İstanbul'un çevresel doğal radyoaktivitesinin tayini ve doğal radyosyonların yıllık etkin doz eşdeğeri

Abstract

Bu çalışmada genel olarak çevresel doğal kaynaklarda bulunan doğal radyoaktivite değerlerinin veya doğal radyonüklit konsantrasyonlarının tayin edilmesi ve bu doğal kaynaklardan yayınlanan farklı tipteki radyasyonların ölçülmeleri ile de bir yerin veya bölgenin doğal temel radyasyon seviyelerinin tespit edilmesi amaçlanmıştır. Bunun için İstanbul şehri seçilmiş ve şehir radyolojik açıdan ele alınarak incelenmiştir. Bölüm 2' de doğal radyoaktivitenin tanımı yapıldıktan sonra doğal radyonuklidlerin bulunduğu toprak su ve hava gibi doğal radyoaktivite ortamları ve çevresel doğal gama radyasyon kaynaklan hakkında teorik bilgiler verilmiştir. Bölüm 3' te çalışmanın amacını teşkil eden İstanbul şehri radyolojik açıdan ele alınmış, şehir toprağı, çevresinde bulunan deniz sulan ile halk içme suyu ihtiyacını sağlayan göller ve şehir şebeke sulan, şehrin yüzeye yakın atmosferinde bulunan doğal radyonuklidlerin aktivite konsantrasyonları ve halk taralından tüketilen gıdalardaki doğal radyoaktivite düzeyleri gama spektrometrik analiz yöntemiyle tespit edilmiştir. Ayrıca, şehir içinde ve çevresinde doğal gama radyasyon ölçümleri alınarak şehrin doğal temel radyasyon seviyesi belirlenmiştir. İstanbul evlerinde alınan radon ölçümleri ile yapı malzemelerinden kaynaklanan 222Rn konsantrasyonları tayin edilmiştir. İncelenen toprak, hava, su ve gıdalar gibi insan yaşamı için gerekli olan bu doğal ortamlarda bulunan doğal radyoaktif maddeler ve doğal radyasyon kaynaklarından insanın hangi oranlarda dozlara maruz kalındığı belirlenmiş ve yıllık etkin doz eşdeğerleri saptanmıştır.

Natural radioactivity originates from extraterrestrial sources as weH as from radioactive elements in the earth crast. The radioactivity of the earth includes the primordial (terrestrial) radionucüdes such as ^"ü, ^'U, 232Th, *°K and "Rb. The majority of these are isotopes of the heavy elements. Terrestrial sources of radiation are the radionucKdes with very long half-life, which exist within the earth since its formation several billion years ago have not substantially decayed. Extraterrestrial sources orginate cosmic rays which come from outer space. Cosmic rays which have high energy entering in the earth's atmosphere from outer space are known as primary cosmic rays. Primary cosmic rays produce neutrons, protons, pions, kaons and cosmogenic nucüdes such as 3H, 7Be, 10Be, 22Na and Na when they interact with the nuclei of atoms present in the air. Cosmogenic radionuclides are known as secondary cosmic rays too. Natural radionuclides exist in soil, air,water, food and building materials. Penetrating radiations and radioactive materials pervade the natural environment. Main types of radiations are gamma rays, alpha and beta particles and neutrons. People exposure to terrestrial and extraterrestrial radiations during their life. The exposure of man to these radiations occurs from sources outside of the body (tbis is external exposure) and upon the decay of radionuclides taken into the body through ingestion and inhalation (this is internal exposure). Natural sources are very important for the assesment of radiation doses in man, because the natural ionizing radiation is the largest contributor to the annual effective döşe equivalent. The main objective of this study is to evaluate istanbul city from the point of radioligical view. For this purpose, it was determined natural concentration of radionuclides in soil samples taking from different areas of the city and hiçe sea water around the city, lake water which supply drinking water to the city, the atmosphere near to the surface of the city and foods. Additionally, yearly radiation doses from these sources taken by the people who living in istanbul was calculates. This study continued two years. Natural radioactivity in istanbul soil samples, Öne of the objectives of this study was to identify and determine natural radionuclides activity concentration in soil samples collected from 35 locations in and around the istanbul city. 105 soil samples were collected at 0-10 cm depth level. The samples were collected from uncultivated fields. The collected samples were weighed individually, dried, pulverized, homogenized and sieved to l mm mesh. Meshed soil samples were transfered to Marinelli beakers which have 1000 mi capacity. Soil samples are weighed and carefiılly sealed for 30 days to reach secular equilibrium of radium and radon and their respective progeny. After that ali soil samples were counted in a high resolution gamma-ray spectrometer to analyze radionuclides. The counting time for each sample and background was 50000 seconds. To calculate the concentrations of radionuclides, the detector eflSciency as a function of energy for the same counting geometry was considered. The concentrations of different radionuclides were calculated based on the measured detector efficiency. Radionuclides which are present in İstanbul soil samples are 226Ra, 214Pb and 214Bi wbich are from *"U series and ^Ac, 212Pb, 212Bi and ^Tl which are from ^Th series, <°K and 137Cs. According to results of analysis the main activity concentration of 226Ra is 27 Bq kg"1. The average activity concentration of 238U and 232Th series were found respectively 20 Bq kg"1 and 33 Bq kg"1. 