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.     

Albedo factors of 279, 320, 511 and 662 keV backscattered gamma photons

The backscattered peak and albedos are important for the estimation of exposure distribution and for better understanding the phenomenon of the backscattering of gamma photons. To characterize the backscattering probability of gamma photons interacting with different atomic numbers (Z), number (A _N), energy (A _E) and dose (A _D) albedos are experimentally evaluated. The response function converts the observed pulse-height distribution of a NaI(Tl) scintillation detector to a true photon spectrum. For each of the incident gamma photon energies, the number and energy albedos show an increase with the increasing target thickness, and finally saturate. The energy albedos are found to be decreasing with the increase in the atomic number of the target material and incident gamma photon energy. The dose albedos do not differ significantly from the energy albedos for the chosen incident gamma photon energies.     

Experimental and simulated validation of the energy dependence of saturation thickness of multiple scattered gamma rays

Saturation thickness for multiple scattering gamma rays from multiple sources has been measured experimentally and simulated using the Monte Carlo N-Particle (MCNP) Code. Experimental measurements were performed using a collimated beam of gamma-rays from ⁵⁷Co, ²⁰³Hg, ¹³³Ba, ²²Na, ¹³⁷Cs, ⁶⁵Zn and ⁶⁰Co sources. The gamma rays were directed at rectangular aluminium targets of varying thickness. A NaI (Tl) scintillation detector placed at a backscattering angle of 180° was used to detect the scattered photons. The measured and calculated saturation thickness increases with increasing energy of incident gamma-rays. Experimental and simulated values are compared and are in good agreement.     

Energy and intensity distributions of 279 keV multiply scattered photons in bronze — an inverse response matrix approach

An inverse response matrix converts the observed pulse-height distribution of a NaI(Tl) scintillation detector to a true photon spectrum. This also results in extraction of intensity and energy distributions of multiply scattered events originating from interactions of 279 keV photons with thick targets of bronze. The observed pulse-height distributions are a composite of singly and multiply scattered events in addition to bremmstrahlung originating from slowing down of Compton and photo-electrons in thick targets. To evaluate the contribution of multiply scattered events, the spectrum of singly scattered events contributing to inelastic Compton peak is reconstructed analytically. The optimum thickness (saturation depth), at which the number of multiply scattered events saturate, has been evaluated in different energy bin meshes chosen for scintillation detector response unfolding. Monte Carlo calculations based upon the package developed by Bauer and Pattison (Compton scattering experiments at the HMI (1981), HMI-B 364, pp. 1–106) supports the present experimental results.      

Portable gamma spectrometry with cerium-doped lanthanum bromide scintillators: Suitability assessments for luminescence and electron spin resonance dating applications

Cerium-doped lanthanum bromide (LaBr₃:Ce) crystals offer a range of improved scintillation properties over traditional NaI:Tl crystals for in situ gamma spectrometry. At present, however, it remains unclear whether the internal radioactivity of LaBr₃:Ce detectors compromises their suitability for low-level activity radioisotope measurements of natural sedimentary deposits, such as those required in luminescence and electron spin resonance (ESR) dating. In this study we investigate the suitability of a commercial LaBr₃:Ce detector for measuring individual concentrations of ⁴⁰K, ²³⁸U and ²³²Th using predefined ‘energy windows’ from gamma ray spectra. Performance tests have been undertaken using reference materials with well-constrained radioisotope concentrations (the Oxford calibration blocks) and compared with results obtained for a NaI:Tl detector of the same geometry. These tests reveal that the LaBr₃:Ce detector has a non-negligible intrinsic activity that needs to be accurately quantified prior to measuring any gamma ray spectra in the field. Compared to the NaI:Tl detector, the energy resolution of the LaBr₃:Ce detector is improved by a factor of two, or more, for the main indicator isotope photopeaks in the ⁴⁰K, ²³⁸U and ²³²Th decay series. Signal-to-noise ratios for the LaBr₃:Ce detector show a 25-35% improvement over those of the NaI:Tl detector. In addition, the LaBr₃:Ce detector is characterised by suitable energy linearity over the full spectral range of interest for the ⁴⁰K, ²³⁸U and ²³²Th decay series. Replicate gamma ray measurements made with the LaBr₃:Ce and NaI:Tl detectors for 20 natural sedimentary samples from the Lower Tejo River basin, Portugal, and the Duero River basin, Spain, yield consistent radioisotope concentrations and gamma dose rate estimates. These results are encouraging and suggest that LaBr₃:Ce detectors can provide suitable estimates of individual radioisotope concentrations in low-level activity (0.5-1.5 Gy/ka) environments, providing that their intrinsic activity is adequately measured and subtracted from field spectra. Our comparison also reveals that subtraction of the intrinsic activity from LaBr₃:Ce spectra produces a significant reduction in the precision with which radionuclide concentrations can be determined using the ‘energy windows’ approach. This shortcoming necessitates longer counting times in natural sedimentary environments and overshadows the practical advantages that LaBr₃:Ce detectors might otherwise offer for luminescence and ESR dating applications.     

