1 / 29

Isobarentrennung bei Teilchenenergien unterhalb 1 MeV/amu mit einem TOF Detektor

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

gino
Download Presentation

Isobarentrennung bei Teilchenenergien unterhalb 1 MeV/amu mit einem TOF Detektor

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. Tandem-AMS: Measurement principle

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

  4. 36Cl vs. 36S: stopping power Stopping Power bei Ein = 18 MeV

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

  6. The TOF Detector

  7. — — Calculated separationof36Cl (radionuclide) – 36S (stable isobar) 15 10 Residual Energy [MeV] Energy loss in silicon nitride 18 MeV initial energy Measured at VERA 5 Thickness of silicon nitride layer [µg/cm2] 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, 1999 -- Volume 475/1, pp 596-599

  8. 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

  9. 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.).

  10. Advantages of higher energy • Beam emittance smaller: E0.5 • Small angle scattering smaller: E1 • Relative energy straggling smaller: E ~ E0.5, • however: (E/E) ~ E0.5

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

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

  13. 36Cl: angular scatter for different energies

  14. 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

  15. TOF at VERA

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

  17. Silicon nitride foils for energy loss • To reduce compressive stress: not stochiometric Si3N4, but ~Si1.0N1.1 • (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 219-220(2004)415-419: Si3N3.1H0.06 • More (physical) straggling and scattering than carbon foils. • Much more homogenous. D.R. Ciarlo, Biomedical Microdevices 4:1(2002)63-68

  18. Silicon nitride foils have no energy loss tails

  19. Separation of 36Cl and 36S at 28 MeV

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

  21. Post stripping with Q3D

  22. Silicon nitride foils (6 µm) with Q3D 175 MeV 176Yb23+ 176Hf23+ Hf suppression: 76 176Yb24+ 76106 counts 176Hf22+ 176Hf counts 1001 counts Position along focal plane [arb. units] Energy/Charge lower higher

  23. DTOF - isobar separation at ~200 MeV13 MV tandem accelerator in Munich

  24. DTOF - isobar separation at 200 MeV13 MV tandem accelerator in Munich No tails! Short TOF High energy Long TOF Low energy

  25. DTOF - isobar separation at 175 MeV13 MV tandem accelerator in Munich 176Yb 176Hf

  26. 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 41Ca and 36Cl. At large tandems, long-lived natural radioisotopes can be tackled which were not yet accessible by AMS at all.

  27. Isobar suppression with energy loss foils • Separation of32Si/32S (18 MeV) with • carbon foils: ~105 • 2.9 MV terminal • voltage Fig. 2. Overlay of ESA scans for silicon and sulfur ion beams after energy degradation through a 100 µg/cm2 carbon foil. The dotted lines represent the slit width allowing the silicon beam into the spectrograph. D.J. Treacy Jr. et al., Nucl. Instr. and Meth. in Phys. Res. B 172(2000)321-327

  28. Stable isobar suppression • 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.

  29. Achievable energy with charge stateswith more than 5% yield Using the formula of Sayer et al., 1977 10Be carbon foil —, gas - - - 36Cl carbon foil —, gas - - - 182Hf carbon foil —, gas - - - E~U1.3 12+ 10+ Energy achieved E [MV] 8+ 11+ 10+ 9+ 4+ 8+ 7+ 3+ Terminal voltage U[MV]

More Related