Scintillation-detecting instruments

SPA technology requires only pipetting steps and there is no need to use scintillation cocktails or to perform a separation, and thus it is ideally suited for automation by robotic liquid handling systems. However, as a consequence of the nonseparation SPA method and due to the screening of colored synthetic or natural compounds, it is essential that the detection instrument accurately assesses the level of color quenching and corrects the observed count rate (cpm) to the true activity (dpm) [49]. 

A liquid scintillation counter. This instrument is used to detect radiation and measure disintegrations per minute quickly and accurately. 

Other uses are to produce phosphorescence and fluorescence in organic compounds and for scintillation screens on instruments used to detect radiation. Radium salts were used in the past to paint the dials of luminous clock faces that glow in the dark. 

Geiger counter. Also known as a scintillation counter. A device used to detect, measure, and record radiation. The instrument gets its name from one of its parts, the Geiger tube, which is a gas-fiUed tube containing coaxial cylindrical electrodes. 

There are two ways to operate the stop-flow mode. The by-level mode, which is based upon the defined count zone(s), operates by signaling the instalment to continue running until a radioactive peak above a designated level is detected. The instrument will then stop and count the predefined fraction (in seconds), which is defined in the count zone. A second way to operate the stop-flow mode is the by-fraction mode. The instalment will stop and count every fraction (in seconds) within the predefined count zone, regardless of whether any radioactivity is detected or not. The fraction size in the stop-flow mode and the volume of liquid scintillant used for counting can be automatically calculated by the instalment or the parameters can be defined by the user. 

Run Module The run module is used to set up the LC conditions, including the pump (e.g., gradient method), UV methods (e.g., wavelength), and autosampler (e.g., sequence setup). Apart from the LC, the method for radiochemical detection and the volume of liquid scintillant for radioactivity counting is also programmed in this module. The autosampler (as set up by the mn module) is used to trigger the Start Run for all other instruments through external contact closures. 

The sample is purified by distillation to separate the tritium-containing water from both non-radioactive and radioactive impurities. Various substances can cause scintillations by means other than radionuclide emission - by chemical fluorescence or luminescence - or interfere with ( quench ) detection of scintillations due to radionuclides. Even after purification, both processes are inevitable, but to a limited extent. Luminescence due to visible light will decay when the sample is stored in a darkened region of the LS system before the sample is counted. The degree of quenching, notably due to water in the sample, is determined instrumentally by reference to comparison sources and recorded, so that any deviation from the quenching observed for the tritium standard can be taken into account. 

Another instrument often used to detect levels of radioactivity is a scintillation counter, which takes advantage of the fact that certain substances, such as zinc sulfide, give off light when they are struck by high-energy radiation. A photocell senses the flashes of light that occur as the radiation strikes and thus measures the number of decay events per unit time. 

This instrument was developed as a centrifugal version of the Allen and Svarovsky s [13] x-ray gravitational sedimentoineter in order to reduce the analysis time and measure down to smaller sizes [14,1.5]. fhe x-rays are generated by an air cooled low power x-ray tube and, after passing through the suspension, they are detected by a scintillation counter. The signal is... 

Figure 3-6. A refrigerated scintillation spectrometer. The large bottom cabinet is a deepfreeze unit that contains the sample holders and photomultiplier detection units. The typewriter is used for data print out. (Courtesy of Packard Instrument Company, Inc.)...

In gas-filled as well as scintillation detectors, the observed count rate is typically less than the actual decay rate of the radionuclide. The efficiency of detection may differ from particle to particle under identical conditions using the same type of detector. The factors that affect the efficiency of detection are operating voltage, resolving time, geometry of the instrument used in relation to the position of the sample with respect to the detector, scaler, energy resolution, absorption by cells, and sometimes constituents of the sample itself. 

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