Full-Energy peak efficiency of an NaI(Tl) detector with coincidence summing correction showing the effect of the source-to-detector distance

The coincidence summing effect is considered to be one of the most important problems in γ-ray spectrometry. It has played an important role in the detector calibration process, especially at low source-to-detector distances, which are the typical conditions when the environmental samples have to be measured. In this work, the γ–γ coincidence summing correction factors (CSCF) for a 3″×?3″ NaI(Tl) γ-ray detector have been calculated at different distances from the detector end cap. These factors with NaI(Tl) γ-ray detectors have rarely been discussed in the previous literature, due to the lack of the separation process of the close gamma energy lines in the scintillation detector spectrum. The γ–γ CSCF were calculated for ⁶⁰Co, ¹³³Ba and ¹⁵²Eu radioactive point sources, which show the simple and the complex decay scheme of several radionuclides. In order to obtain the CSCF, the following parameters must be estimated by the numerical simulation method (NSM): the total efficiency (ε_T), the full-energy peak efficiency (ε_P) and the effective solid angle (Ω_eff). These parameters were calculated numerically depending on the direct mathematical method and efficiency transfer method. The obtained CSCF was used to correct the measured efficiency curves, which were measured at different distances, then compared to the ETNA and EFFTRAN programs, as well as the CSCF. The results show a good agreement, especially at large source-to-detector distances.     

Effects of gamma irradiation on some chemicals using an NaI (Tl) detector

The present work was carried out to find out the gamma ray shielding properties and to study the effects using an NaI (Tl) detector using radioactive sources ⁵⁷Co, ¹³³Ba, ¹³⁷Cs, ⁵⁴Mn, ⁶⁰Co and ²²Na at energies 122, 356, 511, 662, 840, 1170, 1275 and 1330?keV, for some chemicals, namely, sodium thiosulfate (Na₂S₂O₃), benzoic acid (C₇H₆O₂), sodium hydroxide (NaOH), poly vinyl alcohol (PVA) (C₂H₄O), potassium nitrate (KNO3), naphthalene (C₁₀H₈). Mass attenuation coefficient (µ_m) values obtained from the experiment were used to determine the effective atomic numbers (Z_eff) and effective electron densities (N_eff), atomic cross-sections (σ_t) and electronic cross-sections (σ_e); it will be observed from the present work that the variation in the obtained values is only due to the increase or decrease in the gamma ray energy and the chemical composition of the sample. It was seen that the calculated and obtained values showed good agreement. The investigated data are useful in the electronic industry, plastic industry, building materials and agriculture fields. From the present work it was found that the PVA could be used as a better gamma shielding material.     

Scintillation properties of pure and Ce3+-doped SrF2 crystals

In this paper results of scintillation properties measurements of pure and Ce³⁺-doped strontium fluoride crystals are presented. We measure light output, scintillation decay time profile and temperature stability of light output. X-ray excited luminescence outputs corrected for spectral response of monochromator and photomultiplier for pure SrF₂ and SrF₂-0.3 mol% Ce³⁺ are approximately 95% and 115% of NaI–Tl emission output, respectively. A photopeak with a 10% full width at half maximum is observed at approximately 84% the light output of a NaI–Tl crystal after correction for spectral response of photomultiplier, when sample 10 × 10 mm of pure SrF₂ crystal is excited with 662 keV photons. Corrected light output of SrF₂-0.3 mol% Ce³⁺ under 662 keV photon excitation is found at approximately 64% the light output of the NaI–Tl crystal.     

