Khalid Jamil

Gamma-ray activity and dose rate of brick samples from some areas of North West Frontier Province (NWFP), Pakistan

It is important to measure natural radioactivity due to gamma-rays from building materials and consequently to determine the dose rate from these materials. This helps to implement precautionary measures whenever the dose rate is found to be above the recommended limits. A knowledge of gamma radioactivity is required by the building construction association to adopt preventive measures to mitigate or minimize the harmful effects of ionizing radiation. The aim of this work was to measure the gamma activity due to 226Ra, 232Th and 40K in bricks from the Peshawar, Nowshera and Mardan districts of NWFP of Pakistan. The radiological doses to the occupants from the brick floor of a wooden house were calculated. The average gamma dose equivalent rate was calculated to be 0.14 mSv per year, which is well below the external gamma dose limit of 0.46 mSv per year as given in UNSCEAR [2].

Track-registration-and-development characteristics of CR-39 plastic track detector

Abstract CR-39 plastic track detector (Homalite) has been tested for its response to light particles (protons, alpha particles) and heavy ions (238U, 32S, 252Cf fission fragments, 19F, 16O and 12C) having a wide spectrum of energies. Perpendicular and oblique geometries were employed, and the exposed detectors were etched in stages in 6N NaOH at 70 ± 1°C. The track parameters like etch pit diameter, etched track length, and etching velocity along the track (Vt) were obtained at the end of every etching stage. Both, Optical-Microscope (O.M.) and Scanning-Electron-Microscope (SEM) were utilized in these measurements. It has been found that the Etch-Rate Ratio (ERR = Vt/Vg) for CR-39 is related to REL, the Restricted Energy Loss (in units of MeV cm2g-1) of charged particles through the relation, ERR = 7.51 × 10-8 (REL)2.36 for REL values exceeding 2 × 103MeVcm2g-1. Similarly, the results show that ERR = 7.24 × 10-7 (Z2eff/β2)1.92 for Z2eff/β2 values exceeding 3 × 103. The Restricted Energy Loss (REL) has been found to be related to Z2eff/β2 through the relation (REL) ωo = 200 eV = 2.62 (Z 2 eff /β 2 ) 0.81 It has also been observed that the prolonged etching of CR-39 in stages (short intervals) enables us to make a rough classification between ‘light’ and ‘heavy’ reaction products such as those emitted in high energy nuclear interactions. The rate of track etching is significantly enhanced when the etchant becomes saturated with the etched products.

Swelling in CR-39 and its effect on bulk etch-rate

Abstract The effect of swelling in CR-39, during the etching process has been studied using ‘thickness-measurement’ method. The values of the bulk etch-rate ( V B ) determined by this method have been corrected in the context of irreversible swelling produced in various CR-39 samples. The results have been compared with those obtained using the ‘mass-measurement’ as well as the ‘fission fragment track diameter’ methods. The values of experimental V B have been obtained by taking the arithmetic average as well as by fitting the Gaussians to the results of a number of CR-39 samples. It is shown that the ‘thickness-measurement’ method is as good as the other methods of determining the bulk etch-rate provided sufficiently large number of CR-39 samples are used for obtaining the mean value from the Gaussian fit of the experimental data.

Experimental and Monte Carlo simulation studies of open cylindrical radon monitoring device using CR-39 detector

There are several methods of measuring radon concentrations but nuclear track detector cylindrical dosimeters are widely employed. In this investigation, the consequence of effective volumes of the dosimeters on the registration of alpha tracks in a CR-39 detector was studied. In a series of experiments an optimum radius for a CR-39-based open cylindrical radon dosimeter was found to be about 3 cm. Monte Carlo simulation techniques have been employed to verify the experimental results. In this context, a computer code Monte Carlo simulation dosimetry (MOCSID) was developed. Monte Carlo simulation experiments gave the optimum radius of the dosimeters as 3.0 cm. The experimental results are in good agreement with those obtained by Monte Carlo design calculations. In addition to this, plate-out effects of radon progeny were also studied. It was observed that the contribution of radon progeny (218Po and 214Po) plated-out on the wall of the dosimeters increases with an increase of dosimeter radii and then decrease to 0 at a radius of about 3 cm if a point detector has been installed at the center of the dosimeter base. In the code MOCSID different types of random number generators were employed. The results of this research are very useful for designing an optimum size of radon dosimeters.

