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Specific activity and derived intervention levels for Cesium-137 in Costa Rican export goods: Tuna fish, coffee and powdered milk
Radiation
Pergamon
Measurements, Vol. 23, No. 4, pp.731-736, 1994 Copyright 0 1994 Ekvicr Science Ltd Printed in Great Britain. All rights renewed 1350-4487/94 $7.00 + .oo
1350-4487(93)E0002-4
SPECIFIC ACTIVITY AND DERIVED INTERVENTION LEVELS FOR CESIUM-137 IN COSTA RICAN EXPORT GOODS: TUNA FISH, COFFEE AND POWDERED MILK LUISGUILLERMO LOR~A and PATRICIA MORA Nuclear Physics Laboratory, School of Physics, University of Costa Rica, San Jose, Costa Rica (Received 26 July 1993; in final form 25 October 1993)
Almtract-Cesium-137 is a fission product of *W and *r’)Pu.After a major nuclear accident, it is released into the atmosphere and in the far field region it will produce radioactively contaminated food and
drinking water. This paper will study the specific activity of %s in three Costa Rican export products: tuna fish, coffee and powdered milk. The average specific activities found are as follows: 0.89 + 0.41, 1.16 f 0.76 and 4.53 & 2.00 Bq kg-‘. They represent low values compared to their derived intervention levels: 1.25 x lo’, 1.25 x 10’ and 2.44 x lo5 Bq kg-‘, respectively.
1. INTRODUCTION
stipulate a dose of 5 mSv/y (UNSCEAR, 1982; WHO, 1988). The amount of 13’Csfound in coffee and powdered milk may come from nuclear accidents, the latest being the Chernobyl nuclear power plant accident. We presume that the tuna fish might get its 13’Csfrom nuclear tests in the South Pacific. Other y-emitting nuclides resulting from a nuclear accident, such as ‘“Cs and 13’1,cannot be studied quantitatively in this paper, since more than 6 years have passed since the last accident (1986) and their longest half-life is approximately 2 years. i3’Cs with a half-life of 30 years, is a fission produc; of uranium and plutonium. This element is soluble in the corporal fluids, and after its ingestion, it is distributed almost evenly throughout the whole body. The effective half-life in the human body is from 70 to 110 days depending on age. The critical organ for this element is the entire body, and the equivalent committed dose in a period of 50 years should not exceed 8.1 mSv/Bq (UNSCEAR, 1982).
A POPULATION close to the location of a nuclear accident will be, during the first minutes, highly exposed to internal and external irradiation by inhalation of radioactive products present in the atmosphere. The radioactive cloud scattered by the wind contaminates both aquatic and terrestrial ecosystems. Therefore, man is exposed to external irradiation from the materials deposited on the ground, to internal irradiation caused by breathing in suspended material in the air and subsequently, by its ingestion when taking food and water (IAEA, 1991; ICRP, 1977). In places far away from the site of the accident, the means of contamination are solely the intake of food and water (IAEA, 1991). Figure 1 shows the major pathways to man resulting from the release of radioactive elements in the atmosphere. As a consequence of nuclear accidents, such as Chernobyl in 1986, the International Atomic Energy Agency placed a low level counting system in the Nuclear Physics Section of the School of Physics at the University of Costa Rica. This was in order to measure the specific activity of nuclides present in the Costa Rican diet and in accordance with the table of consumption, to compare those results with their derived intervention levels (DILs), accurately measured for each one of the nuclides. The purpose of this paper is to present information as to the specific activity of 13’Cs in tuna fish meat, coffee, and powdered milk and furthermore, to compare these results with the DILs for these products as proposed by the World Health Organization and the Food and Agriculture Organization, which
2. METHODS AND MATERIALS 2.1. Derived intervention levels The derived intervention levels (DILs) are those that guarantee the maximum amount of nuclide activity that man can consume without exceeding the reference level of dose established internationally for a year, to avoid any detrimental effect on health (WHO, 1988). The two organizations that mark the trends which establish the DIL values are, as already stated, the World Health Organization (WHO) and the Food and the Agriculture Organization of the 731
L. G. LORIA
and P. MORA
UWORbOllOd otmorphrrlo rrloorr
ot rodionuolldxx I Rolytrr
hti0ut
(wofhoutl
ml&Ion
I
I
I
I I I
I
Ingatlon
I
7
D8op roll
I
lrrlgotlon
I I
I
rxtomal lxpoouro
I
lngo8tlon
1
FIG. 1. Major pathways of radionuclides to man in the event of an uncontrolled release of radioactivity [International Chernobyl Project, 19911.
