Radioactivity with, or without, the sources

Posted: 13 November 2020

This blog post is based on the teaching materials I use in a CPD session that runs anything from an hour to a full day depending on the level of hands-on and the depth to which we explore the ideas, the pedagogy and how to embed it all in the curriculum. Because this post is intended to be reference and notes on teaching radioactivity it will always read in a fashion more akin to notes than journalistic prose.  As you can see it’s a huge amount of stuff – If you want to use any of it — please feel free…


What is “radioactive” anyway

The tricky thing here is that essentially everything is radioactive to a degree – all living things contain radioactive isotopes, it’s how carbon-dating works after all! The Radiation Protection Division of the Health Protection Agency defines a radioactive substance as having a specific activity 400 Bq/kg. Typical naturally occurring radioactive materials range from about 2 Bq/kg up to 300 Bq/kg background.
As always there’s a really good XKCD cartoon all about it. Click for Bigger!


The terms radiation’ and radioactivity’ are often interchangeable in the public mind. Because of its invisibility, radiation is commonly feared yet it is a natural substance — it is extremely likely that without the mutations caused by ionising radiation living creatures would not have evolved to the point of wondering about the universe.

We need to confirm that radiation” just means giving out and that this topic should be…..

Nuclear Radiation:

Nuclear radiation is the topic that deals with electromagnetic waves and items of matter that have their origin in the nucleus of the atom.

Lets have a quick atomic aside:

Note: the number of protons is the defining feature of the atom and how it ends up behaving in a chemical sense.

The neutrons can be thought of as Padding”


Protons are positive, and repel. What holds the nucleus together?
Enter stage left: The Strong Force.

It never seems to occur to students up until this point — what holds those repelling protons together? – Turns out there’s a sub-atomic glue” that only works over tiny ranges holding it all together. (Why/How? — might as well ask why anything, at some point we have to deal with the fact that the universe is beautiful, amazing, and weird)

We call this glue the strong force” and the interplay between the strong and the electromagnetic force is critical to the stability of atoms…

Here’s a thing you have to accept about the universe: Every thing tends toward being in a state where it’s energy store” is as empty as possible. Why? — see above point about weird universe.

Too much padding” (neutrons) and the balance shifts toward the repulsion — the electric-magnetic store is too full) and the probability of something causing the state to change increases. We say that the nucleus becomes unstable when it is significantly likely to spontaneously change. Too few, and the strong force store” is too full, and again, the stability decreases.

This kind of abstract balancing of the books leads us to the classic plot that shows the stable isotopes of the elements — there are versions in Lego too :



The terms ‘radiation’ and ‘radioactivity’ are often interchangeable in the public mind. Because of its invisibility, radiation is commonly feared and many misconceptions abound. A quick 5 question True-false discussion will highlight a few of them:
True or false
1.Radioactive substances make everything near to them radioactive.
2.Once something has become radioactive, there is nothing you can do about it.
3.Some radioactive substances are more dangerous than others.
4.Radioactive means giving off radio-waves.
5.Saying that a radioactive substance has a half life of three days means any produced now will all be gone in six days
The answers:
1. False-ish – There’s a misconception between irradiation and contamination that warrants discussion here – see later!
2. True-ish – You can wait until it’s not longer distinguishable from background radiation if you like – this is pretty much what we do when disposing of nuclear waste – we put it somewhere inaccessible and hope nobody disturbs it for thousands of years. Working out signage that works in ten thousand years is a serious issue
3. True – but it also depends on where you’re exposed, what kind of radiation it is etc.
4. Fase. It means “giving off a thing”. we didn’t know what alpha, beta, gamma were so it was hard to classify stuff that gave it off.
5. False. It’s an exponential decay, just like so many other things in the universe, so it will keep halving every three days till you can no longer tell the difference from the rest of the environment.
Lets look at a story that’s useful for illustrating some misconceptions;

Story: Chernobyl

In April 1986, a serious accident occurred at the nuclear power station at Chernobyl in Russia. A week later, radiation detectors (Geiger counters) in Britain recorded higher  than usual levels of radiation. Britain is more than 1000 miles from Chernobyl!                     
Explain what reached the Geiger counters in Britain to make them record extra counts.

