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Tracers
Published in Werner Käss, Tracing Technique in Geohydrology, 2018
Counting tubes are chambers filled with special ‘counting gases’, with an internal electrical field, usually between the electrically conducting counting tube which acts as the (negative) cathode and the anode, usually a wire. If ions (e.g. ß-particles) enter this field, they are accelerated. When they hit the gas molecules, an ion avalanche is released, which is registered as an electrical pulse, γ-rays can also be registered, if they release electrons due to interaction in the counting gas or counting tubewall. The radioactive sample to be measured is either outside the chamber (Fig. 78 left) or in special cases, it is placed directly in the counter. The least expensive, but also least sensitive detectors can be inserted directly in the water with the dissolved radioactive tracer (Fig. 78 right). For particle radiation, the counting efficiency is high, provided that the particles actually enter the sensitive inner chamber of the counter. For γ-radiation, the efficiency ratio is usually low, since the mass of the counter with which the y-radiation can cause an interaction is small.
Total Particle Counts
Published in James Agalloco, Phil DeSantis, Anthony Grilli, Anthony Pavell, Handbook of Validation in Pharmaceutical Processes, 2021
Counting efficiency is an expression of the probability that an OPC will sense, and therefore count, a particle passing through the particle counter’s sample volume. This probability is a function of size up to a certain critical size above which all particles are normally sensed and counted. Figure 31.9 displays the plots of counting efficiency versus particle size. Note that although the signal produced by the particles is distributed symmetrically about the nominal most sensitive threshold, the exponential relationship between particle size and signal returned causes the counting efficiency curve to be asymmetrical.
Determination of Radon-222 Content of the Atmosphere
Published in James P. Lodge, Methods of Air Sampling and Analysis, 2017
The alpha counter should be calibrated with a standard such as 242Cm of known disintegration rate distributed over a geometric area similar to the area of the air filter sample. The “counting efficiency” is the ratio of the count rate of the standard to the known disintegration rate. Suitable standards are commercially available and should be traceable to the National Bureau of Standards.
Generation, characterization, and comparison of human exhaled and technical aerosols for the evaluation of different air-purifying technologies against infectious aerosols
Published in Journal of Occupational and Environmental Hygiene, 2022
Thomas Penner, Simon Berger, Jennifer Niessner, Achim Dittler
Figure 11 exemplarily shows particle size distributions of an aerosol generated with the AGK 2000 from a 4% NaCl solution measured by SMPS and OPC. The spike in particle concentration below 20 nm for the SMPS does not occur in every measurement and is likely an artifact caused by leftover aerosol in the DMA in consecutive measurements. The size distribution measured by SMPS shows a mode at around 80 nm, which is not covered by the OPC. Instead, the OPC measurement shows a mode at around 300 nm and a steep drop in concentration for smaller sizes, while the SMPS measurement indicates a significant amount of particles in that size range. This drop in particle concentration below a size of around 300 nm is often observed when measuring with optical particle counters. These observations can be explained by the counting efficiency of OPCs in that size range. The counting efficiency is the probability that an OPC will accurately detect a particle in the instrument’s detection area. OPCs typically use a trigger threshold to exclude light scattered by the air from being counted as particles. As the intensity of scattered light by small particles decreases with particle size, they are increasingly harder to differentiate from background noise by exceeding the trigger threshold. About studies measuring human exhaled aerosol using OPCs (Haslbeck et al. 2010; Schwarz et al. 2010, 2015) the following conclusions can be drawn:
Determining the cutoff diameter and counting efficiency of optical particle counters with an aerodynamic aerosol classifier and an inkjet aerosol generator
Published in Aerosol Science and Technology, 2020
Steven Tran, Kenjiro Iida, Kumiko Yashiro, Yoshiko Murashima, Hiromu Sakurai, Jason S. Olfert
The cutoff diameter of an OPC is the diameter at which the counting efficiency is 50%. The counting efficiency of an OPC drops as particle sizes decrease due to the amount of light scattered by the particle being below the limit of the detector. Current standards for evaluating the counting efficiency of OPCs are outlined in ISO 21501-4 (ISO 2018). The procedure to evaluate the counting efficiency involves the use of two populations of calibration particles (often polystyrene latex [PSL]); one with a size close to the minimum detectable particle size of the OPC and another that is 1.5 to 2 times larger than the minimum detectable particle size. The counting efficiency is determined by taking the ratio of the concentrations measured by the particle counter being tested and a reference particle counter (either a condensation particle counter paired with a differential electrical mobility classifier or a calibrated OPC).
A direct-reading particle sizer with elemental composition analysis for large inhalable particles
Published in Aerosol Science and Technology, 2022
James Sipich, Christian L'Orange, Kimberly Anderson, Christopher Limbach, John Volckens, Azer Yalin
The counting efficiency (ηcount) represents the fraction of all particles entering the device which are successfully counted and sized. The number of entering particles is found as the number of scattering events in the top beam while the number of particles counted and sized is found as the number of particles yielding a pair of scattering signals that can be paired to compute a transit time: