S.A. Durrani

A fast-neutron dosimeter with nearly flat dose equivalent response using CR-39 detectors with various combinations of radiators

CR-39 foils of thickness 500 μm, placed behind several combinations of radiators, have been irradiated with monoenergetic neutrons of energies ranging from ∼ 100 keV to ∼ 19 MeV at normal incidence from a Dynamitron accelerator. The radiator configurations and thicknesses used are: 1. Stack 1. 150 μm Polyimide (PI) + 800 μm Polystyrene (PS) + 5 mm Polyethylene (PE) 2. Stack 2. 100 μm Aluminium (27Al) + 800 μm Polystyrene (PS) + 5 mm Polyethylene (PE) 3. Stack 3. 50 μm Iron (56Fe) + 800 μm Polystyrene (PS) + 5 mm Polyethylene (PE) where the first, or primary, radiator in each case (viz., PI or Al or Fe) is the one closest to the CR-39 detector, followed by polystyrene and polyethylene, as one proceeds outwards towards the neutron source. The polymeric radiators have been chosen on the basis of their hydrogen contents, which are as follows: CR-39, ∼ 48%; polyimide, ∼ 19%; Polystyrene, 50%; and Polyethylene, 66.6%, by atomic ratio. The dose equivalent response of the detector has been studied by using conventional pre-etching for 6h followed by electrochemical etching for up to 3h. The results obtained are compared with calculated values. The thicknesses and compositions of the radiators are chosen so as to suppress the CR-39 response below ∼ 4 MeV by preventing the recoils of hydrogen nuclei, out of the hydrogen-rich radiators (viz., PS or PE), from reaching the post-etch surface of the detector. When 27Al and 56Fe are used as primary radiators, they contribute towards the enhancement of the detector response for neutron energies above ∼ 6 MeV through (n,p) and (n,α) reactions, while the thickness of the configuration suppresses the response below ∼ 4 MeV.

The effect of crystal composition on fission-track annealing and closure temperatures in geological minerals: implications for the cooling rates of terrestrial and extraterrestrial rocks

Abstract Fission track closure temperatures determined for a given type of mineral show a wide spread in their values, which may be attributable, in part, to variations in crystal composition, etching conditions and the method by which closure temperatures are calculated from the experimental annealing data. Our experiments suggest that the fission track annealing and closure temperatures of the minerals apatite and olivine are dependent upon their chemical composition. The resistance to annealing of fresh fission tracks is found to increase with the chlorine content of apatite and the Fe: Mg ratio in olivine. Calculated closure temperatures show a corresponding dependence on crystal composition. The results of closure temperature calculations have also been found to depend strongly on the numerical method of treating groups of tracks in a given isothermal interval. Only a slight increase in the calculated closure temperature is, however, obtained when the radioactive decay of a track-producing species with a short half-life (such as 244Pu in meteorites; t 1 2 ∼82 Myr) is included in the calculations. These results have an important bearing on the assessment of fission track closure temperatures and for the interpretation of the cooling histories of both meteorites and terrestrial rocks.

Charged-particle radiography of insects, using accelerated alpha-particles and plastic SSNTDs

Abstract Alpha-particle of energy 10 MeV/nucleon were used for the radiography of insects. Of the two types of plastic detectors tried, viz. CR-39 and CN-85, the former was found to give better contrast between the object radiograph and the α-irradiated background area. The range of 40 MeV α-particles in these detectors is long enough to produce several radiographs on successive layers of the plastic, revealing the internal organs of the insect. It may be noted that the CN-85 detector does register α-particles of total energy 40 MeV, but the registration efficiency is lower than that of ordinary α-particles.

Interpretation of Mason's equation in terms of measurable electrochemical-etching parameters governing the dielectric breakdown phenomenon

Abstract A new interpretation of Masons (1959) equation, as applicable to the electrochemical etching (ECE) of tracks in polymers, is presented in terms of physically meaningful track-etching parameters. The values of these parameters have been experimentally obtained by irradiating CR-39 foils with heavy ions (238U and 129Xe) and α-particles of various energies, and applying chemical pre-etching (CPE), followed by ECE in steps, with increasing electric field until the ‘treeing’ breakdown is initiated. A value of 1.19 MV cm-1 is obtained for the intrinsic electrical breakdown stress of CR-39. It is found that the track radius r used in Masons equation is related to the radius rϑ of a sphere, inscribed within the etch cone, whose lowest point is at a fixed distance l0 above the cone tip, with lo ⋍ 1.75 μm for α-particle tracks of energies 2–5.5 MeV, such that r= 0.43 + rϑ (in μm).

