As scintillation cocktail to produce light (about 1 photon/keV

As early as 1966, H. P. Kallmann noted that certain organic materials fluoresced under ultraviolet light (Elster and Geitel, 1899). Kallmann also showed that aromatic solvents with certain dissolved solutes were efficient scintillation sources when subjected to nuclear radiation and from here on the LSC detection method was further developed.The LSC as we know it today is a widely-used method for the detection of beta emitting radionuclides. The measurement by LSC involves the mixture of the aqueous sample with an appropriate scintillation cocktail. The radionuclides are usually dissolved in a scintillation cocktail, but in some cases suspensions or whole filters are mixed with the cocktail and measured. Scintillation vials must be transparent at the wavelength of the used scintillator and resistant to the solvent. The liquid scintillator is prepared by dissolving of scintillation material in suitable organic solvent. Several liquid scintillation cocktails are nowadays commercially available (Ultima GoldTM, HiSafe®III etc.). Due to the close contact of the radionuclide to22the detector medium high efficiencies can be achieved, so that even nuclides with low energy beta radiation, such as tritium and 241Pu, can be measured with high efficiencies.The scintillation process and light that is produced are different for the alpha and beta decay processes. When alpha decay occurs in a liquid scintillation cocktail, the alpha particles interact with the scintillation cocktail to produce light (about 1 photon/keV of original decay energy). The emitted photons from the sample will be led over reflectors to a photo multiplier tube (PMT), where the detection will take place. Approximately 10 photons per keV of beta particle decay energy are produced in a liquid scintillation process (Lannuziata, 2003).The liquid scintillation counting efficiency for beta particles is dependent on the original energy of the beta decay. For most beta particles with decay energy above 100 keV, the counting efficiency is 90–100 %, but for lower energy beta decays the efficiency is normally in the range of 10–60 % depending on the degree of quench in the sample. The counting efficiency is approximately 100 % for almost all alpha decays using a liquid scintillation cocktail. Because of the slower pulse decay time, alpha particles can be distinguished from most other nuclear decay radiations with the liquid scintillation analyser.Because of the complex processes involved in the energy transfer in the liquid scintillation detector, various disruptions in the energy transfer can occur. These mechanisms are described as quench effects. These quench effects all contribute to a spectral shift to lower energies and to a decrease in the total count rate. There are several ways to correct quench effects; with an internal standard, the sample channels ratio method and with the external standard technique (Horrocks, 1974).The oldest method for quench correction is the internal standard technique. It involves the addition of a known amount of the nuclide in high specific activity to the same sample that is being measured. The efficiency is calculated by taking the difference of the count rates before and after the addition. The main disadvantages with internal standard techniques are that they require additional sample manipulation and precision addition of an accurately labelled standard of the same material as the sample.The Liquid Scintillation Counter used for the measurements was the Wallac Quantulus 1220™ from PerkinElmer Life and Analytical Sciences. The Wallac Quantulus 1220™ is a device for low level measurements; due to its unique detector shielding, consisting of a passive and an active shield, low background levels are achieved. The passive shield consists of three layers; first a lead shield, asymmetrically placed around the detector, with a maximum thickness of 20 cm above the measuring position. The lead shield will absorb high energy cosmic radiation (mostly coming from above) and gradually transform it to low energy23radiation, which will not disturb the LSC measurements. The second layer consists of cadmium, which absorbs low energy (thermal) neutrons and X-rays produced in the lead shield by fluorescence reactions. Any X-ray fluorescence produced in the cadmium is shielded by a third layer, made of copper. The active shield consists of a tank, filled with a mineral oil based scintillator solution. Two photomultiplier tubes (G-PMT, circuited in coincidence), are used to detect scintillations in the tank (Figure 2). The sample itself is also surrounded by two photomultipliers (?-PMT, also circuited in coincidence).Figure 2. Scintillation detectorIonizing radiation moving through the active guard creates scintillations, which are detected by the photomultiplier tubes of the guard detector. The pulse activates a logical signal. If this signal is coincident with a pulse in the ?-PMT it can be used to inhibit the analogue to digital conversion of the pulse. If a quantum from the outside reaches the guard scintillator tank, it will create a scintillation event which is coincidently detected by both G-PMT. If this quantum creates a scintillation event in the sample as well, which is detected by the ?-PMT, the anti-coincidence circuit will inhibit this signal, and consequently reducing the background radiation. The voltage pulse from the ?-PMT will then be supplied to an analogue to digital converter (ADC) and a multi-channel analyser (MCA). The programme incorporated in Quantulus enables the simultaneous measurement of four spectra, each with 1024 channel resolution. The pulse amplifiers yield a linear pulse height spectrum. The analogue to digital24conversion is logarithmic. The data storage is managed by a personal computer using the provided Quantulus software.

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