United Nations (FAO). Through the Alimentarius Codex, both organizations recommended a DIL of 1000 Bq/kg for 13’Cs in export goods resulting from an intake of 500 kg with an annual dose of 5 mSv. Specific activities that are below the established level allow for free commerce of the export goods (WHO, 1988). The DIL is symbolically expressed by means of the following relationship (Randell, 1988): DIL = RLD/md
(Bq/kg)
(1)
where: RLD = reference level of dose (Sv/y); m = mass of the food consumed annually d = dose per unit intake (Sv/Bq).
(kg/y);
The criterion that conditions the reference level of dose, establishes that measures to restrict the distribution of food be introduced if the dose to the
individual exceeds an effective dose equivalent to 5 mSv after exposure (Randell, 1988; Waight, 1989). The value of 5 mSv, established internationally, does not correspond to the 5 mSv of effective equivalent dose to the population recommended by the International Commission on Radiation Protection. On the contrary, it corresponds to a comparative value of the natural background radiation average of 3 mSv and of the level of 5 mSv for radon established by the WHO (IRCP, 1977). The human diet is a distinguishing trait of each country, and could be further stated that it is distinctive of each individual. However, the WHO, regarding calculations, has proposed a global diet on an annual average consumption of 550 kg of food per person (Randell, 1988). Table 1 shows the food consumption patterns for different diet types (WHO, 1988).
Table 1. Food consumption patterns for different diet types (per capita in kg per year) Diet type
Cereal
Roots and tubers
Vegetables
Fruit
Meat
Fish
Powdered milk
African Cent. American Chinese Mediterran. European Far Eastern N. African S. American
127.4 113.2 171.7 188.2 121.1 206.6 161.9 129.5
134.8 46.0 85.8 19.3 72.7 28.4 20.0 67.6
25.9 38.8 85.2 91.6 86.7 54.3 63.4 34.2
45.2 98.6 5.5 101.5 81.4 48.3 63.9 83.2
16.8 42.3
15.0 18.7 8.9 8.4 20.2 24.4 7.3 14.4
28.9 82.2 1.9 74.3 154.9 33.6 77.2 70.7
15.0 30.4 75.3 21.5 24.0 48.4
CESIUM-137
733
IN COSTA RICAN EXPORT GOODS
Table 2. Comparison of the maximum and worldwide normalized nourishment consumption with the Costa Rican consumption (per capita in kg per year)
Reg. maxim. Normalized Costa Rica
Cereal
Roots and tubers
Vegetables
Fruit
Meat
Fish
Milk
205 140 126
135 100 12
90 60 21
100 70 100
15 50 33
2s 15 06
15s 105 121
Table 2 shows the maximum and normalized consumption of food, as well as the typical Costa Rican consumption, for different diet types (WHO, 1988). Table 3 shows the radionuclides of importance present in the food chain as a consequence of reactor and nuclear plant accidents (ICRP, 1977). The nuclides included in the table are divided into two groups in accordance with the dose per unit intake. Group I includes 2MP~and the actinides that have a factor of 10m6Sv/Bq, while Group II includes those y -emissive nuclides, such as i3’Cs, whose factor is of the order of lo-’ Sv/Bq (Randell, 1988; Waight, 1989). Table 4 presents the DIL for seven groups of food for human consumption with their respective doses per unit intake factor. The levels that are included were selected based on the worldwide and normalized consumption of each product, as if each one incorporated a dose of 5 mSv/y. Throughout a nuclear accident, not only does a single food become contaminated, but there can also be a number of nuclides present in the same food. Therefore, the new DILs should be evaluated again to guarantee that the total level of dose does not exceed 5 mSv annually. The new DIL can be expressed by the following relationship (WHO, 1988):
goods. These samples were dried for a period of 24 hours with the purpose of eliminating the moisture. Once the tuna fish is thoroughly dry, the meat must be ground until the fiber is no larger than 2 mm, while the powdered milk, as well as coffee beans are analyzed in their original form. For each one of the samples their mass, fresh as well as dry, is recorded. Approximately 1 kg is placed in a polyethylene container, better known commercially as a “marinelli” with a volume of one liter, and its geometry allows for its insertion into the y -spectrometry system’s detector (IAEA, 1989; Loria and Jimenez, 1992).
