View
215
Download
0
Category
Preview:
Citation preview
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)
14C26Al
129I
210Pb
236U
244Pu
AMS isotopes where stable isobar suppression is needed
10Be
36Cl
41Ca55Mn
60Fe
146Sm
182Hf
36Cl vs. 36S: stopping power
Stopping Power bei Ein = 18 MeV
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
The TOF Detector
Res
idua
l Ene
rgy
[MeV
]S
epar
atio
n /
of s
trag
glin
g
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
5
10
15—
—
Thickness of silicon nitride layer [µg/cm2]
3617 Cl
3616 S
Energy loss in siliconnitride18 MeV initial energy
Measured at VERAMeasured at VERA
Calculated separationof
36Cl (radionuclide) – 36S (stable isobar)
Comparison to other methodsE 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: E0.5
• Small angle scattering smaller: E1
• Relative energy straggling smaller: E ~ E0.5, however: (E/E) ~ E0.5
Facilities used for AMS
15 MV TandemTU and LMU München
Germany
3 MV TandemUniversität Wien
Austria
0.5 MV TandemETH ZürichSwitzerlandS
mal
l
Big
Calculated separation of 36Cl – 36Sfor different terminal voltages
Terminal voltage [MV]
Se
para
tion
/
of s
tra
ggl
ing
carbon foil —, gas - - -
36Cl: 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 36Cl and 36S (28 MeV) after various SiN foil thicknesses
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, 55 mm• amorphous (i.e. no channeling)• Döbeli et al., NIM B 219-220(2004)415-419: Si3N3.1H0.06
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 36Cl and 36S at 28 MeV
TOF at a big tandem
15 MV TandemTU and LMU München
Germany
Separation of 182Hf from 182Wat 200 MeV
Post stripping with Q3D
thick foil
Silicon nitride foils (6 µm) with Q3D
176Hf23+176Yb23+
176Hf22+
Position along focal plane [arb. units]
Energy/Chargehigher lower
176 H
f cou
nts
1001counts
76106counts
176Yb24+Hf suppression: 76
175 MeV
TOF - isobar separation at ~200 MeV13 MV tandem accelerator in Munich
TOF 3.5 m Ionization chamber
TOF - isobar separation at 200 MeV13 MV tandem accelerator in Munich
Long TOFLow energy
Short TOFHigh energy
No tails!
TOF - isobar separation at 175 MeV13 MV tandem accelerator in Munich
176Yb
176Hf
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.
Isobar suppression with energy loss foils
D.J. Treacy Jr. et al.,Nucl. Instr. and Meth. in Phys. Res. B 172(2000)321-327
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.
• Separation of 32Si/32S (18 MeV) with carbon foils: ~105
• 2.9 MV terminal voltage
Standard methods use different energy loss whenions pass through matter (gas, foils):
• Active measurement of energy loss (ionization chamber)• Energy measurement after passive absorber
Physical limitations:• Energy straggling: (E/E) ~ E0.5
• Small angle scattering: E1
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 stateswith more than 5% yield
10Be carbon foil —, gas - - - 36Cl carbon foil —, gas - - - 182Hf carbon foil —, gas - - -
Terminal voltage U [MV]
Ene
rgy
achi
eved
E [M
V] E~U1.3
Using the formula of Sayer et al., 1977
3+
4+
10+
12+
11+
10+
9+
8+7+
8+
Recommended