Source self-absorption

Radioactive substances are deposited on a backing material in thin deposits. But no matter how thin, the deposit has a finite thickness and may cause absorption of some particles emitted by the source. Consider the source of thickness t shown in Fig. 8.13. Particle 1 traverses the source deposit and enters the detector. Particle 2 is absorbed inside the source so that it will not be counted. Therefore, source self-absorption will produce a decrease of the counting rate r. 

Source self-absorption may be reduced to an insignificant amount but it cannot be eliminated completely. It is always important for charged particles and generally more crucial for heavier particles ip, a, d, heavy ions) than for electrons. 

Figure 8.13 Source self-absorption. Particles may be absorbed in the source deposit.

Figure 8.14 Diagram used for the calculation of the source self-absorption factor for betas.

How would the result of Prob. 8.9 change if the backscattering factor was known with an error of +1 percent, the efficiency with an error of +0.5 percent, and the source self-absorption factor with an error of 1 percent ... 

The subject of relative and absolute measurements is presented in Chap. 8. The solid angle (geometry factor) between source and detector and effects due to the source and the detector, such as efficiency, backscattering, and source self-absorption are all discussed in detail. 

Self-absorption of the source. Self-absorption of the source to be measured is, in principle, not a problem, as long as the absorption is identical for all samples and standards. Self-absorption can be important for low-energy y-rays or sources containing components of high atomic number. 

Figure 6. Tempcraiure dependence of the fluorescence lifetime of BMPC in 1 1 ethanol-mcihanol. Measurements were carried out at the LENS laboratory of Florence by a picosecond apparatus using as an excitation source (at 380 nm) a dye laser pumped by a frequency-doubled cw Nd-YAG laser and recording the fluorescence time jirofiles by a streak camera. Since the overall insuumental response time was 75-80 ps, decays with t>200 ps, observed at T<130 K, were analyzed without deconvolution. At 177, 178 and 193 K, the lifetimes were roughly estimated as i=(FWHM -77 ), where FWHM was the width at half maximum of the decay. Because of the rather high sample absorbances (An,x=2), self absorption may have reduced the lifetimes to some extent.

The expression is known as the transmission integral in the actual formulation, which is valid for ideal thin sources without self-absorption and homogeneous absorbers assuming equal widths F for source and absorber [9]. The transmission integral describes the experimental Mossbauer spectrum as a convolution of the source emission Une N(E,o) and the absorber response exp —cr( )/abs M - The substitution of N E,d) and cr( ) from (2.19) and (2.20) yields in detail ... 

High intensity, microwave powered emission sources have recently been developed that are reported to provide substantially higher DUV output than classical electrode discharge mercury lamps 76). These sources suffer from self-absorption of the intense 254 nm emission but have a relatively high output in a band between 240 and 280 nm. They are extended sources of finite size rather than point sources, and they must also be an integral part of a tuned, resonant microwave cavity. Consequently, extensive condenser design work would be required in order to utilize the microwave powered sources in projection printers. 

As we have seen, a narrow line source is required for AAS. Although in the early days vapour discharge lamps were used for some elements, these are rarely used now because they exhibit self-absorption. The most popular source is the hollow-cathode lamp, although electrodeless discharge lamps are popular for some elements. 

For resonance lines, self-absorption broadening may be very important, because it is applied to the sum of all the factors described above. As the maximum absorption occurs at the centre of the line, proportionally more intensity is lost on self-absorption here than at the wings. Thus, as the concentration of atoms in the atom cell increases, not only the intensity of the line but also its profile changes (Fig. 4.2b) High levels of self-absorption can actually result in self-reversal, i.e. a minimum at the centre of the line. This can be very significant for emission lines in flames but is far less pronounced in sources such as the inductively coupled plasma, which is a major advantage of this source. 

At present 60Co sources are made in England, Canada, and the United States. The primary source manufactured in the United States is the BNL standard source (5), which is in the form of a strip of cobalt metal about % inch wide, 1 foot long, and 60 mils thick, doubly encapsulated in thin stainless steel jackets. The total amount of self-absorption of 7-rays in this source is less than 10%—i.e., over 90% of the 7-rays are available for absorption in target material. 

Other sources of error, particularly in quantitative Raman analysis, include laser self-absorption effects leading to attenuation of some spectral bands. Similarly diffuse reflectance of the laser light, which is dependent on the particle size of the formulation components, may increase or decrease the collection volume. However, normalisation techniques can be used to overcome some of these effects [35]. 

For samples that emit beta particles, the sample must be evenly distributed, with defined and uniform thickness. Quantifying geometry and self-absorption of beta particles is unreliable for an unevenly deposited source. 

This experiment examines the count rate as a function of sample thickness. All other variables are held constant (except for a small change in source-detector distance). As the sample becomes thicker, more of the beta particles are absorbed in the sample itself. This is called self-absorption, and is shown in Figure 4.1. In thin samples, self-absorption is relatively small or negligible, but in thick samples it is measurable and must be considered when calculating the counting efficiency. 

All these limitations do not exist in HR-CS AAS. Firstly, the radiation intensity of the source is always high enough to provide a significantly better SNR than the LS in conventional AAS. In a first approximation the radiation intensity, and hence the SNR for all lines, is of the same magnitude, although it degrades somewhat in the far UV. Secondly, the resolution of the monochromator is such that only the center of the line is detected by the analytical pixel. As the source emits a continuum, and the analytical pixel is adjusted in such a way that it is always in the line center, none of the phenomena associated with LS exist, for example, line shift, self absorption, or the presence of other lines emitted by the lamp [3]. 

For counting low-energy j3 radiation, the crystal is substituted by a scintillating liquid, and the sample is dissolved in the liquid (internal-source liquid scintillation counting). The method is also used to measure weak X-ray and y-ray emitters. Under these conditions, self-absorption of the radiation in the sample, absorption of the radiation in the air and the window of the detector, and backscattering of p particles are avoided. 

Surface-barrier detectors are very useful for detection and measurement of a particles. The internal counting efficiency for a particles is = 1.0. If the geometry G of the arrangement of a source and detector is well-defined and self-absorption S in the sample can be neglected, absolute activities of a emitters can be determined. The performance of surface-barrier detectors is conventionally tested by recording the spectrum of a calibration source, e.g. a Am source. 

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