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 W_R 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, t_½. 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: t_1/2 = 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 W_R. 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 W_R 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.