Gamma-ray Backscatter Research Articles

Two-channel comparative method of energy spectrum evaluation in measurements using gamma-ray backscattering

The energy spectrum in a thickness measurement using gamma-ray backscattering depends on the measured thickness only at the lower part. The paper presents the method of evaluation of the spectrum by comparing its two parts (the lower and the upper one). The ratio of the output signals in both parts depends on the measured thickness only, but does not depend on the activity of the radiation source.

Backscattering of 137Cs gamma rays by finite barriers

Angular differential number and energy albedo were measured at a scattering angle of 60° for narrow beam of 137Cs gamma rays normally incident on finite thickness scattering media of various atomic numbers. Measurements were made using a collimated 3 cm 3 Ge(Li) detector with suitable electronics and a 400 channel pulse-height analyzer. The calculation of the energy spectrum of the scattered photons was carried out by using a response matrix with 51 energy intervals over the range 0·04−0·3 MeV.

The distribution of backscattered gamma ray photons in the scattering medium

Monte Carlo calculations were performed to simulate the behaviour of photons scattered back from a semi-infinite medium. The source considered was point isotropic on the surface of the medium, and the media simulated soils of various densities. The values of the number albedo obtained agreed well with those quoted in the literature. The radial distribution was found to fall off exponentially, with a dependence on distance from the source practically independent of source energy and material, provided the photo electric absorption cross section at the source energy is negligible. This conclusion is supported by calculation from results quoted in the literature. The distributions of the initial direction cosine relative to the radius joining source and point of exit, and of the number of scatters, were calculated and tabulated.

Measurement of the density of the top inch of asphalt paving by gamma radiation

An instrument was developed to measure, non-destructively, the density of the top 2.5 cm layer of an asphalt paving. The instrument is based on the principle of gamma ray backscattering, but differs from the usual instrument in that the depth of measurement is only 2.5 cm. This is achieved by using a low energy source, Am 241, with a source to detector distance of 5.5 cm, and a NaI (Tl) detector. The instrument was checked in the field against the method of coring, cutting and weighing, and the root-mean-square difference between the methods was found to be 0.02 g/cc, which is less than 1%.

Determining Optimum Conditions for Measuring by Gamma-ray Backscattering

The paper is concerned on the calculation of the optimal thickness of measured material from the point of view of measurement error. The minimum relative error of a measurement is determined as a function of the gamma radiaiton linear attenuation coefficient and the thickness of the material used. Various types of fluctuations are considered and attention is paid to the background detected in the same arrangement without measured material. The results are summarized into simple figures. The conclusion is made that the optimal thickness increases with poorer quality of the measuring instrument.

Discrimination of Liquid Level Using Backscattered Gamma-rays

Discrimination of Liquid Level Using Backscattered Gamma-rays

Design optimization of the surface type gamma-ray backscatter soil density gauges using conventional components with the dual-gauge principle

The dual-gauge principle has been optimized for conventional gauge components by varying the four parameters of gamma-ray source energy, source collimation length, source-detector separation and detector efficiency spectrum in a prototype gauge to obtain a maximum quality factor. Quality factors obtained from over 400 dual-gauge combinations ranged from 0.019 to 1.344, indicating that dual-gage quality is strongly dependent on gauge design. The significance of rigid, stable construction to overall gauge performance is indicated. Optimum dual-gauge designs were achieved with quality factors at least four to five times larger than commercial gauges using the air-gap nomograph calibration.

A quality factor concept for evaluation of the surface type gamma-ray backscatter soil density gauges

The major sources of error for the surface-type, gamma-ray scatter soil density gauges are identified as sensitivity to soil composition, surface roughness, and counting rate measurement errors. Methods of measuring these errors are outlined and a quality factor is proposed which properly combines these errors into a single parameter suitable for gauge evaluation. Typical values of these errors and the quality factor are reported for commercial gauges. Results are also given for the air-gap ratio method and the air-gap nomograph method. A method for determining the optimum air-gap height based on obtaining the maximum quality factor is demonstrated for a specific commercial gauge.

Studies of the surface type gamma-ray backscatter soil density gauges using collimation and spectrometry techniques

A prototype gauge was constructed which allowed evaluation of the effects of energy discrimination and source and detector collimation and collimation angles on gauge performance. A NaI (T1) scintillation detector and a multichannel analyzer were used to provide the energy discrimination capability and rotatable lead collimators were constructed for collimation purposes. In addition, source-detector separation was varied over a small range. Because of possible competing effects, the different design parameters were varied simultaneously and both single- and dual-gauge results evaluated. The purpose was to monitor and interpret the effects of energy discrimination and of collimation more than to obtain the best possible design for field applications. The effects on individual errors and on quality factor are displayed and discussed for single gauges, and the best dual gauges obtained are reported. A triple-gauge concept is introduced which would eliminate errors due both to composition variations and to surface roughness.

A moving gamma-source method of measuring soil densities

The conventional type of gamma-ray backscatter density gauge has a characteristic (counting-rate vs density) comprising a portion with a positive slope and a portion with a negative slope, the maximum counting rate being obtained with a specific density ϱ m, which depends on the geometry of the gauge and, in particular, on the source-detector separation d. It is shown that these considerations lead to a different type of density gauge in which d is varied for maximum response. This moving-source type of density gauge possesses a number of advantages, but imposes certain design difficulties.

