On microdosimetry of neutrons of selectable energy in mixed (n,y) fields

Biological damage of tissue due to intermediate energy neutrons is generally known to be very important in radiobiology and radiation protection. However, there is no suitable method to determine the quality of these neutrons in particular in the working environment of mixed (n,y) radiation fields. In this thesis, an attempt is made to develop a dosimeter based on microdosimetric principles which has the capability for such a purpose. With this object the basic concepts of microdosimetry are reviewed and discussed with emphasis on their application for radiation protection and in designing of the dosimeter. Microdosimetry based on low pressure tissue-equivalent proportional counters (TEPCs) is a powerful technique for determining microscopic distributions of energy deposition and quality of ionizing radiations. However the energy deposition spectra of intermediate energy neutrons in mixed fields of fast neutrons can only be measured using TEPC in co-axial double cylindrical form by an appropriate choice for the thickness of the common tissue-equivalent (TE) dividing wall separating the inner and outer counters and by appropriate use of coincidence/anti-coincidence pulse arrangements. An analytical calculation for the response of the inner counter operating in coincidence/anti-coincidence modes with the outer counter was developed. However there will be some events, due to fast neutrons, which will contribute to the signals from intermediate energy neutrons and which cannot be removed by anti-coincidence. For these analytical corrections must be made. Also, the events associated with the dividing wall inherent in the system can contribute to the response of the inner counter and must be corrected by calculation. The calculation was possible due to the fact that recoil particles from intermediate energy neutron interactions have effective stopping powers and projected ranges which differ significantly from the continuous slowing down approximation (CSDA) values. By incorporating these the basic CSDA formulae for energy deposition spectra of neutrons could be extended down to intermediate energy neutrons of about 1 keV. A prototype co-axial double cylindrical TEPC capable of separating the component of neutrons (≤ 850 keV) in mixed (n,y) radiation fields was manufactured and tested. The thin wall dividing the inner and outer counters was fabricated from the standard A-150 TE plastic with the thickness equivalent to the range of 850 keV protons. The operational characteristics of the dosimeter were studied to determine its applicability for use in microdosimetry. The gas gain of the inner and outer TEPCs was measured at various simulated mean chord lengths and applied voltages. The results can be expressed according to Campion's equation within a given range of the electric field strength. The resolution of the inner TEPC measured at the operating voltages is in agreement with the theoretical prediction. A series of microdosimetric experiments were performed with mixed fields of 60Co gamma-rays and neutrons from the UTR-300 nuclear reactor and from 252Cf and 241Am-Be radioactive sources. Discrimination against fast neutrons of energy > 850 keV was achieved using an anti-coincidence unit specially designed for better efficiency of data acquisition. Discrimination against fast electrons due to photon interactions was also achieved. Spectra with anti-coincidence are dominated by slow protons and electrons. Their mean lineal energies are higher than those of spectra without anti-coincidence. The quality factor and dose equivalent for spectra with anti-coincidence are higher than the spectra without anticoincidence indicating the importance of intermediate energy neutrons in mixed fields. The quality factor and the corresponding dose equivalent corrected for saturation of lineal energy corresponding to 2 nm of ionization spacing is consistently higher than those derived from the absorbed dose based formulae, the biophysical implications of which are discussed. Suggestion for future developments for microdosimetry of intermediate energy neutrons in mixed fields are made and discussed

Quantum mechanical calculation of the optical absorption of silver and gold nanoparticles by density functional theory

Problem statement: Metal nanoparticles confine the motion of conduction electrons and exhibit a strong optical absorption of electromagnetic radiation in the UV-vis-NIR region. The absorption is classically derived from the collective oscillations of free electrons in a metallic nanostructure as a consequence of incident electromagnetic radiation polarizing the particle optically embedded in a dielectric matrix. These oscillations, known as the localized surface Plasmon resonance has been modelled by Gustav Mie in 1908 using the Maxwell's equations. Nevertheless, the electrodynamics approach cannot account for the electronic transitions often displayed in experiment as a broad UV-vis optical absorption spectrum originated from the conduction electrons of metal nanoparticles. A quantum mechanical approach is required to address the optical absorption spectra of metal nanoparticles systemically. Approach: In this study, an attempt was made to calculate the optical absorption spectra of conduction electrons of metal nanoparticle quantum mechanically using the density functional theory. The particle was an isolated spherical metal nanoparticle containing N atoms confined in a face-centered cubic lattice structure. When light strikes the particle, the occupied ground-state conduction electrons absorbed the energy and excite to the unoccupied higher energy-state of the conduction band. In this development, we used time-independent Schrodinger equation for the ground-state energy of Thomas-Fermi-Dirac-Weizsacker atomic model for the total energy functional and the density function in the Euler-Lagrange equation is algebraically substituted with the absorption function. The total energy functional was computed numerically for silver and gold nanoparticles at various diameters. Results: The results showed broad absorption spectra derived from the occupied ground-state conduction electrons at the orbital {n = 5 and l = 0 or 5s} for silver and {n = 6 and l = 0 or 6s} for gold, which excite to the unoccupied higher energy of conduction band at the orbital {n≥6 and l = 0 or 1} for silver and {n≥7 and l = 0 or 1} for gold. A nonlinear red-shift of the absorption peak λmax, appearing at 404.79, 408.36, 412.55, 415.73, 418.42 and 420.96 nm for silver and at 510.28, 520.91, 533.11, 542.35, 549.74 and 556.04 nm for gold when the particle diameter varies at 4, 5, 7, 10, 15 and 25 nm respectively. The quantum confinement effect of the conduction bands is stronger for silver and gold nanoparticles of less than about 20 nm in diameter. Conclusion: The optical absorption spectra of silver and gold nanoparticles have been successfully calculated using a quantum treatment and this calculation could be extended to other transition metal nanoparticles of interest in nanoscience and nanotechnology

