3.5. Accelerator Mass Spectroscopy -AMS -

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1 3.5. Accelerator Mass Spectroscopy -AMS - AMS is a method which counts radioactive particles ( 14 C) rather than measuring the characteristic decay activity.

2 Comparison Traditional 14 C dating and AMS In 14 C dating you count 14 C activity With AMS you count 14 C number A( A( C) C) λ N( ln y C) N( 14 C) N( 14 C) ε ε Decay constant λ versus efficiency ε of device including ionization in sources and transmission in accelerator.

3 Comparison with traditional technique AMS is technically more demanding than a radiocarbon dating experiment with LSC, but it is more accurate, and requires smaller samples! C particles in original sample AMS LSC 1000 cts/min 1000 cts/14y 500 cts/min cts/min approximately 4 orders of magnitude improvement!!!

4 Method sample preparation oxidizer ion source beam separator and accelerator system detector system complex organic molecules C/CO C C 3+

5 Summary C-beam production The 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, 14 N does not interfere with 14 C measurements. However, the molecular ions 1 CH - and 13 CH - are produced, and are accelerated with the 14 C -. The accelerated ions encounter a position defining slit, which causes only a fine beam of ions to pass through entering the accelerator.

6 sample preparation Mechanical methods to pulverize material to form Alternative method is chemically a carbon pellet suitable separating and oxydizing carbon for use in sputter source. to use CO with subsequent Cs charge exchange in ion source. sample preparation needs experience!

7 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.

8 simulation1; sputter source 1 C, 13 C, 14 C Carbon molecules Heavier junk particles

9 magnetic separation system B-field accelerating potential V radius r centripetal force : electrostatic energy : v m q v B r 1 q V m v r B V m q 14 1 : Lorenz force : kinetic energy r r 1 14 ; r1 r r 14

10 Injector magnet The 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 to only allow ions of a certain mass to pass. slits

11 simulation ; injector magnet 1 C, 13 C, 14 C Carbon molecules Heavier junk particles

12 The accelerator The ions then enter the accelerator and are attracted to the high voltage in the terminal (> 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 a background interference. If it is an atom, it becomes positively charged and is accelerated towards the ground potential.

13 Inside the tank The charging and acceleration system

14 The stripper gas stripper or foil stripper? most likely charge state between q + and 3 +. Total energy E(q+1) V

15 simulation 3; tandem accelerator 1 C, 13 C, 14 C Carbon molecules Molecule fragments (will be separated out by next dipole magnet)

16 detection system The 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 14 C detector then measure the current of each of the separate beams. This provides information which can be utilized to obtain the amount of 14 C and its ratio in comparison to 1 C and 13 C.

17 simulation 4; analyzing system r( 1 C) r( 13 C) r( 14 C) 1 C, 13 C, 14 C

18 Example: Magnetic Separation Assume a V MV tandem! What is the separation of 1 C, 13 C and 14 C in the charge state q3 + expressed in terms of radius r for a fixed magnetic field of B1 Tesla? V q E B q E m r m r B q E V B r kin kin kin ) (1 ; q m + with ma kg; q q C; 1 ev J A: mass number; q 3: charge; V: terminal voltage in MV

19 Separation in magnetic field E kin J J r A kg C 0.1 T 10 1 J r ( q + 1) q B A V 1.36 A V: terminal voltage in MV B: magnetic field in T q: electrical charge A: mass number for 1 C: r m for 13 C: r m for 14 C: r m

20 ΔE-E gas-counter system is based on measurement of energy loss and total energy of incoming ions in gas. U+500V U g +300V

21 separation and identification r( 14 C) r( 13 C) r( 1 C) With good separation and particle identification a nearly background free spectrum can be achieved. Potential background sources are room background radiation, cosmic rays, leakage of molecules. Two-dimensional gas counter spectrum for radiocarbon 14 C analysis

22 Counting efficiency and sample size counting efficiency is the fraction of 14 C ions detected in the final detector from a sample put in the ion source. For 14 C: ε c 1 % N det ε c N sample Assume the previous 1 g piece of wood with C atoms, this translates into a total number of counts N det of 14 C. (It takes about a week to sputter the sample completely away.) Minimum sample size with 10% statistics: N det 10 ±10 cts of 14 C N sample ( 14 C) 10 4 atoms of 14 C, N sample ( 1 C)N sample ( 14 C)/ atoms of 1 C 1g has part, min. sample needs to be 0.15 mg.

23 Comparison again! How long does the traditional LSC technique take to analyze the same sample size with equally good statistics of 10%? N LSC λ N sample ( 14 C) ln T 1 N sample ( 14 C) N LSC ln cts / week of 14 C to accumulate 10% statistics with the same sample size requires 10 4 times longer counting time 180 years. As claimed before 4 orders of magnitude improvement! (for orders of magnitude increase in costs: 100k$ 10M$)

24 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.