Monday, 19 November 2012

Radioactivity

Marie Curie - winner of two Nobel prizes
Recently I did a bit of a general post on atoms and atomic nuclei, which got me thinking about radioactivity and nuclear magnetic resonance (NMR). The first of these, radioactivity, I am going to post about here, the other, NMR, I'll do in a post shortly.

While Rutherford was undoubtedly one of the great contributors to the study of radioactivity, I think that the most famous must be Marie Curie, Nobel Prize winner in both Physics and Chemistry, the only person to win two prizes in the sciences.

Curie discovered polonium and radium. These elements provided Rutherford with the materials he need to do his great works. She came up with the name Radioactivity. She was also instrumental in the development of the use of X-rays in medicine. The Red Cross actually made her head of its radiological service.


Today there are numerous schools and institutes bear her name. Her research and long term exposure to radioactive substances would eventually cause her cancer that would ultimately kill her. Cancer research and cancer charities now carry her name.

Albert Einstein once remarked that she was probably the only person not corrupted by fame. What a superstar!

Marie Curie I think you are great.

I also think Radioactivity is great because it features the nucleus of the atom, that magnificently tiny speck at the centre.

Apart from Hydrogen, which is a bit of a special case in many ways, all nuclei consist of combinations of protons and neutrons. It is the protons in the nucleus which determine what element we are dealing with. For example, any atom with 6 protons in the nucleus is Carbon. If it had 7 protons it would not be Carbon, it would be Nitrogen.

Carbon typically has 6 protons and 6 neutrons, but it can have 8 neutrons, this is C14. Nitrogen usually has 7 protons and 7 neutrons, so it is N14. N14 is very similar in mass to C14, but it is the difference of just 1 proton that changes everything and gives us two very distinct elements. There are NO two elements with the same amount of protons. Though there are some with the same amount of neutrons.

Isotopes are variants within a single element, by this we mean that an element can have atoms with a different number of neutrons, eg Carbon 12, Carbon 13 and Carbon 14 are said to be isotopes of Carbon.

Some isotopes are found to be unstable and are known to be radioactive. They can undergo a reaction resulting in particles being ejected from the nucleus. The most common types of radiation, are alpha, beta and gamma radiation. There are certainly more than this, including proton emission and neutron emission.

The names alpha, beta and gamma are from the Greek alphabet and were used by Rutherford and Villard to describe the different types of radiation they had witnessed in experiments. At the time they had no idea what they actually were. It took Rutherford 7 years to prove that alpha particles were actually the same as a Helium nucleus. The beta particle was known to be far more penetrating than the alpha particle, but again it took a considerable time to prove it was in fact and electron.

The realization that a beta particle was an electron was a major discovery because it showed that electrons can originate from a nucleus, even though a we know that the nucleus only contains neutrons and protons. This in turn lead to the idea that a neutron can decay into a proton and an electron that is then ejected as a beta particle.


It was Villard who discovered gamma rays while studying radium in 1900 and it was Rutherford who gave it its name in 1903, these are not particle but are high energy light waves, so energetic that they can go through a fair amount of lead shielding!

The discovery of radioactivity and its associated properties lead to a rather astonishing idea and it was that it should be possible to use this behaviour to calculate just how old the earth is! The idea was dreamed up by Rutherford, who you may have gathered was a bright chap, in 1905, the same year Einstein was publishing is ideas on Special Relativity.

Rutherford realized that radioactivity was due to some atoms changing into lighter elements and emitting alpha, beta or gamma radiation. They also discovered that particular "isotopes" of a specific element decay at a distinctive rate, now called its half life.

Imagine you had a billion atoms of Uranium 235, the half life is the time it takes for 0.5 billion of them to decay into something else. It turns out that Uranium and thorium have very long half lives and so stay for long long periods, which is why they are so difficult to manage when we take them out of nuclear reactors.

Rutherford figured that it may be possible to work out the age of the earth from the relative proportions of radioactive materials in rock samples. This is slightly trickier than you might think because some of the particles that are produced by the radioactive process are themselves radioactive and decay! So you end up with a sort of family decay tree, with parents, children, grand children and so on.

In the following 50 years this method was investigated and refined, there are about 40 different techniques that have been used working with a fairly large variety of radioactive materials.

The results put the earth at about 4.5 billion years old. Which is only about 1/3 the age of the known universe, but similar in age to the sun, it was thought to have formed within 100 million years of so after the sun.

100 million years! You pop it in a sentence as if it is nothing, but think about that for one minute. 100 million years after the sun fired up with think the earth formed. The earth has been here for about 4.5 Billion years! the is a period of time that is just so far beyond our imagination.

Atoms on earth have been undergoing radioactive decay for about 4.5 billion years. That is loads of atoms undergoing loads of decay and it is still going on. We still have radioactive elements in the earth. For all that time, this process has been going on and on and on.

It really does stagger my brain. I think I need to go for a lie down!

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