Free Flow Traffic Noise in Three Malaysian Cities

This paper reports a study on free flow traffic noise in three cities in Peninsular Malaysia. It is found that generally the noise level is high and correlates with the type of traffic. A few suggestions on control measures are made

Calculation of the Effective Stopping Power of Ions Generated by Neutrons in Tissue Constituents

This paper reports the calculation of stopping powers of heavy charged particles generated by neutrons in four-element tissue constituents for particle energies from 0.1 keY to 1.0 MeV. At low projectile energies of less than 30 keV/amu where the nuclear stopping phenomenon is more dominant than the electronic stopping phenomenon, the effective stopping power values are higher than the continuous slowing-down approximation (CSDA) values, from which the deviation is dependent upon the target mass and the energy and mass of the projectiles

Band gap of cubic and hexagonal CdS quantum dots - experimental and theoretical studies

CdS quantum dots of face centered cubic (fcc) and hexagonal close packed (hcp) structures were synthesized from sulphur source of sodium sulphide and thioacetamide respectively via microwave-hydrothermal method. The synthesized quantum dots were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-visible spectrophotometry. The average particle size in the range 8.5 - 12.5 nm increases with the increase of microwave exposure time from 10 to 40 min. Particles with hcp structure are larger than those with the fcc structure. The band gap in the range 2.54 - 2.65 eV decreases with the increase of microwave exposure time and the particles with the hcp structure have larger band gap than those with the fcc structure. The band gap of the CdS quantum dots were also derived from time independent Schrodinger equations for CdS system and calculated using the density functional theory (DFT). There is good agreement between the measured and calculated band gap values. The results also reveal that the band gap decreases with the increase of particle size due to the quantum size effects

Probing the microscopic worlds by ionizing radiation

A journey from the femto-scale world to the nano-scale world is full of mysteries and enlightening phenomena that inspire further scientific and philosophical debates. Among others, nuclear reactions, radioactivity, atomic structure and molecular bonds are the essence of microscopic worlds, for which accurate descriptions can only be through quantum physics. The fundamental aspect of nuclear radiation is its ability, in a single event, to transfer energy to the nucleus or to orbital electrons. Understanding such interactions and devising techniques that allow them to be manipulated and controlled remains one of the greatest challenges in research that underpins the development of new technologies. This lecture discusses the fundamental and social aspects of nuclear radiation and reviews the author's experiences in nuclear technologies in three disciplines of physics. For research in nuclear physics, neutron interactions with nuclei were exploited by irradiating marine sediment samples with thermal neutrons in the nuclear reactor. The neutron activation analysis technique was used to determine the concentration of anthropogenic and non-anthropogenic elements to monitor pollution levels. In atomic physics research, interactions of charged particles with tissue matters were calculated prior to the development of a microdosimeter that is able to measure the quantity and quality of radiation in a simulate tissue volume of nm dimensions. In molecular physics research, the desired chemical changes by gamma rays in molecules were used to synthesize functional materials including ionic exchange membranes by radiation grafting, conducting polymers and polymer gels by radiation-induced polymerization and metal nanoparticles by radiation reduction process. This journey has provided exciting prospects where ionizing radiation could be exploited in molecular engineering for peaceful use

Effects of gamma radiation treatment and plasticizer on alkaline solid polymer electrolytes

Alkaline solid polymer electrolyte films have been prepared by the solvent-casting method. Gamma radiation treatment and propylene carbonate plastisizer were used to improve the ionic conductivity of .the electrolytes at ambient temperature. The structure of the irradiated electrolytes changes from semi-crystalline to amorphous, indicating that the crosslinking of the polymer has been achieved at a dose of 200 kGy. The ionic conductivity at room temperature of PVA/KOH blend increases from 10-7 to 10-3 Scm-1 after the PVA crosslinking and when the plasticizer concentration was increased from 20 to 30%

The amazing effects and role of PVP on the crystallinity, phase composition and morphology of nickel ferrite nanoparticles prepared by thermal treatment method

Nickel ferrite nanocrystals were prepared from an aqueous solution containing metal nitrates and various concentrations of poly(vinylpyrrolidone) followed by calcination temperature. X-ray diffraction (XRD) analysis was performed to determine the degree of crystallinity of the ferrite nanoparticles. By transmission electron microscopy, the morphology and average particle size of the nickel ferrite nanoparticles were evaluated which had good agreement with the XRD results. Fourier transform infrared spectroscopy suggested the presence of metal oxide bands in all samples as well as the effective elimination of organic constituents after calcinations. Measurements of the extent of magnetization of the nickel ferrite nanoparticles synthesized in different concentrations were obtained at room temperature using a vibrating sample magnetometer

Determination of Effective Atomic Number of Rubber

This paper reports a simple technique to determine the effective atomic number of rubber materials. The gamma ray attenuation coefficient of rubber was measured with high energy resolution Si(Li) detector at low gamma ray energies and the effective atomic number was determined by the functional ratio of attenuation coefficients at different energies. This study could provide a guide to an understanding of the quality of rubber based on its composition