40K concentration values ranged from 117-1204 Bq kg"1 with an average of 342 Bqkg1. 137Cs doesn't exist in nature naturally. it is a production of fallout radioactivity. 137Cs was found to be very high in some soil samples such as Kınalı Ada, Habibler Koyu and Kanallar Orman içi. We supposed that these districte were effected more than other districts during the Chernobly accident in 1986. 137Cs activity concentration values ranged from 1,5 Bq kg"1 to 81 Bq kg"1. Natural radioactivity in istanbul water samples, Öne of the aim of this study is to determine natural radioactivity of sea and lake water in and around of istanbul city. Also, the average concentrations of natural radionuclides in drinking water are estimated from measurement and used to estimate the annual effective döşe equivalent. Several natural radioactive elements are present in water. Especially, 222Rn and the long lived radium isotope 226Ra have been found in concentrations that are significant from the point of view radiation doses. During this study, samples were taken from different water sources such as sea water, lake water and drinking water analyzed. The gross alpha and gross beta radioactivity values for all water samples were determined. Also, activity concentrations of radionuclides like 40K, 226Ra and 137Cs were determined by using gamma spectroscopy. Radioactivity concentration of 40K in sea water has been found higher than lake water. Radioactivity concentration of ^K in water samples changes depending on the rate of salinity. The salinity in sea water is higher than lake water. 222Rn and 226Ra activity concentration in lake water which supply dirinking water to the city were determined using Radon Lucas Detectors. The average of activity concentrations of 222Rn and 226Ra in Istanbul tap water samples were found to be 0,029 Bq r1 and 0,022 Bq l"1. The amount of two radionuclides concentration are important for ingested and inhaled radiation doses. The annual effective dose equivalent due to 222Rn in Istanbul tap water were determined as 0,016 jiSv per year. This value were found as 5,5 jxSv per year for ^«Ra. Natural radioactivity in atmosphere near to surface of Istanbul, The natural radioactive materials in atmosphere are terrestrial and exstraterrestrial origin. Main terrestrial sources are uranium and thorium and their progenies and ^C. The atmosphere contains natural and man-made aerosols which originate from many sources. In addition to the air pollutants introduced as the result of human activities, meteorites, volcanic activitity, dust stroms and forest fires contribute quantities of gases and suspended solids. Nuclear energy and particularly, the testing of nuclear weapons extend atmosphering contaminants. Cosmogenic radionuclides increase the radioactivity in atmosphere too. In this study, we also fried to determine gross alpha and gross beta radioactivity and activity concentration of radionuclides which occur in atmosphere near to surface of Istanbul. For the measurement of the concentration of the radioactivite dust in the atmosphere proportional counters weed used. Radioactive dust was collected on Wattman filters by using an air sucking pump. Filters were counted in a proportinal counter. According to the results of counting, the annual average gross alpha and gross beta radioactivity values in air of Istanbul in 1994-1995 are respectively 8,4xl0"3 Bq m"3 and 2,4xl0"2 Bq m"3. Particularly, the measurements of gross alpha and gross beta radioactivity which were taken in winter months were found higher because of using fueloil and coal for heating purposes. Radioactive nuclides in the biosphere enter the human body through ingestion and inhalation. The absorbed doses from internal exposure in body is estimated as well as the effective dose equivalent for the radionuclides considered, 7Be and ^a. xiii The absorbed doses are derived from the measurement of the concentrations of the radionuclide concerned or the most abundant stable isotope of the element. Radionuclides determined in Istanbul air in 1994 are 226Ra, 38U, 232Th, 137Cs, 7Be and ^a. The mean annual activity concentrations of these radionuclides are respectively 6,7xl0"3 Bq in3, 4,65xl0"3 Bq m-3, l,72xl0"3 Bq m'3, 0,13xl0"3 Bq m'3, 2,53xl0"3 Bq m"3 and 0,08xl0"5 Bq m"3. The annual effective dose equivalent from the man exposure to cosmogenic radionuclides which are 7Be and ^a in atmosphere near to surface of Istanbul are 0,78 nSv per year and 0,023 u,Sv per year. Indoor 222Rn concentration in Istanbul houses, Radionuclides occuring naturally in building materials are sources of external and internal radiation exposure in dwellings. External radiation exposure is caused by gamma radiation originating from 23*U and ^^h and their decay products and 40K. Internal radiation exposure is due to radon and its products which are exhaled from building materials into the room air. Knowledge of radiation levels in buildings is important in the assesment of population exposure, as most indivuduals spend a large portion of their time indoors. Two different methods were generally used for measuring the exhalation rate from building materials. They are passive and active methods. We used passive method in this study. 