Title: A gamma-backscatter density gauge with differential-mode counting
Abstract: The majority of gamma-backscatter density gauges use integral-mode counting with pulse-analysis circuits so that only gamma-photons are detected over an energy range, where Compton scattering is predominant, and, by comparison, photoelectric effect and pair production can be neglected. Neverthless, differential-mode counting has been employed, and instruments of this type, when calibrated, have been useful. In the present work an investigation is made of backscatter gauges using differential-mode counting using a narrow window. In the range where Compton scattering is predominant, the performance is similar to that with integral-mode counting, except that the sensitivity is very much reduced, necessitating the use of a stronger radioactive source and, as necessary, heavier shielding. However, if the spectra of the backscattered gamma-radiations are examined it will be noted that with geometry such that multiple scattering is always obtained, there is a peak intensity at about 80 keV for a 137Cs source. Therefore, by using a photon detector with the window set on this energy peak, the setting should be the same for all materials and moreover the sensitivity should be high, and this proved to be the case. It is also found that the (I vs ϱ) characteristic, where I is the counting rate and ϱ the density of the soil under investigation, gives an I maximum at a constant ϱ, independent of the source-detector separation (d). This is a quite different phenomenon to that found at higher energies, both for differential- and integral-mode counting. Regular curves are obtained. On the other hand, the (Idn vs d) characteristics display similar properties to those obtained for integral-mode counting, namely a shift of the Idn maxima to a different source-detector separation (d) with change of density (ϱ) and with change of n value, but the (I vs d) relationship is not the same as for integral-mode counting. It is shown that over a useful range (ϱ = 1.0 to 2.25 g/cm3) the Compton mass absorption coefficient (k2) is approximately linearly related to the density, and by introducing a new parameter (k3 = k2/ϱ), a formula is derived for dm (the source-detector separation at an Idn maximum). This formula allows a determination of soil densities by measuring dm experimentally. The accuracy possible is of the order of ± 2%. One of the most significant features of differential-mode counting with the window set at about 0.08 MeV and integral-mode counting with the discriminator set at 0.400 MeV is their different behaviour with hydrogen-containing materials. In the latter case hydrogen-containing materials give two Idn maxima, one satisfying the usual formula, namely dm = (n−1)/k2ϱ, where ϱ is the bulk density, and a second maximum which appears to be characteristic of the hydrogen content. Differential-mode counting with the window set at about 0.08 MeV only shows an Idn maximum corresponding to the bulk density. Theoretical explanations of these experimental facts are given in the paper.
Publication Year: 1971
Publication Date: 1971-06-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 8
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