Procedure for determining the specific activities of radionuclides in fish and fishery products by gamma spectrometry

Transcription

1 Procedure for determining the specific activities of radionuclides in fish and fishery products by gamma spectrometry G--SPEKT-FISCH-0 Authors: G. Kanisch A. Krüger Federal coordinating office for fish and fishery products, crustacean, shell fish and aquatic plants (Leitstelle für Fisch und Fischereierzeugnisse, Krustentiere, Schalentiere, Meereswasserpflanzen) ISSN Version November 015

2 G--SPEKT-FISCH-0-01 Procedure for determining the specific activities of radionuclides in fish and fishery products by gamma spectrometry 1 Scope The procedure described in the following is suitable for gamma-ray-spectrometric analysis fish samples in case of higher contamination e. g. after nuclear weapons tests or accidents in nuclear facilities to get a rapid overview on the extent of contamination. It offers the main advantage to determine the specific activities of iodine isotopes, which is not possible with the procedure G--SPEKT-FISCH-01 due to the volatility of iodine. The procedure described in the following may also be used for gamma spectrometric analysis of samples of fish that have to be monitored according to the German Precautionary Radiation Protection Act (StrVG) in the IMIS-routine programme and the Guideline for the Monitoring of Emissions and Immissions of Nuclear Installations during Regular Operation. Sampling The basic sampling is described in the procedure G--SPEKT-FISCH-01. An amount of 1,5 kg fresh sample is required, which is filled into a 1-liter-Marinelli-beaker (reentrant beaker). Special care must be taken if fish is sampled in areas with higher contamination. The freshly collected fish are killed quickly at the point of sampling and packed into plastic bags, which are stored on ice inside a suitable container. Freezing of the sample should be avoided as they should be measured as direct after sampling as possible. If freezing of the sample cannot be avoided, special care must be taken to include tissue fluids that may be liberated during the defrosting process in the sample. Furthermore, it must be distributed homogenously in the sample during the measurement process. In case of import goods, crustaceans and shellfish and also tinned fish (fish commodities) are sampled besides fish. Samples are obtained from importers operating in the respective Federal State. Only the consumable parts considered as generally consumed by the public are analysed. For correct labeling of samples, it is referred to procedure G--SPEKT-FISCH-01 and the literature (1). Version November 015

3 G--SPEKT-FISCH Analysis 3.1 Principle of the procedure Fresh fish is filleted first. Fillets and tinned fish is chopped and homogenized. The specific activity is of gamma-ray emitting radionuclides including iodine is determined inside a 1-liter-Marinelli-beaker using a gamma-spectrometer. 3. Sample preparation All item used for sample preparation, e. g. cutting boards, knives and Marinellibeakers, must be cleaned using cleaning solution prior each use. Fish are cleaned like in common household. Afterwards, they are filleted and bones and skin are removed as much as possible. The fish flesh obtained is finely cubed, a preserving agent, e. g. sodium azide is added and both are thoroughly mixed (see below). The use of a mixer or meat grinder must be avoided in cases of higher contamination. In case of tinned fishery products it is recommended that the included liquids (e. g. sauces) are processed according to their generally consumption by the public. When they are not consumed, they may be discarded; otherwise they must be uniformly distributed inside the sample. The filling is not cleaned, but the texture may require mincing. The addition of preserving agents takes place before homogenization of the sample. Generally the preparation is only stable for a short time. The addition of preserving agents decelerates microbial processes inside the preparation by which gases are eased that lead to a swelling of the preparation. During measurement it is strictly advised that the cap is only loosely placed on top of the beaker. Additionally, the measurement chamber should be coated with foil or the beaker should be placed inside a large plastic bag. At longer idle times before analysis, the preparation must be stored in the fridge. 3.3 Radiochemical separation No radiochemical separation is required. Version November 015

