Thermal Neutron Dose Research Articles

Lethal effect of thermal neutrons on hypoxic Ehrlich ascites tumour cells in vitro.

Ehrlich ascites tumour cells were irradiated in vitro with thermal neutrons under aerobic and hypoxic conditions, and the survival of their reproductive capacity was assayed in vivo. Only a slight hypoxic protection was observed for thermal neutron irradiation with an oxygen enhancement ratio (OER) of 1.2, as compared with OER of 3.3 for 60Co-γ-rays. Absorbed dose of thermal neutrons was calculated by assuming that the energies of recoiled nuclei were completely absorbed within a cell nucleus. This led to a conclusion that the RBE of thermal neutrons were 1.3 and 3.6 for the oxygenated and the hypoxic cell fractions, respectively.

Neutron leakage measurements from a medical linear accelerator.

The McCall method has been used to measure neutron leakage from the Mevatron 77, 18- and 15-MV photon beams. Gold foil activation has been used employing a beta counting technique for the 18-MV beam and a gamma counting technique for both the 18- and 15-MV beam. The two counting techniques were used to evaluate their relative merit. The measurements were made at various locations in the patient-treatment plane for different field sizes. The results show that the thermal-neutron dose equivalent contributes only about 1%-2% of the total neutron dose equivalent. At 100 cm, the neutron dose equivalent for the 18-MV beam is approximately six times that of the 15-MV beam, slightly exceeding the 0.1% of the useful beam criteria used by some of the regulatory agencies. In light of the uncertainty in fluence to dose equivalent conversion factors, the increased dose equivalent above 0.1% is insignificant.

Optical properties of neutron-irradiation-induced defects in single-crystal tellurium near the fundamental energy gap

Abstract Undoped tellurium single crystals have been irradiated at room temperature with thermal neutron doses between 2·1 × 1016 and 3·4 × 1017 nth cm−2 at a flux ratio ⊘th/⊘fast of about 5. Photoconductivity measurements as a function of energy, temperature and modulation frequency provide evidence for radiation-induced defect states centred at an energy 10–13 meV above the valence band edge. These exhibit a distribution in energy with a full width at half maximum of 20 meV, depending on neutron dose and annealing treatment. The corresponding defect structures show an acceptor-like character similar to those observed in dislocation-rich crystals. From the rise of the Fermi energy between 2·15 and 5 K after annealing the irradiated crystals, we conclude that the iodine atoms created by neutron transmutation doping have donor-like character. This proposal is confirmed by the results of simultaneous measurement of the optical absorption and the electrical resistivity.

Intercomparison of personnel dosimetry for thermal neutron dose equivalent in neutron and gamma ray mixed fields.

In order to consider the problems concerned with personnel dosimetry, an Intercomparison of personnel dosimetry, especially dose equivalent responses of personnel dosimeters to thermal neutron, was carried out in neutron and gamma ray mixed fields.The dose equivalent responses and their standard deviations of individual dosimeter were almost independent on cadmium ratio and gamma ray contamination in the radiation fields. However, a difference of about 4 times was observed among the relative thermal neutron dose equivalents of individual dosimeter normalized to the ICRP recommended value.It is suggested that the standardization of dose evaluation method is required from the practical point of radiation protection and safety.

Neutron dosimetry with boron-doped CR-39 plastic.

As a highly sensitive, simple and non-radioactive neutron dosimeter, CR-39 plastic plates doped with a boron compound: ortho-carborane were prepared. After thermal neutron irradiation, the plates were etched in an aqueous solution of 30%KOH, at 60°C for 2 to 16 h. The etch-pits generated by 10B(n, α)7Li reactions were then counted using an optical microscope or an automatic track counting system. The density of the etch-pits on an irradiated plate increases with the etching time. When the etching time is kept constant, the etch-pit density is proportional to the irradiated thermal neutron fluence. The proportional constant is termed “sensitivity”, which is 4.2 x 10−4 for a plate containing ortho-carborane at a concentration of 0.5% by weight and for etching time of 16 h. By considering background counts, a thermal neutron dose of 0.025 mSv can be measured with this plate. The plates are insensitive to visible-, UV-, X-, β- and γ-rays and are easy to handle because the detector and converter are incorporated. There is no possibility of underestimating the dose equivalent due to fading. Furthermore, the isotopes of boron are not radioactive and thus are radio- logically safe.

Inter-laboratory standardization of glass dosimeters

Intercalibration charts for various glass dosimeters have been prepared using Fleischer's glass as reference standard. Calibration constants have been determined using two different irradiation thermal neutron doses. The geometry factor and uranium content for these glasses are determined. The U-content of the dosimeters varies from 0.35±0.02 ppm to 41.93±1.0 ppm and the geometry factor G lies within the range 0.57–0.77.

Neutron dosimetry by the spark counting of tracks in boron-doped film.