A novel method for non-destructive Compton scatter imaging based on the genetic algorithm

Compton scattering tomography is widely used in numerous applications such as biomedical imaging, nondestructive industrial testing and environmental survey, etc. This paper proposes the use of the genetic algorithm (GA), which utilizes bio-inspired mathematical models, to construct an image of the insides of a test object via the scattered photons, from a voxel within the object. A NaI(Tl) scintillation detector and a 185 MBq ¹³⁷Cs gamma ray source were used in the experimental measurements. The obtained results show that the proposed GA based method performs well in constructing images of objects.     

Investigation of the reaction Dy164 (n, γ) Dy165

The level structure of Dy¹⁶⁵ has been investigated at the Karlsruhe research reactor using thermal neutron capture in Dy¹⁶⁴. The target was Dy₂O₃ enriched to 92.71% Dy¹⁶⁴. Neutrons were either monochromized by Bragg reflection from a lead crystal or transmitted through a bismuth single crystal filter. High-resolution measurements of the capture spectrum have been performed by means of a Ge(Li) 5-detector pair spectrometer and a Ge(Li) anti-Compton assembly. A large number of new lines has been observed. Cascade relationships were studied by using several techniques: a double coincidence apparatus with 4″ Ø×5″ NaI(Tl) detectors, a coincidence system containing a scintillation detector and a 34 cm³ Ge(Li)-diode as well as a two-parameter system involving the pair spectrometer (with NaI (Tl) as the primary detector) and a single 4″Ø×6″ NaI (Tl) counter. In the framework ofNilsson's model and simple microscopic pictures of vibrational states the results are consistent with the following spectroscopic interpretation: 0 keV, 7/2⁺[633]; 108.2 keV, 1/2^?[521]; 184.3 keV, 5/2″[512]; 533.5 keV, 5/2″ [523]; 538.6 keV, 7/2⁺[633] +Q₂₂; 570.3 keV, 5/2^?[512]+Q₂₂+1/2^?[510]; 573.6 keV, 3/2^? [521]+1/2^?[521]+Q₂₂; 1103.3 keV, 1/2^? [521]+Q₂₂+3/2^? [521]. For comparison preliminary results are given for the isotonic nucleus Yb¹⁶⁹.     

Scintillation properties of Ce3+-doped YCl3 and YBr3

The dependence of the scintillation properties of Ce³⁺-doped YCl₃ and YBr₃ on activator concentration (0.5, 1 and 2 mol%) has been studied. The radioluminescence spectra of both materials contain asymmetric bands with maxima located at 3.13 eV (383 nm) for YCl₃:Ce³⁺ and 2.84 eV (422 nm) for YBr₃:Ce³⁺. The scintillation pulse decay curves for both materials are described by two components with decay constants of 37 and 640 ns for YCl₃:Ce³⁺ and 36 and 450 ns for YBr₃:Ce³⁺, the fractions of the faster component being 86 and 79 per cent, respectively. The dependences of the light yield of the studied materials on Ce³⁺ concentration pass through a maximum near 1 mol% of the activator, and the maximum light yields (relative to NaI:Tl) are 8700 photons per MeV for YCl₃:Ce³⁺ and 20,600 photons per MeV for YBr₃:Ce³⁺.     