4 G--SPEKT-FISCH Measuring the activity 4.1 General The basics of gamma spectrometry and safety measures in case of higher levels of radiation are discussed in the basic chapter -SPEKT/GRUNDL of this procedures manual and in the literature (1, ). 4. Determining the detection efficiency 4..1 Calibration in solution For determining the energy-dependency of the detection efficiency, reference is made to the basic chapter -SPEKT/GRUNDL of this procedures manual. The calibration is carried out with a suitable calibration preparation, e. g. an aqueous solution of a traceable standard or a resin containing traceable amounts of radioactive compounds. 4.. Correction for self-attenuation Because of the small difference in densities between water and fish flesh, a density correction for fish flesh may be omitted. This is due to the fact the density of the aqueous calibration solution is close to that of water (1 g cm -3 ) and fish flesh contains from water by approx. 80 %. 4.3 Background The basics of measuring the background effects are described in the procedure G--SPEKT-FISCH-01. In cases of specific activities two orders larger compared to the routine measuring program, the background effect must be determined with smaller measurement duration but higher frequency, e. g. each working day. For samples of fish, iably determining the K-40 background count rate of the background effect is important in order to be able to use the K-40 activity determined in fish for verifying the correctness of the subsequent sample spectrum analysis. 4.4 Gamma spectrometric measurement For the detailed measurement procedure it is referred to section 4.4 of the procedure G--SPEKT-FISCH-01. Instead of the cylindrical beaker described there, a 1-liter-Marinelli-beaker is used. Version November 015

5 G--SPEKT-FISCH Calculation of the results 5.1 Equations When determining specific activities of radionuclides in fish flesh, the gamma lines of different radionuclides may interfere. Therefore, determining specific activities of radionuclides from a single gamma line may not always be possible by easy-to-use methods. If interferences occur or multiple-line radionuclides need to be analysed, least squares fitting algorithms or, in the interference-free case, weighted means of the single line activities have to be used for calculation of the specific activities and measurement uncertainties. In this case, reference is made to the basic chapters -SPEKT/INTERF and -SPEKT/GRUNDL of this procedures manual. For calculation of specific activities from single gamma lines it is referred the procedure G--SPEKT-FISCH-01. If a net count rate R n,r associated with the gamma-ray energy of the radionuclide r has been detected, the resulting specific activity a r, ative to fresh mass (FM) and the date and time of sampling, is calculated according to equation (1): ar e r t A Rn, r f1 f f3 r t A e (1) p mf Rn, r with: f3 Herein are: r t c 1 e r tc R n,r net count rate of the line of radionuclide r in s -1 ; procedural calibration factor in s kg -1 ; f 1 correction factor for coincidence summing; f self-attenuation correction factor for fish flesh ative to water, f = 1; f 3 correction factor for the decay of the activity of the radionuclide r during the measurement; p emission probability of the gamma line of the radionuclide r; m F mass of fresh fish flesh used for measurement kg; t A time period between sampling and beginning of the measurement in s; t c period of measurement in s; r decay constant of the radionuclide r in s -1. The net count rate of the gamma-line of the radionuclide r is calculated according to equation (): R n, r Rg, r R T, r R0, r () with: R g,r gross count rate of the line of the radionuclide r in s -1 ; R T,r background count rate underneath the line of the radionuclide r, e. g. as counting rate of a trapezoidal background, in s -1 ; R 0,r net count rate at the line of the radionuclide r in a background spectrum in s -1. Version November 015

6 G--SPEKT-FISCH-0-05 The generalized expression to calculate the uncertainty of the net count rate is given in equation (3), with the coefficients µ k to be determined according to equation (4): Rn, r 0 Rn, r 1 Rn, r u (3) with coefficients: 0 0 ; 1 1 tm R R T,r R0,r t m T,r 0,r ; u R u (4) Using the trapezoidal method (assuming a linear background continuum) the coefficient µ is given by equation (5): R T,r b 1 1 R T,0,r b0 1 R0, r 1 (5) tm L tm t0 t 0 L0 Herein are: t 0 duration of the background measurement in s; b, b 0 line widths of sample and background spectra at the respective peak baselines, in channels; L, L 0 numbers of channels for the sample and for the background spectrum, respectively, over which the background continuum to the left and to the right of the peaks are estimated; R T,0,r background continuum count rate at the line of the radionuclide r within the background spectrum, in s -1. Equation (5) is a sufficient approximation for the background estimated by an empirical background step function. The µ k-coefficients only need to be inserted into equation (3) to receive the standard uncertainty of the net count rate u(r r). The standard uncertainty of the net count rate u(r n,r) calculated by equations (3) and (5) is given by equation (6): Rn, r Rn, r R T,r b 1 1 R T,0,r b0 u Rn, r µ 1 R0, r 1 (6) tm tm tm L tm t0 t0 L0 When no count rate occurs at corresponding gamma ray energy of background spectrum equation (6) is reduced by the last two terms. The combined standard uncertainty of the specific activity a r using equation (1) is calculated as: u r A a a u u e (7) r r u R n, r R n, r t Version November 015