Determination of equilibrium factor between radon and its progeny using surface barrier detector for various shapes of passive radon dosimeters

Abstract In the field of radon dosimetry, it is customary to measure radon ( 222 Rn) concentration while potential health hazard is due to the radon short-lived progeny. When radon is in secular equilibrium, the measured activity of radon equals the activity of radons progeny. However, in practical cases an inequilibrium between radon and its progeny exists which is measured in terms of the equilibrium factor. To determine the equilibrium factor between radon and its progeny in a closed environment various shapes of passive dosimeters based upon solid state nuclear track detectors (SSNTDs) are employed. In order to observe the dependence of equilibrium factor upon shapes or effective volumes, experiments have been performed replacing the SSNTDs with a surface barrier detector in Karlsruhe Diffusion Chamber (KDC), Pen-type and Box-type dosimeters. Using the collected alpha spectra, the equilibrium factor has been determined for a radon-air mixture in a custom designed radon chamber simulating a closed environment of a room. The results show that the radon equilibrium factor is about 0.20 for different shapes of dosimeters studied in this research. It is concluded that the determination of equilibrium factor between radon and its progeny does not depend upon effective volume or shape of the passive dosimeters using alpha spectroscopic data acquired by surface barrier detector.

Improvement in the sensitization of the plastic track detectors by ultraviolet irradiations

Abstract The track registration properties of CN-85 and CR-39 plastic track detectors have been investigated under the influence of u.v. irradiations. The length and width of the etched tracks increase considerably due to u.v. absorption in the solid state nuclear track detectors. Post-irradiated CR-39 and CN-85 detectors show more u.v. sensitization than the pre-irradiated ones. The pre-irradiated CN-85 detectors, however, exhibit no significant enhancement in the sensitization, while CR-39 renders pronounced effect in both the modes. One can speculate that u.v. is preferentially absorbed in the damaged regions of the detectors.

Experimental and simulation study of neutron dosimetry at various neutron energies

Abstract A Monte Carlo Neutron Photon (MCNP) transport code has been employed to simulate CR-39 plastic track detector as neutron dosimeter at various neutron energies. In each simulation a monoenergetic neutron source of a particular energy was embedded in the center of a sphere made of CR-39. Surrounding the source there were concentric shells of 2 μm CR-39 track detectors. The code, MCNP, was run on personal computer for 7.5×106 histories. The number of proton recoils in each shell of 2 μm of CR-39 were determined. The simulation results show that apart from proton recoils (for En > 1.0 MeV), about 50% recoils are due to heavy charged particles i.e. Oxygen and Carbon in CR-39. This indicates that etched tracks are not only due to recoil of protons but also due to recoil of heavy charged particles. The upper limits of the track registration efficiencies have been determined as a function of neutron energies. These simulation results have been experimentally verified using CR-39 track detectors at various known energies of neutrons. The proton and heavy charged particle recoil tracks in CR-39 were made visible by etching in NaOH solution, at 70±1°C for 1.75 hours.

Annealing of heavy ion latent damage trails in muscovite mica and CR-39 plastic track detectors

Abstract Thermal stability of latent damage trails due to (a) 2380 MeV-238U ions in CR-39 polycarbonate, and (b) 3975 MeV-238U ions, in Muscovite mica track detector has been studied. The exposed mica detectors were annealed in the temperature range of 80°C – 850°C and afterwards etched in 48% HF acid. The CR-39 detectors were annealed in the temperature range of 50° – 300°C, and the etching was carried out in 6N aqueous solution of NaOH kept at 70 ± 1°C. Variations in (a) the geometry, and (b) the dimensions of the etched-channel-profiles, were systematically studied and attempts have been made to quantify the shrinkage of the latent damage trails.

Light charged particle registration characteristics of CR-39 plastic track detectors

Abstract CR-39 plastic sheets have been tested for their response to protons, deuterons, and alpha particles. The enlargement of the latent damage trail has been studied (a) under varying experimental conditions, and (b) as a function of energy, restricted energy loss, and Z2 crr/β2 of the light charged particles. The critical angles of etching of CR-39 and its response curves have been obtained. Scanning-Electron-Microscopy (SEM) has been employed for studying the profiles of the etched channels.

Some important considerations in the use of solid state nuclear track detectors for radon gas concentration measurements

Abstract Experiments have been carried out concerning the usefulness of an alpha sensitive plastic SSNTD for radon gas concentration measurements. In this respect, the effects of factors like (a) radon source concentration, (b) exposure time, (c) size of the tube, and (d) post-exposure handling etc, have been studied. In addition to the alpha sensitive plastic detectors, a surface barrier detector has also been employed to confirm the results. It has been observed that the track density is an increasing but non-linear function of the exposure time and of the size of the tubes containing the detectors. Also, the time required for radon concentration to attain equilibrium in the tubes appears to be dependent upon the strength of the radon source. Theoretical models have been developed to explain this behaviour of the etched track density. These results clearly indicate that the use of these systems for the absolute measurements of radon concentration is not quite straightforward.