. This then gives the likelihood of cancer or genetic damage from long term, low level exposure. The unit is the sievert (Sv), in J/kg, though the röntgen equivalent man (rem) is still in use in some parts of the world - 1 Sv = 100 rem. 1 Sv represents a 5.5% chance of developing cancer from that radiation.
Values of WR are given in the table below.
| Radiation | Energy E, MeV | WR |
| X-rays, 𝛽 particles, 𝛾 rays, muons | 1 | |
| Neutrons | <1 | 2.5 + 18.2·e−[ln(E)]²/6 (range 2.5 - 20.6) |
| 1 - 50 | 5.0 + 17.0·e−[ln(2·E)]²/6 (range 20.6 - 5.5) | |
| >50 | 2.5 + 3.25·e−[ln(0.04·E)]²/6 (range 5.5 - 2.5) | |
| Protons, charged pions | 2 | |
| 𝛼 particles, heavy nucleii | 20 |
The equivalent dose works well for when the whole body is exposed to an external radiation source.
Some examples of exposure levels can be found in the following table. For those interested in calculating their exposure more accurately, the EPA have a useful online calculator.
| Source | Effective dose, mSv | |
| Dose from eating 1 banana | 0.0001 | |
| Hourly dose at 1m distance from 1kg of depleted Uranium | 0.001 | |
| Max dose from airport X-Ray scanner | 0.0025 | |
| Background radiation, daily dose | 0.01 | |
| Hourly dose on surface of 1kg of depleted Uranium | 0.012 | |
| US artificial radiation exposure to the public, annual limit above background | 1 | |
| X-ray | 0.01 - 1.5 | |
| Round trip flight across the USA | 0.12 | |
| Annual dose from Radon gas in homes | 0.2-3 | |
| Annual dose from cosmic radiation | 0.3 - 0.8 | |
| Annual dose from all natural sources | 1.8 - 7.7 | |
| CT scan | 2 - 16 | |
| Hourly dose from depleted uranium in contact with the skin | 2.5 | |
| US occupational health average annual limit above background | 10 | |
| EU occupational health limit above background, over 1 year | 20 | |
| Smoking 30 cigarettes a day over 1 year | 36 | |
| US occupational health limit above background, over 1 year | 50 | |
| Lowest annual dose to give an increase in incidence of cancer | 100 | |
| Accumulated dose to cause a fatal cancer in 5% of people | 1000 | |
| Single dose to cause non-fatal radiation sickness | 1000 | |
| Single dose that would kill half the people within a month | 5000 | |
| Single dose, fatal within weeks | 10000 | |
Effective Dose
The effective dose takes the modelling one step further, to deal with irradiation of just parts of the body, either due to shielding of the radiation source, or through ingestion of radioactive material, especially 𝛼 particle emitters whose short range means that their effect is highly localized. The weighting factors from the ICRP for various tissue types are given below (Note that in the USA an older version of the ICRP tables is still used, together with some of the older approaches for assessing exposure).
| Organ | Tissue Weighting Factor | |
| Skin | 0.01 | |
| Bone surface | 001 | |
| Salivary glands | 0.01 | |
| Brain | 0.01 | |
| Bladder | 0.04 | |
| Liver | 0.04 | |
| Oesophagus | 0.04 | |
| Thyroid | 0.04 | |
| Gonads | 0.08 | |
| Bone marrow | 0.12 | |
| Colon | 0.12 | |
| Lungs | 0.12 | |
| Stomach | 0.12 | |
| Breasts | 0.12 | |
| Rest of body | 0.12 | |
| Total | 1.00 | |
Committed Dose
Finally, for radioactive sources that are in the body, e.g. inhaled or ingested uranium, we have the committed dose. This is the effective dose applied over the life of the radioactive source - taken as 50 years for adults, or 70 for children, unless the radioactivity is reduced over a shorter period, e.g. by the source having a short half life, or it being expelled from the body.
Time of Exposure
In most of the discussions above, we have been talking as though the radiation happens outside of time, in one instantaneous exposure. But, or course, this is not the case. When measuring the output of a radiation source, this is normally measured on a time basis, e.g. mSv/hour, and then the actual exposure is calculated by multiplying this rate by the actual time of exposure. If the source has a short half life in relation to the exposure time, so the radiation rate cannot be considered to be constant, the same approach can be taken but making allowance for the changing output over time.
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