3.5. Accelerator Mass Spectroscopy - AMS -

AMS is a method which counts radioactive particles (14C) rather than measuring the characteristic decay activity.

Comparison Traditional 14C dating and AMS

In 14C dating you count 14C activity

With AMS you count 14C number

)(5730

2ln)(

)()(

1414

1414

CNy

CA

CNCA

⋅=

⋅= λ

1.001.0)(14

−≈⋅

εεCN

Decay constant λ versus efficiency ε of device including ionization in sources and transmission in accelerator.

Comparison with traditional technique

6·105 14C particles in original sample

AMS

1000 cts/2min

500 cts/min

LSC

1000 cts/14y

1.4·10-4 cts/min

AMS is technically more demanding than a radiocarbon dating experiment with LSC, but it is more accurate, and requires smaller samples!

approximately 4 orders of magnitude improvement!!!

Methodsample

preparation oxidizer ion source detectorsystem

beam separatorand accelerator

system

complexorganicmolecules

C/CO2 C-- 14C3+

Summary C-beam productionThe carbon in the sample is converted to nearly pure carbon in the laboratory. The prepared sample is placed in an evacuated chamber, where it is bombarded with positive cesium ions (Cs+). Cesium lowers the work function of the material, allowing the release of negative carbon ions (C-). Because the N- ion is unstable, 14N does not interfere with 14C measurements. However, the molecular ions 12CH2

- and 13CH- are produced, and are accelerated with the 14C-. The accelerated ions encounter a position defining slit, which causes only a fine beam of ions to pass through entering the accelerator.

sample preparation

Mechanical methods to pulverize material to form a carbon pellet suitable for use in sputter source.

Alternative method is chemicallyseparating and oxydizing carbon to use CO2 with subsequent Cs charge exchange in ion source.

sample preparation needs experience!

Cs sputter ion source

Bombardment of sample pellet with Cs beam causes carbon atom or molecule release with charge exchange by pick up of electrons from cesium atoms which can easily be ionized.

simulation1; sputter source

12C, 13C, 14CCarbon moleculesHeavier junk particles

magnetic separation system

VBr

vmVq

Bvqrvm

⋅⋅

=

⋅⋅=⋅

⋅⋅=⋅

2qm

energy kinetic: 21 :energy ticelectrosta

force Lorenz : :force lcentripeta

22

2

2

B-field

radius raccelerating potential V

141412

2

12

14 93.01412;

1214 rrr

rr

⋅=⋅=⎟⎟⎠

⎞⎜⎜⎝

⎛=

Injector magnetThe ion beam from the source enters an injector magnet, which bends the beam. Heavier ions are bent less than lighter ones, because of higher momentum. The second slit is calibrated toonly allow ions of a certain mass to pass.

slits

simulation 2; injector magnet

12C, 13C, 14CCarbon moleculesHeavier junk particles

The acceleratorThe ions then enter the accelerator and are attracted to the high voltage in the terminal (>2 MV). The ions are accelerated to a sufficient velocity. As they traverse a gas canal they are stripped of some of their electrons. If the ion is a molecule, it breaks apart, eliminating abackgroundinterference. If it is an atom, it becomes positively charged and is accelerated towards the ground potential.

Inside the tank

The charging andacceleration system

The stripper

most likely charge statebetween q=2+ and 3+.

Total energy E=(q+1)·V

gas stripper or foil stripper ?

simulation 3; tandem accelerator

12C, 13C, 14CCarbon moleculesMolecule fragments (will be separated out by next dipole magnet)

detection systemThe ion beam, now positively charged (3+) passes through a position- defining slit to obtain a concentrated beam, containing minimal impurity interference. The beam then is subjected to a final magnet, separating the isotopes of carbon from the previously uniform beam. Two Faraday Cups and one 14C detector then measure the current of each of the separate beams. This provides information which can be utilized to obtainthe amount of 14C and its ratio in comparison to 12C and 13C.

simulation 4; analyzing system

r(12C)r(13C)

r(14C)

12C, 13C, 14C

Example: Magnetic SeparationAssume a V=2 MV tandem! What is the separation of 12C, 13C and 14C in the charge state q=3+ expressed in terms of radius r for a fixed magnetic field of B=1 Tesla?

VqEBqEm

r

mrBqE

VBr

kinkin

kin

)1(2

2;

2qm 22222

+=⋅⋅⋅

=

⋅⋅⋅

=⋅⋅

=

with m=A·1.67·10-27kg; q=|q|·1.6·10-19C; 1 eV = 1.6·10-19 J

A: mass number; |q|=3: charge; V=2: terminal voltage in MV

Separation in magnetic field

ABq

VAqr

TCJ

r

JJEkin

⋅=⋅

⋅⋅+⋅=

⋅⋅⋅⋅⋅⋅

=

⋅=⋅⋅⋅⋅=

−

−

−−

36.1)1(

144.0

1.0106.131028.1kgA·1.67·102

1028.1106.11024

19

1227-

12196

for 12C: r12=4.71 mfor 13C: r13=4.90 mfor 14C: r14=5.09 m

V: terminal voltage in MVB: magnetic field in Tq: electrical charge A: mass number

ΔE-E gas-counter system

U=+500V

Ug=+300V

is based on measurement of energy loss and total energy of incoming ions in gas.

separation and identification

Two-dimensionalgas counter spectrum for radiocarbon 14C analysis

With good separation and particle identification a nearly background free spectrum can be achieved. Potential background sources are roombackground radiation, cosmic rays, leakage of molecules.r(12C)

r(13C)

r(14C)

Counting efficiency and sample sizecounting efficiency is the fraction of 14C ions detected in the final detector from a sample put in the ion source.

For 14C: εc=1 %

Assume the previous 1 g piece of wood with 1.5·1010 14C atoms, this translates into a total number of counts Ndet=1.5·108 of 14C. (It takes about a week to sputter the sample completely away.)

samplec NN ⋅= εdet

Minimum sample size with 10% statistics: Ndet=102±10 cts of 14C Nsample(14C) =104 atoms of 14C,

Nsample(12C)=Nsample(14C)/1.3·10-12=7.7·1018 atoms of 12C12g has 6.023·1023 part, min. sample needs to be 0.15 mg.

Comparison again!

CofweekctsN

CNT

CNN

LSC

samplesampleLSC

1424

14

21

14

/102.210565730

2ln

)(2ln)(

−⋅=⋅⋅

=

⋅=⋅= λ

to accumulate 10% statistics with the same sample size requires 104 times longer counting time ≡ 180 years.

As claimed before 4 orders of magnitude improvement!(for 2 orders of magnitude increase in costs: 100k$→10M$)

How long does the traditional LSC technique take to analyze the same sample size with equally good statistics of 10%?

Applications of AMS

There is a rich field of applications for AMS due to the increased efficiency and accuracy of radiocarbon dating. It ranges from geology, hydrology, oceanology, climatology and environmental studies to history and archaeology. AMS is now also being used for a number of other radioisotopes to enhance the sensitivity of corresponding dating methods. The limitations are the possible background counts from isotopes in the same mass range which cannot be separated.