7 G--SPEKT-FISCH-0-06 The ative standard uncertainty of the procedural calibration factor u () is determined according to equation (8), where the uncertainty of the decay correction may be neglected: u u f u f u u p u m (8) 1 F 5. Worked example The following example is used to calculate the specific I-131 activity in 1 kg fresh fish flesh (FM) using only the intensity of the gamma-ray energy at 364,5 kev. The following values are used: m F = 1,00 kg; R n,i-131 = 0,5115 s -1 ; = 0, s -1 ; R T,I-131 = 0,00196 s -1 ; b/(l) = 0,53; p = 0,81; f 1 = 1,000; f = 1,0; u (m F) = 0,004; u () = 0,09; u (p ) = 0,00985; u (f 1) = 0; u (f ) = 0. The standard uncertainties of the following input quantities are negligible: t A = s; t I-131 = s; t m = 1800 s; I-131 = 9, s -1. This leads to the correction factor f 3 for the decay during the duration of the measurement: 7 9, , f 3 1, , , e 1 e The specific activity in the fresh fish flesh (FM) results from equation (1): ai-131 1,0 1,0 1, , e 0, ,81 1,0 0, kg 70,006 1,0593 0, kg 37,666 1 kg Version November 015

8 G--SPEKT-FISCH-0-07 The ative standard uncertainty of the procedural calibration factor is calculated by equation (7): u 0,0 0,0 0,09 0, ,004 0, The standard uncertainty of the net count rate is obtained by equation (6) with neglecting the last two terms: u 0,5115 0, n, I-131 s 1, s R 1 0,53 The ative standard uncertainty of the specific activity is calculated according to equation (7), from which the third term is neglected: u - 1, ,5115 a 0, , 0454 I-131 u ai ,666 0, , , kg 37,666 0, , kg 37,666 0, kg 1,70 1 kg Finally, the specific I-131 activity is obtained as: a I-131 = (39,6 ± 1,7) kg -1 (FM) 5.3 Consideration of uncertainties The combined standard uncertainties of the specific activity amount to be 0 % or less except that the activities are so small that the ative uncertainties of the net counting rate already clearly exceed 0 %. The standard uncertainty of the self attenuation correction factor does not contribute anything to this. For multi-line radionuclides such as Cs-134, the uncertainty contribution of the coincidence summing correction factors (see basic chapters SPEKT/GRUNDL and -SPEKT/SUMESC of these measuring instructions) has to be taken into account, which may lead to additional few percents in case of significant corrections. Version November 015

9 G--SPEKT-FISCH Characteristic limits of the procedure 6.1 Equations For the calculation of the detection limit # ar and the decision threshold * a r of the specific activity of the radionuclide r it is referred to section 6.1 of the procedure G--SPEKT-FISCH Worked example Using the values of the input quantities from section 5. and the quantiles of the normal distribution, k 1- = 3,0 und k 1- = 1,645, the coefficient µ is calculated first. In this case, only the first term of equation (5) is used, because no corresponding line was observed in the background effect spectrum. 0, ,53 s 1, s 1800 a I , ,5613 kg s Rn, I-131 0,5115 * Using this value, the decision threshold of the specific activity a I-131 is calculated according to equation () of the procedure G--SPEKT-FISCH-01 considering procedural calibration factor: * a I ,5613 3,0 1, kg 0,84 1 kg The detection limit according to equation (3) of the procedure G--SPEKT-FISCH-01 the detection limit of the specific activity a is: # I ,645 0, , ,645 0,84 73, ,1946 # a I-131 0,84 1, , ,9974 1, ,645 3,0 1 kg 0, , ,30067 kg 0,5837 kg Version November 015

