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Asked by fish238 to Clara, Daniel, Simon, Thomas on 19 Mar 2014.
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Clara Nellist answered on 19 Mar 2014:
Radioactive decay is the process by which unstable atoms will give off energy to try to be in a more stable form. There are three main types of radiation: alpha decay, beta decay, and gamma decay (there are more, but these three were discovered first).
Alpha decay is when the nucleus releases a helium nucleus (two protons and two neutrons). The alpha particle is extremely stable, so this is a very common nucleus to emit.
Beta decay is when a proton changes into a neutron and in the process releases an electron (or anti-electron) and a neutrino.
The last is gamma, which i’ll leave for now because it doesn’t deal with a change in the nucleus of our original atom.
The reason this process is able to release energy is because it takes energy to bind protons and neutrons together. This is called the strong nuclear force and it binds protons and neutrons together in a nucleus, but it only works on a very short distance. Also, charges that are the same repel each other (just like opposite attract). Therefore, because protons are positive, more force is required to hold larger nuclei, with more protons, together. So since it takes less energy to bind two smaller nuclei together, than one big one, this energy is release when they spilt. The same in beta decay when the proton is changed into a neutron (which is a neutral particle and so doesn’t contribute to the repelling of the protons).
But also, when we say that something is radioactive, it’s not that every atom is continually emitting radiation, but that some atoms are. It is a random process, so we don’t know when exactly one atom will emit the radiation, but with enough atoms, we can predict when *an* atom will emit it.
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Thomas Elias Cocolios answered on 19 Mar 2014:
The radioactive decay processes describe the change of an element to another. As such, a radioactive particle decays away into something generally less radioactive, or even stable, and does not keep on emitting more radiation.
The reason that a radioactive sample (e.g. waste) gives off radiation continuously is because you have many of those radioactive particles giving off their own radiation one at a time. It looks like it is radiating all the time, but it is actually dying down with time, according to an exponential decay law (discovered by Ernest Rutherford when he was at McGill University and for which he received the Nobel Prize). The idea is that since the decay process is a question of probability, the more atoms you have, the more likely you are to see a decay. However, once many decays have occurred, you have less probability to see radiation coming off. The probability is different for each radioactive isotope and we quantify this in term of the half-life, that is the measured time it takes for half of your isotopes to decay. Half-lives range from pico-seconds to millions of years! (some half-lives are actually longer than the age of the universe, but those have not really been measured, as you can guess).
The energy contained in the nucleus comes from the famous relation E=mc^2. This relates mass and energy together. In the case of the nucleus, what happens is that in order to hold the protons and neutrons together, some of their respective mass is given away to what is called the ‘binding energy’. This process is related to the short-range strong force that Clara refers to. Depending on your number of neutrons and protons, those particles will take different configurations that will result in a sensibly different binding energies. If you are not in the best configuration, the nucleus will eventually tend to go a more tightly bond state, releasing energy in the process, according the alpha, beta, gamma descriptions that Clara gave, until the system takes the most tightly bound configuration, known as the stable elements. We say that those lie on the ‘valley of stability’, picturing the radioactive isotopes up on a hill and slowly rolling down to the stable river in the middle.
This is the picture that I would like to build up in LEGO blocks to have a visual representation of all the isotopes together.
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Simon Albright answered on 19 Mar 2014:
The energy is in the particle itself.
Atomic nuclei weigh less than the sum of their parts but atomic nuclei could weigh even less if they decay. The universe likes to minimise the energy of a system (like a ball rolling down a hill instead of staying at the top).
For alpha decay there’s so many particles in the nucleus it can’t hold itself together, by splitting up and spitting out a helium nucleus (alpha particle) it lowers the total energy.
For beta decay there are too many neutrons, neutrons weigh a teeny bit more than protons so if there are too many of them it spits out an electron and lowers the energy that way.
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