Preamplifier scintillation detector

The concentration of 222Rn in air was determined with a radon measurement detector. The detector allows realizing continuous radon monitoring. It consists of an electronic unit and a scintillation cell. The electronic unit contains power supply, amplifier, discriminator, timer, counter, and indicator. The scintillation cell contains the zinc sulfide scintillator, photomultiplier, preamplifier, high voltage power supply and chamber with a volume of 200 mL over the scintillator. This chamber is filled with the gas to be analyzed. The air is either pumped or diffuses into the scintillation cell. The scintillation count is processed by electronics, and radon concentrations for predetermined intervals are stored in the memory of the device. 

Other Components and Techniques. Other components of a liquid scintillator detector include (1) electronics, (2) a photomultiplier tube, (3) a preamplifier, and (4) a pulse-height analyzer. Description of these components and discussion of relevant topics such as (1) efficiency of scintillation counting, (2) quenching, (3) counting statistics, (4) assay optimization, and (5) radiation safety can be found in an earlier edition of this textbook. ... 

The gamma ray scintillation spectrometer (Fig. 1) consisted of two single channel analyzers coupled to a common sodium iodide well detector, preamplifier, amplifier and scalers. By setting each analyzer for the appropriate energy the two isotopes were determined. 

The broadening of the photo peak has many causes such as inhomogeneities in the crystals and variations in charge, or light, collection and noise in the preamplifier. However, for scintillation detectors the main cause is found in the PMT where nonuniformity in the photocathode, fluctuations in the high voltage imposed on each dynode, and statistical variations in the small number of photoelectrons formed at the photocathode are all contributing factors. 

Current mode preamplifiers can provide reliable counting up to 10 counts per second. They are specially suited for use with fission chambers.Voltage sensitive preamplifiers, with low gain, have high input impedance and low output impedance. They are useful for use with scintillation detectors, and also for matching of cable impedance. 

The second type of event is the absorption of lower energy X-ray photons within the bulk of the silicon photodiode. Here the overall conversion efficiency is determined simply by the energy required to produce an electron-hole pair, which in silicon is 3.6 eV. Thus the OCE for this type of detection is 275 electrons/keV. The signals from the silicon are much faster than those from the scintillator, and will, in practice, be limited by the risetime of the preamplifier used ( 60ns). The basic principle of operation of the hybrid detector is shown in Figure 1. 

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... 

As with the preamplifier, the scintillation amphfier need not be of such a demanding low noise specification as would be needed for semiconductor systems. In the manufacturers catalogues, a distinction is commonly made between amplifier , suitable for low-resolution spectrometry, and spectroscopy amplifier intended for high-resolution spectrometry using semiconductor detectors. Typical simple amplifier modules provide pole-zero cancellation and automatic base line restoration. The pulse shaping time options provided are often limited on such instruments and may need to be selected internally. Because of the faster rise time of scintillation pulses, the time constants provided are usually within the range 0.2 to 2 or 3 (its. 

Scintillation Crystal-Photodiode Coupled Detector. The performance of this detector, which was developed more recently than its counterparts, has been found to be satisfactory. A typical scintillation crystal-photodiode detector comprises a CsI(Tl) scintillation crystal, a P-N junction photodiode coupled to the crystal, and a preamplifier for the low-level signal amplification. 

The incident X-ray photon is converted to visible light in the scintillation crystal, which then falls into the PN junction photodiode. Generated electron-hole pairs are collected at the junctions. Since the generated current is usually weak, a low-noise preamplifier is required. The voltage output is proportional to the energy and fluence of the X-ray incidence on the detector. 

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