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V. V. Parkhomchuk, S.A. Rastigeev BINP, Novosibirsk, Russia. PowerPoint PPT Presentation


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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.

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V. V. Parkhomchuk, S.A. Rastigeev BINP, Novosibirsk, Russia.

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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 m/dm=84000

  • 13CH, 12CH2 m/dm ~1000

    (About 108molecular isobars

    for each negative radiocarbon ion in modern samples)


Mass spectrum of the modern sample

low energy spectrum

11orders of magnitude

at the exit of

AMS facility


Electrostatic filter

E/q

M/q

ME/q2

Magnetic filter

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

  • 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

14C

  • modern sample (carbon fabric),

  • “dead” sample (graphite MPG)


2D ToF spectrum at the exit of AMS

  • modern sample (carbon fabric),

  • “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 background

  • 14N background for BINP AMS,

  • 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 sample14C/12C~10-12

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


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