In order to help minimise the impact of our language on the misconceptions held by our students here’s a slide from my training on language;


Roentgen was exploring the path of electrical rays passing from an induction coil through a partially evacuated glass tube. Although the tube was covered in black paper and the room was completely dark, he noticed that a screen covered in fluorescent material was illuminated by the rays. He later realised that a number of objects could be penetrated by these rays, and that the projected image of his own hand showed a contrast between the opaque bones and the translucent flesh. He later used a photographic plate instead of a screen, and an image was captured. In this way an extraordinary discovery had been made: that the internal structures of the body could be made visible without the necessity of surgery.

By 1896 an x-ray department had been set up at the Glasgow Royal Infirmary, one of the first radiology departments in the world. The head of the department, Dr John Macintyre, produced a number of remarkable x-rays: the first x-ray of a kidney stone; an x-ray showing a penny in the throat of a child, and an image of a frog’s legs in motion. In the same year Dr Hall-Edwards became one of the first people to use an x-ray to make a diagnosis – he discovered a needle embedded in a woman’s hand. In the first twenty years following Roentgen’s discovery, x-rays were used to treat soldiers fighting in the Boer war and those fighting in WWI, finding bone fractures and imbedded bullets. Much excitement surrounded the new technology, and x-ray machines started to appear as a wondrous curiosity in theatrical shows.


It was eventually recognised that frequent exposure to x-rays could be harmful, and today special measures are taken to protect the patient and doctor. By the early 1900s the damaging qualities of x-rays were shown to be very powerful in fighting cancers and skin diseases

•1896 — Henri Becquerel discovers that rocks that contain uranium emit radiation

In 1896 Henri Becquerel was using naturally fluorescent minerals to study the properties of x-rays, which had been discovered in 1895 by Wilhelm Roentgen. He exposed potassium uranyl sulfate to sunlight and then placed it on photographic plates wrapped in black paper, believing that the uranium absorbed the sun’s energy and then emitted it as x-rays. This hypothesis was disproved on the 26th-27th of February, when his experiment failed” because it was overcast in Paris. For some reason, Becquerel decided to develop his photographic plates anyway. To his surprise, the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity.

Becquerel used an apparatus similar to that displayed below to show that the radiation he discovered could not be x-rays. X-rays are neutral and cannot be bent in a magnetic field. The new radiation was bent by the magnetic field so that the radiation must be charged and different than x-rays. When different radioactive substances were put in the magnetic field, they deflected in different directions or not at all, showing that there were three classes of radioactivity: negative, positive, and electrically neutral.

The term radioactivity was actually coined by Marie Curie, who together with her husband Pierre, began investigating the phenomenon recently discovered by Becquerel. The Curies extracted uranium from ore and to their surprise, found that the leftover ore showed more activity than the pure uranium. They concluded that the ore contained other radioactive elements. This led to the discoveries of the elements polonium and radium. It took four more years of processing tons of ore to isolate enough of each element to determine their chemical properties.

Ernest Rutherford, who did many experiments studying the properties of radioactive decay, named these alpha, beta, and gamma particles, and classified them by their ability to penetrate matter. Rutherford used an apparatus similar to that depicted in Fig. 3-7. When the air from the chamber was removed, the alpha source made a spot on the photographic plate. When air was added, the spot disappeared. Thus, only a few centimeters of air were enough to stop the alpha radiation.

Because alpha particles carry more electric charge, are more massive, and move slowly compared to beta and gamma particles, they interact much more easily with matter. Beta particles are much less massive and move faster, but are still electrically charged. A sheet of aluminum one millimeter thick or several meters of air will stop these electrons and positrons. Because gamma rays carry no electric charge, they can penetrate large distances through materials before interacting–several centimeters of lead or a meter of concrete is needed to stop most gamma rays.

•1908/9 — Marie Curie discovers radium, polonium and thorium — good biog here:

•1903 — Becquerel, and the Curies get Nobel Prize

Story: Why green?

