Sherif S. Nafee

A theoretical approach to calibrate radiation portal monitor (RPM) systems

Radiation portal monitor (RPM) systems are widely used at international border crossings, where they are applied to the task of detecting nuclear devices, special nuclear material, and radiation dispersal device materials that could appear at borders. The requirements and constraints on RPM systems deployed at high-volume border crossings are significantly different from those at weapons facilities or steel recycling plants, the former being required to rapidly detect localized sources of radiation with a very high detection probability and low false-alarm rate, while screening all of the traffic without impeding the flow of commerce [Chambers, W.H., Atwater, H.F., Fehlau, P.E., Hastings, R.D., Henry, C.N., Kunz, W.E., Sampson, T.E., Whittlesey, T.H., Worth, G.M., 1974. Portal Monitor for Diversion Safeguards. LA-5681, Los Alamos Scientific Laboratory, Los Alamos, NM]. In the present work, compact analytical formulae are derived and used to calibrate two RPM systems with isotropic radiating sources: (i) polyvinyltoluene (PVT) or plastic and (ii) thallium-doped crystalline sodium iodide, NaI(Tl), gamma-ray detector materials. The calculated efficiencies are compared to measured values reported in the literatures, showing very good agreement.

New analytical approach to calibrate the co-axial HPGe detectors including correction for source matrix self-attenuation

To calibrate the co-axial HPGe semiconductor detectors, we introduce a new theoretical approach based on the Direct Statistical method proposed by Selim and Abbas (1995, 1996) to calculate the full-energy peak efficiency for cylindrical detectors. The present method depends on the accurate analytical calculation of the average path length covered by the photon inside the detector active volume and the geometrical solid angle Ω, to obtain a simple formula for the efficiency. In addition, the self attenuation coefficient of the source matrix (with a radius greater than the detectors radius), the attenuation factors of the source container and the detector housing materials are also treated by calculating the average path length within these materials. (152)Eu aqueous radioactive sources covering the energy range from 121 to 1408 keV were used. Remarkable agreement between the measured and the calculated efficiencies was achieved with discrepancies less than 2%.

New analytical approach to calibrate the NaI (Tl) detectors using spherical radioactive sources

A new theoretical approach was used to calibrate and calculate the full-energy peak efficiency of the NaI (Tl) detectors based on the direct statistical method proposed by Selim and Abbas for cylindrical detectors. In addition, the self-attenuation of the source matrix, the attenuation by the source container and the detector housing materials were considered in the mathematical treatment. Results were compared with those measured by a cylindrical NaI (Tl) detector with resolution (FWHM) at 662 keV equal to 7.5 %. (152)Eu aqueous radioactive spherical sources covering the energy range from 121 to 1408 keV were used. In comparison, the calculated and the measured full-energy peak efficiency values were in good agreement.

Calibration of the 4π γ-ray spectrometer using a new numerical simulation approach

The 4pi gamma-counting system is well suited for analysis of small environmental samples of low activity because it combines advantages of the low background and the high detection efficiency due to the 4pi solid angle. A new numerical simulation approach is proposed for the HPGe well-type detector geometry to calculate the full-energy peak and the total efficiencies, as well as to correct for the coincidence summing effect. This method depends on a calculation of the solid angle subtended by the source to the detector at the point of entrance, (Abbas, 2006a). The calculations are carried out for non-axial point and cylindrical sources inside the detector cavity. Attenuation of photons within the source itself (self-attenuation), the source container, the detectors end-cap and the detectors dead layer materials is also taken into account. In the Belgium Nuclear Research Center, low-activity aqueous solutions of (60)Co and (88)Y in small vials are routinely used to calibrate a gamma-ray p-type well HPGe detector in the 60-1836keV energy range. Efficiency values measured under such conditions are in good agreement with those obtained by the numerical simulation.

Effects of Very Low Dose Fast Neutrons on Cell Membrane And Secondary Protein Structure in Rat Erythrocytes.

The effects of ionizing radiation on biological cells have been reported in several literatures. Most of them were mainly concerned with doses greater than 0.01 Gy and were also concerned with gamma rays. On the other hand, the studies on very low dose fast neutrons (VLDFN) are rare. In this study, we have investigated the effects of VLDFN on cell membrane and protein secondary structure of rat erythrocytes. Twelve female Wistar rats were irradiated with neutrons of total dose 0.009 Gy (241Am-Be, 0.2 mGy/h) and twelve others were used as control. Blood samples were taken at the 0, 4th, 8th, and 12th days postirradiation. Fourier transform infrared (FTIR) spectra of rat erythrocytes were recorded. Second derivative and curve fitting were used to analysis FTIR spectra. Hierarchical cluster analysis (HCA) was used to classify group spectra. The second derivative and curve fitting of FTIR spectra revealed that the most significant alterations in the cell membrane and protein secondary structure upon neutron irradiation were detected after 4 days postirradiation. The increase in membrane polarity, phospholipids chain length, packing, and unsaturation were noticed from the corresponding measured FTIR area ratios. This may be due to the membrane lipid peroxidation. The observed band shift in the CH2 stretching bands toward the lower frequencies may be associated with the decrease in membrane fluidity. The curve fitting of the amide I revealed an increase in the percentage area of α-helix opposing a decrease in the β-structure protein secondary structure, which may be attributed to protein denaturation. The results provide detailed insights into the VLDFN effects on erythrocytes. VLDFN can cause an oxidative stress to the irradiated erythrocytes, which appears clearly after 4 days postirradiation.