Thin cellulose nitrate films are doped with a boron compound. After thermal neutron irradiation, the films are etched in an aqueous solution of 10% NaOH, at 50 degrees C, for 1.5 hr, during which the 27 microns thick films are reduced to about 7 microns. The etch-pits caused by 10B(n, alpha)7Li reactions are punched twice at 600 V and then counted at 500 V. The ratio of the spark density to the thermal neutron fluence was found to be 1.0 X 10(-4) for a boron concentration of 1%. After due consideration of background counts, a thermal neutron dose of 0.3 mrem (3 X 10(-6) Sv) can be measured with this system.

Ｌｉ２Ｂ４Ｏ７（Ｃｕ）素子による自動読取型個人被ばく線量計の開発 ＩＩ 中性子線量当量評価法

The evaluation method of neutron dose equivalent was studied on the basis of the albedo type neutron dosimetory to design the personnel dosimeter. The dosimeter was composed of three 6Li210B4O7(Cu) TL elements and one 7Li211B4O7(Cu) element.The equations for assessing thermal, epithermal and fast neutron dose equivalents were derived by 252Cf, 241Am-Be and PuO2 neutron sources.The minimum detectable amount of 6Li210B4O7(Cu) element to thermal neutron was 0.02 mrem. The neutron dose equivalent and the gamma one were evaluated separately within about 20% error in the mixed radiation field.

Reduction in Proliferative Life Span of Human Diploid Cells after Exposure to a Reactor Radiation Beam

Cultured human diploid fibroblasts were exposed to various doses of thermal neutrons delivered from a 5-MW KUR reactor. Irradiated cultures showed an apparent shortening of the life span, the magnitude of which was dependent on dose. The dose vs life-span response curve did not have an initial shoulder. The D0 of the exponential slope was 260 rad. Furthermore, the dose-survival response curve for colony formation had no shoulder, and its D0 was 55 rad. The relative biological effectiveness (RBE) of the high-LET particles to X rays for late effects was estimated at 3.0, though the RBE values for acute effects were dose dependent.

Attenuation of reactor thermal neutrons in a bulk shield of ordinary concrete

This work is concerned with the study of the distribution and attenuation of doses of thermal neutrons emitted directly from the core of 235U research reactor in ordinary concrete shields. In practice it is not possible to identify the reactor thermal neutrons in the emitted continous neutron spectrum, therefore, measurements were carried out by using a direct and cadmium filtered beam of reactor neutrons. All measurements were performed using Li 2B 4O 7:Mn thermoluminescent dosimeters. The data obtained were analysed and the dose distributions of reactor thermal neutrons were evaluated. A group of isodose curves were constructed which give directly the shape and thickness of the shield required to attenuate the intensity of doses of reactor thermal neutrons to specific values. In addition, the thermal neutron relaxation lengths in ordinary concrete were derived for disc collimated beam and infinite plane monodirectional sources.

Quantitative determination of fissionable materials in human hair

A technique for quantitative neutron-induced autoradiography (NIAR) of human hair samples containing small amounts of plutonium or uranium is described. It requires irradiation of hairs in intimate contact with two thin mica sheets by a suitable dose of thermal neutrons to provide adequate numbers of fission fragments. The technique has been used to examine scalp hair voluntarily provided by 35 humans intermittently exposed to both soluble and insoluble uranium materials. The results indicate that in situations of low intermittent exposure the method is preferable to presently used blood, urine, or whole body monitoring methods. Limitations of the method are discussed.

Release behavior of xenon from uranium dicarbide.

The initial burst and the diffusion of fission produced Xe were investigated on UC2 irradiated to thermal neutron doses between 2.1 x l014 and 1.3 x 1016 fission/cm8. In heating-runs with 5 and 10°C/min and below 1,000°C, the burst revealed three steps at around 200, 400 and 600°C (in 5°C/min heating run), to which activation energies were obtained. It was found that the temperatures and the activation energies for each burst were close to those appearing in recovery processes of fission induced defects. In diffusion process above 1,000°C, on the other hand, the activation energy of Xe in UC2 increased with increasing fission dose. In the specimens once heated up to 1,500°C, however, the diffusion coefficient could be expressed as, D≃10-1 exp (— 344±50 kJ/RT) (cm2/s), which was independent of the level of the fission dose. An enhancement and a suppression of the diffusion were suggested also in the dicarbide as was done previously for the monocarbide of uranium.

Change in mechanical properties of nickel alloys KhN78T and KhN75TYuR as a result of high-temperature irradiation

1. High-temperature (710–750 °C) reactor irradiation by a dose of l · 1021 neutrons/cm2 (E ≥ 1 MeV) with a thermal-neutron dose of 2 · 1021 neutrons /cm2 for alloys KhN78T and KhN75TYuR leads to a marked change in mechanical properties, resulting mainly in high-temperature radiation embrittiement. 2. For irradiated specimens of alloy KhN78T, the effect of a notch is manifest in intensification of the softening process. It is manifest it alloy KhN75TYuR specimens, moreover, by a reduction in the temperature of the beginning of the transition to intergranular fracture. 3. At temperatures up to 500 °C, alloy KhN78T (slight precipitation-hardening) is superior to dispersion-hardened alloy KhN75TYuR with respect to level of ductility and reaction to notching.