2.3. Gamma spectrometry
Figure 2 shows the block diagram of the y-spectrometry system which is used for the analysis of an energy spectrum emitted by the nuclides present in the sample (Salazar and Loria, 1987). By this method, it is possible to measure the specific activity of the nuchdes that are present in the sample without the need to separate them from the matrix where they are contained. The radiation from the samples interacts with a high purity germanium detector (Tennelec) under any of the already known processes: photoelectric effect, Compton effect and pair production, producing an g kf) DIL*(i,S) = (2) electrical voltage pulse proportional to the given energy of the incident photon. The pulse is con1 cg (ff)lNIWf) i/ formed by a linear amplifier to a size and shape where g (i, f) is a function that represents a specific compatible with the multichannel analyzer (Canberra pattern of contamination. S-100) which classifies them according to their Since this function appears in the numerator as voltage or energy in the corresponding channels, thus well as in the denominator of equation (2), it can be obtaining the emission energy spectrum. Through a expressed in relative terms, being the ratio of the computer program which identifies the energy photospecific activity found in different foodstuffs. peaks, it estimates the specific activity and the minimum detectable amount of the radioisotopes present in the sample. 2.2. Recollection and preparation of samples An exact identification of the photopeaks present The samples of tuna fish, coffee and powdered milk in the spectrum is essential to the measurement of ywere supplied by the exportation companies of these emissions. Therefore, the y -spectrometry system must be calibrated, which is done by using known radionuclides with well-defined energies, usually in the Table 3. Nuclides of importance in the food chain and their typical emissions range of 122 to 1460.7 keV. The calibration consists of establishing a lineal relationship between the chanNuclides Type of decay Half-life nels of the multichannel analyzer and the emission Strontium-90 27.70 y energy of the nuclides used. Certified nuclides with a Iodine-131 8.05 d I!,; Cesium- I 34 2.05 y relatively long half-life in watery solution with the Cesium- 137 30.00 y s-1 Y same geometry as the sample are used to measure the Plutonium-239 a 24,OOO.OO y efficiency of the detection system. The efficiency for a
L. G. LORfA and P. MORA
734
Table 4. Derived intervention levels in Bq/kg, classifiedaccording to the conversion factor of dose in Sv/Bq Nuclide
Factor
Cereal
Roots and tubers
Vegetables
Fruit
Meat
Milk
Fish
Pu-239 Sr-90 I-131 cs-134 cs-137
10-b 10-E IO--* 10-n IO-*
35 3500 3500 3500 3500
50 5000 5000 5000 5000
80 8000 8000 8000 8000
70 7000 7000 7000 7000
100 10,000 10,000
45 4500 4500 4500 4500
350 35,000 35,000 35,000 35,000
given energy can be tested using equation (Salazar and Loria, 1987): e [E] = A VA,, e-“‘1
the following
(3)
where: A = activity A,, =
t =
f = 1 =
of the standard measured experimentally in Bq; activity of the standard on the date of fabrication in Bq; time elapsed since the date of fabrication of the day of the measurement of the standard activity; absolute transition probability of a y-decay at that specific energy; and decay constant of the standard nuclide.
The measured values of each one of the patterns are represented by graphs as a function of energy, establishing a relationship between the variants of the form (IAEA, 1989): In t = k, + k, In E
(4)
where: c = absolute efficiency at a given energy; k, and k, = constants to be determined; and E = energy in keV.
10,000 10,000
When the y-spectrometry method is used, a lower limit of detection (LLD) is associated with each specific activity measurement which corresponds to the minimum amount of the specific activity that can be detected. An accepted expression for this limit, which contains a 95% degree of confidence for detecting the present activity, is given by the equation (IAEA, 1989; Lauria, 1988): LLD = 4.66&[~ [Elf]-’
(5)
where S, is the estimated standard error of the net count rate. The detection limit is a way of determining the operation capability of a y measuring system without the influence of a sample. It is also reliant on the assumption that the count rate in the energy area taken for the specific nuclide and the count rate in the region or regions taken for background are essentially equal. When a sample is being measured in terms of the specific activity of a nuclide, the term usually associated with the detection limit is the minimum detectable concentration (MDC), which is expressed through the following equation (IAEA, 1989): CMD = LID/W where W is the mass of the sample (kg).
Biar rupply I
I Data rtorr I FIG. 2. Block diagram of a typical gamma-ray spectrometer system.
