Tag Archives: radioactivity

Tritium Illumination

Radioluminesence is the emission of light due to bombardment by ionising radiation, the most common example of which is tritium illumination. Tritium is an isotope of hydrogen, made up of one proton and two neutrons (i.e. it is hydrogen-3). It has a half-life of 12.3 years and decays by emitting beta particles (high speed electrons) to form helium-3.

In a tritium illumination light source the tritium gas is trapped in a glass tube that has been coated on the inside with a phosphor. When the gas decays, the electrons produced strike the phosphor and their kinetic energy is transferred into light energy. By choosing different phosphors, different colours of light are produced.


Novelty keychains containing tritium light sources.

Because tritium illumination requires no power source and lasts for a long time, it is commonly used in situations where long-term but low-power lighting is required. For example, tritium illumination is used on watchfaces, compasses, instrument dials and gunsights. The encapsulation of the tritium source prevents any radiation risk, and if a tritium light source is broken open then simply leaving the area and allowing the gas to disperse will mitigate any health risks.

The most radioactive parts of the UK

The average radioactive background dose in the UK is 2.7 millisieverts. Of this 2.7 mSv, 1.35 mSv comes from radioactive radon gas leaking out of the ground.

This radioactive radon (Rn-222) is produced by the decay of uranium-238, after a series of intermediate non-gas stages that cannot escape from rocks.

Because radon has such a large effect on the annual radiation dose that someone receives, it is closely monitored. In the UK, this monitoring is done by the Health Protection Agency (HPA). One of the things that the HPA does it produce radon maps, showing which areas of the UK have the highest presence of radon.

The map is graded by the percentage of homes in that area which have a level of radon beyond the action level of 200 becquerels per cubic metre (200 radon decays per second per cubic metre).

There are a number of important radon hotspots in the UK. The most noticeable one is Cornwall in the south-west where the average UK background dose is 7.8 mSv, nearly three times the national average. This is due to the presence of igneous granite, which naturally contains more uranium (10-20 parts per million) than other rocks.

Radioactive areas tend to be hilly, where igneous rocks have been forced to the surface or left behind by the erosion of softer sedimentary rocks (the Chiltern Hills are particularly radioactive, for example). The Yorkshire Dales sit on top of an underground deposit of pink granite called the Wensleydale Granite that lies underneath the Askrigg Block, and the Peak District features many granite outcroppings.

Potassium iodide pills are radioactive

Matter is made of atoms, and atoms are made of protons, neutrons and electrons. The protons are positively charged, the electrons are negatively charged and (as their name suggests) the neutrons are neutral, having no charge. For every atom, the number of protons and electrons is the same so that the positive and negative charges cancel each other out, leaving the atom neutrally charged overall.

The number of protons inside the nucleus at the centre of an atom decides what element it is. Different atoms with the same number of protons and a different number of neutrons are known as isotopes. For example, there are three naturally occurring isotopes of carbon: carbon-12, carbon-13 and carbon-14. Most (98.9%) of the natural carbon is carbon-12 and the remaining 1.1% is made up of stable carbon-13 and radioactive carbon-14.

Potassium also has three natural isotopes: K-39, K-40 and K-41. The potassium-40 isotope, which makes up 0.0117% of all naturally occurring potassium, is a radioactive beta emitter with a half-life of 1.25 billion years. Therefore anything that contains potassium, including the potassium iodide pills that idiotic US West Coasters are stockpiling (thereby making them unavailable to those who might actually need them) is radioactive.

More than 10% of the average person’s annual background dose of radiation comes from food. Any food that contains a large amount of potassium will be radioactive: bananas, each of which contains about 450 milligrams of potassium, are radioactive enough to be detected by scanning equipment at ports designed to stop nuclear smuggling. The most radioactive food is the Brazil nut, but this is due to the presence of radium, rather than potassium.

You can buy potassium iodide pills from Amazon.

Understanding radioactive dose

If someone says something is “very radioactive”, what do they actually mean? How do you measure “radioactiveness”? There are many terms used in physics to describe radioactive decay and each has a specific use.

(Throughout this post I’ll be using polonium-210 as an example. Polonium-210 is most famous as the radioactive poison used to murder Russian dissident Alexander Litvinenko.)

