ТЕПЛОФИЗИКА ВЫСОКИХ ТЕМПЕРАТУР, 2014, том 52, № 5, с. 677-690
THERMOPHYSICAL PROPERTIES OF RUBIDIUM AND LITHIUM HALIDES
BY y-RAY ATTENUATION TECHNIQUE
© 2014 S. Ammiraju, R. Madhusudhan1, K. Narender2, K. G. K. Rao3, N. G. Krishna2
department of Physics, Varadha Reddy College of Engg., Warangal-506009-India 2Department of Physics, Kakatiya University Warangal 3CIC, Kakatiya University Warangal, India e-mail: madhuammiraju@yahoo.co.in
Abstract—The temperature dependence of linear attenuation coefficient, density and thermal expansion of rubidium halides (RbCl, RbBr and Rbl) and lithium halides (LiCl, LiBr and LiF) has been studied by y-ray attenuation technique. The y-ray attenuation studies have been carried out using a y-ray densitometer. The mass attenuation coefficients p.m of rubidium and lithium halides have been determined using y-beam of different energies viz. (0.0595, 0.662, 1.173 and 1.332 MeV) respectively. The variation of density and coefficients of temperature dependence of density have been measured using Cs (0.662 MeV) source. The values of density at different temperatures have been used to estimate the values of linear attenuation coefficients p of the alkali halides studied in the present work for other y-energies. The variation of thermal expansion of alkali halides studied in the present work has been compared with the results obtained from other methods. The variation in these thermophysical properties have been represented by linear equations. Volume thermal expansion coefficients and mass attenuation coefficients pm of these compounds for the different energies have been reported and compared with data calculated by empirical and experimental method.
DOI: 10.7868/S0040364414020021
INTRODUCTION
The interaction of high energy photons with matter is important in radiation, medicine and biology, nuclear engineering and space technology. The knowledge of physical parameters such as mass attenuation coefficients, linear attenuation coefficients, atomic and electronic crosssections plays an important role in understanding the physical properties of composite materials like inorganic compounds. They are invaluable in many applied fields, such as nuclear diagnostics, radiation protection, nuclear medicine and radiation dosimetry. The mass attenuation coefficient is a measurement of how strongly a chemical specie or substance absorbs or scatters radiation at a given wavelength, per unit mass. Mass attenuation coefficient is a fundamental tool to derive many other photon interaction parameters. The linear attenuation coefficient (^L) describes the fraction of a beam of X-rays or y-rays that is absorbed or scattered per unit thickness of the absorber. The researchers have focused on the studying of photon interaction parameters with matter [1-3].
The study of thermophysical properties such as density and thermal expansion of solids and their temperature dependence is very important for understanding variation of other properties like elastic constants, refractive indices, and dielectric constants, thermal conductivity, diffusion coefficients and other heat transfer dimensionless numbers as a function of
temperature. Temperature dependent thermal expansion of solids is of technical importance as it determines the thermal stability and thermal shock resistance of the material. In general the thermal expansion characteristics decide the choice of material for high temperature applications in science and technology. The density and thermal expansion of solids at high temperature can be determined by number of methods like Archimedean method, pycnometry, dilatometry, and electromagnetic levitation, Method of maximal pressure in gas bubble, method of sessile drop, hydrostatic weighing, high temperature electrostatic levitation [4] and gamma ray densitometry [59]. Thermal expansion studies on alkali halides have been reported by several workers using X-ray diffraction [10-11], dilatometry [12-13], Fabrey-Perot interference method [14] and by other theoretical models [1522]. Such studies on alkali halides by y-ray attenuation technique are lacking. Using y-ray attenuation technique, Drotning [23] measured thermal expansion of solid materials at high temperatures. He studied thermal expansion of several solids at high temperatures and such studies have been extended by him to study the thermal expansion of metals of low density and glasses in the condensed state [24]. The gamma ray attenuation technique is a method utilizing the gamma beam only as a probe, which is neither in physical nor in thermal contact with the sample. The gamma ray attenuation technique used for determination of ther-
mal expansion and densities offers several advantages over other methods, and it is particularly advantageous at high temperatures as thermal losses are minimized. This technique also ensures the elimination of incompatibility of sample and probe materials. In measurement of density and thermal expansion by this method, only the solid or molten materials of the samples are involved, eliminating the free liquid surface at high temperatures.The y-ray attenuation technique has been used to carry out the studies on temperature dependence of y-ray attenuation, density and thermal expansion of rubidium halides (RbCl, RbBr, and Rbl) and lithium halides (LiCl, LiBr, and LiF). In the present communication, we report mass attenuation coefficient, the linear attenuation coefficient of y-radiation of different energies, density and thermal expansion of rubidium halides (RbCl, RbBr, and Rbl) and lithium halides (LiCl, LiBr, and LiF) as a function of temperature. A y-ray densitometer and a programmable temperature controlled furnace (PTC) which can reach up to 1300 K have been fabricated in our laboratory based on design proposed by Drotning [23] to carry out the work. The results on temperature dependence of coefficient of linear thermal expansion of alkali halides obtained in the present work have been compared with experimental and theoretical results available in literature.