Study of radiation damage in mineral epidote using nuclear track and thermoluminescence (TL) methods

Abstract The effect of natural radiation damage, resulting from the decay of the uranium and thorium content of epidote, on its thermoluminesce (TL) properties has been studied. Correlations between the fossil fission track density (FFTD) and the TL properties, and between thermal annealing and TL sensitivity, have been made for several different samples of mineral epidote. It is found that while there is no significant correlation between the fossil fission track density and the natural TL output or TL sensitivity, there is an inverse correlation between the TL sensitivity (i.e. the TL response per unit test dose) and artificially induced α-damage. Annealing for 1 hour at different temperatures results in an increase in the TL sensitivity; the maximum sensitivity is observed for an annealing temperature of 500–600 °C, whereafter it begins to decline with increasing temperature of annealing. Interpretation of these observations in terms of effects of radiation damage and high temperatures on the crystal structure is discussed.

Track registration properties of Lexan and CR-39 for 12C and 16O ions in the energy range relevant to neutron radiotherapy

Abstract In our studies of depth-dose profiles in water phantoms irradiated with neutrons used in radiotherapy, where we have employed a system of CR-39, LR 115 and Lexan plastics for recording the various types of charged particles produced in neutron interactions with tissue-equivalent materials, the need arose to establish the registration properties of low-energy heavy ions such as 12C, 14N and 16O in these plastics. Samples of Lexan and CR-39 were therefore exposed to low-energy 12C and 16O ions (0.05 – 0.65 MeV/nucleon) produced by the Harwell Tandem accelerator. We find that it is possible to produce etchable tracks of 12C and 16ions in this energy range in Lexan. We also report the etching characteristics of these tracks in both Lexan and CR-39.

Fission tracks and thermoluminescence (TL) properties of natural sphene crystals

Abstract As a mineral with a significant U-content, the natural radiation damage in sphene is produced predominantly by the α-particles and recoiling nuclei resulting from the decay of its uranium (and thorium) impurities. The amount of damage can be estimated by measuring the fossil fission track density (FFTD) on an internal surface of a crystal. Annealing for lh at different temperatures is found to yield an inverse correlation between FFTD and the TL “sensitivity” (i.e. TL output per unit γ-ray test dose). An increase in the TL sensitivity is observed at annealing temperatures above ∼ 400 °C, reaching a maximum at ∼ 600 °C, followed by rapid decline. At annealing temperatures beyond ∼ 700 °C, the TL properties of the crystals are fully restored, and the sensitivity increases steadily, eventually tending to saturate. When preannealed samples of sphene were separately exposed to (a) thermal neutrons from a reactor to induce fission, and (b) a beam of ∼ 40 MeV α-particles from a cyclotron, the TL sensitivity against annealing temperature exhibited broadly the same features as above.

The relationship between the pre-etched track length and the ECE track-spot size

Abstract It is known that electrochemical etching (ECE) occurs in two stages. First, an etch-cone is formed by ordinary chemical etching; and then, in the second stage, the initiation and propagation of the ‘treeing’ process takes place. The geometry of the pre-etched track pit has an important effect on the magnitude of the field strength developing at the track tip. Hence, in order to determine the etching conditions -- in particular, the length of the pre-etched track before applying ECE --, it becomes necessary to study the kinetics of the track-pit development for a given charged particle and detector. A systematic attempt has been made to verify the above concepts, in the light of the Mason (1959) equation, by studying the initiation, growth and termination of the ‘treeing’ process in the case of α-particle tracks in Makrofol E, subjected to chemical pre-etching (CPE) followed by ECE. The ECE-spot diameters and ECE ‘efficiency’ have also been studied as a function of the duration of CPE and the pre-removed layer thickness. The results have been applied to derive optimum etching conditions for α-particle discrimination in CR-39.

The scintillator-filled etch-pit method for neutron dosimetry: Part II — Fast neutrons, using intrinsic tracks

Abstract The scintillation method of Harvey and Weeks (1982) has been extended, in Part II, to the counting of “intrinsic tracks” made in the CR-39 detector by fast neutrons from either portable sources or accelerators, without moderation. The pits of the chemically etched intrinsic tracks are filled with a scintillator material and counted as in Part I. An optical photodensitometer has also been used to measure the scintillator-filled etch pits. The effect on the detector response of varying the incident neutron energy has been studied, as have various combinations of pre- and post-ECE chemical etching in order to optimize the response. A linear response of count rate vs neutron fluence has been obtained on the log-log scale up to a certain fluence value, which is eventually limited by the overlapping of the etch-pit openings.

The scintillator-filled etch-pit method for neutron dosimetry: Part I — Thermal neutrons, with (n,α) converters

Abstract The scintillation method of Harvey and Weeks (1982) has been extended to the counting of etched tracks in CR-39 made by thermal and fast neutrons. In Part I, we report our work on thermal neutron detection. Here, CR-39 detectors, palced in contact with both Li2B4O7 and BNl converter screens, have been exposed to neutrons from portable sources, with suitable moderation. The chemically etched (n,α) pits are filled with scintillator material, and the light pulses caused by a strong readout α-source are detected by a photomultiplier. The linearity of response with this method has been tested for tracks made by α-particles of different energies (1 - 5.5 MeV). The response remains linear so long as serious overlapping of etch pits does not occur. The method promises to offer a quick and automatic technique for neutron dosimetry.