(6)
CESIUM-137
735
IN COSTA RICAN EXPORT GOODS
FIG. 3. Gamma-ray spectrum of tuna fish.
3. RESULTS Figure 3 shows the corresponding emission spectrum of one of the tuna fish samples collected during a timing of 60,000 s. Table 5 presents DILs for 13’Csin tuna fish meat, coffee and powdered milk for Costa Rica, taking into account the real consumption of each one of the goods which incorporates an effective dose equivalent to 5 mSv/y (Ministry of Health, 1982, 1991) Table 6 presents the average and maximum values of the specific activity measured for 13’Cs in coffee, tuna, and powdered milk, utilizing 16 samples for the first two products and 10 samples for the powdered milk.
4. CONCLUSIONS From an economic point of view, the values measured for the specific activity of i3’Cs in the studied products, compared with the normalized International Derived Intervention Levels and the National Levels, are extremely low. Furthermore, the measured levels are below 1000 Bq/kg, for “‘Cs determined by the Alimentarius Codex, which allows for free commerce of the goods. From the point of view of radiological protection of the population, the activities found for ‘r’Cs in the Table 5. Costa Rican derived intervention levels (DILs) for coffee, tuna fish and powdered milk in Balka Product Coffee
Tuna fish Powdered milk
1.25 x lo5 1.25 x 10’ 2.44 x 105
Table 6. Average and maximum values of the specific activity measured for “‘Cs in coffee, tuna fish and powdered milk, expressed in Bq/kg Product
Average value
Maximum value
coffee
1.16kO.76 0.89 f 0.41 4.53 f 2.00
3.12&0.16 1.33 * 0.58 7.61 * 1.11
Tuna fish Powdered milk
three products represent an equivalent dose that varies in a range of 0.01 to 2.5 ~SV. The consumption of the mentioned products does not convey any risk to the health of the Costa Rican population since the effective equivalent dose is around four orders of magnitude lower than the established levels for the natural background radiation and for radon. The methodology used, and the equipment available, allows the measurement of the specific activity of any y-emitter isotope, and the establishment of the DILs for these emitters in the Costa Rican diet, before the outcome of a nuclear accident. As a consequence of the new recommendations dictated by the International Commission on Radiation Protection, new DILs will have to be calculated if the reference level of doses changes, because the limiting value of dose for the public has been recently established as 1 mSv (ICRP, 1991). Acknowledgements-The authors hereby thank the International Atomic Energy Agency, for the training session provided by the expert Ing. Luis Conti, from the Nuclear Energy Commission of Brazil, and for the scholarships granted. Furthermore, we are grateful for the equipment donated to the Nuclear Physics Laboratory, obtained through the bilateral pledge of Technical Assistance (COS 2/002). We wish to express our deep appreciation to the Vicerectory of Investigation of the University of Costa Rica for its support given throughout our work.
REFERENCES IAEA (1989) Measurement of Radionuclides in Food and the Environment. Technical Reports Series No. 295. International Atomic Energy Agency, Vienna. IAEA (1991) The International Chernobyl Project. Technical Report. International Atomic Energy Agency, Vienna. ICRP (1977) Recommendation of the International Commission on Radiological Protection Annals of ICRP, Publication 26. Pergamon Press, New York. ICRP (1991) Recommendations of the International Commission on Radiological
Protection
Annals of ICRP,
Publication 60. Pergamon Press, New York. Lauria, D. (1988) Determination of U-238 and U-234 in mineral waters of Planalto de Pocos de Caldas. Cienciu e Culrura 40(9) 906-908 (Brazil). Loria L. and JimCnez R. (1992) Quantification of the concentration of natural and artificial gamma radioactive elements in soils of the Peninsula de
736
L. G. LORfA Nicoya, Guanacaste,
utilizing low level techniques.
Technologia en Marcha 14(11), 61-72. Ministry of Health (1982) National Survey of Nutrition.
Ministry of Health, San Jose, Costa Rica. Ministry of Health (1991) First National Survey of Apparent Food Consumption. Ministry of Health, San Jose, Costa Rica. Randell A. (1988) FAO Recommended Limits for Radionuclide Contamination in Foods. Food and the Agriculture Organization of the United Nations, Rome, Italy. Salazar A. and Loria G. (1987) Spatial distribution of the thermic and epithermic flux of a californium source. Ciencia y Technologia 11(l), 65-75.
and P. MORA UNSCEAR (1982) Ionizing Radiation: Sources and Biological Effects. United Nations Scientific Committee on the E-&&s of Atomic Radiation, United Nations, New York. Waight D. (1989) Setting derived intervention levels for food. In: Int. Symposium on Environmental Contamination Following a Major Nuclear Accident. World Health Organization, Geneva. WHO (1988) Derived International Levels for Radionuclides in Food: Guidelines for Application After Widespread Radioactive Contamination Resulting from a Maior Radiation Accident. World Health Organization, Geneva.
