Written and developed by Prof. Tor Bjørnstad (IFE/UiO)
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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.
- 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.
- Due to the density of ions, the probability of recombination of an ion and electron is greater for α than β.
- Recombination may produce a ground state molecule or an excited molecule.
- 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.
e-+X+ 1X*(exited singlet)
e-+X+ 3X*(exited triplet)
β produce mainly singlets while α produce mainly triplets.
- The excited singlet will undergo fluorescence and emit a photon in a very short time.
- 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 3X* molecules colliding.
3X* + 3X* 1X* + 1X* + phonons
This is triplet annihilation.
- The 1X* decays rapidly but has been delayed by the lifetime of the 3X* molecules, i.e. produces "delayed fluorescence".
- The enhanced delayed fluorescence contribution for α produces a longer tail (30-40 ns) to the output pulse compared with β.
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.