example: accelerator mass spectrometry (ams) · example: accelerator mass spectrometry (ams) how is...

Example: Accelerator mass spectrometry (AMS)

Today’s schedule:

• What is AMS used for?

• How does it work?

• Assignments: dead-line 21 December

26Al

10Be

14C36Cl

3H59Ni

• Archeology• Geology • Medicine• Food chemistry• Radiation protection• Ecology• Radioecology• Aerosol science• Pharmaceutical development• Bomb-pulse dating

Example: Accelerator mass spectrometry (AMS)

How is 14C created?

O2

O2

O2

O214CO2

14CO2

14CO2

14CO2 14C + O214CO2

14C 14N + -radiationT1/2=5730 years

Modern carbon 99% 12C1% 13C

10-10% 14C

Cosmic rays+

14N (nitrogen-14)14C (carbon-14)

60 000 million atoms

14 decays/minute1 gram of modern carbon 14C 14N + -radiation

”Modern”carbon:99% 12C1% 13C

10-10 % 14C

How to measure 14C? (T1/2=5730 years)

Decay measurement:• several days of measuring time• 1 g carbon

AMS measurement:• < 1 hour of measuring time• 10 µg - 1 mg carbon

Why not conventional mass spectrometry?

The 14C signal will drown in a background of interfering isobars

(ions with M=14, e.g. 13CH)!

BR=mv/q=(2mE)1/2/q

Two types of AMS systems

• Single Stage AMS (SSAMS): first commercial system installed in 2004 in Lund

The exercise is on SSAMS!

• Tandem AMS (usually called just AMS): developed in the late 1970ies

Ion source

Detector

Accelerator Mass Spectrometry (AMS)Tandem AMS

• Measures 14C/13C/12C: gives activity ( age)• Measuring time: ca 20 min/sample• Detection limit: <1 attomole (10–18 mole) 14C• Sample size: 10 g - 1 mg carbon

Counting atoms with tandem-AMS:refined mass spectrometryExample: a large 3 MV system

Ion sourcewith carbonsamples

3 MV tandem accelerator

Mass separator

Dipolmagnets

Velocityfilter

Particledetector

AMS removes interfering isobars!

Important properties:•Negative ion source: suppresses certain atomic isobars•Stripping process: breaks up molecules•High energy: every particle can be identified

14C3+

14Cq+,13CH+, 13CH2+

No 13CH3+

or higher!

14C–

13CH–

BR=mv/q=(2mE)1/2/q

12C–

13C–

14C–

13CH–

Removes electrons:•Changes charge state from negative to positive•Breaks up molecules

Ion source• Single-charged negative ions• High-intensity beam (current: tens of A)• Stable beam current• Fast switching between samples

– Multi-sample source• Low memory effect

Cs-sputtering ion source (solid samples), e.g. SNICS from NEC

(see www.pelletron.com)

Sample holders with carbon samples

Ion source wheel for 40 carbon samples

Cs sputtering ion source

Injector (from ion source to accelerator):

separating masses• Electrostatic lenses• High and reproducible transmission

– large-diameter vacuum tubes– spacious vacuum chambers inside the magnets– using as wide apertures as possible– excellent ion optics

• Injection– Most commonly: Sequential injection: Change ion

energy or magnetic field in the low-energy dipole magnet

– Simultaneous injection

Accelerator: breaking up

isobaric molecules

• High and reproducible transmission• Gas stripping preferred over foil

stripping• High vacuum• Very stable terminal voltage

Post-acceleration system:excluding molecular fragments

• Dipole magnet for selection of mass, energy and charge state

• Velocity filter

Aperture

+

-

E

B

Velocity filter (or cross-field analyser, or Wien filter)

Electrostatic field E applied at right angle to the beam and a magnetic field B orthogonal to both E and the beam.

For the undeflected particles the two forces must be identicalFE = FB

orqE = qvB

Velocity selector!

FB

FE

Alternative to velocity filter: The electrostatic analyser (ESA) –

Selection criteria: Energy over charge

An ESA consists of two parallel cylindrical or spherical conducting plates with a large potential difference. With a plate separation (d) and potential difference (∆U), an ion of charge q, kinetic energy T and velocity v follows a circular trajectory with radius r:

r = 2Td/q∆U

Particle detection

• Detectors: Silicon detectors, ionization chambers, time-of-flight systems, gas-filled magnets, X-ray detectors

Single Stage AMS at Lund University

Ion sources

ElectrostaticdeflectorSelects E/q

Mass selectionmagnet

12C / 13C / 14C

250 kV acceleration

Faraday cups

(off-axis)Measure 12C, 13C

ElectrostaticdeflectorSelects E/q

DetectorMeasures 14C

Gas Stripper

Breaks up molecules

DipolemagnetSelects

m Eq

• Measures 14C/13C/12C: gives specific activity• Measuring time: ca 2 - 20 min/sample• Detection limit: <1 attomole (10–18 mole) 14C• Sample size: 10 g - 1 mg carbon

Ion source

Electrostatic analyser

250 keV acceleration tube

Ar stripper

Control room

In the SSAMS system at the Lund University an ion beam consisting of carbon ionsshould be transported from the ion source to the detector. Carefully study thereferences on next page to answer the following questions:

1. What unit generates the ion beam in the SSAMS system and how does it work?

2. What units are used for steering and shaping the beam in the SSAMS system, and how do they work?

3. What units are used to discriminate between different ions in the SSAMS system (i.e. to separate the different isotopes and to remove molecular interferences)?

4. What other units are found along the path of the ions in the SSAMS system?

Exercise Single Stage Accelerator Mass Spectrometry (SSAMS)

Work in pairs, hand in a briefly written report by 21 December 2012 (or before!) to kristina.stenstrom@nuclear.lu.se.

The report will be graded.

Literature needed to complete the exercise:

• Slides from lecture

• A description of the Lund SSAMS system, including what units it consists of, can be found at:

GM Klody et al. “New results for Singe Stage Low Energy Carbon AMS” Nuclear Instruments and Methods B 240 (2005) 463-467.READ THIS CAREFULLY!

• Also read the following where you find more information about different electrostatic and magnetic devices used in accelerator systems:

K Stenström: Beam transport. Available at http://www.nuclear.lu.se/fileadmin/nuclear/beamtransport2011_02.pdf

READ THIS CAREFULLY!

• More information about AMS systems (e.g. how ion sources function can be found at):

http://www.pelletron.com/AMS.htm#SSAMS