Example: Accelerator mass spectrometry (AMS)

Today’s schedule:

• What is AMS used for?

• How does it work?

• Assignments: dead-line 21 December Example: Accelerator mass spectrometry (AMS) • Archeology • Geology • Medicine • Food chemistry 3H 14C 36 • Radiation protection Cl • Ecology 10 Be 59Ni • Radioecology • Aerosol science 26Al • Pharmaceutical development • Bomb-pulse dating How is 14C created?

Cosmic rays 14 14 + C (carbon-14) Modern carbon O2 CO2 14 99% 12C N (nitrogen-14) 1% 13C 14CO 10-10% 14C 14CO O2 2 14 14 2 C + O2 CO2 O2 14 CO2

O2

14C 14N + -radiation

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

”Modern”carbon: 99% 12C Decay measurement: • several days of measuring time 13 1% C • 1 g carbon 10-10 % 14C

14 decays/minute 1 gram of modern carbon 14C 14N + -radiation 60 000 million atoms

AMS measurement: • < 1 hour of measuring time • 10 µg - 1 mg carbon Why not conventional mass spectrometry?

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

The 14C signal will drown in a background of interfering isobars (ions with M=14, e.g. 13CH)! Two types of AMS systems

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

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

The exercise is on SSAMS! Accelerator Mass Spectrometry (AMS) Tandem AMS

Ion source

Detector Counting atoms with tandem-AMS: refined mass spectrometry Example: a large 3 MV system

3 MV Ion source tandem accelerator with carbon Velocity samples filter Particle Dipolmagnets detector

Mass separator • 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 AMS removes interfering isobars! BR=mv/q=(2mE)1/2/q

14C3+

12 – C 14Cq+, 13 – 14 – C C 13CH+, 13CH2+ 14 – 13 – C CH No 13CH3+ 13 – CH or higher!

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

Important properties: •Negative ion source: suppresses certain atomic isobars •Stripping process: breaks up molecules •High energy: every particle can be identified 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) Ion source wheel for 40 carbon samples

Sample holders with 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 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. Aperture

+ E FB

F B E -

For the undeflected particles the two forces must be identical

FE = FB or qE = qvB Velocity selector! 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

Dipole Gas 250 kV magnet Stripper acceleration Selects Breaks up molecules mE q Mass selection magnet 12C / 13C / 14C

Faraday cups Electrostatic (off-axis) deflector Measure 12C, 13C Selects E/q

Electrostatic Detector deflector Measures 14C Selects E/q Ion sources • 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 Ar stripper 250 keV acceleration tube

Electrostatic analyser Ion source Control room 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.

In the SSAMS system at the Lund University an ion beam consisting of carbon ions should be transported from the ion source to the detector. Carefully study the references 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? 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