ION SELECTION IN
ACCELERATOR MASS SPECTROMETER
BINP SB RAS
V. V. Parkhomchuk,
S.A. Rastigeev
BINP, Novosibirsk, Russia.
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
For modern carbon
12C/ 14C ~1012
The amount of 14C for dead samples
decreases through decay with a half-life of 5730 years.
Atomic and molecular isobars of
radiocarbon
• 14N
• 13CH, 12CH2
m/dm=84000
m/dm ~1000
(About 108 molecular isobars
for each negative radiocarbon ion in modern samples)
Mass spectrum of the modern sample
low energy spectrum
11 orders of magnitude
at the exit of
AMS facility
Electrostatic filter
E/q
M/q
Magnetic filter
ME/q2
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.
AMS method
is based on the direct rare isotope counting
•
•
•
•
•
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
•negative 14N ions not stable
• stripping destroys molecules
BINP AMS facility
Basic features of BINP AMS facility
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
• 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
recharging in accelerator tubes
time-of-flight detector
ToF spectrum at the exit of AMS
14C
a) modern sample (carbon fabric) ,
b) “dead” sample (graphite MPG)
2D ToF spectrum at the exit of AMS
a) modern sample (carbon fabric) ,
b) “dead” sample (graphite MPG)
16O
background
a) 16O background,
b) 16O
carbon background
a) 13C background, b)
12C
background
14N
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.
Moreover, the magnesium vapors stripper had no the
observable influence on vacuum condition
14N
a)
background
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.
2D TOF spectrum
Modern sample
14C/12C~10-12
“dead” sample
(graphite MPG)
14C/12C~2*10-15