Alpha\Beta-Discrimination

Back to Main In modern LSC equipment it is possible to discriminate between the α - and β -emissions. The resolution of the α -peaks is relatively poor due to the small number of excitations produced but the background associated with the α -emissions is very small. The ability to discriminate between α- and β- particles lies in the small difference in pulse shapes. The following steps explain the difference in the pulse profile. > e - +X + →X * (exited singlet) > e - +X + →X * (exited triplet) > β produce mainly singlets while α produce mainly triplets. > 3 X * + 3 X * → 1 X * + 1 X * + phonons > This is triplet annihilation. By electronic analysis of the descending portion of the amplifier-integrated pulses, it is possible to almost completely (99.95+%) separate α -pulses from β -pulses.
 * 1) The initial interaction of the α -particle is much stronger than the β -particle due to a) the double charge, and b) the low velocity. The α -range is much less than β -range so that the ionisations produce a very high density track. Compared with the β -particle, the α -particle produces fewer excitations (~ 0.4%). Since it is the excitations which contribute to the pulse, equal energies of α and β give an α -pulse height approximately 10% of the β -pulse height. It is the greater density of ions and electrons which will produce the difference in the pulse profile.
 * 2) Due to the density of ions, the probability of recombination of an ion and electron is greater for α than β.
 * 3) Recombination may produce a ground state molecule or an excited molecule.
 * 4) Quantum mechanics postulates the number of states (orientations) is given by 2s + 1, where s = spin. Spin of singlet (S) = 0, spin of triplet (T) = 1. Hence, for a singlet, the number of states = 1 whereas for a triplet, the number of states = 3.
 * 1) The excited singlet will undergo fluorescence and emit a photon in a very short time.
 * 2) The excited triplets have a longer lifetime due to the low probability of changing spin from 1 to 0. The concentration of excited molecules is such that there is a probability of two 3 X * molecules colliding.
 * 1) The 1 X * decays rapidly but has been delayed by the lifetime of the 3 X * molecules, i.e. produces "delayed fluorescence".
 * 2) The enhanced delayed fluorescence contribution for α produces a longer tail (30-40 ns) to the output pulse compared with β.