Gamma-ray Attenuation Measurements Research Articles

Impact of air entrapment on capillary absorption in porous building materials

When the water content of a porous material is high, air entrapped in the pore space is expected to affect water transfer through the pores. To understand the effects of air entrapment on water transfer in porous building materials in the high-water-saturation region, we examined the water transfer characteristics corresponding to significantly small air entrapment effects. First, we conducted two sets of water uptake experiments. In the first experiment, using three building materials, the time evolution of the amount of water absorption was measured at a low air pressure near vacuum (several kPa). In the second experiment, the water content profile during water uptake was measured using the gamma-ray attenuation method. The experimental results showed that low air pressure accelerated the water uptake by the brick and aerated concrete specimens, whereas water uptake by the calcium silicate board specimens was not significantly affected. These differences among materials were analyzed from a pore structure viewpoint. Moreover, gamma-ray attenuation measurements confirmed that the obtained water content profile was qualitatively similar at atmospheric and low air pressures, although the low air pressure increased both the water content of the material after capillary absorption and the wetting front propagation rate. Finally, simultaneous water and air transfer calculations based on the air and liquid water balance in a material reproduced the measured water absorption rates well, confirming that air entrapment and pressure development in the pores can significantly reduce the rate of water uptake and water content after capillary absorption. The calculation results also indicated that the air pressure in a material did not significantly increase at early water uptake stages where local water content was not high, which supported the general assumption that treating the liquid-water transfer in porous building materials as a one-component flow is valid in most cases.

Density, Thermal Expansion, Enthalpy, Heat Capacity, and Thermal Conductivity of Calcium in the Temperature Range 720–1290 K

Thermophysical properties of solid and liquid calcium (99.75 wt % pure) were experimentally studied with high accuracy in the temperature range 720–1290 K using dilatometer measurements, gamma-ray attenuation measurements (the gamma-ray method), high-temperature drop calorimetry, and the laser flash method. The behaviors of the density, enthalpy, and thermal conductivity of calcium in the fusion–crystallization region were studied. The enthalpy of fusion was measured as 8075 J/mol, the relative density change upon fusion as 3.3%, and the relative change in thermal conductivity upon fusion as 26%. The results were compared to the respective values in the related literature. The measurements at temperatures above 720 K either significantly amend the available literature data, or are currently unique. The constancy of the heat capacity of liquid calcium in the temperature range 1115–1290 K was verified. Fitting equations were derived, and recommended values of the investigated properties of calcium were tabulated for 720–1290 K, the temperature range where calcium is in the condensed state.

Fiber- reinforced concrete containing nano - TiO2 as a new gamma-ray radiation shielding materials

In this study, the compressive strength, ultrasonic pulse velocity (UPV), impact resistance, gamma-ray radiation shielding properties, and investigation of microstructure by scanning electron microscope (SEM) analysis of concrete specimens produced from magnetite aggregate Fe3O4 containing 0.6% of polypropylene fiber (PPF) and various percentages of 0, 2, 4, 6, and 8% of nano titanium dioxide (nano - TiO2) were evaluated. Also, the gamma-ray attenuation measurements were using two sources of 137Cs and 60Co with three gamma-ray energies of 662, 1173, and 1332 keV are conducted on the specimens. Some gamma-ray radiation shielding parameters such as the half-value layer (HVL), tenth-value layer (TVL), the mean free path (MFP), linear attenuation coefficient (μ), and mass attenuation coefficient (μm) were also examined in the gamma-ray attenuation. The results show that by increasing the (wt.) % of nano - TiO2 by 0–8%,(a), the compressive strength and the UPV initially increased and then decreased,(b) The impact resistance for the first crack and the ultimate failure was firstly increased and then decreased, (c) The μ and the μm continuously increased, (d) The HVL, TVL and the MFP decreased,(e). The percentage of the ray transmission is decreased so that specimens containing 8% of nano - TiO2 had the lowest ray transmission percentage at every photon energy,(f) A lower thickness of the absorbent material was needed to decrease the photon energy.

Reduction of the buildup contribution in gamma ray attenuation measurements and a new way to study this experiment in a student laboratory

In continuation of our investigation into the buildup phenomenon appearing in gamma ray attenuation measurements in laboratory experiments we study the dependence of the buildup factor on the area of the absorber in an effort to reduce the buildup of photons. Detailed measurements are performed for up to two mean free paths of 60Co γ-rays through a lead absorber. A semi-empirical model on the attenuation of γ-rays studied in the laboratory, which includes the buildup effect, is developed. Finally, we propose a new procedure for studying laboratory γ-ray attenuation, which takes the buildup phenomenon into consideration.

