V. V. Parkhomchuk, S.A. Rastigeev

BINP, Novosibirsk, Russia.

ION SELECTION IN ACCELERATOR MASS SPECTROMETER

BINP SB RAS

The BINP AMS is mainly dedicated for dating of archaeological and geological samples

by measurements

of the ratio between carbon isotopes.

AMS can be used for many others applications.

isotopic abundance ratio in carbon samples

13C/ 14C ~1010

The amount of 14C for dead samples decreases through decay with a half-life of 5730 years.

12C/ 14C ~1012

For modern carbon

Atomic and molecular isobars of radiocarbon

• 14N m/dm=84000

• 13CH, 12CH2 m/dm ~1000

(About 108 molecular isobars for each negative radiocarbon ion in modern samples)

Mass spectrum of the modern sample

low energy spectrum

at the exit of AMS facility

11 orders of magnitude

Electrostatic filter E/q

Magnetic filter ME/q2 M/q

Problems:The scattering and charge exchange processes

allow the unwanted particles to pass through electrostatic and magnetic filters. The ions can interact with molecules

of residual gas and parts of vacuum chamber.

• 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

AMS method is based on the direct rare isotope counting

•negative 14N ions not stable• stripping destroys molecules

BINP AMS facility

Basic features of BINP AMS facility

• 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

1 - pressure tank, 2 - accelerator tube,

3 - cascade generator, 4 - middle energy electrostatic filter,

5 - magnesium vapors stripper, 6 - ion source,

7 - low-energy magnetic analyzer,

8- high-energy magnetic analyzer

recharging in accelerator tubes

time-of-flight detector

ToF spectrum at the exit of AMS

a) modern sample (carbon fabric) , b) “dead” sample (graphite MPG)

14C

2D ToF spectrum at the exit of AMS

a) modern sample (carbon fabric) , b) “dead” sample (graphite MPG)

a) 16O background, b) 16O

16O background

carbon background

a) 13C background, b) 12C background

Without tandem terminal filter:

With tandem terminal filter:The 14N and 14C ions energy have the same values at the exit of accelerator only after five steps recharging process.

14N background

Moreover, the magnesium vapors stripper had no the

observable influence on vacuum condition

14N background

a) 14N background for BINP AMS,

b) 14N background

SUMMARY

• The effective suppression of 14N background in BINP AMS is demonstrated.

• Shown that the influence of ion background in the measured radiocarbon concentration of 14C/12C ~ 2 * 10-15 (graphite MPG) is insignificant.

Modern sample14C/12C~10-12

“dead” sample(graphite MPG) 14C/12C~2*10-15

2D TOF spectrum