ALPHA DECAY

This process tends to take place for larger unstable nuclei. The strong nuclear force cannot extend over the whole nucleus so occasionally the electrostatic repulsion gets a look in spitting out the helium nucleus that is the a particle.

Often the a particle is associated with one or more g photon. These are at specific energies which uniquely identify the isotope. This is an indicator of quantitized energy levels of the nucleides themselves - presumably the nucleus in some way resembles the outer atom!

As nuclei have changed and both photons and a particles have energy, when we total the masses on either side of the equation, we find that the mass of the right is less than the left, the parent.

mass difference = mass parent - ( mass daughter + mass alpha )

To calculate the energy split between the daughter and alpha particles ( remember, a "parent" nucleus disintegrates into "daughter" nucleus and emission ) we look at the conservation of momentum.

Taking the parent as stationary,

MD. VD = ma.va

Square both sides

( MD. VD )2 = ( ma.va )2 and fiddle gives

MD. VD2 / ma.va2 = Ek daughter / Ek alpha = ma / MD

Add one to both sides and fiddle a bit more gives

( Ek alpha + Ek daughter ) / Ek alpha = ( ma + MD ) / MD = Total available energy / Ek alpha = M parent / MD

Inverting this gives the maximum kinetic energy of the alpha particle in terms of the total available energy.

Ek alpha = ( MD / M parent ) . Total available energy

eg Consider Bi-212 alpha decaying without an accompanying gamma emission.

Bi-212 decays to Tl-208 with an energy release of Q = 6.20 MeV

then the maximum Ek of alpha = ( 208/212 ) . 6.20 MeV = 6.08 MeV

Similarly Ek of daughter = ( 4/212) . 6.20 MeV = 0.12 MeV

This simple partitioning of energy only occurs if no gamma ray is to be produced - a process depending on the internal rearrangement of the nucleus.

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