H.A. Khan

EFFICIENCY CALIBRATION OF SOLID STATE NUCLEAR TRACK DETECTORS.

Abstract The track registration efficiencies of various glasses, plastics and mica for fission fragments of various average energies and for alpha particles have been calculated on the basis of a series of new experiments in which exposures were made in “2π” and other geometries. The effects of temperature and ultra-violet light on the registration efficiencies have been studied. Lastly, certain modifications in standard formulae for source-detector geometry, when applied to SSNTD, are proposed.

Natural radioactivity from the building materials used in Islamabad and Rawalpindi, Pakistan

Abstract Studies have been carried out to determine the natural radioactivity in the building materials used for construction of dwellings in Islamabad and Rawalpindi, Pakistan. The measurements have been carried out on samples of bricks, cement, gravel aggregates, sand, soil and paints using a HPGe based gamma-ray spectrometer. In all the materials 40 K, 226 Ra and 232 Th have been found; in paints activity was below the detectable limits. The overall activity (sum of activities) from 40 K, 226 Ra and 232 Th in brick and soil samples was the highest and the lowest was found in gravel aggregate samples. The average activity of 40 K varies from 51.3 ± 3.8 Bq kg −1 (gravel aggregates) to 631.2 ± 6.5 Bq kg −1 (bricks); 226 Ra varied from 21.5 ± 0.5 Bq kg −1 (sand) to 43.2 ± 0.6 Bq kg −1 (bricks); 232 Th varied from 9.9 ± 0.6 Bq kg −1 (gravel aggregates) to 56.2 ± 1.7 Bq kg −1 (soil). Radium equivalent activities with external and internal hazard indices have been also determined. The maximum value for the radium equivalent activity, 206.35 Bq kg −1 , was found in soil, but it is less than the permitted value of 370 Bq Kg −1 . All the materials have values for both external and internal hazard indices of

Annealing of fission tracks in tektites: corrected ages of bediasites

Abstract Systematic annealing experiments have been carried out in an endeavour to obtain quantitatively reliable correction factors for the ages of tektites which may have been subjected to heat. The standardisation of etching conditions, which is a prerequisite for making such corrections, is described. The application of the correction factors so obtained to a differentially-annealed bediasite gives self-consistent results. Inferences as to the possible thermal history of the bediasite are drawn from a careful analysis of the etch-pit diameter vs. track density histograms of natural as well as freshly irradiated samples. The mechanism of track shrinkage and fading is examined in some detail.

The effects of high gamma doses on the response of plastic track detectors

Abstract Systematic studies have been made concerning the effects of high doses of gamma rays on the track-registration properties of various plastic Solid State Track Detectors (SSTDs). The detectors were first exposed to 252Cf fission fragments and then to gamma doses ranging between 30 and 160 megarad. The results show that both the length and the width of latent damage trails are affected when exposed to gamma doses of the order of a few megarads. Furthermore, it was found that the general etching velocity, Vg, increases with increasing gamma dose. The above mentioned results suggest that: (a) gamma doses below the megarad region do not affect the track-registration characteristics of plastic SSTDs, and (b) the change in track parameters (in the megarad region) can be employed as an index of gamma dose. This new type of gamma dosimeter, due mostly to its extremely small size and flexibility, offers greater advantages over other detectors for in-core gamma dosimetry.

PROLONGED ETCHING FACTOR IN SOLID STATE TRACK DETECTION AND ITS APPLICATIONS.

Abstract It has been observed that the number density of etch pits resulting from spontaneous or induced fission within a solid state track detector (SSTD) is a function of the duration of etching, other conditions being constant. As the etching proceeds, the net number of etch pits observed per unit area continues to increase. This necessitates a modification of the conventional equations governing the observed etch-pit densities in SSTD. A prolonged etching factor f(t) is defined, and its value measured as a function of time under various specific etching conditions, in a number of materials of interest, including ‘reference glass’. Furthermore, the theoretical relationship of this factor with the general velocity of etching in a given material, and with the average range R of fission fragments in it, is utilized in deriving self-consistent and very satisfactory values of R in the above-mentioned materials.

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].

Measurement of spontaneous-fission decay constant of 238U with a mica solid state track detector

Abstract The spontaneous-fission decay constant, λ f , of 238U is a basic parameter of interest in fields ranging from cosmochronology to reactor physics. Over the years, a number of determinations have been made, yielding a series of values which differ by a factor of two between the extremes, viz. (5.3 ± 0.9) × 10−17 yr−1 found by Perfilov1 and (11.9 ± 1.0) × 10−17 yr−1 measured by Gerling et al 2. It was therefore decided to make a fresh measurement of λ f using the fission track analysis technique.

Anisotropic etching of charged-particle tracks in crystals

Abstract It is well-known that highly-energetic heavily charged particles arising both from the fissioning of heavy elements and from cosmic and solar radiations leave tracks which have been found in glasses and crystal-line minerals recovered from lunar and meteoritic material. It has been shown that an appropriate analysis of these tracks can provide informationabout periods since solidification and the thermal and dynamic history of the specimen, as well as insight into the nature and temporal variation of the cosmic ray flux.1-2 An understanding of the track registration characteristics of crystals is clearly an important pre-requisite to an appropriate interpretation of the natural tracks. 3-6 This letter describes briefly some observations we have recently made on one particular aspect of this-the anisotropy in track distributions which seems to result from the inherently anisotropic structure of crystals. This effect seems so far to have received little attention.

An important precaution in the etching of solid state nuclear track detectors

Abstract The effects of etch product layer on (a) the etch pit diameter and (b) the general etching velocity, V g , have been studied in a number of Solid State Nuclear Track Detectors. Observations were made under various etching conditions and attempts were made to reduce the adverse effects of the etched product layer. The results show that (a) the effect of the etch product formation is more serious in the case of oblique tracks (as compared with perpendicular or near perpendicular ones) and (b) the method of mechanical brushing of the detector surface during the etching process is quite effective in removing this layer.

Preliminary radon concentration-survey in some houses of Islamabad

Abstract Measurements of radon and its daughter products have been carried out in 13 houses in the city of Islamabad (Pakistan). Both CR-39 and CN-85 detectors were installed in bed rooms, kitchens and drawing rooms of the chosen houses and were exposed to radon and its daughters over a period of about four months. All the exposed detectors were processed employing the optimized etching conditions. Significant variation in radon concentration has been observed in the houses under investigation.