Isobarentrennung bei Teilchenenergien unterhalb 1 MeV/amu mit einem  TOF Detektor Peter Steier, Robin Golser, Walter Kutschera, Alfred Priller, Christof Vockenhuber, Katharina Vorderwinkler, Anton Wallner Institut für Isotopenforschung und Kernphysik der Universität Wien, Währinger Straße 17, A-1090 Wien, Österreich 55. Jahrestagung der Österreichischen Physikalischen Gesellschaft, Wien, 27. September 2005

Tandem-AMS: Measurement principle

AMS Isotopes AMS isotopes where stable isobar suppression is not needed (no stable isobar or stable isobar does not form negative ions) 14 C 26 Al 129 I 210 Pb 236 U 244 Pu AMS isotopes where stable isobar suppression is needed 10 Be 36 Cl 41 Ca 55 Mn 60 Fe 146 Sm 182 Hf

36 Cl vs. 36 S: stopping power Stopping Power bei E in = 18 MeV

Isobar identification with a particle detector Energy required:  1 MeV/amu Ionization Chamber From: Finkel and Suter Advances in Analytical Geochemistry 1 (1993) 1-114

The  TOF Detector

Residual Energy [MeV] Separation /  of straggling Simulation using a Mathematica™ package from Robert A. Weller, “General purpose computational tools for simulation and analysis of medium-energy backscattering spectra”, AIP Conference Proceedings -- June 10, Volume 475/1, pp — —— — Thickness of silicon nitride layer [µg/cm 2 ] Energy loss in silicon nitride 18 MeV initial energy Measured at VERA Calculated separation of 36 Cl (radionuclide) – 36 S (stable isobar)

Comparison to other methods  E with ionization chamber TOF has a better energy resolution TOF can handle higher background count rates “Post stripping”, i.e. electrostatic or magnetic separation after energy-loss foil Post stripping can suppress the isobar, i.e. reduce background count rate in detector. TOF can use all charge states: higher efficiency possible Gas filled magnet Gas filled magnet suppresses isobars Gas filled magnet can use all charge states Charge state fluctuations and angular scattering deteriorate resolution. Full stripping Extreme energies needed Inverse PIXE, i.e. characteristic X rays of projectile Some tests, low efficiency, not yet fully explored

Why we use  TOF for our measurements Energy resolution of  TOF can be made arbitrarily high by longer flight path. Physical limitations (energy straggling) can be studied without interfering technical limitations (detector noise, etc.).

Advantages of higher energy Beam emittance smaller: E  0.5 Small angle scattering smaller: E  1 Relative energy straggling smaller:  E ~ E 0.5, however: (  E/E) ~ E  0.5

Facilities used for AMS 15 MV Tandem TU and LMU München Germany 3 MV Tandem Universität Wien Austria 0.5 MV Tandem ETH Zürich Switzerland Small  Big

Calculated separation of 36 Cl – 36 S for different terminal voltages Terminal voltage [MV] Separation /  of straggling carbon foil —, gas - - -

36 Cl: angular scatter for different energies

Disadvantages of large tandems More charge state ambiguities Lower yield of the individual charge states Large machines are more complex About half of all AMS facilities are based on 2-3 MV tandems

 TOF at VERA

Separation of 36 Cl and 36 S (28 MeV) after various SiN foil thicknesses

Silicon nitride foils for energy loss To reduce compressive stress: not stochiometric Si 3 N 4, but ~Si 1.0 N 1.1 (density: 3.4 instead of 3.44) Silson Ltd, Northampton, England: 50 to 1000 nm, 5  5 mm amorphous (i.e. no channeling) Döbeli et al., NIM B (2004) : Si 3 N 3.1 H 0.06 D.R. Ciarlo, Biomedical Microdevices 4:1(2002)63-68 More (physical) straggling and scattering than carbon foils. Much more homogenous.

Silicon nitride foils have no energy loss tails

Separation of 36 Cl and 36 S at 28 MeV

 TOF at a big tandem 15 MV Tandem TU and LMU München Germany Separation of 182 Hf from 182 W at 200 MeV

Post stripping with Q3D

Silicon nitride foils (6 µm) with Q3D 176 Hf Yb Hf 22+ Position along focal plane [arb. units] Energy/Charge higher lower 176 Hf counts 1001 counts counts 176 Yb 24+ Hf suppression: MeV

 TOF - isobar separation at ~200 MeV 13 MV tandem accelerator in Munich

 TOF - isobar separation at 200 MeV 13 MV tandem accelerator in Munich Long TOF Low energy Short TOF High energy No tails!

 TOF - isobar separation at 175 MeV 13 MV tandem accelerator in Munich 176 Yb 176 Hf

Conclusions  TOF allows to exploit the energy loss difference for isobars to the physical limit imposed by energy straggling (however on the cost of efficiency losses due to straggling). Foils of sufficient homogeneity exist, produced from silicon nitride. For AMS with 3-MV tandems, suppression of stable isobars is possible for 41 Ca and 36 Cl. At large tandems, long-lived natural radioisotopes can be tackled which were not yet accessible by AMS at all.

Isobar suppression with energy loss foils D.J. Treacy Jr. et al., Nucl. Instr. and Meth. in Phys. Res. B 172(2000) Fig. 2. Overlay of ESA scans for silicon and sulfur ion beams after energy degradation through a 100 µg/cm 2 carbon foil. The dotted lines represent the slit width allowing the silicon beam into the spectrograph. Separation of 32 Si/ 32 S (18 MeV) with carbon foils: ~ MV terminal voltage

Standard methods use different energy loss when ions pass through matter (gas, foils): Active measurement of energy loss (ionization chamber) Energy measurement after passive absorber Physical limitations: Energy straggling: (  E/E ) ~ E  0.5 Small angle scattering: E  1 Technical limitations: Inhomogeneities of foils produce additional energy straggling and low energy tails. Electronic noise, incomplete charge collection, etc. Stable isobar suppression

Achievable energy with charge states with more than 5% yield 10 Be carbon foil —, gas Cl carbon foil —, gas Hf carbon foil —, gas Terminal voltage U [MV] Energy achieved E [MV] E~U 1.3 Using the formula of Sayer et al.,