Reassessment of (182)Hf AMS measurements at VERA

Autor(en)

Oliver Forstner, Hubert Gnaser, Robin Golser, Dag Hanstorp, Martin Martschini, Alfred Priller, Johan Rohlén, Peter Steier, Christof Vockenhuber, Anton Wallner

Abstrakt

The radioisotope (182)Hf (t(1/2) = 8.9 Ma) is of great interest for astrophysical applications as a chronometer for the early solar system or as possible live supernova remnant on earth. However, AMS measurements of (182)Hf are seriously influenced by the presence of the stable isobar (182)W, which cannot be separated at typical AMS energies. Previous studies revealed a possible suppression of (182)W against (182)Hf by extracting the negatively charged pentafluoride HfF(5)(-) from the ion source, leading to a detection limit for (182)Hf/(180)Hf in the order of 10(-11). However, this suppression behavior is in contrast to theoretical calculations of the electron affinity and recent measurements using SIMS instruments, where the achieved suppression cannot be reproduced. The aim of our study is to determine the effects of ion source background as well as further investigate the suppression of tungsten against hafnium by extracting negatively charged fluoride ions from different sample materials. The previously reported suppression factor of about 6000 could be increased to 36000 by careful tuning of the ion source using HfF(4) as sample material. The trend of the theoretical electron affinities could be reproduced using atomic tungsten and hafnium instead of HfF(4) as sample material. This supports the assumption that the major contribution of the tungsten background is not sputtered from the target matrix but comes from somewhere else in the ion source. Measurements from the second ion source show a higher background of tungsten and a lower suppression factor, i.e. careful design of the ion source is crucial. Moving the sputter beam over the target surface extending over the wheel holding the targets revealed the highest tungsten background was detected outside the sputter target position. Further investigations are necessary to locate the origin of the tungsten background in the ion source. Possible sources are the material used for the ion source construction or contaminations in the cesium used for sputtering.

Organisation(en)

Isotopenphysik

Externe Organisation(en)

Technische Universität Kaiserslautern, University of Gothenburg, Eidgenössische Technische Hochschule Zürich

Journal

Nuclear Instruments & Methods in Physics Research. Section B. Beam Interactions with Materials and Atoms

Band

269

Seiten

3180-3182

Anzahl der Seiten

3

ISSN

0168-583X

DOI

https://doi.org/10.1016/j.nimb.2011.04.022

Publikationsdatum

2011

Peer-reviewed

Ja

ÖFOS 2012

103014 Kernphysik

Link zum Portal

https://ucris.univie.ac.at/portal/de/publications/reassessment-of-182hf-ams-measurements-at-vera(dafe5c52-7947-4b2c-bcfd-fd0731e9dedc).html