LSC Principles

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Tsble 1: Typical radionuclodes for LSC counting



            β energy



Half-Life Decay Emax Emin Monitor Specific Activity
125I 60 days γ Auger electrons
0.035 MeV
γ-probe
2167 Ci/mA
3H   12.3 years β 0.019 MeV
0.035 MeV
Swabs
28.8 Ci/mA
14C   5730 years β 0.156 MeV 0.006 MeV
β Counter
62.4 Ci/mA
25S 87.4 days β 0.167 MeV
0.049 MeV
β Counter
1494 Ci/mA
32P 14.3 days β 1.709 MeV
0.69 MeV
β Counter
9311 Ci/mA


uantitative measurements of beta-radiation, alpha-radiation and low-energy gamma-radiation with traditional gas or solid state detectors may be difficult due to the self-absorption of the radiation in the sample itself and further absorption in the counter window. Therefore, low-energy radiation is, in many cases, best detected and measured with liquid scintillation counting where absorption loss is absent. Here, the sample and the detector (the liquid scintillator) is intimately mixed in a homogeneous solution. In addition, this leads to nearly 100% counting geometry (only the liquid surfaces against the container wall and the air above the liquid account for a small loss in counting geometry).


A liquid scintillator consists of a solvent and a primary scintillant. In some cases a secondary scintillant is also added. The function of these substances is described below.


The solvent: The primary function of the solvent is to solubilize the radioactive sample, preferentially into a homogeneous solution. An equally important function is to absorb and transfer the energy deposited by the radioactive decay event to the primary scintillant. The process is as follows:


The β-particles from the radioactive substance ionize and excite molecules in the solution, - first and foremost the solvent molecules because of its high concentration. The excitation energy is transferred from one solvent molecule X to another. Some of these excitation transfer chain reactions result in the encounter with a molecule of the primary scintillant Y. The excitation is transferred to this molecule:



[math]\beta+X_{1}\rightarrow X_{1}*+X_{2}\rightarrow X_{2}*+X_{3}\rightarrow X_{3}*...\rightarrow X_{n}* + Y \rightarrow Y*[/math]