So, radioactive materials are always depicted as glowing green – as in the Simpsons:

It’s almost certain that this misconception arises from one of the most tragic stories surrounding the topic. When Radium is  mixed with phosphorescent copper-doped zinc sulfide, the radioactive decay of the radium causes the zinc-sulphide to emit a characteristic green glow. Though the use of radio luminescent paint was mostly phased out by the mid-1960s, there were hundreds of “Radium Girls” that suffered from the exposure to radioactive paint.The Radium Girls were female factory workers who contracted radiation poisoning from painting watch dials with self-luminous paint. The painting was done by women at three different United States Radium factories, and the term now applies to the women working at the facilities: one in Orange, New Jersey, beginning around 1917; one in Ottawa, Illinois, beginning in the early 1920s; and a third facility in Waterbury, Connecticut.

The women in each facility had been told the paint was harmless, and subsequently ingested deadly amounts of radium after being instructed to “point” their brushes on their lips in order to give them a fine tip; some also painted their fingernails, face and teeth with the glowing substance. The women were instructed to point their brushes in this way, because using rags or a water rinse caused them to use more time and material, which was made from powdered radium, gum arabic and water.

Five of the women in New Jersey challenged their employer in a case over the right of individual workers who contract occupational diseases to sue their employers under New Jersey’s occupational injuries law, which at the time had a two-year statute of limitations, but settled out of court in 1928. Five women in Illinois who were employees of the Radium Dial Company (which was unaffiliated with the United States Radium Corporation) sued their employer under Illinois law, winning damages in 1938

The radium paint manufactured early in the 20th century has lost most of its glow now, but it’s still radioactive. The isotope of radium used has a half life of 1200 years, but the chemical phosphor that makes it glow has broken down from the constant radiation—so if you have luminescent antiques that barely glow, you might want to have them tested with a Geiger counter and take appropriate precautions. You certainly shouldn’t have them in school without RPA permission. Though the radiation emitted is alpha, and relatively harmless from an irradiation perspective – the risk of contamination and ingesting or inhaling the radium is quite high – in which case it becomes a serious cancer risk.The myth is kept alive by the phenomenon of Cherenkov glow, which arises when a charged particle (such as an electron or proton) from submerged sources exceeds the local speed of light through the surrounding water.

Miracle cures:

As with anything that is little understood, there is often a lot of quackery on the fringes of the scientific experimentation. Some, like this invigorator” have died out….

The days of radioactive miracle cures are not yet gone – there are those that will sell you a wristband that has a “negative ion infused ION CORE™ producing over 1,500 negative ions per second”. The only way this can be true, without a power source, is through radioactive decay.
zen loop duo
Radium was even included into kitchenware — in this case the source is kept away from the water, which is only irradiated and not contaminated so it is perfectly drinkable, in others the water became contaminated:

In several places on Dartmoor there are old mineshafts that are now sealed-off because the granite releases a lot of Radon (gaseous alpha emitter — BAD!) and the threat to health is significant. In other parts of the world the quackery continues and you can pay to stay in them. Its brochures attract visitors with the phrase “the unmedical approach to arthritis.” Presumably such wording avoids classification as a medical claim and the legal constraints that would go along with it.


And so on to the nuts and bolts of it all — How do we Detect it?

We could sit on top of Ben Nevis, getting bored and looking at fog and invent the cloud chamber…

Which has lovely tracks and patterns like this….

Or we could use the ionising properties to make sparks, as with the spark-gap:

Or we could use the now accepted Geiger-muller tube. They are very simple devices — essentially an anode in a can with a thin end such that alpha can penetrate one of the walls of the container.

The chamber is partially evacuated and a high voltage is applied between the anode and the wall of the can. Radiation ionising the low pressure gas causes a spark. We count the sparks and call that rate of sparks per second the measured activity. (in per-seconds or Becquerels, same thing)

Here’s a nice little sim:

How to do it without a GM tube

In short – if you have sources, you must have a way to test them – ie you have to have a GM tube of sufficient quality to safely test for contamination. There are lots of options in the standard scientific catalogues – it’s great to have a cheaper second too and there are plenty of neat devices available on Ebay, amazon and others.

You can also get vintage kit and these are really good looking and would lend a real sense of drama to your lesson!

Other tools – Enter the Ionisation Chambers

So, I credit Nick Mitchener for this entirely – he really ought to get a LOT more recognition . He’s one of the few people I would expect to have an idea about a bit of physics kit when nobody else does — Give him a follow over at:

Anyway — what you’re doing here is re-creating the GM tube, just using a Darlington pair to make it work at a much lower potential difference. They’re not as good but you can make loads cheaply and hand them out to students to use.

