GB 14883.10-2016National food safety standard - Examination of Radioactive Materials for Foods - Determination of Cesium-137 (English PDF)

4.1 Low Background gamma Spectrometer System

Low background gamma spectrometer system shall satisfy the following requirements:

a) Detector: coaxial high-purity germanium or germanium (lithium) detector. In terms of 60Co 1332.5 keV gamma-ray full energy peak, its energy resolution shall be less than 3 keV; its relative efficiency shall be higher than 15%.

b) Shield: the primary shield shall be multi-layer material heavy metal shield,

5.1.1 Energy scale

In accordance with energy scale, use gamma radioactive source (4.4) to conduct energy scale of the low background gamma spectrometer system (4.1). Record the characteristic gamma- ray energy of the scale source, and corresponding full energy peak location. Through computer processing, or drawing on a cartesian coordinate paper, or conducting the least square fitting of the data, a graph of the relations between energy and the location may be obtained.

5.1.2 Full energy peak detection efficiency scale

Measure 137Cs standard radioactive source. In order to reduce system errors, the net area of the full energy peak being measured shall at least be more than 100,000 counts.

In accordance with Formula (1), calculate its detection efficiency at 661.6 keV gamma-ray full energy peak.

Where,

E---detection efficiency of 137Cs standard radioactive source at 661.6 keV gamma-ray full energy peak;

N---net area of 661.6 keV gamma-ray full energy peak, expressed in (count);

A ---activity of 137Cs standard radioactive source, expressed in (Bq);

T ---measured time of 137Cs standard radioactive source, expressed in (s);

B---branch ratio of 137Cs 661.6 keV gamma-ray, 84.62%.

5.2 Sampling

Sampling shall comply with the stipulations in GB 14883.1.

5.3 Preparation of Sample for Measurement

5.3.1 Grain sample: take 500 g of sample, then, evenly spread it in an enameled plate or a stainless-steel plate. Place it in the oven, at around 70 °C, dry it for around 5 h.

Then, weigh it; calculate the dry mass/fresh mass ratio. In terms of granular grain, dry it, then, directly place it into a sample box, which is already cleaned on the outside and the inside; tamp it. In terms of finely powdered grain, use a sampler to press it, so that the height of the sample is the same as the height of 137Cs standard radioactive source.

Record the dry sample mass and the height of the sample to be determined; calculate its apparent density.

5.3.2 Vegetable sample: take around 3 kg of sample; remove the inedible part. Wash

E---same as Formula (1);

F---total correction factor of measurement efficiency, %, please refer to Appendix A;

please refer to GB/T 16145 for more accurate calculation methods;

B---branch ratio of 137Cs 661.6 keV gamma-ray, 84.62%;

W---equivalent fresh sample mass of the sample being measured, expressed in (kg) or (L);

lambda---137Cs’s disintegration constant, expressed in (a-1); lambda = 0.693/T0, T0 is 137Cs’ radioactive half-life, 30 a;

t---time interval from sampling to measurement, expressed in (a).

9.1 Reagents

9.1.1 Diammonium hydrogen phosphate [(NH4)2HPO4].

9.1.2 Ammonium nitrate (NH4NO3).

9.1.3 Ammonium molybdate [(NH4)6Mo7O24-4H2O].

9.1.4 Bismuth trioxide (Bi2O3).

9.3.1 137Cs standard solution: around 1 x 103 disintegrations / (min-mL), contains 0.1 mol/L hydrochloric acid solution of 0.1 mgCs+/mL.

9.3.2 Cesium carrier solution (10 mgCs+/mL): weigh-take 12.67 g of cesium chloride, then, place it in a small beaker. Add water to dissolve it, then, add 3 drops of concentrated hydrochloric acid. Quantitatively transfer it into 1 L volumetric flask; use water to dilute to the scale. Calibration may use one of the following two methods:

a) Calibration method 1 -- cesium perchlorate method: accurately absorb 4.00 mL of cesium carrier solution, then, place it in a 125 mL conical flask. Add 1 mL of nitric acid and 5 mL of perchloric acid. Evaporate it, till it emits white smoke for a few minutes. Take it down, then, cool it down to the room temperature. Add

15 mL of absolute ethanol, shake it up. Then, in ice bath, cool it down for a few minutes. Extract and filter the sediment in G4 sand core glass crucible, which is already weighed. Use 10 mL of absolute ethanol to rinse it once, then, at

105 °C, dry it for 15 min. Cool it down in the dryer, then, weigh it.

b) Calibration method 2 -- tetraphenylboron-cesium method: take 2.00 mL of cesium carrier solution, then, place it in a 100 mL beaker. Add 20 mL of water and 1 mL of 6 mol/L acetic acid solution; mix it up. Add 10 mL of 3% sodium tetraphenylborate solution; slightly heat it up. Then, cool it down to room temperature. Extract and filter it on G5 sand core funnel, which already reaches a constant mass. Use 20 mL of 1% acetic acid solution to rinse the beaker.