FTIR and optical properties for irradiated PVA–GdCl3 and its possible use in dosimetry

ABSTRACT There are various chemical and physical changes caused when polymers are exposed to a source of radiation. In the present work, four films of polyvinyl alcohol (PVA) doped with 2, 4, 6 and 8 wt% of gadolinium chloride (GdCl3) have been irradiated by low-dose fast neutrons using Am-241/Be-9. The structural properties of the synthesized films have been investigated pre- and post-irradiation on the basis of Fourier transform infrared technique. The optical properties of the PVA–GdCl3 films have been investigated for doses up to 1.2 KGy. The energy gap of the synthesized films showed a neutron dose dependence and was decreased by 0.81 eV after exposing to 1.2 kGy. At a wavelength of 1000 nm, the refractive index of the PVA doped with 8 wt% GdCl3 has been increased by nearly 16%. The results suggested the possible use of PVA--GdCl3 as a dosimeter for the neutron beams.

New numerical algorithm method to calibrate the HPGe cylindrical detectors using non-axial extended source geometries

The knowledge of the full-energy peak efficiency for a specific source-detector arrangement is often required in various fields of research and applications, such as the analysis of nuclear waste or environmental samples, where both require modeling because it is not practical to prepare a standard that matches the physical and nuclear properties of every waste or environmental item. Therefore, a new numerical algorithm method (NAM) is proposed in the present work to calibrate the co-axial HPGe cylindrical detectors. Cylindrical sources are used in the calibration process placed perpendicularly to the detectors axis. The self-attenuation and the coincidence summing effects at low source-detector distance are also included in the algorithm. A remarkable agreement between the measured and the calculated efficiencies is achieved with discrepancies less than 3%.

An approach to evaluate the efficiency of γ-ray detectors to determine the radioactivity in environmental samples

This work provides an approach to determine the efficiency of γ-ray detectors with a good accuracy in order to determine the concentrations of either naturally occurring or artificially prepared radionuclides. This approach is based on the efficiency transfer formula (ET), the effective solid angles, the self- absorptions of the source matrix, the attenuation by the source container and the detector housing materials on the detector efficiency. The experimental calibration process was done using radioactive (Cylindrical & Marinelli) sources, in different dimensions, that contain aqueous 152Eu radionuclide. The comparison point to a fine agreement between the experimental measured and calculated efficiencies for the (NaI & HPGe) detectors using volumetric radioactive sources.

Calculating the ambient dose equivalent of fast neutrons using elemental composition of human body

The sphere of International Commission on Radiation Units and Measurements (ICRU) consists of 4-elemental compositions of 76.2% oxygen (O), 11.1% carbon (C), 10.1% hydrogen (H), and 2.6% nitrogen (N) whereas there are 26 elemental compositions in the human body. In this work, human body elemental composition has been used to calculate the ambient dose equivalent rate of fast neutrons. 241Am-Be of 185?GBq (5?Ci) was utilized as neutron source. In addition, the conversion coefficients in International Commission on Radiological Protection publication 116 (ICRP 116) was used to verify from the results of using elemental compositions in the human body. The calculated results have been compared to those measured by a neutron monitor. The mean values of discrepancies from the measured values were within ~8%. Moreover, systematic comparisons have been carried out with values published in literature. This work concluded that the elemental compositions in the human body could be used to design a phantom that has the same elemental composition of human body.

Probability Correction Approach to Calibrate High-Purity Germanium Cylindrical Detectors Using Parallelepiped Sources

Abstract The calibration of high-purity germanium gamma-ray cylindrical detectors using bar (parallelepiped) sources is carried out analytically using the probability correction approach. Improved expressions for the source self-attenuation coefficient have been included in the present algorithm based on the accurate calculation of all possible path lengths covered by the gamma ray inside the bulky source. Moreover, the full-energy peak attenuation coefficient μp is included in the present algorithm. The sources were positioned at long distances from the detector window so that the coincidence summing effects could be neglected. Remarkable agreement between the measured efficiency values and the corrected efficiency values calculated by the present technique was observed. The percentage relative differences for the results calculated in this way from experimental values are at least 25% smaller than those observed and reported by the direct mathematical method in previous work.