You could also use some of the nice sims that are out there:

Alternative sources

If you have a GM tube of sorts but no sources  then you have

Gas Mantles

Thoriated gas mantles are radioactive and can be used as a source — keep them in plastic bags and treat as any other radioactive source

Thoriated welding rod

Thoriated welding rods are radioactive and can be used as a source — keep them in plastic bags and treat as any other radioactive source

Uranium Glass

Uranium glass is glass which has had uranium, usually in oxide diuranate form, added to a glass mix before melting for coloration. The proportion usually varies from trace levels to about 2% uranium by weight, although some 20th-century pieces were made with up to 25% uranium. It fluoresces brilliantly under UV light


On to the Radioactive stuff then: Alpha!

Excellent Sim:

All the info:

Beta Decay

Excellent Sim:

All of the info:

Aside – Neutrinos!

Neutrinos are a by-product of the beta decay process that just doesn’t come up at GCSE — it pops up at A-level though! I’m not going to go into to them much save to show you an AWESOME picture of a detector. The little boat has two adults in it, they are fixing some of those light collimater/detectors that look for the very rare flashes of light that sometimes happen when neutrinos pass through stuff:


Radioactivity all around!

Background radiation is all over the place – If you inflate a balloon, statically charge it (by rubbing on your top) and hang it up it will attract any nearby ions. Most of these will be radioactive daughter products. After 20 mins, carefully deflate the thing and pop it under a GM tube — you’ll discover it is now significantly above background!

This is the breakdown of the isotopes you’ve probably collected (sorry, can’t find the source to attribute)

All sorts of stuff is naturally radioactive – People are radioactive – Typically 7000 Bq. So it’s dangerous to sleep with somebody”! However, most of the resulting radiation is absorbed within the owners’ body.

National Background radiation:

The primary decay series causing background radiation is as follows, highlighted in red is radon — a common cause of difficulty in older UK homes:



Your school should have an RPO who can best advise you – the course is really fun and worth doing if you can:

Key things are

1.Display signage:

2.Handle sources correctly:

3. Take precautions:

The sources you will meet in schools

The most common sources are the closed-cup type in these nifty wooden boxes. Always remember your primary safety feature is distance, never handle a source directly and always put back in the box as soon as you have finished using it!

The much less common type are the more modern aluminium sources:

You may also have protactinium — if this smells then it’s time to dispose of it — Please check CLEAPS guidance!

Classic demos

Magnetic steering

Alpha and Beta decay can be steered using a magnetic field — the range of alpha is such that this is hard to see outside of a cloud chamber — Beta on the other hand lends itself to the task. Here’s an example set-up, Vary the position of the detector, with and without the magnetic field present:

Uses of radioactivity


One of the best simulations out there is the “inside story” one from the IoP:

Here’s a few real PET scanners one:

Industrial uses

Beta decay — thickness monitoring:

Alpha – Smoke detector

Radioisotope dating

Here’s a nice analogy — Pour water into the bottle such that the level is constant — this represents the constant level of C14 in a living organism. When the plant dies” you stop pouring. The level will then drop exponentially

Half life

Big misconception — two half-lives is not the time it takes for all the substance to decay. Each half-life halves the number of isotopes from previous….

Fission vs fusion

Fission is essentially the ballistic splitting of unstable, heavy, nuclei — it’s not radioactive decay so much as hit a big lump that’s about to fall apart with a thing and it will split”. The trick to reactors, bombs and the like is controlling the number of neutrons that fly out and collide with other nuclei. A controlled Fission reaction = reactor. Uncontrolled = Boom.

Fusion is the process going on in the sun — the combination of light elements to form heavier ones – at time of writing we have not managed to make a commercial fusion powerstation.

There is LOADS of social science around nuclear power etc:

Power station locations.

Fun things to do:

Stop motion


These images are all made by placing radioactive things over photographic film. In some cases there is stuff in the way.

This boot picture was taken using a boot contaminated at fukishima:

You can recreate these using paper that is UV sensitive:


Make 3d models

Chain reactions

Chain Reaction | Pepsi Max | #LiveForNow – YouTube

AND Finally – Lego!

Build Your Own Universe poster

That’s it for now!

Triangle mountains