Effect of collimator size and absorber thickness on soil bulk density evaluation by gamma-ray attenuation

Representative measurements of the soil bulk density (ρs) can be affected depending on the geometry selected for gamma-ray attenuation (GRA) measurements. In this study, the effect of collimator size (2–4mm diameters) and absorber thickness (2–15cm) on ρs measurements of sandy and clayey soils was investigated. In summary, the results obtained for the 137Cs show that ρs remained fairly constant for values of thickness larger than 7cm (both soils). The very same result was observed for collimator sizes 2–4mm. Regarding the 241Am source, ρs values presented variations with the collimator size (both soils) for the different thicknesses. The best values of ρs were obtained for samples smaller than 5cm and also for the small collimator diameters.

Gamma-ray attenuation measurements as a laboratory experiment: some remarks

In this article we make some significant remarks on the experimental study of theabsorption of gamma radiation passing through matter. These remarks have to do with theseemingly unexpected trend of the measured intensity of radiation versus the thickness ofthe absorber, which puzzles students and its explanation eludes many laboratory assistants.

Water vapour sorption experiments on hardened cementitious materials: Part I: Essential tool for analysis of hygral behaviour and its relation to pore structure

This paper forms the first part of a series. In this first part, a broad range of normal and high-performance (HP) hardened cement pastes and concretes is studied under both laboratory and in-situ conditions. Water vapour desorption–adsorption experiments are carried out by means of the saturated salt solution method on very thin specimens. The effect of various parameters and in particular of the mix-composition is studied on the so-called water vapour sorption isotherms (WVSIs). It is found that the presence of aggregates does not influence the curves. Likewise, the peculiarities of HP materials are highlighted: within the high relative humidity (RH) range, significant lower water contents are measured than for normal materials, and large RH changes induce only slight variations of water content. More generally, desorption isotherms can be partitioned in several ranges, which are influenced or not by W/C: a unique partition is pointed out, valid for every material tested. Moreover, the hysteretic behaviour of the materials is investigated by plotting scanning isotherms within various RH ranges and by performing first and second desorption–adsorption cycles. Furthermore, a pore structure analysis is carried out from WVSIs. The bulk porosity accessible to water, the C–S–H ‘‘gel’’ porosity, the BET specific surface area, and the BJH pore size distribution are thus assessed, along with the C–S–H ‘‘gel’’ amount. In addition, a master curve is exhibited as regards the average adsorbed water layer thickness vs. RH, for RH ≤ 63.2%. Moisture profiles in structural elements exposed to various drying conditions in laboratory and in natural environments (RC test specimens and bridge deck) are assessed by means of gamma-ray attenuation measurements. A perfect consistence is pointed out between the water contents measured by this technique on 70- to 150-mm thick cylinders and those provided by the desorption experiments previously mentioned (on 1- to 3-mm thick specimens). The very similar moisture profiles and the high degrees of liquid water saturation recorded in HP materials, whatever the mixture, the age and the environmental conditions, confirm that these materials are weakly sensitive to environmental hygral changes within a broad RH range.

Evolution of the Extragalactic Infrared-to-Ultraviolet Background and Gamma-Ray Attenuation

We describe a model for the evolution of the cosmic background radiation field from infrared to ultraviolet wavelengths tuned to observations of star formation and including the effects of reprocessing by dust and gas. With this model, we can compute the attenuation of gamma rays from extragalactic sources. The attenuation length for the vast majority of extragalactic gamma-ray sources (blazars and gamma-ray bursts at redshifts of order unity) depends on the evolving optical-to-ultraviolet background, whereas gamma-ray attenuation measurements of nearby sources-such as Mrk 501 probe the infrared background. We focus on the cosmological aspects of the model and discuss the effects of changing cosmological parameters, star formation rate, initial mass function, and dust opacity on the resulting gamma-ray attenuation. A plausible choice of parameters leads to fair agreement between our model in the infrared and the observed attenuation of gamma rays from Mrk 501.

Effect of collimator size and absorber thickness on gamma ray attenuation measurements

Mass attenuation coefficients have been measured in extended media of soil and water for 662 keV gamma rays under different collimation conditions. A correlation effect due to absorber thickness and collimator size has been observed.

Effect of collimator size and absorber thickness on gamma ray attenuation measurements for bakelite and perspex

The mass attenuation coefficient (μ m) of 662 keV gamma rays have been measured in the extended media of bakelite and perspex under different collimation conditions. The increase in attenuation coefficient is seen with increase in sample thickness as well as with collimator size due to the contribution of multiple scattered photons in the uncollided beam of 662 keV gamma rays.

