Electronics scintillation systems

Samples for liquid scintillation counting consist of three components viz. (1) the radioactive material, (2) a solvent, usually aromatic, in which the radioactive substances is dissolved or suspended, and (3) one or more organic fluorescent substances. Components (2) and (3) make up the liquid scintillation system. The P particles emitted from the radioactive sample (most of the radioisotopes used in biochemical research are P emitters) interact with the scintillation system, producing small flashes of light or scintillations. The light flashes are detected by a photomultiplier tube (PMT). Electronic pulses from the PMT are amplified and registered by a counting device called a scaler. 

Breakdown in control and stability of the immediate detector environment with regard to cleanliness, temperature level, power supply, and radiation background interferes with reliable radiation detector operation. Electronic components function best at a cool, constant temperature in a dust-free environment. Special low-temperature and power-supply-stability controls are needed to stabilize the response of gamma-ray spectrometers and liquid scintillation systems. 

The prototype system used in the trial was made by General Electric Medical Systems (Milwaukee, WI). The system used a 18 x 23 cm detector incorporating an amorphous silicon thin-film transistor bonded to a cesium iodide crystal scintillator. Other than the detector and associated electronics, the system was identical to one of the GE s clinical units, the DMR. For this reason, the GE DMR was chosen as the film unit for the trial. The digital and film units had almost identical dimensions, allowing the same patient positioning to be used on both. The only noticeable difference was the thickness of the digital detector on the prototype, compared with the film cassette used on the DMR. 

Unlike the semiconductor detector system where electronic noise can be a major cause of poor resolution, this is not a significant factor in scintillation systems, but we include it here for completeness. 

As with all detectors, the pulse of current at the output, in this case the PMT anode, must be integrated to provide the signal. Because electronic noise is usually not a problem, preamplifiers for scintillation systems need not have a particularly low noise specification. AU three types of preamplifier - voltage, current and charge-sensitive - are in common use. Charge-sensitive preamplifiers are often offered for routine use but low cost voltage-sensitive types are also common. For normal gamma spectrometry... 

An ion beam causes secondary electrons to be ejected from a metal surface. These secondaries can be measured as an electric current directly through a Faraday cup or indirectly after amplification, as with an electron multiplier or a scintillation device. These ion collectors are located at a fixed point in a mass spectrometer, and all ions are focused on that point — hence the name, point ion collector. In all cases, the resultant flow of an electric current is used to drive some form of recorder or is passed to an information storage device (data system). 

Scintillators are also used in the detectors of CT scanners. Here an electronic detector, the photomultiplier tube, is used to produce an electrical signal from the visible and ultraviolet light photons. These imaging systems typically need fast scintillators with a high efficiency. 

The availability of integrated circuits paved the way for the next generation ofX-ray inspection system. These units featured a side-shooting fan beam of X-rays incident on an extended array of scintillators optically coupled to photodiodes or phototransistors. The resulting low-level electric currents were then amphfied, integrated, and electronically sampled and digitized. Such systems were under development by ScanRay and Picker in 1977. By 1979, Picker was marketing... 

The combination of the picosecond single electron bunch with streak cameras, independently developed in 1979 at Argonne National Laboratory [55] and at University of Tokyo by us [56], enabled the very high time resolution for emission spectroscopy. The research fields have been extended to organic materials such as liquid scintillators [55-57], polymer systems [58], and pure organic solvents [59]. The kinetics of the geminate ion recombination were studied [55,57,59]. 

The modified TPX offers low efficiency for generating scintillation pulses. One reason for the low light output is that the TPX host polymer does not have an extended system of -electronic bonding. Alternative materials are polymers consisting of p-tert-butylstyrene and 4-vinylbiphenyl (31). 

Of the three commonly used X-ray detectors—(1) Geiger counter, (2) scintillation counter, and (3) proportional counter—the latter is used most frequently for electron-probe microanalysis. In the wavelengths from 1 to 10 A, sealed proportional counters may be used. For longer-waveleiigtli analysis—in the range from 10 to 93 A—the thinnest possible detector window is required to limit spectral attenuation. Nitrocellulose windows have proved successful. Nondispersive detection systems using cooled Li-dnfted Si are also applicable. 

Fig. 1. A magnetic + Si(Li) combination conversion-electron spectrometer based on an "old" Siegbahn-SIStis magnet. 1) beam, 2) target,3) target-changing system, 4) collimator and current measurement, 5) Faraday-cup, 6) Pb shield, 7) anti-positron baffle, 8) detector, 10) cold fingers, 13) cylindrical plastic scintillator 14) light guide, 16) P.M. tube.

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