Non-destructive evaluation of scientific and biological samples by scattering of 145 keV gamma rays

The objective of present experiment is to assign effective atomic number (Z_eff) to samples of scientific interest (oxides of lanthanoids, also called rare earths, and alloys of lead and tin of known composition) and to measure stable iodine content of tissue (biological sample). An HPGe semiconductor detector, placed at 70° to the incident beam, detects gamma photons scattered from the sample under investigation. The experiment is performed on various elements with atomic number satisfying, 6 ≤ Z ≤ 82, for 145 keV incident photons. The intensity ratio of Rayleigh to Compton scattered peaks, corrected for photo-peak efficiency of gamma detector and absorption of photons in the sample and air, is plotted as a function of atomic number and constituted a fit curve. From this fit curve, the respective effective atomic numbers of the scientific samples are determined. The agreement of measured values of Z_eff with theoretical calculations is found to be quite satisfactory. The measured intensity ratio from phantom (KI solutions, simulating thyroid content of stable iodine) varies linearly with KI concentration and provides stable iodine content of tissue.     

Nuclear radiation shielding and mechanical properties of colemanite mineral doped concretes

ABSTRACT

The present research focused on the investigation of photon and fast neutron shielding parameters of colemanite mineral doped and undoped concretes. The fabricated concretes have been exposed to gamma rays at 59.5 and 81?keV energies and the measurements have been carried out with NaI(Tl) detector. The parameters of effective atomic number (Z_eff) and electron density (N_el) have been determined experimentally and theoretically. The exposure buildup factor (EBF) and energy absorption buildup factor (EBF) have been computed utilizing the Geometric progression (G–P) fitting method. In addition to the photon shielding parameters, the macroscopic effective removal cross-section calculations for fast neutron (Σ_R) were performed. As a result, it was observed that the concretes doped with colemanite mineral are not very effective in gamma radiation shielding. On the contrary, it was observed that concretes with colemanite were more effective in shielding fast neutrons and the fast neutron removal cross-section values increased with increasing colemanite concentration in the concrete. Additionally, compressive strength values (MPa) of concretes were tested using ALFA TESTING (B001-PC) 200 tons capacity device.     

Experimental evaluation of multiple Compton backscattering of gamma rays in copper

The gamma ray photons continue to soften in energy as the number of scatterings increases in thick target, and results in the generation of singly and multiply scattered events. The number of these multiply scattered events increases with an increase in target thickness and saturates beyond a particular target thickness known as saturation depth. The present experiment is undertaken to study the saturation depth for 279 and 320 keV incident gamma ray photons multiply backscattered from copper targets of varying thickness. The backscattered photons are detected by a Nal(Tl) gamma detector whose pulse-height distribution is converted into a photon spectrum with the help of an inverse matrix approach. To extract the contribution of multiply backscattered photons only, the spectrum of singly scattered photon is reconstructed analytically. We observe that the numbers of multiply scattered events increases with an increase in target thickness and then saturate. The saturation depth is found to be decreasing with increase in incident gamma energy.     

GdBr3:Ce in glass matrix as nuclear spectroscopy detector

An alumina-silica glass cylinder with cerium-activated gadolinium bromide, 2.5 cm in diameter and 3 cm long was cast to form a scintillation detector for spectral analysis of nuclear radiation, especially gamma rays. The detector, operated with a conventional photomultiplier tube and associated power supply, amplifier, and multichannel analyzer, shows for the first time for this material system a set of spectra that include well-defined full-energy gamma-ray peaks from 59.5 keV to 2500 keV. The full-energy peaks were characterized in terms of pulse height linearity, resolution, and counting efficiency. Energy spectral resolution also was demonstrated for alpha particles, beta particles, and ²³⁹Pu/Be neutrons. Gamma-ray detection efficiency was comparable to a conventional NaI(Tl) detector of similar volume but peak resolution at 662 keV in this new detector type was currently only 27%, compared to about 7% for the NaI(Tl) detector. Improved light output and thus resolution is being sought by optimizing glass compositions and processing.     

