RADIOCARBON ANALYSIS OF SAMPLES ON 1 MV AMS SPECTROMETER AT ION CHARGE STATE 3+ S.A. Rastigeev , A.R Frolov, A.D. Goncharov, V.F. Klyuev, E.S. Konstantinov, V. V. Parkhomchuk,A.V. Petrozhitskii, BINP,NSU, Novosibirsk, Russia.

BINP AMS facility The AMS is mainly dedicated for radiocarbon dating of archaeological and geological samples, for biomedical, environmental and climatological applications by measurements of the ratio between carbon isotopes. AMS can be used for many others applications. The ratio between isotopes in sample can be less than 10-15. So, the counting methods are used for detection of such low radiocarbon concentration.

Basic features of BINP AMS facility Schematically stages: The ion extraction from the sample The rejection of the primary isotopes The beam acceleration The rejection of the isobaric ions The rare isotope counting The ion energy selection just after molecular destruction  effective filtration of the molecular fragments, because energy of fragments always less then ion energy (at this moment). The magnesium vapor target as a molecule destroyer  localized molecular destruction 2D time of flight detector  accurate recognition of each ion

Charge state distribution After 1MeV carbon beam is passed through a target, the ions of three charges are formed in the main: 1+, 2+ and 3+ in a percentage ratio of approximately 30%, 50% and 20%, respectively.

Molecular isobars for radiocarbon ions. About 108 molecular isobars for each negative radiocarbon ion in modern samples

Molecular isobars problem for 1+ and 2+ selection The molecular background can be suppressed by many orders of magnitude by the stripping process. 14M2+ The efficiency of the destruction of molecular ions increases with increasing thickness of the charge-exchange target. However, the increase in target thickness is limited by a number of effects worsening the efficiency of selection, such as: energy loss and scattering of ions on a thick target or deterioration of vacuum in accelerating tubes leading to parasitic charge exchange and ion scattering in the accelerating field region. In natural samples, the content of impurities (H, Li, B, N, O et al.) can radically differ.

(7Li+)2 problem for 2+ charge selection The increase in the thickness of charge-exchange target does not reduce this background, since the lithium ions are already in the atomic state. In natural samples, the content of impurities (H, Li, B, N, O et al.) can radically differ.

3+ charge selection The absence of molecular isobars allows to achieve complete ion selection by the difference of ion trajectories in electromagnetic fields. Ion selection can be achieved for any natural samples by reliable filtration of molecular ion fragments.

The influence of HE magnet on ion selection. Counts rate of ions by TOF detector depending on the current of the magnet located just before the detector.

The influence of 1800 terminal bend on ion selection. Counts rate of ions by TOF detector depending on the 1800 HV terminal bend voltage.

The influence of energy spread of the ion source on ion selection. Counts rate (a) and time-of-flight (b) of ions registered by TOF detector depending on the voltage of ion source.

The ion flight time spectrum for modern and dead samples. 10-12 (14C/12C) 10-16 (14C/12C) Radiocarbon concentration in the modern sample (1) and graphite MPG (2).

Sample preparation and sputtering Cs ionizator Pressed samples in sample wheel. For AMS analysis, all samples must be converted to so-called “graphite”. For these purposes, a sample is combusted in vacuum. Then the carbon from formed CO2 gas catalytically deposited on iron powder. The Fe-C mixture is pressed in aluminum sample holder (cathode for sputter ion source) for AMS analysis. 

Algorithm for measuring of the radiocarbon concentration on BINP AMS The cycle of AMS-analysis of samples is represented as follows. For each sample, the 14C ions are counted four times (10 seconds each) and twice the 13C currents are measured. After that, the samples wheel is turned to the next sample for process repetition. Measuring of whole graphitized sample wheel (20 samples) takes about 15 minutes. For a set of statistics the wheel are moving to the second turn, third, etc. Typically, the measurement will take approximately 5 hours, with a statistical error of measurement for modern samples les then 1%. The process of isotope measuring and sample changing (wheel rotation) is fully automated.

The typical radiocarbon analysis of the prepared samples

The radiocarbon age of peat sediments (Novosibirsk region) , depending on the depth from surface level. Such analyzes are necessary to obtain a timescale for sediments. Such results are quite revealing, since in the absence of mixings, deposits should be observed dependence - the deeper the ancient.

Use AMS for biomedical studies Low-concentrated 104/cm3 14C aerosol was inhaled in mice. The aerosol was directly registered in mice lungs, liver, kidneys and brain by BINP AMS.

Old fauna of Russia dated by BINP AMS 781 ± 65 years 15809 ± 138 years 19034 ± 165 years 5281 ± 77 years 6865 ± 89 years 3813 ± 72 years 5190 ± 80 years 4775 ± 75 years 18141 ± 169 years 3161 ± 78 years All the photos were taken from: ru.wikipedia.org

Canvas from A.S. Popov museum, dating by AMS. Radiocarbon age: Canvas sample 2 : 203 ± 49 years Canvas sample 3 : 544 ± 36 years ??? Canvas sample 4 : 213 ± 40 years Canvas sample 5 : 176 ± 40 years Canvas sample 6 : 134 ± 39 years Mean value: 180 ± 21 years Most likely: 1730-1809 AD

SUMMARY When carrying out the radiocarbon analysis of the samples on a 1MV AMS spectrometer with the separation of the charge state 3+, the influence of the isobar background in the initial natural samples on the dating results is significantly reduced, but the counting efficiency of the radiocarbon ions decreases approximately by a factor of 2. It is shown, that a reliable separation of the radiocarbon ions and the accompanying background ions is carried out at the BINP AMS complex. Over the last year, about 1000 samples were analyzed at BINP AMS.