Self-absorption correction

CutshallNH, Larsen IL, Olsen CR. 1983. Direct analysis of 210Pb in sediment samples Self absorption corrections. Nucl Instrum Methods 206 309. 

The classic example of a radiometric titration concerns the titration of some unknown material with a radioactive reagent to give a radioactive precipitate. In this case, the activity of the supernatant or the precipitate can be followed as a function of titrant volume, as shown in Figure 4.10. In this type of titrations, the tracer must have a long half-life and must emit high-energy 3 or y rays so as to minimize self-absorption corrections (assuming, as is common practice, that the supernatant or precipitate is removed from the system and counted in an external sample counter after the addition of each volume of titrant). 

Figure 6-7 Self-absorption correction curve 10 mg/ planchet is taken as the standard density.

For all sources (planes, disks, lines, and small volumes) where 0 is less than 20 . Figures to be used 10-6 for self-absorption correction (multiply by F ), 10-2, and 10-3. Correct dose rate or flux ratios on left-hand scale of Pig. 10-3 as follows ... 

In the dawn of the history of nuclear science, the neutron flux density (/) of Ra-Be source was only 10 -10 n cm s The total activity produced by such neutron sources via (n,y) reaction was very low and so was the specific activity of the radionuclides. The Szilard-Chalmers process, however, could dramatically increase the specific activity the improvement could reach orders of magnitude. In the measurement of P radioactivity, which was a frequent task in early days of nuclear science, samples with low specific activity brought sometimes troublesome problems of self-absorption corrections. By the introduction of the Szilard-Chalmers process, however, this difficulty could be avoided, because the measurement could be performed within small statistical errors using a sample with high specific activity. Therefore, the Szilard-Chalmers process became one of the useful means of preparation of radioisotopes for measurement, as Szilard and Chalmers (1934b) recognized the importance of this technique in their early work. 

For narrow collimation, the self-absorption correction factor for a slab of thickness d is obtained by integrating the exponential attenuation function ... 

Calculations according to O Eq. (30.43) require the knowledge of the photon attenuation coefficients /r of the sample matrix (see Sect. 11.5.1 in the Appendix of Vol. 1). Experimental calibrations are readily conducted by placing an unirradiated sample between the activated sample and the detector, thus determining the attenuation coefficients for an unknown or unlisted matrix. By using an irradiated standard identical to the sample, as is possible in the comparator method, the need for geometry, self-shielding, and self-absorption corrections is eliminated, and thus the potential errors are reduced. 

If we consider making a mathematical self-absorption correction to the measured gamma spectrum peak count rate, R, we can use the following simple equation ... 

In a routine service laboratory, there will inevitahly he occasions when the sample is of unknown composition. A particular real, if uncommon, example is the measurement of °Pb in a lead-rich dust. Even without the presence of the lead, the low energy of the Pb gamma-ray (45keV) means that in any sample of reasonable size a self-absorption correction is needed. A procedure, which was followed in that case, is based upon an empirical determination of fit using an external source ... 

Oresegun, M.O., Decker, K.M. and Sanderson, C.G. (1993). Determination of self-absorption corrections by computation in routine gamma-ray spectrometry for typical environmental samples, Radioact. Radiochem., 4, 38-45. 

However, if a larger sample is chosen, can appropriate self-absorption corrections be made ... 

SELF ABSORPTION CORRECTION The correction to a count rate that needs to be made to take into account the loss... 

Neutron diffraction was carried out on the liquid samples on the D4c diffractometer [6] at the ILL, using neutron wavelengths of 0.5, 0.33, and 0.29 A. A Cu(220) monochromator was used for 0.5 A and a Cu(331) monochromator was used for both the 0.33 and 0.29 A neutrons. In order to avoid harmonic contamination a rhodium filter was inserted between the monochromator and the sample when measuring at 0.5 A. Diffraction measurements were taken at the three wavelengths on the empty furnace, the empty container, the liquid sample, nickel powder in a vanadium can for calibration, a vanadium bar for normalisation, and powder in a silica container for a self absorption correction. All measurements were taken at ambient temperature, except the sample measurement, where a vanadium furnace was used to heat the sample to 800 °C. The essential diffraction parameters are shown in Table 7.1. Significant care was taken in minimising the background to avoid the possibility of an A1 peak coincident with the InSe first peak [13]... 

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