M. Sohrabi

THE STATE-OF-THE-ART ON WORLDWIDE STUDIES IN SOME ENVIRONMENTS WITH ELEVATED NATURALLY OCCURRING RADIOACTIVE MATERIALS (NORM)

Direct observations and studies of the radiobiological and epidemiological effects of ionizing radiation from naturally occurring radioactive materials (NORM) on man, in particular in areas with elevated NORM, are becoming of prime concern in radiation protection. This is due to existing discrepancies in the application of the linear no-threshold theory in obtaining radiation risks at low doses by extrapolation from high dose to low dose using dose and dose-rate effective factors. Many areas in the world have elevated NORM caused either by the geological and geochemical structure of the soil, or by the radioactive content of the water flowing from hot springs and/or due to technologically enhanced radioactivity as well as due to cosmic rays. Such areas, with relatively large cohort sizes, have been the subject of intensive dosimetry, radiobiological and epidemiological studies. It is the purpose of this article to review: sources of NORM and human exposure, needs and problems in study of areas with elevated NORM; the criteria for their classification; some areas with elevated NORM and the results of related studies, and some conclusions and recommendations for unification of an approach in future studies aimed at obtaining better estimates of human radiation risk factors from the effects of ionizing radiation.

Discovery of an “internal heating effect” during electrochemical etching of polymeric dosimeters

Abstract An “internal heating effect” was discovered during the electrochemical etching (ECE) of some polymeric dosimeters such as polycarbonate (PC) and allyl diglycol carbonate (CR-39) employing a new multi-chamber ECE system capable of etching a large number of foils separately using small-volume chambers in parallel. The rate of heating is high enough to raise the etchant temperature significantly in a short period of time, e.g. from 25°C to 47°C in 20 min in CR-39. The rate of heating was smaller in polycarbonate. This heating effect was experimentally and theoretically investigated and discussed to be due to the “dielectric loss” in the polymer. The rate of heating is proportional to the polymer loss index (ϵ′ tan δ), square of field strength, and frequency. Significant dimensional changes found in the foils, especially at high field strengths, were discussed to be due to the “electrostriction” phenomenon. In this paper, the results on the above findings and the effects of parameters are reported and discussed with a hope to partially understand some unknowns and to stimulate further research on the ECE mechanisms.

Measurement of photoneutron doses in and out of high-energy X-ray beam of a SATURNE-20 medical linear accelerator by ECE polycarbonate detectors

Abstract Photoneutron contaminations in and out of high energy X-ray beams of the medical linear accelerator SATURNE 20 (CGR) of the Radiotherapy Department of Omeed Hospital in Isfahan, Iran, have been determined using 250 μm polycarbonate (PC) dosimeters, in strips or in sheets, processed by electrochemical etching (ECE) using specially designed ECE chambers to etch larger sheets. A two dimensional or topographical distribution of neutron contamination was also determined in a full size beam. The neutron dose equivalents (Hn) in the beam of 18 MV X-rays at 80 cm FSD were determined to be linear functions of X-ray dose equivalents (Hx) up to 1400 cSv. The distribution of the Hn at different X-ray doses showed bell-shape profiles with maxima at the isocenter. The ratios of dose equivalents of neutrons to those of X-rays increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10×10, 20×20, 30×30, and 40×40 cm2 respectively. Although such neutron dose equivalents can be corrected for patient treatment, it can cause radiation protection problems for workers where the design of the facility is not well planned.

Efficient detection and spectrometry of alphas from radon daughters in polycarbonate

Abstract Alphas with energies up to even 5.99 MeV and 7.69 MeV respectively from 218po and 214Po (radon daughters) were directly registered in polycarbonate (PC) with also demonstrating the full energy spectrum by applying a pre-etching (PE) process to a point of Bragg peak followed by electrochemical etching (ECE). The efficiencies were found to be up to 70%. The registered energy range for each PE time, the mean alpha track diameter and the alpha spectrum from radon daughters are presented and discussed.

Indoor radon level measurements in some regions of Iran

Abstract To determine the effective dose equivalent of the public in Iran due to 222 Rn and its daughters ( 218 Po and 214 Po) from natural radiation environment, a national radon monitoring program was developed at the Radiation Protection Department of the Atomic Energy Organization of Iran (AEOI). The AEOI passive radon diffusion dosimeters were applied in this program. About 250 dosimeters applied in about 206 randomly selected houses in some regions of Iran including Ramsar (a high natural radiation area), Tehran, Babolsar and Gonabad have been processed and analyzed for radon level evaluation. The polycarbonate foils have been electrochemically etched using our new design of multi-chamber electrochemical etching system applying our optimized ECE conditions. The mean radon levels in the above cities were determined to be respectively 578, 80, 88 and 84 Bq.m -3 , leading to average effective dose equivalents of 17.6, 2.44, 2.68 2.56 mSv/year. Gamma dosimetry were also carried out with TLD and film badge in houses of high natural radiation areas of Ramsar together with Rn dosimeters. Some houses were found with very high gamma and Rn exposures. In this paper, some preliminary results of such measurements are reported and discussed.

