ESTIMATION OF EFFECTIVE DOSE CAUSED BY 40 K , 90 Sr AND 137 Cs IN DAILY FOOD

This paper presents Cs, Sr and K activity concentrations in daily food and an annual effective dose caused by these radionuclides. Samples were taken during the period 28 October 2004–23 June 2005 once a month in a students’ canteen of Vilnius Gediminas Technical University (VGTU). The weight of samples varied from 1,37 kg to 2,26 kg, and an average weight was 1,89 kg. The volume of liquids varied from 500 cm to 1000 cm, and the average was 816 cm. The average of activity concentration of Sr in daily food from the students’ canteen was (0,03±0,01) Bq/kg, of Cs – (0,02±0,01) Bq/kg and of K – (34±3) Bq/kg. An annual effective dose caused by these radionuclides was estimated using measured activity concentrations in daily food and dose coefficients. An annual effective dose caused by Sr was in the range of (1,9–14)⋅10 Sv, by Cs – (0,47–6,2)⋅10 Sv and by K – (6,8–21)⋅10 Sv.

An average activity concentration of 137 Cs before the accident at Chernobyl NPP in the soil of Lithuania was 10,0 Bq/kg, of 90 Sr -5,8 Bq/kg, and the distribution of these radionuclides was not homogenous. After the accident at Chernobyl NPP approximately 43 % of 137 Cs was deposited in the territory of Lithuania and some of this amount passed into feed.
The results of measurements show that in the regions of Lithuania over which contaminated plume passed an average density of 137 Cs activity concentration was 1190 Bq/m 2 . In other regions of Lithuania the density varied from very low up to 8430 Bq/m 2 . Contamination with 90 Sr in the Lithuanian territory varied slightly after the accident at Chernobyl NPP [1].
The influence of radionuclides on the environment and its components is different. The influence of some of the radionuclides was estimated. However, it varied in time or depended on local conditions. Dose estimation of all possible sources is important for the population [2].
Radionuclides pass from the atmosphere to the ground surface with rain or dry deposition. They accumulate in plants through their roots and deposit from the air to the soil. Radionuclides come into the organism of animals from feed that was contaminated with these radionuclides. In a human organism radionuclides accumulate by a food chain [3][4][5].
The aim of this work was to measure the activity concentrations of man-made radionuclides 137 Cs, 90 Sr and a natural one 40 K in students' food, and to estimate an annual effective dose caused by these radionuclides.

Sampling and preparation for measurements
Samples were taken in VGTU students' canteen once a month (usually at the end of a month) during the period 28 October 2004-23 June 2005. Each sample was constituted of two parts: liquids and solids.
For breakfast, pancakes with curd, various salad and tea were usually selected, for lunch -various soup and meat dishes, for dinner -various dishes from potatoes, tea and so on, including bread, popular drinking products, such as Coca-Cola, Sprite and others.
Preparation of samples for measurement was performed in a way explained below.
1. The volume of liquids was measured using a 1 000 ml measuring flask, solids were weighed using electronic balances with resolution of 0,01 g.
2. Weighed solids were ground and weighed again. Such an operation indicates the lost weight of a sample during these procedures. After that solids were dried in a drying oven at 105 °C temperature, then ashed for 3 hours at 300 ºC temperature and for 15 hours at 400 ºC temperature [6].
3. After measuring the volume of liquids, a solution was placed into an evaporating bowl and dried to a dry mass. The dry residue was ashed using the same procedure as that for solids. Then liquid and solid ashes were mixed, and the activity concentration of radionuclides in the ashes was measured.

Radiochemical method for separation of 90 Sr
The method of determining 90 Sr by measuring 90 Y counts after extraction with 10 % di(2-ethyl-hexyl) phosphoric acid (HDEHP) in toluene and counting by a liquid scintillation spectrometer was used [6]. Ashes of samples were dissolved in 1 M hydrochloric acid, pH was used in the range of 1,0-1,2 for extracting 90 Y from a solution with HDEHP. After this yttrium was re-extracted using 3 M nitric acid, and precipitations of Y(OH) 3 were performed. Then the beta counts from dissolved precipitations were measured [6].