Characterization and identification of equilibrium and transfer moisture properties for ordinary and high-performance cementitious materials

In this paper some aspects of equilibrium and transfer moisture properties of high-performance materials are presented and compared with ordinary cement pastes and concretes. First, the equilibrium moisture properties of the hardened materials are described by means of water vapour sorption isotherms, which illustrate the hysteretical behaviour of the materials. Experimental results of drying shrinkage versus relative humidity (RH) are also reported here. These experimental data are in good agreement with the numerical results provided by a thermodynamic modelling based on capillary stresses and hygromechanical couplings. In particular the linearity of the strains-RH curve over a wide range is pointed out in both cases. Isothermal drying process at RH = 50% has experimentally and numerically been studied. After identification of the intrinsic permeability of the materials from experimental weight losses, numerical moisture profiles were compared with gamma-ray attenuation measurements. The influence of the initial moisture state of the materials that results from self-desiccation in particular was pointed out on the evolution of the moisture profiles as a function of time.

Improved void fraction determination by means of multibeam gamma-ray attenuation measurements

Gamma-ray densitometry is a frequently used non-intrusive method for determining void fraction in two- and multi-phase gas liquid pipe flows. The traditional gamma-ray densitometer using a 137Cs source and a scintillation PMT detector has proved itself reliable and robust. This paper presents a method using a low energy source ( 241Am), which offers the advantages of reduced size due to reduced shielding requirements, compact detectors, and lesser dependence on flow regime, due to its multibeam measurement configuration. These are important aspects with regard to future subsea and down-hole fluid flow measurement applications. The performance of single-beam and the compact multi-beam low-energy gamma-ray measurement principles was compared. Consideration of the measurement volume, defined by the detector area and the radiation beam, demonstrated the flow regime dependency of single-beam gamma-ray measurement principles. With the multi-beam low-energy gamma-ray measurement principle, the dependence on flow regime is negligible when several detector responses are combined. Use of phantoms and one movable detector verified the multi-beam gamma-ray measurement principle. The detector responses at several positions around the pipe were obtained for different flow regimes and void fractions.

Magnetic resonance imaging of the Lake Agassiz–Lake Winnipeg transition

As part of the Geologic Survey of Canada (GSC) Lake Winnipeg Study, we have successfully imaged the Lake Agassiz to Lake Winnipeg transition in Section 4 of Core Namao 94-900-122a (i.e., from 313 cm to 465 cm), using a newly-developed magnetic resonance imaging (MRI) technique called SPRITE (Single-Point, Ramped Imaging with T1 Enhancement). Whole core, gamma-ray attenuation measurements have been used to calculate the bulk porosity of the sediment at 1 cm intervals for comparison with the SPRITE images. Image contrast and image intensities observed in the SPRITE images of Section 4 are related to local porosity and magnetic susceptibility variations. In general, regions of the core with low signal intensity contain high porosity and low magnetic susceptibility. The best contrast between sediment layers is observed from regions of the core with high magnetic susceptibility. High signal intensity is observed from regions with low porosity and/or high magnetic susceptibility.

Calibration and application of marine sedimentary physical properties using a multi-sensor core logger

We examined geophysical data from a Multi-Sensor Core Logger (MSCL), a logging device providing continuous measurements of gamma-ray attenuation, p-wave travel time, and magnetic susceptibility on marine sediment cores. In the first part we focused on the gamma-ray system and compared two different calibration methods. From the gamma-ray attenuation, we calculated densities and porosities by incorporating mass weighted attenuation coefficients. The application of an iteration method reduces the error of the density and porosity estimates compared to GRAPE data. In addition, we derived equations to calculate water content and dry bulk density from gamma-ray attenuation measurements. Comparison with physical properties determined on discrete samples revealed a very good correlation of both data sets ( r = 0.99). This correlation is valid for sediments from substantially different geological settings (e.g., turbidites, hemipelagic muds, and opal-rich sediments). In the second part we applied our data to marine geological questions. For sediments from the Antarctic Polar Frontal Zone, there is indication that the content of biogenic opal can be assessed using a correlation of density and p-wave velocity. For sediments from the Bengal Fan, the relationship between the MSCL acoustic impedance (the product of density and p-wave velocity) and the grain-size distribution in discrete samples can be used to predict clay and sand/silt ratios for sediment cores from the shelf and upper continental slope.

Computation of hydraulic conductivity of porous materials using computer-assisted tomography

The spatial distribution of porosity and continuity of pore space in glass bead and soil aggregate systems has been characterised by application of computer-assisted tomography to gamma-ray attenuation measurements. It has been demonstrated that a useful empirical relationship between hydraulic conductivity and pore size and continuity for the glass bead system as measured by CAT scanning can be derived.

