OPTIMIZING THE COUNTING CONDITIONS FOR CARBON-14 FOR THE SAMPLE OXIDIZER-LIQUID SCINTILLATION COUNTER METHOD

ABSTRACT. Radiocarbon is a pure beta emitter with a half-life of 5730 yr and a maximum energy of 156 keV. 14C is pro-duced in nuclear reactors through neutron-induced reactions with carbon, nitrogen, and oxygen isotopes and is mostly released into the environment in gaseous releases such as carbon dioxide, 14CO2. Nowadays, 14C is one of the major contrib-utors to the airborne radioactive releases in Finland. In recent years, 14C measurements in the environment have become a part of the monitoring program in the vicinities of Finnish nuclear power plants. The sample preparation for the liquid scintillation counting (LSC) was done with a 307 Sample Oxidizer by PerkinElmer. The dried environmental samples were combusted completely in an oxygen atmosphere to carbon dioxide and water. The 14CO2 was absorbed by the special reagent CarboSorb E (3-methoxypropylamine) and mixed with the LS cocktail PermaFluor E+. The apparatus is almost fully automatic and the combustion of a sample takes only a few minutes. Its disadvantage is the small amount of sample that can be combusted, with a 1-g upper limit for samples. The small volume per sample is challenging when the 14C concentrations are quite low and one would like to obtain a representative sample from the environment. The samples were counted with a low-level liquid scin-tillation counter Quantulus 1220. One of the most interfering phenomena in LSC is the quenching. In our case, it was extremely important since the CarboSorb E is a very strongly quenching agent. When combusting the actual samples, most of the CarboSorb E was used to absorb the CO2 from the sample and the quenching of the sample was low. When preparing a background sample by combusting only a cellulose cup, almost no CarboSorb E was consumed and the background spec