Pergamon
Measurements, Vol. 23, No. 4, pp.731-736, 1994 Copyright 0 1994 Ekvicr Science Ltd Printed in Great Britain. All rights renewed 1350-4487/94 $7.00 + .oo
1350-4487(93)E0002-4
SPECIFIC ACTIVITY AND DERIVED INTERVENTION LEVELS FOR CESIUM-137 IN COSTA RICAN EXPORT GOODS: TUNA FISH, COFFEE AND POWDERED MILK LUISGUILLERMO LOR~A and PATRICIA MORA Nuclear Physics Laboratory, School of Physics, University of Costa Rica, San Jose, Costa Rica (Received 26 July 1993; in final form 25 October 1993)
Almtract-Cesium-137 is a fission product of *W and *r’)Pu.After a major nuclear accident, it is released into the atmosphere and in the far field region it will produce radioactively contaminated food and
drinking water. This paper will study the specific activity of %s in three Costa Rican export products: tuna fish, coffee and powdered milk. The average specific activities found are as follows: 0.89 + 0.41, 1.16 f 0.76 and 4.53 & 2.00 Bq kg-‘. They represent low values compared to their derived intervention levels: 1.25 x lo’, 1.25 x 10’ and 2.44 x lo5 Bq kg-‘, respectively.
1. INTRODUCTION
stipulate a dose of 5 mSv/y (UNSCEAR, 1982; WHO, 1988). The amount of 13’Csfound in coffee and powdered milk may come from nuclear accidents, the latest being the Chernobyl nuclear power plant accident. We presume that the tuna fish might get its 13’Csfrom nuclear tests in the South Pacific. Other y-emitting nuclides resulting from a nuclear accident, such as ‘“Cs and 13’1,cannot be studied quantitatively in this paper, since more than 6 years have passed since the last accident (1986) and their longest half-life is approximately 2 years. i3’Cs with a half-life of 30 years, is a fission produc; of uranium and plutonium. This element is soluble in the corporal fluids, and after its ingestion, it is distributed almost evenly throughout the whole body. The effective half-life in the human body is from 70 to 110 days depending on age. The critical organ for this element is the entire body, and the equivalent committed dose in a period of 50 years should not exceed 8.1 mSv/Bq (UNSCEAR, 1982).
A POPULATION close to the location of a nuclear accident will be, during the first minutes, highly exposed to internal and external irradiation by inhalation of radioactive products present in the atmosphere. The radioactive cloud scattered by the wind contaminates both aquatic and terrestrial ecosystems. Therefore, man is exposed to external irradiation from the materials deposited on the ground, to internal irradiation caused by breathing in suspended material in the air and subsequently, by its ingestion when taking food and water (IAEA, 1991; ICRP, 1977). In places far away from the site of the accident, the means of contamination are solely the intake of food and water (IAEA, 1991). Figure 1 shows the major pathways to man resulting from the release of radioactive elements in the atmosphere. As a consequence of nuclear accidents, such as Chernobyl in 1986, the International Atomic Energy Agency placed a low level counting system in the Nuclear Physics Section of the School of Physics at the University of Costa Rica. This was in order to measure the specific activity of nuclides present in the Costa Rican diet and in accordance with the table of consumption, to compare those results with their derived intervention levels (DILs), accurately measured for each one of the nuclides. The purpose of this paper is to present information as to the specific activity of 13’Cs in tuna fish meat, coffee, and powdered milk and furthermore, to compare these results with the DILs for these products as proposed by the World Health Organization and the Food and Agriculture Organization, which
2. METHODS AND MATERIALS 2.1. Derived intervention levels The derived intervention levels (DILs) are those that guarantee the maximum amount of nuclide activity that man can consume without exceeding the reference level of dose established internationally for a year, to avoid any detrimental effect on health (WHO, 1988). The two organizations that mark the trends which establish the DIL values are, as already stated, the World Health Organization (WHO) and the Food and the Agriculture Organization of the 731
L. G. LORIA
and P. MORA
UWORbOllOd otmorphrrlo rrloorr
ot rodionuolldxx I Rolytrr
hti0ut
(wofhoutl
ml&Ion
I
I
I
I I I
I
Ingatlon
I
7
D8op roll
I
lrrlgotlon
I I
I
rxtomal lxpoouro
I
lngo8tlon
1
FIG. 1. Major pathways of radionuclides to man in the event of an uncontrolled release of radioactivity [International Chernobyl Project, 19911.