Evaluation of target photon dose mixed in mono-energetic neutron fields using 7Li(p,n)7Be reaction

Target photons mixed in the 144, 250 and 565 keV mono-energetic neutron calibration fields were measured using a cylindrical NaI(Tl) detector with 7.62 cm both in diameter and in length. The ambient dose equivalent H*(10) of the photons was evaluated by applying the “G(E) function” to the measured pulse height spectrum. Neutrons induce photons by nuclear reactions in the NaI(Tl) detector and affect the pulse height spectrum. In order to eliminate the influence of these neutron events, the time-of-flight technique was applied with operating the accelerator in the pulse mode. The ratios by the ambient dose equivalent H*(10) of the photons to the 144, 250 and 565 keV neutrons were evaluated to be 3.3%, 4.7% and 0.9%, respectively. Although high energy photons ranging from 6 to 7 MeV are emitted by the ¹⁹F(p,αγ)¹⁶O reactions, the dose of the target photons is low enough to calibrate neutron dosemeters except for ones with high sensitivity to the photons.     

Calibration of NaI(TI) spectrometer for radiocesium volume-samples. Some results from Transylvania region

A four-channel NP-424 type gamma analyzer coupled with a large NaI(Tl) scintillator was calibrated using a¹³⁷Cs point source. The volume-samples were plexiglas cylindrical boxes (D=76mm, H=40mm) directly placed on the NaI(Tl) detector. Measurements for 64 indoor positions at full-662 keV-peak were made. Sample selfabsorbtion was determined considering a linear attenuation for gamma rays and measuring the absorption factor for maximum thickness of the volume sample (h0=40mm). The obtained results for two types of environmental samples (soil and milk-powder) are compared with the data acquired from other Romanian laboratories, in the frame of the Environmental Radioactivity Action (ERA) which standardized such types of measurements. The same samples were measured with a calibrated GeLi detector (INAN-Louvain la Neuve) and the results were in good agreement with our measurements. Using this NaI(Tl) calibrated spectrometer some radiocesium deposits (56 localities) from Transylvanian region were measured. The results show that a large deposit exists in the direction NE-SW in connection with the radioactive cloud passing. For Alba region were measured more than 40 kBq/m² of cesium deposit.     

Measurement of attenuation cross-sections of some fatty acids in the energy range 122–1330 keV

The mass attenuation coefficients (μ_m) have been measured for undecylic acid (C₁₁H₂₂O₂), lauric acid (C₁₂H₂₄O₂), tridecylic acid (C₁₃H₂₆O₂), myristic acid (C₁₄H₂₈O₂), pentadecylic acid (C₁₅H₃₀O₂) and palmitic acid (C₁₆H₃₂O₂) using ⁵⁷Co, ¹³³Ba, ¹³⁷Cs, ⁶⁰Co and ²²Na emitted γ radiation with energies 122, 356, 511, 662, 1170, 1275 and 1330 keV, respectively. The accurate values of the effective atomic number (Z_eff), atomic cross-section (σ_t,), electronic cross-section (σ_e) and the effective electron density (N_eff) have great significance in radiation protection and dosimetry. These quantities were obtained by utilizing experimentally measured values of mass attenuation coefficients (μ_m). A NaI(Tl) scintillation detector with 8.2% (at 662 keV) resolution was used for detecting of attenuated γ-photons. The variation in Z_eff and N_eff of fatty acids with energy is discussed. The experimental and theoretical results are in good agreement within 2% deviation.     

A gamma-ray scattering technique for estimation of density and moisture content of wood

A densitometer system, operating in a non-destructive and non-invasive way, is presented to investigate the density and moisture variation in wood samples of Pinus roxburghii (known as chir pine). The inherent ability of Compton scattering can ascertain quantitative information of electron density of the target material. A well collimated beam of 662?keV gamma rays from ¹³⁷Cs radioactive source has been used to extract the information of density from interior of sample by recording scattered spectra with NaI(Tl) scintillation detector. An inverse matrix approach for unfolding of observed pulse-height distribution to a true photon spectrum is used, by choosing bin mesh (E^1/2) of 0.025 (MeV)^1/2, for the measurements of scattered spectra. The regression lines, obtained from data of scattered spectra, provide the density and moisture content of selected wood sample. The adjusted R² value 0.984 for fitted curve, justifies the linear dependence of scattered intensity upon wood density. Also, Compton scatter measurements results that the technique is less sensitive beyond wood thickness of ～110?mm due to overlying scatter components in the measured spectrum.