Background Rejection Techniques

In order to optimize the cosmic gamma-ray signal measured with a detector, a number of background-suppression techniques are usually employed. The most common are outlined in the following. 

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Examples of fluorophores that are used as labels in fluo-roimmunoassay and their properties are listed in Table 9-3. Initially, background fluorescence from drugs, drug metabolites, and protein-bound substances, such as bilirubm, limited the utility of this technique. However, this problem has largely been overcome by the use of rare earth (lanthanide) chelates and background rejection (time-resolved)... 

I Wieder. Background rejection in fluorescence immunoassay. In Proc 6th Int Conf Immunofluorescence and Related Staining Techniques. Vienna Elsevier, 1978,... 

Hard X-rays detectors can also be divided in two broad classes gas filled proportional counters (Ubertini, 1987) and scintillation detectors, the latter type being successfully implemented at energies higher than a few tenths of keV. Currently, the availability of space-qualified, high pressure gas cells and the outcome of the fluorescence gating technique for background rejection, have made proportional counters efficient up to < 200 keV. 

Opto-galvanic spectroscopy of flames has important analytical applications. If an atomic absorption flame (Sect.6.5.3) is irradiated by a tunable laser a change in the current between two electrodes, placed in the outer parts of the flame, can be detected. A typical arrangement is shown in Fig. 9.10. A sensitivity exceeding that obtainable in atomic absorption spectroscopy (Sect.6.5.2) can be achieved for elements seeded into the flame. Two-step excitation improves both sensitivity and background rejection. The opto-galvanic technique can also be used for studying normal flame constituents such as O, H and OH [9.60]. 

One of the most fundamental techniques for background rejection is the use of passive high-Z materials (usually lead) to shield the central detector elements from... 

But there is a price to be paid. Good Workmanship acceptance criteria for conventional techniques must, to a certain extent, be conservative, in order to compensate for the inherent "limitations" of conventional NDT. And, what is worse, the degree of conservatism is more or less unknown. Therefore the question can be asked "if the historic background of present NDT practice would not exist, what would we like to know today about a weld to be able to accept or reject it "... 

Software mechanisms have been developed in order that a representation of the true boundaries of the particle is established. Images of touching particles are either software segregated or rejected from the analysis. Out-of-focus objects are also rejected by software techniques. A binary array, where the shadowed pixels are 1 in a background of 0, is often employed to permit rapid data storage and image analysis (Figure 7). 

Since shortly after its inception in 1955, AAS has been the standard tool employed by analysts for the determination of trace levels of metals. In this technique a fine spray of the analyte is passed into a suitable flame, frequently oxygen-acetylene or nitrous oxide-acetylene, which converts the elements to an atomic vapour. Through this vapour is passed radiation at the right wavelength to excite the ground state atoms to the first excited electronic level. The amount of radiation absorbed can then be measured and directly related to the atom concentration a hollow cathode lamp is used to emit light with the characteristic narrow line spectrum of the analyte element. The detection system consists of a monochromator (to reject other lines produced by the lamp and background flame radiation) and a photomultiplier. Another key feature of the technique involves... 

Frequency-response analysis [1] is the most widely used technique for impedance testing. Similar to the lock-in technique, it can extract a small signal from a very high background of noise, automatically rejecting DC and harmonic responses. The difference is that a frequency-response analyzer (FRA) correlates the input signal with the reference sine waves. To achieve faster measurements, FRAs are usually equipped with separate analyzers for each input chaimel. 

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