The activity of a radioactivity substance is the number of decays that it undergoes per second: one becquerel (Bq) is one decay per second. Polonium-210 has an activity of 166 terabecquerels per gram (166 TBq/g) which means that each gram of Po-210 undergoes 166 trillion decays per second. But knowing how many radioactive decays a substance undergoes isn’t going to tell us how dangerous it is. Standing one kilometre away from a 1 TBq source is very different to standing one metre away from a 1 TBq source.

The absorbed dose, measured in grays* (Gy), is a measure of the amount of energy deposited by a radioactive source into each kilogram of mass (one gray is one joule per kilogram).

Every time a Po-210 nucleus decays it emits a particle with an energy of 5.3 MeV, which is equivalent to 8.50×10−13 joules. 1 gram of polonium-210, emitting 166 trillion of these particles per second is equivalent to 141 watts, easily enough to run a laptop or two standard 60 W lightbulbs. After a day one one-thousandth of a gram of polonium-210 would have released 12 200 joules of energy, about the same amount of energy as a twenty-five kilogram mass travelling at 70 mph. This 12 200 joules, divided evenly amongst the mass of an 80 kg human being would be more than 150 Gy, where anything more than 5 Gy at any one time is usually fatal.

Absorbed dose isn’t perfect for measuring the danger posed by a radioactive source as it doesn’t take into account where the radiation is absorbed, nor the type of radiation.

The equivalent dose only takes into account the organ or tissue being affected but the effective dose, measured in sieverts† (Sv) is designed to compensate for these failings and attempts to reflect the biological rather than the physical effects of radiation. It is calculated by combining the absorbed dose and two dimensionless factors: one to account for the type of radiation and one to account for the organ or tissue being irradiated. These factors, Q and N, combined together are called the radiation weighting factor.

Q (sometimes called the quality factor) accounts for the type of radiation being absorbed. It is equal to 1.0 for all photons, electrons, positrons and muons; 2.0 for protons and pions; 5.0 to 20 for neutrons according to their energy; and 20 for alpha particles and the heavier products of nuclear fission. N accounts for the tissue or organ that is being irradiated. N is greatest for bone marrow, the colon, the lungs, heart or stomach; and lowest for the skin.

Looking back at the 150 Gy absorbed dose for one milligram of Po-210 we end up with a table that looks like this:

150 Gy of alpha radiation incident upon the lungs or stomach (360 Sv) is approximately 250 times more damaging biologically than 150 Gy of electron or positron radiation received to the skin (1.5 Sv). For comparison, the average dose for a resident of the UK, due to natural background sources is about 2.6 millisieverts and a dose of more than 3 Sv kills fifty percent of people within thirty days.

* The gray is named after Hal Gray, a British physicist who created the field of radiobiology.
† The sievert is named after Rolf Maximilian Sievert, a Swedish medical physicist who studied the biological effects of radiation.

How does the damage caused by exposure to radiation vary as the dose of radiation increases?

Most people assume that if you double the amount of radiation you double the damage caused, and that there is no threshold below which no damage is done. This is called the Linear No Threshold (LNT) model and is represented by the graph below:

The LNT model has been the subject of some disagreement in recent years. The American Nuclear Society said in a 2001 Position Statement* that:

“There is substantial and convincing scientific evidence for health risks at high dose. Below 10 rem (which includes occupational and environmental exposures) risks of health effects are either too small to be observed or are non-existent.”

This linear threshold model holds that there is a limit below which no damage is caused, but that damage then increases linearly beyond that limit.

There are other possible models. Damage may increase in an exponential way, with very low damage at low doses but increasing amounts of damage at higher doses.

In a logarithmic model the damage would be very large at first, but taper off as the dose increases.

So which model is correct?

The LNT model remains the most commonly used by regulatory bodies, but there is growing interest in threshold models and in the idea of radiation hormesis, the idea that a small dose of radiation is actually good for the body by somehow stimulating the body’s repair systems. I think the most likely candidate is a J-shaped curve with a significant threshold but then a fairly rapid linear increase in damage caused.

* American Nuclear Society, Health Effects of Low-Level Radiation, Position Statement 2001.