Backscattering of Gamma-Rays from Semi-Infinite Stratified Slabs of Alternating Tin and Aluminum Layers

The backscattering of γ-rays has been investigated from semi-infinite stratified slabs of alternating layers of tin and aluminum. The energy spectrum and the angular distribution of backscattered photons, as well as the number and energy albedos were obtained for 60Co and 137Cs point isotropic sources. They were compared with the data obtained with single layers of tin and aluminum. The stratified slabs of alternating layers were assumed in this study to be equivalent in property to a homogeneous mixture of tin and aluminum, and the effective atomic numbers were computed on this basis. The resulting values are found to agree with the effective atomic numbers corresponding to the albedo values obtained for the alternating layers.

Improvements in the method of measuring the density of soil using nuclear back-scatter

The effect of soil type on the accuracy of the counting rate/density characteristic of the gamma-ray back-scatter gauge can be very much reduced by limiting the scattering angle < 1 2 π . This may be achieved by using a scintillation counter in place of the Geiger counter and introducing circuit which cuts off at an energy ( E min) given by E min = E [1+α(1− cosø m )] , α being E (m 0c 2 ) and ø m the maximum scattering angle to be allowed. The method reduces the volume of ground investigated by a factor of about 20 times as compared with the standard instrument. However, the use of a scintillation counter and the same radioactive source, because of the higher detection efficiency obtained, compensates for this volume effect and allows the same counting rate and, therefore, the same precision of measurement to be obtained.

Backscattering of Gamma-Rays from Semi-Infinite Stratified Slabs of Alternating Tin and Aluminum Layers

Backscattering of Gamma-Rays from Semi-Infinite Stratified Slabs of Alternating Tin and Aluminum Layers

Backscattering of Gamma Rays from Polyethylene, Aluminum and Lead Slabs

Backscattering of Gamma Rays from Polyethylene, Aluminum and Lead Slabs

Back-scattering of low-energy gamma rays

Experimental data are obtained on the spectral angular distribution of γ-rays back-scattered by different scatterers-paraffin wax, graphite, aluminium, iron, zinc, cadmium and lead (semi-infinite media) in an isotropic point source geometry. The energies of the primary y-rays were 0·145,0·190,0·279,0·320,0·396,0·661 and 0·765 MeV. The experimental values of the numerical and energy albedos A and AE are in good agreement with the calculated values obtained by the Monte Carlo method. The results of the work are presented in the form of tables and graphs of A, A E and A/IA E as functions of Z and E γ .

Experimental Differential Number, Energy and Exposure Albedos for Semi–Infinite Media, for Normally Incident Gamma Photons

Differential angular and energy distributions of backscattered gamma rays have been measured for photons normally incident on essentially semi-infinite scattering media of various atomic numbers. T...

Backscattering of Gamma Rays from Polyethylene, Aluminum and Lead Slabs

Spectra of backscattered radiation from finite thick slabs of polyethylene, aluminum and lead were obtained by means of a scintillation spectrometer. A point 60Co γ-souree was placed in close contact with a scattering slab. Variations of number and energy albedo values with increasing slab thickness are given as experimental formulas and graphical representations.

Measuring the density of thin asphalt or concrete layers with a gamma-ray back-scatter apparatus

One drawback of radioactive back-scatter apparatus for measuring density is the difficulty of defining adequately the depth-sampling characteristic. In this paper the concept of the mean depth to which useful gammas penetrate is introduced. Provided the instrument has been designed to measure the absorption of back-scattered gamma radiation, the density determined in the presence of small density gradients will be the average density of the layer lying above the mean depth. The derived density of this shallow layer may be slightly erratic due to the presence of layers with a different density at depths between one and three times the mean depth to which the gamma rays penetrate. Allowances are calculated for such errors for a particular instrument used in the experiments described. For this particular apparatus with a radium 226 source, the mean depth of penetration of the useful gamma radiation was found to be between 1.2 and 1.8 in. for materials with densities ranging from 150 to 100 lb/ft 3. This apparatus was found useful for measuring the increase of density during compaction of a 2 1 2 - in. asphalt base-course, and for measuring the density variation near to the surface across layers of bituminous-bound materials.

Differential Dose-Rate Measurements of Backscattered Gamma Rays from Concrete, Aluminum and Steel

The angular distribution of backscattered dose-rate ratio was measured for ‘semi-infinite’ concrete, aluminum and steel slabs irradiated by plane-parallel beams of Co60 and Cs137 gamma photons. The photons were incident on the slabs at angles, measured from the normal to the slab, of arccos 1.00, 0.75 and 0.50. For the necessary sensitivity, the backscattered dose rate was measured by a digital dosimetry system using a plastic scintillator as the detector.An empirical formula for differential dose-rate ratio was derived from the experimental data. Comparisons are made with two other experiments and a semi-empirical formula fitted to a Monte Carlo calculation. The experiments generally agree to within 20%. The values based upon the Monte Carlo calculation are generally 20% to 35% lower than the experimental values.

Backscattering of Gamma Rays from Point Sources by an Infinite-Plane Concrete Surface

Backscattering factors, as fractions of the direct dose rate, are obtained for point sources of gamma radiation, specifically Cs137 and Co60, placed near a plane interface between vacuum and concrete. The method is based on the application of albedo principles, using the Chilton-Huddleston formulation for albedo. The results are considered practically applicable to air-concrete or air-ground interface situations, provided the source-detector, source-interface, and detector-interface distances are within certain limits. The lower limit is in theory the order of a mean free path of the source radiation in concrete, although under certain circumstances the present results are valid for distances even less. The upper limit appears to be on the order of a few dozen feet, but further precise experimental work is needed to establish this.