B.S. Sandhu

Response function of NaI(Tl) detectors and multiple backscattering of gamma rays in aluminium.

The response function, converting the observed pulse-height distribution of a NaI(Tl) detector to a true photon spectrum, is obtained experimentally with the help of an inverse matrix approach. The energy of gamma-ray photons continuously decreases as the number of scatterings increases in a sample having finite dimensions when one deals with the depth of the sample. The present experiments are undertaken to study the effect of target thickness on intensity distribution of gamma photons multiply backscattered from an aluminium target. A NaI(Tl) gamma-ray detector detects the photons backscattered from the aluminium target. The subtraction of analytically estimated singly scattered distribution from the observed intensity distribution (originating from interactions of primary gamma-ray photons with the target) results in multiply backscattered events. We observe that for each incident gamma photon energy, the number of multiply backscattered photons increases with increase in target thickness and then saturates at a particular target thickness called the saturation thickness (depth). Saturation thickness for multiply backscattering of gamma photons is found to decrease with increase in energy of incident gamma-ray photons.

Incoherent scattering of gamma photons for non-destructive tomographic inspection of pipeline.

A scanner system, operating in a non-destructive and non-invasive way, is presented for pipeline to determine its location in land soil, wall thickness, type of liquid flowing and crack/blockage position. The present experiment simulates a real case where pipe corrosion (wall thinning) under insulation can be known from the study of incoherent scattering of 662 keV gamma photons. The incoherent scattered intensity, obtained by unfolding (deconvolution) the experimental pulse-height distribution of NaI(Tl) scintillation detector with the help of inverse response matrix, provides the desired information. The method is quite sensitive for small change (approximately 1 mm) in the thickness of pipe wall, locating a defect of 1mm width under insulation and a small change (approximately 0.1 gm cm(-3)) in the density of liquid flowing through pipe.

Coherent and incoherent scatterings for measurement of mandibular bone density and stable iodine content of tissue

The aim of present study is to investigate the feasibility of gamma ray scattering for measurements of mandibular bone density and stable iodine content of tissue. Scattered spectra from solutions of K2HPO4 in distilled water (a phantom simulating the mandibular bone) and KI in distilled water filled in a thin plastic vial (a phantom simulating the kinetics of thyroid iodine) are recorded for 59.54 and 145 keV incident gamma rays, respectively. A high-purity germanium detector is placed at various angular positions to record the scattered spectra originating from interactions of incident gamma rays with the phantom. The measured intensity ratio of coherent to incoherent scattered gamma rays, corrected for photo-peak efficiency of HPGe detector, absorption of gamma rays in air column present between phantom and detector, and self-absorption in the phantom, is found to be increasing linearly with increase in concentration of K2HPO4 and KI in distilled water within experimental estimated error of <6%. The regression lines, obtained from experimental data for intensity ratio, provide the bone density and stable iodine contents of thyroid. The present non-destructive technique has the potential for a measure of mandibular bone density and stable iodine contents of thyroid.

Multiple backscattering on monoelemental materials and albedo factors of 279, 320, 511 and 662?keV gamma photons

The energy and intensity distributions of multiple backscattering of 279, 320, 511 and 662 keV gamma photons emerging from targets of different elements are observed as a function of the target thickness. To characterize the backscattering, probabilities of gamma photons of different energies interacting with different atomic number (Z), energy (AE), number (An) and dose (AD) albedos are experimentally evaluated. The number and energy albedos are plotted as a function of incident gamma photon energy and atomic number (Z) of the target. The numbers of these multiple backscattered events show an increase with increasing target thickness and then saturate for a particular thickness of the target (the saturation thickness (depth)). The energy albedo is found to decrease with increasing incident gamma photon energy. The response function converts the observed pulse-height distribution of a NaI(Tl) detector to a true photon spectrum. Monte Carlo calculations support the present experimental results. The present measurements are carried out to study how the entities the gamma photon number albedo and the energy albedo vary as a function of the incident energy of the gamma photons, and the atomic number and thickness of an irradiated target.

Experimental evaluation of effective atomic number of composite materials using back-scattering of gamma photons

ABSTRACT A method has been presented for calculation of effective atomic number (Zeff) of composite materials, by using back-scattering of 662 keV gamma photons obtained from a 137Cs mono-energetic radioactive source. The present technique is a non-destructive approach, and is employed to evaluate Zeff of different composite materials, by interacting gamma photons with semi-infinite material in a back-scattering geometry, using a 3″ × 3″ NaI(Tl) scintillation detector. The present work is undertaken to study the effect of target thickness on intensity distribution of gamma photons which are multiply back-scattered from targets (pure elements) and composites (mixtures of different elements). The intensity of multiply back-scattered events increases with increasing target thickness and finally saturates. The saturation thickness for multiply back-scattered events is used to assign a number (Zeff) for multi-element materials. Response function of the 3″ × 3″ NaI(Tl) scintillation detector is applied on observed pulse-height distribution to include the contribution of partially absorbed photons. The reduced value of signal-to-noise ratio interprets the increase in multiply back-scattered data of a response corrected spectrum. Data obtained from Monte Carlo simulations and literature also support the present experimental results.

