Current dependent isotope fractionation of radiocarbon measurements

Current dependent fractionation is a limiting factor in the accuracy and precision of radiocarbon AMS measurements. This undesirable effect is present to some degree in all AMS systems and, most likely, has similar causes. Although the effect is more pronounced at higher ion currents (actual values depending on particulars of each system), it can also be a problem when comparing samples and standards yielding significantly different currents. The National Laboratory for Age Determination in Trondheim has a compact multi-element AMS system model 4110Bo from High Voltage Engineering based on a 1MV tandetron. The system has sequential injection and is representative of many recent AMS systems. It shows a more pronounced isotopic fractionation for extracted 12C ion currents of more than 40 µA. We present here the results of our investigation of the causes of isotopic fractionation. For this, the effect of the different ion optical elements of the system, mainly at the injection, on the three carbon isotopes were studied for different ion beam intensities. The coupling between several elements was also tackled. The first priority of this study being to avoid situations that increase isotopic fractionation and, eventually, to improve the AMS system.