Determination of 226Ra in food samples by a new method using polycarbonate detectors

Abstract A new method is introduced in this paper for determination of 226Ra in food stuffs when it is in equilibrium with its decay products in prepared microprecipitates when alphas are detected by polycaronate detectors when electrochemically etched. The microprecipitates were prepared from food samples in particular from high level natural radiation areas of Ramsar, Iran by the collection of radium with lead as Pb/RaSO4. Then radium was coprecipitated by addition of small amounts of a barium carrier leading to thin films or microprecipitates with negligible self absorption for alpha particles. The results compared to those obtained by the emanation method are in good agreements.

Design characteristics of a three-component AEOI Neutriran Albedo Neutron Personnel Dosimeter

Abstract To establish a national personnel neutron dosimetry service in Iran, different parameters of the AEOI Neutriran Albedo Neutron Personnel Dosimeter (NANPD) have been optimized. A NANPD was designed with three dosimetry components to measure (a) direct thermal neutrons, (b) direct fast neutrons and (c) direct neutrons by the detection of the albedo neutrons reflected from the body. The dosimeter consists of one or more Lexan polycarbonate and/or CR-39 foils and two 10 B (n, α) 7 Li converters in a cadmium cover arranged such that to efficiently measure the three neutron dose components separately. Boron converter thickness, its position relative to the beam direction and its distance from the PC foil were studied and they were implemented into the design. The dose response of the dosimeter, its lower detection limit as well as the correction factors related to the field neutrons and albedo neutrons were also determined for a 238 Pu-Be, an 241 Am-Be and a 252 Cf sources. In this paper, the dosimeter design and its dosimetric characteristics are presented and discussed.

Advances on research, development and applications of SSNTDs in radiation protection at AEOI

Abstract Iran has been one of the countries started early in 1967 on research, development and applications of SSNTDs in radiation protection. Since then extensive efforts have been made at the National Radiation Protection Department (NRPD) in AEOI on national development and international cooperation to promote the applications of SSNTDs in this field. Efficient track registration of alphas, recoils and fission fragments in different SSNTD materials, advancements and new findings on chemical and electrochemical etching (ECE); introduction of new neutron personnel dosimeters such as 237 Np/10 μm polycarbonate combination (PC) foils coupled with a spark counting technique, ECE of recoils, Neutriran Albedo Neutron Personnel Dosimetry; neutron depth dose measurements in particular at air-tissue and bone-tissue interfaces; neutron contamination measurements in high energy x-ray beams of medical accelerators; development of a nationwide indoor radon monitoring program and spectrometry of radon daughters in PC; discovery of some phenomena in ECE and education and training can be considered some highlights of such studies, in particular, on the protection of man and his environment against harmful effects of ionizing radiation, some of which are presented and discussed in this paper.

A new dyed ece track identification method for nuclear particle detection

Abstract A new development procedure of highly contrasted red-dyed ECE recoil tracks in polymers such as polycarbonate (PC) and CR-39 has been successfully achieved at the Atomic Energy Organization of Iran (AEOI) for spectrophotometry as applied to neutron dosimetry. The principal rationale in this method has been the provision of highly contrasted, photon-absorbing, large, dyed recoil tracks in an unaffected bulk material. The method consists of: (a) exposing the polymer to charged particles or neutrons; (b) electrochemical etching (ECE) of tracks; (c) acid sensitization; (d) dyeing with an appropriate dye. By investigation of the type, concentration, duration and temperature of the acid and the dye, optimized values of 20% by weight acrylic acid at 75°C for 3.5 h for sensitization, and 3% by weight eosin bluish dye at 95°C for 4 h for dyeing, provided a nearly 100% dyed-track efficiency. Spectrophotometry by UV and i.r., track counting, and optical densitometry were applied to the dyed samples. The results have shown some promise for UV absorbance measurements in routine large-scale applications. In this paper, the results of optimization studies and preliminary application of the technique to neutron dosimetry are presented and discussed.

Broadening registration energy range of alpha tracks in CR-39 under a new ECE condition

Abstract A new single and simple electrochemical etching (ECE) condition is proposed for efficient etching of alpha tracks over a broad energy range, using our new multi-chamber ECE system which is capable of etching any number of chambers in parallel; e.g. 24 chambers used in this study. The principle rationale is the use of a 15N KOH solution at 25 °C, employing 24 kV/ cm at 2 kHz for 6 to 10 hours. Alpha registration efficiencies of 90±10% were obtained in CR-39 up to 5 MeV as well as registering those from radon daughters on a filter. This approach have solved the deficiencies of applying pre-etching of mixed alpha energy tracks and it is practical for many health physics and radiation research applications. In this paper, some preliminary results on: efficiency and track diameter of tracks of alphas up to 5.49 MeV, effect of pre-etching in the same solution, and effect of etching time as well as track characteristics are reported and discussed.