Discussion: A radio-active method for measuring variations in density in concrete cores, cubes and beams

Abstract

Mr Harland's paper is of considerable interest to us since we are using the gamma-ray attenuation technique in work (to be published) on the density distribution in concrete compacted by an immersion vibrator.The technique has been used by a number of workers and will undoubtedly become of increasing value in the field of concrete research and technology. It should, however, be appreciated that there are many pitfalls and that considerable preliminary work is necessary if the results are not to be misleading. In this connexion, we feel attention should be drawn to the work of Preiss(l).We note that Mr Harland has used a source of caesium 137 and a scintillation counter with a sodium iodide crystal under discrimination conditions, and we feel that the reasons for these choices are worthy of more discussion.Caesium 137 is mono-energetic, emitting photons at the relatively low energy of 0·662 MeV. This makes it fairly easy to shield and collimate, whilst its single peak is readily distinguishable if a technique is used in which pulses between two energy levels only are accepted, i.e. channel analysis. It also has a relatively long half-life. More important, however, is that the slope of the curve of energy transmitted versus mass thickness is greater with caesium 137 than with the higher energy isotopes (2). All these factors make caesium 137 a very convenient element to use.If a source and crystal as noted above are used, an energy spectrum of the type shown in Figure I is obtained. The peak at 0·662 MeV is clearly seen, the rest of the curve corresponding to photons scattered within the crystal and elsewhere.It is not difficult to arrive at conditions approaching narrow beam. In any case it will be necessary to apply (1) To use the apparatus with a discriminator set at a low level to minimize electronic noise. This method uses the instrumentation in its simplest form. Stability problems as far as the apparatus is concerned are here reduced to a minimum, but the intensity of scattered radiation received can be high.(2) To use a discriminator set at a higher value. By proper setting up it is possible to obtain a position where the discriminator is least sensitive to drift and where the signal to noise ratio of the detector is at its maximum. The intensity of scattered radiation recorded can be greatly reduced in this case.(3) To use channel analysis, whereby the discriminator as used in (2) has a gate superimposed. Pulses below the discriminator level, or threshold, are rejected as are those above the gate. Most of the scattered radiation is cut out in this method, but the system is also the most sensitive to drift. To minimize the effect of drift it may be better to set the gate to straddle the main energy peak with the lower discriminator at the valley of the spectrum rather than to use a very narrow gate. The intensity of scattered radiation received can then be reduced further by geometrical collimation. This is achieved by applying a heavy shield to the detector. The shield can be used in (1) and (2) as well w...