222Rn measurements were made in 33 places selected in different region of city. The measurements were carried out during 1994 year. Man is exposed to 222Rn and to its short-lived decay products mainly by inhalation. The average concentration of 222Rn was found to be 27 Bq m"3 in Istanbul houses. Annual effective dose equivalent corresponding to the mean value of 222Rn was determined 1,7 mSv per year. Natural radioactivity in foods which consumed in Istanbul, It is known that radioactive elements are present in the soil. They are absorbed by means of plant roots. Natural radionuclides are passed by two ways from plants to man. One of them is direct way and the other way is by animals. One of the intent of this study is to determine natural radioactivity in foods consumed by people who live in Istanbul. For this purpose, many foods samples was collected. These food samples were analysed by gamma spectroscopy for determining radionuclides concentrations such as 226Ra, 23*U and 40K. Gross alpha and gross beta radioactivity values for 45 food samples were ascertained. Gross alpha values for food samples vary between 0,035 Bq kg"1 and 0,77 Bq kg"1. Gross alpha racüoactivity were not determined some İn food samples. xiv Gross beta values were found very high in some samples. The biggest values for beta were ascertained in horicot bean which has 450 Bq per kg. The isotope '"'K emits both gamma and beta rays. The results of activity concentration of ""K which found in foods are found very close to beta values. Gamma activity concen trations of ^ in foods vary between 2,5 and 473 Bq kg"1. The value for horicot bean activity according to gamma energy is 473 Bq kg"1. The radionuclides of 226Ra were found in some food samples. The biggest concentration for 226Ra were ascertained in pepper which were found 3,95 Bq kg"1. ^ and 226Ra are most important radionuclides that are ingested to body. The average activity concetration of '"'K determined for food consuming by people who live in Istanbul is about 61 Bq kg"1. This is 1,27 Bq kg"1 for 226Ra The total annual effective dose equivalent from 40K, 226Ra and ^U existing in food, are respectively 146,5 u,Sv per year, 1,45 uSv per year and 0,36 \iSv per year. Measurement of natural gamma radiation in and around Istanbul In this study, natural gamma radiation was investigated in both rural and urban environment of Istanbul city. Surveys were carried out using a Reuter-Stokes Environment Monitor RSS-111, which consist of a high pressure ionization chamber. Measurements were performed in the open air on the soil and asphalt surfaces and inside residential buildings. Outdoor measurements were monitored for 112 locations in and around the city. The relationship between the indoor and outdoor absorbed dose rates depends essentially on the type of building material used and on its origin. The building materials act as sources of radiation and also as attenuators of outdoor radiation. In wooden and prefabricated houses the effect is negligible and the walls are an inefficient shield with respect to the outdoor sources of radiation, so that the absorbed dose rate is generally lower titan outdoors. In massive houses made by bricks, concrete or stone, the gamma rays emitted outdoors are efficiently absorbed by the walls and indoor absorbed dose rate depends mainly on the actvity concentrations of natural radionuclides in the building materials. The natural outdoor radiation measurements change between 33 nGy per hour and 105 nGy per hour. The mean outdoor gamma radiation measurement is 66 nGy per hour. Contribution of cosmic rays in these measurements were determined 12,2 nGy per hour. XV Radiation levels İn buildings is very important in the assessment of people exposures as most individuals spend a large amount of time indoors. The measurement of average radiation level in air is 80 nGy per hour. Indoor measurements are higher than outdoor measurements. The average annual effective dose equivalent resulting from exposure to environmental gamma radiation for outdoor and indoor were determined respectively 79,35 p.Sv per year and 390,1 [iSv per year. The total (outdoors plus indoor) annual effective dose equivalent from terrestrial and extraterrestrial gamma radiation sources are 467 jxSv per year. This value for world average is 410 u,Sv per year.