The old carbon project: how old is old?

The goal of the old carbon project is to measure the ratio of 14C to 12C in methane at levels below 10−18 by accelerator mass spectrometry (AMS). This research is chiefly motivated by the need for very low background raw materials (methane, petroleum) to manufacture scintillator fluids for large volume neutrino detectors, particularly for solar neutrinos. The 14C activity in such material, introduced by the radioactivity of the reservoir rocks, cosmic rays or later handling, limits the low-energy sensitivity of the neutrino detector. This paper reviews the scintillator requirements for low-energy neutrino observation in terms of the 14C/12C ratio, as well as earlier AMS and decay counting measurements of this ratio at the 10−18 level. Recent experiments to determine the limitations of the heavy element line on the IsoTrace spectrometer for these ratio measurements are reported; analysis of the data obtained to date indicate a maximum interference limit of 14C/12C≈10−19. This progress report will also mention some methods for reducing this interference further.