Wednesday 2 September 2009

Ask a particle physicist with Dr Jeanne Wilson

Some time ago, I posed several questions about particle physics and radioactivity on this blog (now removed). I am happy to say that I have had my questions answered, courtesy of Dr Jeanne Wilson.

Dr Wilson is a former lecturer in physics at Oriel College Oxford and begins a lectureship Queen Mary, University of London next year. She studied physics at Sheffield and Oxford Universities in the UK and specialises in neutrino physics.
You can find a brief outline of her research here - http://www.oriel.ox.ac.uk/content/337/dr-jeanne-wilson-physics-

Jeanne kindly answered 3 questions for me. One of these is reproduced below- the answers to the other two will be available online shortly. These questions and answers are to be the first of a series of articles called "Ask an Expert",where I will ask experts in a range of fields for simple answers to fundamental and difficult questions about life, the laws of nature and the world in general.

I will modify this post to include a link [here] once the full article is available. In the meantime, here is question 1

VN) I wondered why alpha particles are the largest form of radiation? Are larger particles theoretically possible but only occur with a very, very, very long half life or at much higher or lower energy levels than in the universe as we experience it?
JW) Alpha particles are actually helium nuclei – they are made up of two protons and two neutrons, a stable particle. Radioactive decay occurs because the process of emitting the radiation releases energy leaving products in a more stable configuration than what you started with. You start with a nucleus with some intrinsic energy (as Einstein’s famous equation tells us – energy is related to mass) and you end up with a new nucleus and an alpha particle which together have a lower intrinsic energy. However, for this to happen you must negotiate the “energy barrier” provided by the force that holds nuclei together. Energy to step over this barrier could be provided by a external process such as a collision with another particle, but in the case of spontaneous radioactive decay the barrier is not stepped over, but tunnelled through. This phenomenon is called “quantum tunnelling” and is due to the wave-like behaviour of particles. In quantum physics, the wave describes the probability of a particle being in a certain location with a certain energy. If the barrier is narrow enough, the wavefunction extends to the other side of the barrier and there is a small possibility that the alpha will escape from the nucleus.

In principle, you could release something larger than an alpha particle but then the energy barrier is much larger and therefore quantum tunnelling is much less likely. This process is a form of radioactive decay, called spontaneous fission, and can only occur for very heavy nuclei.

-Many thanks to Dr Jeanne Wilson.

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