United Nations (FAO). Through the Alimentarius Codex, both organizations recommended a DIL of 1000 Bq/kg for 13’Cs in export goods resulting from an intake of 500 kg with an annual dose of 5 mSv. Specific activities that are below the established level allow for free commerce of the export goods (WHO, 1988). The DIL is symbolically expressed by means of the following relationship (Randell, 1988): DIL = RLD/md
(Bq/kg)
(1)
where: RLD = reference level of dose (Sv/y); m = mass of the food consumed annually d = dose per unit intake (Sv/Bq).
(kg/y);
The criterion that conditions the reference level of dose, establishes that measures to restrict the distribution of food be introduced if the dose to the
individual exceeds an effective dose equivalent to 5 mSv after exposure (Randell, 1988; Waight, 1989). The value of 5 mSv, established internationally, does not correspond to the 5 mSv of effective equivalent dose to the population recommended by the International Commission on Radiation Protection. On the contrary, it corresponds to a comparative value of the natural background radiation average of 3 mSv and of the level of 5 mSv for radon established by the WHO (IRCP, 1977). The human diet is a distinguishing trait of each country, and could be further stated that it is distinctive of each individual. However, the WHO, regarding calculations, has proposed a global diet on an annual average consumption of 550 kg of food per person (Randell, 1988). Table 1 shows the food consumption patterns for different diet types (WHO, 1988).
Table 1. Food consumption patterns for different diet types (per capita in kg per year) Diet type
Cereal
Roots and tubers
Vegetables
Fruit
Meat
Fish
Powdered milk
African Cent. American Chinese Mediterran. European Far Eastern N. African S. American
127.4 113.2 171.7 188.2 121.1 206.6 161.9 129.5
134.8 46.0 85.8 19.3 72.7 28.4 20.0 67.6
25.9 38.8 85.2 91.6 86.7 54.3 63.4 34.2
45.2 98.6 5.5 101.5 81.4 48.3 63.9 83.2
16.8 42.3
15.0 18.7 8.9 8.4 20.2 24.4 7.3 14.4
28.9 82.2 1.9 74.3 154.9 33.6 77.2 70.7
15.0 30.4 75.3 21.5 24.0 48.4
CESIUM-137
733
IN COSTA RICAN EXPORT GOODS
Table 2. Comparison of the maximum and worldwide normalized nourishment consumption with the Costa Rican consumption (per capita in kg per year)
Reg. maxim. Normalized Costa Rica
Cereal
Roots and tubers
Vegetables
Fruit
Meat
Fish
Milk
205 140 126
135 100 12
90 60 21
100 70 100
15 50 33
2s 15 06
15s 105 121
Table 2 shows the maximum and normalized consumption of food, as well as the typical Costa Rican consumption, for different diet types (WHO, 1988). Table 3 shows the radionuclides of importance present in the food chain as a consequence of reactor and nuclear plant accidents (ICRP, 1977). The nuclides included in the table are divided into two groups in accordance with the dose per unit intake. Group I includes 2MP~and the actinides that have a factor of 10m6Sv/Bq, while Group II includes those y -emissive nuclides, such as i3’Cs, whose factor is of the order of lo-’ Sv/Bq (Randell, 1988; Waight, 1989). Table 4 presents the DIL for seven groups of food for human consumption with their respective doses per unit intake factor. The levels that are included were selected based on the worldwide and normalized consumption of each product, as if each one incorporated a dose of 5 mSv/y. Throughout a nuclear accident, not only does a single food become contaminated, but there can also be a number of nuclides present in the same food. Therefore, the new DILs should be evaluated again to guarantee that the total level of dose does not exceed 5 mSv annually. The new DIL can be expressed by the following relationship (WHO, 1988):
goods. These samples were dried for a period of 24 hours with the purpose of eliminating the moisture. Once the tuna fish is thoroughly dry, the meat must be ground until the fiber is no larger than 2 mm, while the powdered milk, as well as coffee beans are analyzed in their original form. For each one of the samples their mass, fresh as well as dry, is recorded. Approximately 1 kg is placed in a polyethylene container, better known commercially as a “marinelli” with a volume of one liter, and its geometry allows for its insertion into the y -spectrometry system’s detector (IAEA, 1989; Loria and Jimenez, 1992).