Quantitatively transfer it into the sand core funnel. In the end, at 110 °C, dry it;

weigh it, till it reaches a constant mass.

11.1 Sampling and Pretreatment

Sampling and pretreatment shall comply with the stipulations in GB 14883.1.

11.2 Sample Preparation and Determination

11.2.1 Weigh-take 1 g ~ 10 g (accurate to 0.001 g) of food ash sample, then, place it it reaches dryness; weigh it, till it reaches a constant mass. On low background beta meter, measure beta radioactivity of 137Cs. Next, under the same conditions, measure 137Cs supervised-source.

11.3 Calibration of Supervised-source through Standard Source, Drawing

of Counting Efficiency - Mass Curve

11.3.1 Calibration of 137Cs supervised-source through 137Cs standard source

Clean and dry a stainless-steel measuring plate, whose internal surface is smooth.

Use a pencil to draw a circle, which has the same diameter as the sample being measured. Drop 0.1 mL of insulin solution (20 units/mL); make it evenly distributed in the circle. Then, dry it. Accurately add 137Cs standard solution (102 disintegrations/min

~ 103 disintegrations/min) onto the insulin circle. Carefully and evenly spread it, then, dry it. Then, add 1 drop of colloidal solution; evenly spread it on the source. After drying it in the air, 137Cs supervised-source (it would be better to use 137Cs plane standard source in the active zone, which has the same diameter as the sample) may be obtained.

Accurately transfer-take 2.00 mL of cesium carrier (9.3.2) and 1.00 mL of 137Cs standard solution (9.3.1), then, place it in a 50 mL beaker. Follow the operation in

11.2.4 ~ 11.2.5; obtain 137Cs standard source.

Continuously measure the above two sources on the low background beta meter for sample measurement. In accordance with Formula (3), calculate the intensity of the post-calibration 137Cs supervised-source.

Where,

A1---the intensity of 137Cs supervised-source after the calibration through 137Cs standard source, expressed in (dpm);

N1---137Cs supervised-source’s net count rate during calibration, expressed in (cpm);

A2---the activity of added 137Cs standard solution, expressed in (dpm);

N2---standard source’s net count rate after self-absorption and chemical recovery rate correction, expressed in (cpm).

11.3.2 Drawing of counting efficiency - mass curve

Accurately prepare a series of solutions, which contain different contents of cesium.

Respectively add an equivalent amount of 137Cs standard solution (9.3.1), then,

Method-3 -- Cobalt Potassium Ferricyanide

15.1 Cobalt potassium ferricyanide: at room temperature, drop 1 volume of 0.5 mol/L

potassium ferrocyanide {K4[Fe(CN)6]} solution to 2.4 volumes of 0.3 mol/L cobalt nitrite solution. Continuously stir it for 30 min, complete the operation. Conduct centrifugation, then, discard the supernatant.

Use water to rinse the sediment. Thoroughly stir up all the sediment; conduct centrifugation, then, discard the washing liquid. Follow this method to rinse it, till the liquid becomes colorless. Take out the sediment, then, spread it on a watch glass. At

115 °C, dry the sediment, till it becomes purple brown. Then, take it out, cool it down;

grind and sieve it. Fill cobalt potassium ferricyanide powder, which passes through 250

µm, into a bottle; reserve for later usage.

15.2 Cesium iodine bismuthate solution, cesium carrier solution, nitro-hydrochloride, glacial acetic acid, oxalic acid and sodium hydroxide solution are the same as Method

II - ammonium phosphomolybdate method.

17.1 Sampling and Pretreatment

Sampling and pretreatment shall comply with the stipulations in GB 14883.1.

17.2 Sample Preparation and Determination

17.2.1 Same as 11.2.1

17.2.2 Add 1 g of cobalt potassium ferricyanide to the combined solution; continuously

stir it for around 10 min. Use quantitative filter paper to filter all the sediment. Use 30

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