Refining the CO2 absorption method for low level 14C liquid scintillation counting in the ATOMKI

Complete text of publication follows. A new method of chemical sample preparation for liquid scintillation 14C measurements was implemented in the Laboratory of Environmental Studies of the Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI). The developed absorption method is quick, inexpensive and simple. In a special preparation line CO2 samples are absorbed in CarboSorb absorbent. The developed line consists of a gas-bag CO2 container (PG), a freezing finger for easier CO2 transport, a fine- regulation valve (VG), a bubbler filled Hg for gas-flow control (Hg), a puffer bulb for initial pressure reduction (PB), and the vial containing CarboSorb (CS) (Figure 1.). For the exact determination of the amount of the absorbed CO2 gas two independent techniques were applied (direct weighing of vial and measuring of the amount of the rest CO2 gas). Several tests were executed with old borehole CO2 gas (Linde Hungary Ltd. Repcelak, Hungary) without significant content of 14C. This gas has been used as background gas in our gas proportional counter (GPC) system. Tests were also performed on samples of known 14C activities between 29 and 7000 pMC, previously measured by GPC. The 14C activities of all prepared samples were measured by liquid scintillation counter (LSC) including quenching parameter (tSIE). The observed agreement between LSC and GPC data was really good in the case of several samples. We demonstrated the reproducibility of the sample preparation and the stability of the prepared counting mixture. We have determined parameters for measurements in our TRI-CARB 3170 TR/SL liquid scintillation counter (LSC) including optimised 14C window with counting efficiency (65.3 %) and the corresponding 14C dating limit (31.200 yr BP). The combined uncertainty of the determination was about 2 % for recent carbon samples. Thus, the presented method is suitable for 14C determination and dating of geological, hydrological, and environmental (author)