2.3. Gamma spectrometry
Figure 2 shows the block diagram of the y-spectrometry system which is used for the analysis of an energy spectrum emitted by the nuclides present in the sample (Salazar and Loria, 1987). By this method, it is possible to measure the specific activity of the nuchdes that are present in the sample without the need to separate them from the matrix where they are contained. The radiation from the samples interacts with a high purity germanium detector (Tennelec) under any of the already known processes: photoelectric effect, Compton effect and pair production, producing an g kf) DIL*(i,S) = (2) electrical voltage pulse proportional to the given energy of the incident photon. The pulse is con1 cg (ff)lNIWf) i/ formed by a linear amplifier to a size and shape where g (i, f) is a function that represents a specific compatible with the multichannel analyzer (Canberra pattern of contamination. S-100) which classifies them according to their Since this function appears in the numerator as voltage or energy in the corresponding channels, thus well as in the denominator of equation (2), it can be obtaining the emission energy spectrum. Through a expressed in relative terms, being the ratio of the computer program which identifies the energy photospecific activity found in different foodstuffs. peaks, it estimates the specific activity and the minimum detectable amount of the radioisotopes present in the sample. 2.2. Recollection and preparation of samples An exact identification of the photopeaks present The samples of tuna fish, coffee and powdered milk in the spectrum is essential to the measurement of ywere supplied by the exportation companies of these emissions. Therefore, the y -spectrometry system must be calibrated, which is done by using known radionuclides with well-defined energies, usually in the Table 3. Nuclides of importance in the food chain and their typical emissions range of 122 to 1460.7 keV. The calibration consists of establishing a lineal relationship between the chanNuclides Type of decay Half-life nels of the multichannel analyzer and the emission Strontium-90 27.70 y energy of the nuclides used. Certified nuclides with a Iodine-131 8.05 d I!,; Cesium- I 34 2.05 y relatively long half-life in watery solution with the Cesium- 137 30.00 y s-1 Y same geometry as the sample are used to measure the Plutonium-239 a 24,OOO.OO y efficiency of the detection system. The efficiency for a
L. G. LORfA and P. MORA
734
Table 4. Derived intervention levels in Bq/kg, classifiedaccording to the conversion factor of dose in Sv/Bq Nuclide
Factor
Cereal
Roots and tubers
Vegetables
Fruit
Meat
Milk
Fish
Pu-239 Sr-90 I-131 cs-134 cs-137
10-b 10-E IO--* 10-n IO-*
35 3500 3500 3500 3500
50 5000 5000 5000 5000
80 8000 8000 8000 8000
70 7000 7000 7000 7000
100 10,000 10,000
45 4500 4500 4500 4500
350 35,000 35,000 35,000 35,000
given energy can be tested using equation (Salazar and Loria, 1987): e [E] = A VA,, e-“‘1
the following
(3)
where: A = activity A,, =
t =
f = 1 =
of the standard measured experimentally in Bq; activity of the standard on the date of fabrication in Bq; time elapsed since the date of fabrication of the day of the measurement of the standard activity; absolute transition probability of a y-decay at that specific energy; and decay constant of the standard nuclide.
The measured values of each one of the patterns are represented by graphs as a function of energy, establishing a relationship between the variants of the form (IAEA, 1989): In t = k, + k, In E
(4)
where: c = absolute efficiency at a given energy; k, and k, = constants to be determined; and E = energy in keV.
10,000 10,000
When the y-spectrometry method is used, a lower limit of detection (LLD) is associated with each specific activity measurement which corresponds to the minimum amount of the specific activity that can be detected. An accepted expression for this limit, which contains a 95% degree of confidence for detecting the present activity, is given by the equation (IAEA, 1989; Lauria, 1988): LLD = 4.66&[~ [Elf]-’
(5)
where S, is the estimated standard error of the net count rate. The detection limit is a way of determining the operation capability of a y measuring system without the influence of a sample. It is also reliant on the assumption that the count rate in the energy area taken for the specific nuclide and the count rate in the region or regions taken for background are essentially equal. When a sample is being measured in terms of the specific activity of a nuclide, the term usually associated with the detection limit is the minimum detectable concentration (MDC), which is expressed through the following equation (IAEA, 1989): CMD = LID/W where W is the mass of the sample (kg).
