Difference between revisions of "Alpha/beta-Discrimination"
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− | In modern LSC equipment it is possible to discriminate between the | + | ====== Written and developed by [http://www.mn.uio.no/kjemi/personer/vit/torbjor/index.html Prof. Tor Bjørnstad] (IFE/UiO) ====== |
− | + | ||
− | The ability to discriminate between | + | Return to [[Liquid Scintillation Counting|Main]] |
− | + | ||
− | # The initial interaction of the | + | |
− | # Due to the density of ions, the probability of recombination of an ion and electron is greater for | + | |
− | # Recombination may produce a ground state molecule or an excited molecule. | + | In modern LSC equipment it is possible to discriminate between the <span class="texhtml">α</span>- and <span class="texhtml">β</span>-emissions. The resolution of the <span class="texhtml">α</span>-peaks is relatively poor due to the small number of excitations produced but the background associated with the <span class="texhtml">α</span>-emissions is very small. |
− | # 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. | + | |
+ | The ability to discriminate between <span class="texhtml">α- and </span><span class="texhtml">β-</span>particles lies in the small difference in pulse shapes. The following steps explain the difference in the pulse profile. | ||
+ | |||
+ | #The initial interaction of the <span class="texhtml">α</span>-particle is much stronger than the <span class="texhtml">β</span>-particle due to a) the double charge, and b) the low velocity. The <span class="texhtml">α</span>-range is much less than <span class="texhtml">β</span>-range so that the ionisations produce a very high density track. Compared with the <span class="texhtml">β</span>-particle, the <span class="texhtml">α</span>-particle produces fewer excitations (~ 0.4%). Since it is the excitations which contribute to the pulse, equal energies of <span class="texhtml">α</span> and <span class="texhtml">β</span> give an <span class="texhtml">α</span>-pulse height approximately 10% of the <span class="texhtml">β</span>-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 <span class="texhtml">α</span> than <span class="texhtml">β</span>. | ||
+ | #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. <br>e<sup>-</sup>+X<sup>+</sup><math>\rightarrow</math><sup>1</sup>X<sup>*</sup>(exited singlet) <br>e<sup>-</sup>+X<sup>+</sup><math>\rightarrow</math><sup>3</sup>X<sup>*</sup>(exited triplet)<br><span class="texhtml">β</span> produce mainly singlets while <span class="texhtml">α</span> 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 <sup>3</sup>X* molecules colliding.<br><sup>3</sup>X<sup>*</sup> + <sup>3</sup>X<sup>*</sup><math>\rightarrow</math> <sup>1</sup>X<sup>*</sup> + <sup>1</sup>X<sup>*</sup> + phonons<br> This is triplet annihilation. | ||
+ | #The <sup>1</sup>X<sup>*</sup> decays rapidly but has been delayed by the lifetime of the <sup>3</sup>X* molecules, i.e. produces "delayed fluorescence". | ||
+ | #The enhanced delayed fluorescence contribution for <span class="texhtml">α</span> produces a longer tail (30-40 ns) to the output pulse compared with <span class="texhtml">β</span>. | ||
+ | |||
+ | By electronic analysis of the descending portion of the amplifier-integrated pulses, it is possible to almost completely (99.95+%) separate <span class="texhtml">α</span>-pulses from <span class="texhtml">β</span>-pulses. | ||
+ | |||
+ | [[Category:Detection]] [[Category:Alpha_Detector]] [[Category:Gamma_Detector]] [[Category:Scintillation_Detector]] [[Category:Nuclear_Properties]] [[Category:Master]] |
Latest revision as of 10:46, 9 July 2012
Written and developed by Prof. Tor Bjørnstad (IFE/UiO)
Return 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.
- 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.