Comparison of zircon and glass fission-track ages from tephra horizons

Tephras previously dated by the fission-track method used on glass shards have been redated using zircon crystals, which have greater track stability and consequently are expected to yield more reliable ages. The ages of the glass proved to be fortuitously correct in most cases, owing to two compensating errors–unrecognized annealing and inaccurate thermal neutron dose determination. Ages of glass shards should not be regarded as reliable until the glass used has been checked for annealing of the fission tracks. If annealing has taken place, a correction can be applied, but the method is very time consuming. The shards from the Borchers Ash, Kansas, suggested by some geologists as a glass age standard, are totally unsatisfactory because of proven annealing.

Neutron dosimetric measurements using mica and pyrex

Muscovite mica is used as a fission fragment track detector for neutron measurements within the Visiflux of Basrah University. For thermal, intermediate and fast neutron dose equivalent measurements, U-mica, U/Cd-mica and Th-mica systems are used. Hence, the pyrex glass is examined as a fission fragment track detector. Experimental results show that the response of pyrex glass for fission fragment track registration is (0.63±0.03) relative to the response of mica as fission fragment track detector. Consequently, pyrex glass is recommended for neutron dosimetric measurements.

A neutron dosimeter using cellulose nitrate film

A neutron dosimeter using cellulose nitrate film coated on both sides with lithium tetraborate (Kodak CA 80-15 type B film) has been prepared for the surveillance of personnel exposed to thermal and epithermal neutron doses. A calibration of the track density in the film versus absolute thermal and epithermal neutron fluences has been obtained. The response of the dosimeter is found to be (1.06±0.)×10 −3 tracks per thermal neutron and (3.65±0.5)×10 −4 tracks per epithermal neutron for a K/E neutron energy spectrum.

Thermoluminescent Response to Thermal Neutrons of Mixture of CaSO4: Tm and Non-Luminous6LiF

Thermoluminescent property of the mixture of CaSO4: Tm powder and non-luminous 6LiF powder was studied with a view to developing a personnel monitor for thermal neutrons. Its thermoluminescent response to thermal neutrons as a function of the particle size or the mixing ratio of both powders was investigated theoretically and experimentally. The phosphor sample, which was prepared according to the most favorable conditions that the particle size of both powders was between 100–150 μ m and the mixing weight ratio of CaSO4: Tm to 6LiF was 1:1, was found to have the following dosimetric properties. 1. The thermoluminescence output is linear against neutron dose in a dose range from 10 μ rem up to 6 rem.2. The thermoluminescent efficiency remains constant even after 5,000 repetitions of exposure-reading-annealing procedure.3. A thermal neutron dose of 1 mrem can be determined with a standard deviation of 5.2% even under a γ-ray background of 10 mR.

Thermoluminescent Response to Thermal Neutrons of Mixture of CaSO4: Tm and Non-Luminous 6LiF

Thermoluminescent property of the mixture of CaSO4: Tm powder and non-luminous 6LiF powder was studied with a view to developing a personnel monitor for thermal neutrons. Its thermoluminescent response to thermal neutrons as a function of the particle size or the mixing ratio of both powders was investigated theoretically and experimentally. The phosphor sample, which was prepared according to the most favorable conditions that the particle size of both powders was between 100–150 μ m and the mixing weight ratio of CaSO4: Tm to 6LiF was 1:1, was found to have the following dosimetric properties. 1. The thermoluminescence output is linear against neutron dose in a dose range from 10 μ rem up to 6 rem. 2. The thermoluminescent efficiency remains constant even after 5,000 repetitions of exposure-reading-annealing procedure. 3. A thermal neutron dose of 1 mrem can be determined with a standard deviation of 5.2% even under a γ-ray background of 10 mR.

The 238U spontaneous fission decay constant re-determined by fission tracks

The decay constant (λ f 238) for the spontaneous fission of 238U was re-determined by means of a man-made uranium glass of known age (126 yr). The spontaneous U fission tracks that had accumulated since the date of manufacture were counted on internal faces of the glass with an error of less than 1.7%. No thermal annealing of the spontaneous tracks was observed. The U content was determined by induced fission tracks. The value obtained for λ f 238 is (8.57 ± 0.42) × 10 −17yr −1 . Main sources of error are the date of glass melting and the determination of the thermal neutron dose.

The Hanford thermoluminescent multipurpose dosimeter.

A five-element thermoluminescent dosimeter for beta particles, photons and neutrons has been developed using LiF blocks. One LiF block is shielded to provide interpretation of the 1 cm tissue depth dose. A second 7LiF block is unshielded to provide the derma dose interpretation. Two 6LiF and one LiF blocks are used for fast and thermal neutron dose interpretation. The accurate response of the dosimeter to the wide variety of radiation sources at Hanford makes it a good replacement for the Hanford film badge for routine personnel dosimetry.