Reload Index (ZRiChK UMCS)

 

 

STUDY ON APPLICATION OF A STANDARD LIQUID SCINTILLATION COUNTER FOR DETERMINATION OF A SMALL RADIOCARBON CONCENTRATION IN ENVIRONMENTAL MATERIALS

 

ANDRZEJ KOMOSA, PIOTR ZABADAŁA

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

               Radiocarbon (14C) is the beta-emitting natural isotope of cosmogenic origin. It is formed in the upper atmosphere layer as a product of nuclear reaction between cosmic radiation and nitrogen nuclei. Radiocarbon created in this way is diluted by the stable carbon isotopes (12C and 13C). Estimated concentration of 14C in the environment is about 1·10-9 % of mass of carbon isotope mixture. Total amount of radiocarbon present in the environment is about 75.000 kg, what is an equivalent of 1.27·1019 Bq of beta radioactivity [1].

            Owing to its long half-life (5568 years) a radioactive equilibrium between all carbon containing species in a different part of environment can be establish.

            Assumption that radiocarbon production rate is constant and exchange of carbon between living organisms and the environment is stopped in a moment of death, became a basis of radiocarbon dating method. theoretical value. The age of sample material can be calculated by measuring the actual value of 14C activity in the sample and comparing it with the known theoretical value [2].

            Standard liquid scintillation counter is characterized by rather large background, what makes difficulties in determination of small activities. The purpose of the study was to find the optimal conditions for determination of 14C concentration in environmental samples. The spirits samples collected directly from a production line of the factory was a subject of investigation. The samples were of natural origin (as a product of cereal or potato fermentation) and contain large amount of organic carbon. Radiocarbon concentration can be a measure of quality of alcoholic articles assigned for consumption (synthetic alcohol is free from radiocarbon).

            Five liquid scintillation cocktails of different composition was tested using two measuring techniques. First one is the analysis of a spectrum collected within 600 min, the second one is the LOLES method [3] (repeated short-time measurements of a sample and background alternately).

            Seven samples of strong spirits and four of diluted ones were investigated. The results of radiocarbon activity determination in the samples are presented in Table 1.


 

Table 1. The results of radiocarbon concentration measurements in alcohol samples.

 

 

Sample

C2H5OH concentration

[%]

14C activity

[Bq/g C]

14C activity

[Bq/dm3]

unrectified spirit 1

unrectified spirit 2

91.4

91.0

0.178

0.181

80.7

85.9

rectified spirit 1

rectified spirit 2

rectified spirit 3

rectified spirit 4

rectified spirit 5

96.6

96.3

96.3

96.6

96.6

0.173

0.164

0.168

0.174

0.180

87.2

82.4

84.4

87.7

90.7

vodka 1

vodka 2

vodka 3

vodka 4

vodka 5

40

40

40

40

40

0.176

0.178

0.158

0.160

0.161

36.6

37.0

32.8

33.3

33.5

Average value

 

0.179 ± 0.012

 

methanol

 

0.043

 

 

 

            Observed values are similar in every ethanol sample, what confirms good homogeneity of radiocarbon distribution in vegetables (as corn and potato). Theoretical value of 14C concentration is equal to 0.226 Bq/g C [2]. This shows that some radiocarbon fractionation process occurs. For comparison a result of synthetic methanol sample measurement is presented in Table 1 as well.

 

References:

[1]. C.T. Peng, LSC 1992, Radiocarbon 1993, 1.

[2]. G.R. Choppin, J.O. Liljenzin, J. Rydberg, Radiochemistry and Nuclear Chemistry, Butterworth-Heinemann, New York, 1985.

[3]. J.M. Los Arcos, L.R. Barquero, Appl. Radiat. Isot., 47 (1996) 879.