10 7 Catalogue of chemicals and equipment 7.1 Chemicals Cleaning agent: e. g. RBS-50-Super-Flüssigkonzentrat %; Preserving agent: 7. Devices Sampling: ice container / cooling boxes; plastic bags; G--SPEKT-FISCH-0-09 Sodium azide, NaN 3 (required for longer sample storage or in case of long (over night) duration of measurement). deep freezer (ca. -18 C), if the samples have to be stored. Sample preparation: filleting board made of plastic; sharpened filleting knifes; cut-resistant gloves. Measurement: 1-liter-Marinelli-beaker (reentrant beaker); Plastic-foil or bags, as splash guard for detector and interior room of the lead shielding. Version November 015

11 G--SPEKT-FISCH Software supported calculation Example of an Excel-spreadsheet Procedure for determining the specific activities of radionuclides in fish and fishering products by gamma spectrometry G--SPEKT-FISCH-0 Version November 015 Sample identification: I-131 (Evaluation as single line radionuclide by its 364,5 kev-energy) #Number of parameters p: 13 Colors: values from Vbasic k_alpha: quantile for (1-alpha) 3 Excel formulae (user) k_beta: quantile for (1-beta) 1,645 Manual input of values gamma: probability for conf.limits 0,05 Def. Excel-Variable (user) #Keywords Data input: variable names: Uncertainty budget: #Values of parameters p: Unit: Input values partial uncertainty budget StdDev: derivatives budget: in % p1 #Number of gross counts Ng: Ng 94,3 30, , , ,75 p trapezoidal BG count rate 1/s RT 1,96000E-03 0, , , ,106 p3 b/l bl 5,3000E-01 0,00000E ,000 p4 detection efficiency eps 1,77500E-0 5,14750E , , ,094 p5 emission probability, 364,5 kev p_gamm 8,1000E-01 8,00000E-03-46, , ,743 p6 fresh mass of the sample kg mf 1, ,00000E-03-37, , ,78 p7 time period Sampling-->StartMeasum s ta 5,76000E+04 3,763E ,000 p8 half-live of I-131 s ti131 6,9313E+05 1,64000E+0-3,175E-06 0, ,000 p9 factor for coincidence summing correction _f1 1,00000E+00 37, p10 self-attenuation correction factor _f 1, , ,000 p11 duration of measurement s tc 1800,00 0,00000E+00-0, ,000 p1 net count rate of the BG peak 1/s RnBG 0,000E+00 0,00000E+00-34, ,000 p13 duration of BG measurement s tbg 7000,00 0,00000E ,000 (the list may be continued here) Model section c = Factor * Rn auxiliary equations: (formulae) #Gross counting rate Rg: 1/s Rg 5,13461E-01 h1 decay correction counting duration _f3 9,99101E-01 h decay correction Sampling-->StartMes _f4 1,0599E+00 (the list may be continued here) (formulae) #Net counting rate Rn: 1/s Rn 5,11501E-01 #Calibration factor, proc.dep.: *s/kg Factor 73, #Value output quantity: /kg Result 3,7666E+01 0, <-- output value modifiable by VBA #Combined standard uncertainty: /kg uresult 1,7017E+00 #Decision threshold: /kg 0, #Detection limit: /kg 0, further derived quantities (ISO 1199): auxiliary quantity omega Omega 1, best estimate /kg BestWert 3,7666E+01 uncertainty of best estimate /kg 1,7017E+00 lower confidence limit /kg 3,4904E+01 upper confidence limit /kg 4,0968E+01 Calculate! The Excel file can be found on the website of this procedures manual. Version November 015

12 G--SPEKT-FISCH Example of an UncertRadio project The UncertRadio project file can be found on the website of this procedures manual. Literature (1) Leitsätze für Fische, Krebs- und Weichtiere und Erzeugnisse daraus. Enthält auch die Leitsätze für tiefgefrorene Fische, Krebs- und Weichtiere und Erzeugnisse daraus. GMBl Nr. 3-5 S. 451 ff vom Seiten. () ISO 1199:010, Determination of the characteristic limits (decision threshold, detection limit and limits of the confidence interval) for measurements of ionizing radiation Fundamentals and application. Version November 015