Isotopic fractionation during pretreatment for accelerator mass spetrometer measurement of (D3C)(2)O containing C-14 produced by nuclear reaction

摘要

A (D3C)(2)O (d(6)-acetone) target was irradiated with semi-monoenergetic neutrons generated from Be-9(p,n)B-9 reaction with 20 MeV protons to convert C-13 and oxygen nuclides in the target into C-14. With both liquid scintillation counting (LSC) and accelerator mass spectrometry (AMS) we measured the (D3C)(2)O (d(6)-acetone) liquid targets, which were combustible and easy to afford CO2 for the AMS measurements. The C-14 yield measured by the LSC method turned out to be 80 times larger than that by the AMS method. This large discrepancy may be attributed to the loss of C-14 atoms during the sample pretreatment in the AMS method such as combustion and cryogenic trapping of CO2. It means that C-14 newly produced by nuclear reactions can exist in various chemical forms, i.e., C3D6O, CO, CO2, hydrocarbons, etc., and a simple sample pretreatment right after production can cause serious isotopic fractionation. Therefore, using the AMS method, extreme caution in sample pretreatment should be exercised when the C-14 yield produced immediately by nuclear reaction is measured.