Detection techniques particle tracks

Up to date, several experimental techniques have been developed which are capable of detecting some of these particles under ordinary thermodynamic conditions. One can use these methods to keep track of transformations of the particles. For instance, it is relevant to mention here the method of electron paramagnetic resonance (EPR) with sensitivity of about 10 particles per cm [IJ. However, the above sensitivity is not sufficient to study physical and chemical processes developing in gaseous and liquid media (especially at the interface with solids). Moreover, this approach is not suitable if one is faced with detection of particles possessing the highest chemical activity, namely, free radicals and atoms. As for the detection of excited molecular or atom particles... 

The radioactive-tracer technique was perfected for. fluidized-bed application through the development of a computer-aided particle-tracking facility (CAPTF) (Lin, Chen, and Chao, 1985 Moslemian, 1987 Sun, 1989). Considerable effort was expended to develop the efficient photoncounting instrumentation and automated data reduction and processing schemes. In Section 9.2, the principle of radiation detection and a theoretical model of the CAPTF is presented. The instrumentation of the CAPTF and the data reduction schemes are described in Section 9.3. Sample results obtained by the CAPTF are presented in Sections 9.4 and 9.5, and a conclusion is presented in Section 9.6. 

Due to the penetrating power of the emitted SllkeV gamma photons, which can pass through several millimetres of stainless steel, detection is possible from within steel reactors or process vessels. The coincident detection of photons is the principle of techniques such as Positron Emission Tomography (PET), Positron Emission Particle Tracking (PEPT), and Positron Emission Profiling (PEP), which are discussed below. 

A large number of radiometric techniques have been developed for Pu analysis on tracer, biochemical, and environmental samples (119,120). In general the a-particles of most Pu isotopes are detected by gas-proportional, surface-barrier, or scintillation detectors. When the level of Pu is lower than 10 g/g sample, radiometric techniques must be enhanced by preliminary extraction of the Pu to concentrate the Pu and separate it from other radioisotopes (121,122). Alternatively, fission—fragment track detection can detect Pu at a level of 10 g/g sample or better (123). Chemical concentration of Pu from urine, neutron irradiation in a research reactor, followed by fission track detection, can achieve a sensitivity for Pu of better than 1 mBq/L (4 X 10 g/g sample) (124). 

A more common technique employs a nuclear emulsion to detect the radiation. The sample is irradiated in close proximity to a sensitive emulsion, which is subsequently developed, fixed, and examined under the microscope. In this way it is possible to distinguish tracks due to alpha particles, fission fragments, etc. Faraggi et al. 22) and Mayr 54) used this technique to determine boron by the B (n,a)Lr reaction down to a level of 2 X 10" gm. Lithium at the 10" -gm level was determined by Picciotto and van Styvendael 69) by the reaction Li (n.,a)H and Curie and Faraggi 18) studied the distribution of uranium in the surface of polished mineral specimens by the U (n,/) reaction. 

Particles located by ET analysis can be treated in similar fashion, based on the tracks and craters found on the film they can either be manipulated individually or the whole film transferred to SIMS or LA-ICPMS analysis (Figure 5.11). The combination of FT-TIMS is considered one of the most sensitive (FT) and accurate (multicollector TIMS) techniques for detection and isotopic characterization of uranium particles for nuclear forensic applications. 

Particle velocities can also be detected by tracking the trajectory of a single tagged particle over extended time (Godfrey et al., 1999a). However, it is currently very difficult to track particles smaller than about 0.5 mm in diameter, so that the technique has not yet contributed significantly to CFB research. 

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