Nondestructive density determination on marine sediment cores from gamma-ray attenuation measurements

We describe an improved nondestructive technique for density measurement based on gamma-ray absorption that permits construction of high-resolution density profiles of marine sediment cores. The system is capable of resolving vertical density variations with a spatial resolution of 4 mm and absolute accuracy of ± 1%. Comparison with conventional density determination on samples confirms the absolute accuracy. In addition to standard vertical profiling, a core rotation option is included to measure density as a function of rotation angle. This allows the quantification of the variability of density within a layer and detection of inhomogenities like icerafted debris or biogenic structures.

Gamma-ray densitometry: nondestructive analysis of density evolution during ceramic powder processing : Schilling, C.H.; Graff, G.L.; Samuels, W.D.; Aksay, I.A. Pacific Northwest Laboratory, Richland, Washington, (United States), PNL-SA-15179, DE88-006671, 14 pp. (Feb. 1988)

Gamma-ray attenuation measurements enable nondestructive analysis of density heterogeneities which evolve during the processing of ceramics and ceramic composites by powder consolidation methods. A densitometer apparatus has been constructed at the Pacific Northwest Laboratory or use in situ measurement of porosity and composition as a function of time and position during powder compaction and sintering. Attenuation experiments were performed with aqueous suspensions of colloidal ..cap alpha..-Al/sub 2/O/sub 3/ to evaluate the sensitivity of the gamma-ray densitometer and also to investigate the role of pH on compaction processes during sedimentation. One of the concerns with this technique is whether the gamma radiation degrade sample solvents or dispersing agents; no evidence of radiation damage was revealed in aqueous polymer solutions, based on analysis with gas chromatography, ion chromatography, and nuclear magnetic resonance spectroscopy. 29 refs., 6 figs.

Development of a soil water dispersion index (SOWADIN) for testing the effectiveness of soil-wetting agents

Computer-assisted tomography (CAT) applied to gamma-ray attenuation measurements has been used to develop an index termed the soil water dispersion index (SOWADIN), which describes quantitatively the amount and distribution of water in soil columns. The index, which is determined by classifying pixels in a scanned slice into three categories according to their attenuation coefficients, contains two numerical values. The first value corresponds to the water content of the scanned slice and the second value is a measure of the dispersion of the water throughout the slice. Artificially wetted zones were created in soil columns to give one-third of the scanned layer wetted with various patterns of wetted-area distribution. The SOWADIN values obtained accurately reflected the differences in water distribution associated with the different patterns. Application of SOWADIN to columns of a water-repellent sand before and after treatment with a soil-wetting agent clearly illustrates both the increase in water content and improvement in water distribution in the soil column following treatment.

A dual source gamma ray traversing mechanism suitable for the non-destructive simultaneous measurement of bulk density and water content in columns of swelling soil

A traversing mechanism is described in which 100 mCi sources of americum-241 and caesium-137 are alternately brought into line with a sodium iodide (thallium) scintillation counter connected to a single-channel pulse height analyser equipped with two preset base lines. The equipment is capable of scanning a 10 cm diameter soil column equipped with sensors to a height of 200 cm, and facilities are provided for automatic indexing at preset intervals to an accuracy of 0.01 cm. Non-linear counting losses in the spectrometer system are described, and were found to be dependent on the window setting of the pulse height analyser. With the window setting adjusted to cover the apparent spread of the gamma peak, counting losses were negligible. Mass absorption coefficients of various materials were obtained using a compartmented box. Measured mass absorption coefficients of water, silica sand, and a chernozemic soil were 0.201, 0.247, and 0.303 cm2 g-1 respectively for americum-241, and 0.0826, 0.0746, and 0.0728 respectively for caesium-137. The precision of the apparatus was demonstrated by determining the moisture characteristic and bulk density characteristic of 0.5-1.0 mm aggregates of Narrabri soil during absorption and desorption. Gamma ray attenuation measurements and direct volumetric measurements were comparable. The standard deviations in the gamma ray measurements were found to be 0.03 g cm-3 for bulk density and 0.04 cm3 cm-3 for water content. Most of this inaccuracy arose from lack of precision in the americum-241 mass attenuation coefficient for the soil. By contrast, changes in moisture content and bulk density, at any level, of 0.004 cm3 cm-3 and 0.004 g cm-3 respectively, were statistically very significant.

Measurements of gamma-ray mass attenuation coefficients of organic scintillators

Preliminary results of gamma-ray attenuation measurements in organic scintillators are given. Total mass attenuation coefficients of a number of the most important organic scintillators for gamma-rays from 133Ba, 137Cs, 60Co and 65Zn have been determined in narrow-beam experiments. Theoretical mass attenuation coefficients are computed by interpolation from the atomic cross-section data of Grodstein. Satisfactory agreement is obtained between the experimentally determined mass attenuation coefficients and the computed values.