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Author
Date
2024Type
- Doctoral Thesis
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Abstract
A prototype of a compact, low-energy, multi-isotope accelerator mass spectrometer (MILEA) with a maximal terminal voltage of 0.3 MV was built at ETH Zurich in 2017. The presented work eval- uates the capabilities of the new system and thereby investigates ion beam physics at low energies. The findings of this work allowed to optimize the operational parameters for AMS measurements of 10Be, 26Al and uranium at low energies (<1 MeV). The performance of the instrument in terms of measurement stability and -background as well as ionization- and transport efficiency was evaluated.
Detailed investigations into the charge state yields of beryllium at low energies revealed a second maximum of Be2+ at stripping energies below the maximum for the 1+ charge state. This new finding allows for efficient 10Be2+ measurements at a terminal voltage of 220 kV with a beam transmission through the He gas stripper of over 40%. The isobar 10B is effectively suppressed by a degrader foil, while a 23% transmission of 10Be on the high energy (HE) side can be maintained. Further detailed investigations into the break-up of molecular BeO− in the stripper allowed to explain the beam profiles recorded on the high energy side, and allowed to propose an adjustments of the installed degrader foil for a relative improvement of 9% in the HE beam transport. Investigations into different sample matrices showed that a ionisation efficiency of 1% can be achieved after one hour of sputter- ing time by an 1:1 mixture of BeO with Nb by weight. That allowed for efficient measurements of 10Be samples with an overall efficiency of 0.9‰ (10Be detected / 10Be in the sample). The measure- ment precision for 10Be was determined at 0.6% for samples that are not limited by counting statistics.
For 26Al measurements the 2+ charge state is selected with a charge state yield of 60% at a strip-
ping energy of 328.5 keV. Measurements of the molecular dissociation cross-section are performed
that determined the necessary stripper density for routine measurements at 7.7×1016 at/cm2. At
this stripper density 55% transmission through the He stripper is observed. The m/q interference
from 13C1+ is suppressed in a passive gas absorber cell that is located in front of the detector. The
performance of the absorber is compared to stopping power simulations where significant deviations
in the projected range of Al are present. A possible explanation includes Z -oscillation in the real
stopping power at low energies. The transport efficiency of Al from the ion-source to the detector was determined at 48%. The ionization efficiency was determined at 0.8‰ (4 hours of sputtering time). That equates to an overall efficiency of 0.36‰. The measurement precision for 26Al was determined at 0.3% for samples that are not limited by counting statistics.
The measurement of charge state yields of uranium revealed 42% yield of U3+ at a stripping energy of 286.7 keV. However, the necessary stripper density of 1.7×1016 at/cm2 to remove interfering molecules and the deployment of narrow apertures to achieve an abundance sensitivity below 5×10−13 reduces the beam transmission through the stripper to 31%. A HE transport efficiency of 80% is determined that further reduces to an total transport efficiency to 25% (from ion-source to detector). For sputtering times of 10 h an ionization efficiency of 4% is determined. Therefore, the overall efficiency is found to be 1% (atoms detected per atoms in a sample). Isotopic ratios of uranium standard material can be measured with a final precision of 0.4% (0.3%) for the 233U/238U and 236U/238U ratio (235U/238U ratio). Show more
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https://doi.org/10.3929/ethz-b-000671466Publication status
publishedExternal links
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Contributors
Examiner: Synal, Hans-Arno
Examiner: Christl, Marcus
Examiner: Kirch, Klaus Stefan
Examiner: Steier, Peter
Publisher
ETH ZurichSubject
ION BEAMS (ELECTRODYNAMICS); ION BEAM TECHNOLOGY (ELECTRODYNAMICS); CALIBRATION AND TESTING OF INSTRUMENTS (PHYSICS); IONENSTRAHLEN + IONENOPTIK (ELEKTRODYNAMIK); MASS SPECTROMETERS (PARTICLE ACCELERATORS); EICHUNG UND PRÜFUNG VON MESSGERÄTEN (PHYSIK); TANDEMBESCHLEUNIGER + KASKADENBESCHLEUNIGER (TEILCHENBESCHLEUNIGER); MASSENSPEKTROMETER (TEILCHENBESCHLEUNIGER); TANDEM ACCELERATORS + CASCADE ACCELERATORS (PARTICLE ACCELERATORS); AMS; AMS facility report; Aluminium-26; Uranium; Beryllium-10; Charge state distribution; Coulomb explosion; MILEA; Ionization efficiency; Isobar suppressionOrganisational unit
08619 - Labor für Ionenstrahlphysik (LIP) / Laboratory of Ion Beam Physics (LIP)
Funding
197137 - New Developments in Ion Beam Physics (SNF)
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