Example: Accelerator mass spectrometry (AMS) · Example: Accelerator mass spectrometry (AMS) How is...
Transcript of 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 [email protected].
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