Counting of 90 Sr, 137 Cs and 40 K
Counting of 90 Y was carried out because 90 Y is a daughter of 90 Sr and is in equilibrium in a sample. The activity concentration of 90 Y in a sample was measured using a liquid scintillation counter Quantulus 1220-003. Counting was performed by counting the high-energy beta particles of 90 Y (2,27 MeV) by the Cherenkov method (typical background was 0,77 cpm, efficiency -62 %). The chemical yield of 90 Y was determined according to stable yttrium carrier [6].
The activity concentration of 90 Sr in a sample in Bq was calculated according to equation [6]: where A -activity concentration of 90 Sr in sample, in becquerel; N -sample counting rate, in counts per minute; N fbackground counting rate, in counts per minute; N kcounting rate of calibration source, in counts per minute; A k -activity of calibration source, in becquerel.
The activity concentration of 90 Sr in a sample in Bq/kg was calculated according to equation [6]: where Sr A 90 -activity concentration of 90 Sr in sample, in becquerel per kg; A -concentration of 90 Sr in sample, in becquerel; Y -chemical yield of yttrium, in percent; mweight or volume of sample, in kg or m 3 .
Samples for gamma spectrometry were prepared according to the method described in [7] paper. An appropriate volume (50 ml) of a sample was put on a gamma spectrometer with a high-purity Ge detector. The time of counting was estimated according to the activity concentrations of radionuclides in a sample because the time has to be long enough to have an appropriate amount of pulses [7]. Generated spectrum was saved in the spectrometer memory and analyzed using Genie 2000 with mathematical calibration option [7].
For comparison of food products that may lead to increase of activity concentration in a sample from 28 April 2005, average annual activity concentrations of 90 Sr and 137 Cs in different types of raw food products are shown in Table 1 [9]. The data in Table 1 show that the highest activity concentration for 90 Sr can be subject to vegetables, and for 137 Cs -subject to fish. It is believable that the highest activity concentration of 137 Cs in a sample from 28 April 2005 was subject to fish and herring in a sample. In the case of 90 Sr the highest activity concentration was subject to cabbage salad. The range of the activity concentration of natural 40 K measured in the samples of daily food was from (22±2) Bq/kg to (42±3) Bq/kg, and an average activity concentration was (34±3) Bq/kg (Fig 2). The activity concentrations of 40 K were much higher than those of man-made radionuclides. An annual effective dose calculated using the results of measurements and equation (3) are presented in Table 2.
The data given in this paper are comparable with those obtained during measurements of radionuclides in a mixed diet from two canteens of hospitals in Vilnius [10]. Measurements of 28 samples during the period 2001-2002 were made at the Radiation Protection Centre. Change dynamics of the activity concentrations of 90 Sr and 137 Cs measured in the samples is shown in Fig 4. An average intake of 90 Sr was estimated (0,09±0,01) Bq/day, of 137 Cs -(0,12±0,01) Bq/day. Dose estimation was performed in the same way as described in this paper. An average annual effective dose caused by   Analyses of mixed diet samples from Helsinki during 2001 showed that the intake for 90 Sr was 0,12-0,13 Bq/day, for 137 Cs -0,20-0,81 Bq/day in solids, and 0,34-0,36 Bq/day in liquids. An annual effective dose caused by 90 Sr and 137 Cs in a mixed diet for the population of Helsinki is less than 0,01 Sv [11]. The dose for the population of Helsinki is higher to compare with that for Lithuanians because the environment of the Nordic countries was more contaminated during atomic bomb testing and after the accident at Chernobyl NPP. 3. The estimated doses were low and their variation was subject to the components of food.