Radioactivity I

Radioactivity I is the fourteenth lecture within the Properties of Matter subtopic of PH1011. It covers the proprties of alpha, beta and gamma radiation, the effects that these have on their parent atoms, and half-lives.

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Next: Radioactivity II

Alpha, beta, gamma
Alpha radiation involves the emission of an α-particle - a helium nucleus, of two protons and two neutrons. This can be deflected using a magnetic field, and can cause high levels of damage if it manages to pass into avulnerable system. However this is uncommon, as α-particles can be absorbed by a sheet of paper (and therefore cannot penetrate skin). Emission of an α-particle causes its parent element to shift two elements down in the periodic table (due to the loss of two protons) and loose significant mass.

Beta radiation is the emission of an electron from the nucleus; is is deflected in the opposide direction in a magnetic field, and causes far less damage than an alpha particle - but equally is more penetrating (can be stopped by a sheet of aluminium). Emission of an electron from an atom also releases an electron anti-neutrino ( ̅νe). The opposite of thisis electron capture, where the nucleus gains an electron but still releases ̅νe - and positron emission can also occur, still causing a ̅νe to be released.

Gamma radiation involves the release of a gamma ray - not a particle at all, but an electromagnetic wave. These are not deflected by magnetic fields, and can only be stopped by thick lead. γ emission is caused by nuclear rearrangements (electrons changing configuration, for example) and usually occurs alongside other emissions.

Decay
Decays usually occur in series - that is, the initial decay will usually lead to another unstable isotope, and this will in turn decay until a stable atom is reached (usually lead). Radioactive decay is a statistical process; it lessens with time by the relationship dN = -λNdt => dN/N = -λdt. This can be integrated down to give:

N = Noe-λt

The decay rate R is the number of decays per second, and is given by R = -dN/dt = λN = λNoe-λt = Roe-λt.

To find the half life, simply let N = No/2 and substitute that into N = Noe-λt -
 * 1/2 = e-λt
 * t0.5 = ln(2)/λ

Graphing the number of atoms by half life, therefore, gives an exponential decay.

Utilising radiation as power
As particles are emitted with energy, this can be harvested and used as power. The total power available is given by number of decays per second x energy of each or P = R x E


 * eg (mass/molar mass) * Na * λ * energy of each

Summary
α, β-, and γ particles can be emitted spontaneously from an unstable nucleus. The leave with energy which can be used as power.