Investigations of energy dependence of saturation thickness of multiply backscattered gamma photons in elements and alloys - an inverse matrix approach

In Compton scattering experiments employing thick targets one observes that the numbers of multiply backscattered photons increases with increase in target thickness and then saturate at a particular target thickness called the saturation thickness. The energy of each of gamma ray photons continues to decrease as the number of scatterings, the photon undergoes, increases in the sample having finite dimensions. The present experiment is an independent study of energy and intensity distributions of 279-, 320-, 511-, 662 keV, and 1.12 MeV gamma rays multiply backscattered from targets of different atomic numbers and alloys of various thicknesses, and are carried out in a backscattering geometry. The backscattered photons are detected by a NaI(Tl) scintillation detector. The detector response unscrambling, converting the observed pulse-height distribution to a true photon energy spectrum, is obtained with the help of a 12×12 inverse response matrix. The present experimental results confirm that for thick targets, there is significant contribution of multiply backscattered radiations emerging from the targets, having energy equal to that of singly scattered Compton process. The measured saturation thickness (in units of mean free path) for multiply backscattering of gamma photons is found to be decreasing with increase in energy of incident gamma photons.

Experimental calculations of number, energy and dose albedos for various materials using 662 keV gamma rays

ABSTRACT Number, energy and dose albedos are measured at a scattering angle of 180° for a broad beam of 662 keV gamma rays obtained from a radioactive source of 137Cs (having strength in µCi; 1 Ci = 3.7 × 1010 disintegrations per second). The gamma beam is incident on semi-infinite thick targets of variable atomic numbers. The scattering media is divided into three sets, which are pure elements, alloys and composite materials. Experiments are carried out using a 3″ × 3″ NaI(Tl) scintillation detector. To obtain precision in data, the response unfolding of a scintillation detector is used, which converts the observed pulse-height distribution to a true photon spectrum over the energy range of 2.5 to 640 keV. The detector response unfolding results in the true intensity of back-scattered gamma flux by shifting the events resulting from partial absorption of photons to the full energy peak of the spectrum. In the present study, albedo factors are studied as a function of target thickness and their atomic number. The experimentally calculated numbers of back-scattered gamma photon are in good agreement with theoretically generated numbers of multiple back-scattered counts by using a Monte Carlo simulation code. The experimental data on energy and intensity of 662 keV gamma photons are used to evaluate the number, energy and dose albedos for different materials under investigation.

Determination of effective atomic number of biomedical samples using Gamma ray back-scattering

The study of effective atomic number of biomedical sample has been carried out by using a non-destructive multiple back-scattering technique. Also radiation characterization method is used to compare the tissue equivalent material as human tissue. Response function of 3″ × 3″ NaI(Tl) scintillation detector is implemented on recorded pulse-height distribution to boost the counts under the photo-peak and help to reduce the uncertainty in the experimental result. Monte Carlo calculation for multiple back-scattered events supports the reported experimental work.

Non-destructive study of wood using the Compton scattering technique

A simple nondestructive method is presented in this study to characterize woods having different densities, thus estimating the size and depth of inhomogeneities in given wood samples using the Compton scattering technique (CST). This technique uses a collimated beam of 662-keV energy from 137Cs radioactive source, and the scattered flux is detected by an NaI(Tl) detector. To characterize different wood samples on the basis of their densities, both scattering and transmission experiments were performed. The presence of inhomogeneities such as knots in wood was simulated by drilling cylindrical voids of diameter 9mm in the samples and then filling them with a high-density material (aluminum). Furthermore, different sizes of inhomogeneities (Al cylinders) were filled in the wood samples to estimate the depth and size of the inhomogeneity using the CST. A higher linear correlation (R2 ~ 0.96) was found between the scattered intensity and the density of different woods using the CST than that using the transmission (R2 ~ 0.83) method by measuring the density range. An increase of 24.6% in the average scattered intensity was observed at the location where the knot was present, and it was found that an inhomogeneity of the order of ~4mm or more could be detected by the CST.

An experimental study of energy dependence of saturation thickness of multiply scattered gamma rays in binary alloys

The present measurements are carried out to investigate the multiple scattering of 662 keV gamma photons emerging from targets of binary alloys (brass and soldering material). The scattered photons are detected by 51 mm × 51 mm NaI(Tl) scintillation detector whose response unscrambling converting the observed pulse–height distribution to a true photon energy spectrum, is obtained with the help of 10 × 10 inverse response matrix. The numbers of multiply scattered events, having same energy as in the singly scattered distribution, first increases with target thickness and then saturate. The application of response function of scintillation detector does not result in any change of measured saturation thickness. Monte Carlo calculation supports the present experimental results.