Biar rupply I
I Data rtorr I FIG. 2. Block diagram of a typical gamma-ray spectrometer system.
(6)
CESIUM-137
735
IN COSTA RICAN EXPORT GOODS
FIG. 3. Gamma-ray spectrum of tuna fish.
3. RESULTS Figure 3 shows the corresponding emission spectrum of one of the tuna fish samples collected during a timing of 60,000 s. Table 5 presents DILs for 13’Csin tuna fish meat, coffee and powdered milk for Costa Rica, taking into account the real consumption of each one of the goods which incorporates an effective dose equivalent to 5 mSv/y (Ministry of Health, 1982, 1991) Table 6 presents the average and maximum values of the specific activity measured for 13’Cs in coffee, tuna, and powdered milk, utilizing 16 samples for the first two products and 10 samples for the powdered milk.
4. CONCLUSIONS From an economic point of view, the values measured for the specific activity of i3’Cs in the studied products, compared with the normalized International Derived Intervention Levels and the National Levels, are extremely low. Furthermore, the measured levels are below 1000 Bq/kg, for “‘Cs determined by the Alimentarius Codex, which allows for free commerce of the goods. From the point of view of radiological protection of the population, the activities found for ‘r’Cs in the Table 5. Costa Rican derived intervention levels (DILs) for coffee, tuna fish and powdered milk in Balka Product Coffee
Tuna fish Powdered milk
1.25 x lo5 1.25 x 10’ 2.44 x 105
Table 6. Average and maximum values of the specific activity measured for “‘Cs in coffee, tuna fish and powdered milk, expressed in Bq/kg Product
Average value
Maximum value
coffee
1.16kO.76 0.89 f 0.41 4.53 f 2.00
3.12&0.16 1.33 * 0.58 7.61 * 1.11
Tuna fish Powdered milk
three products represent an equivalent dose that varies in a range of 0.01 to 2.5 ~SV. The consumption of the mentioned products does not convey any risk to the health of the Costa Rican population since the effective equivalent dose is around four orders of magnitude lower than the established levels for the natural background radiation and for radon. The methodology used, and the equipment available, allows the measurement of the specific activity of any y-emitter isotope, and the establishment of the DILs for these emitters in the Costa Rican diet, before the outcome of a nuclear accident. As a consequence of the new recommendations dictated by the International Commission on Radiation Protection, new DILs will have to be calculated if the reference level of doses changes, because the limiting value of dose for the public has been recently established as 1 mSv (ICRP, 1991). Acknowledgements-The authors hereby thank the International Atomic Energy Agency, for the training session provided by the expert Ing. Luis Conti, from the Nuclear Energy Commission of Brazil, and for the scholarships granted. Furthermore, we are grateful for the equipment donated to the Nuclear Physics Laboratory, obtained through the bilateral pledge of Technical Assistance (COS 2/002). We wish to express our deep appreciation to the Vicerectory of Investigation of the University of Costa Rica for its support given throughout our work.
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Protection
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L. G. LORfA Nicoya, Guanacaste,
utilizing low level techniques.
Technologia en Marcha 14(11), 61-72. Ministry of Health (1982) National Survey of Nutrition.
Ministry of Health, San Jose, Costa Rica. Ministry of Health (1991) First National Survey of Apparent Food Consumption. Ministry of Health, San Jose, Costa Rica. Randell A. (1988) FAO Recommended Limits for Radionuclide Contamination in Foods. Food and the Agriculture Organization of the United Nations, Rome, Italy. Salazar A. and Loria G. (1987) Spatial distribution of the thermic and epithermic flux of a californium source. Ciencia y Technologia 11(l), 65-75.
and P. MORA UNSCEAR (1982) Ionizing Radiation: Sources and Biological Effects. United Nations Scientific Committee on the E-&&s of Atomic Radiation, United Nations, New York. Waight D. (1989) Setting derived intervention levels for food. In: Int. Symposium on Environmental Contamination Following a Major Nuclear Accident. World Health Organization, Geneva. WHO (1988) Derived International Levels for Radionuclides in Food: Guidelines for Application After Widespread Radioactive Contamination Resulting from a Maior Radiation Accident. World Health Organization, Geneva.