EXPERIMENTAL METHOD
Density of a material can be determined by measured attenuation produced in a collimated beam of gamma radiation by the material. The thermal expansion of the material, change in density for a change in the temperature can be determined by y-ray attenuation studies. The block diagram and the cross-sectional view of y-ray densitometer used in the present work have been presented in reference [25]. Main parameters of the experimental setup are. Source vault and detector housing were made in lead, and collimators were made both in lead and stainless steel (SS) with the outer dimensions as given below.
♦ Source vault — made of lead — 30 cm in length and 27.5 cm in diameter.
♦ Collimators — made of lead — length 5, 7.5, 10 cm and 6 cm in diameter.
♦ Collimators — made of stainless steel — length of 5 cm, 7.5 cm, 10 cm, 15 cm and 6 cm in diameter.
♦ Detector housing — made in lead weighing about 160 kg — 30 cm length and 20 cm in diameter.
♦ A 6 mm collimation is provided to all these items along their axes.
♦ Source, sample and detector placement: Distance from source to the detector is 66.5 cm, distance from source to the sample Face is 32 cm and distance from sample's second face to detector is (34.5 — l) cm where l is thickness of the sample.
The gamma radiation detector used in our study is a sodium iodide — thallium activated detector. The 0.0762 m diameter and 0.0762 m thick crystal is integrally coupled to a 0.0762 m diameter photo multiplier tube (PMT). The PMT has a 14 pin base and can be mounted on two types of PMT preamplifier units. The one used in our study is a coaxial in-line pre-amplifier. The detector has a resolution of 8.5% for 0.662 MeV of 137Cs. The important aspect of fabrication of the experimental setup is designing and fabrication of Programmable Temperature Controlled (PTC) Furnace, such that it can fit exactly into the experimental setup. PTC furnace is an electric muffle furnace consisting temperature sensors along with programmable logic controllers (PLC) used for monitoring and recording the temperature. Type K thermocouple is used in the furnace for wide operating temperatures. The operation of furnace is monitored with the help of electronic panel.
The instrumentation for the furnace has been designed such that feedback and control of the furnace can be handled directly from the control panel. Programmable logic controllers (PLCs) have been used to perform control functions. A controller consists of two basic sections: the central processing unit (CPU) and the input — output interface system. The CPU, which controls all system activity, can further be broken down into the processor and memory system. The input — output system is physically connected to field devices (e.g., switches, sensors, etc.) and provides the interface between the CPU and the information providers (inputs) and controllable devices (outputs). The functional Block Diagram of PTC Furnace is shown in Fig. 1. The PTC uses a Proportional, Integral and Derivative (PID) feedback loop to control temperature. This feedback loop combines three algorithms to calculate the optimal heater power at any given moment. A programmable temperature controlled furnace with sample inside the air tight quartz tube is introduced in the y-radiation path allowing the beam to pass through the sample and to the detector without any interruption. The temperature of the sample is varied to study the attenuation at various temperatures.
The mass attenuation coefficients of the alkali halides have been determined for different y — energies (Am, 0.0595, Cs, 0.662, Co, 1.173 and 1.332 MeV) emitted by 10 mCi Am-241, 30 mCi Cs-137 and 11.73 ^Ci Co-60 radioactive point sources, respectively with the density at room temperature determined by X-ray method [26].
The samples studied in the present work were in the form of pellets. The pellets were prepared wi
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