[Sneap] Caesium Sputter Source and possible contamination from Ti hydride

Thu Mar 6 22:53:14 EST 2008

Chris, Tom and Colin,

We have seen convincing evidence of the poisoning of a Tantalum surface
ioniser by titanium.  In the General Ionex 832 sputter source, the tantalum
frit ioniser faces a large electron suppressor electrode.  This electrode
was originally made of stainless steel and became worn by sputtering over
time.  We replaced it with a home made electrode of a material that
sputtered much less than stainless steel -that is titanium.  With a short
time, (I don't remember whether it was hours or days) the Cs current went to
near zero.  The electron affinity of titanium is very low: 0.079 V.

The titanium suppressor electrode was replace with one made of Molybdenum,
(electron affinity 0.746 V) and the source resumed excellent output for
years.

The poisoning effect is reduced by running the ioniser hotter.  The higher
temperature increases the diffusion of the titanium into the bulk of the
tantalum thereby reducing the amount at the surface where the ionisation
occurs.

Modern ion sources do not use frit ionisers and don't have electron
suppressor electrodes in view of the ioniser.  But they have high sputtering
rates which can contaminate the ioniser.

We have run metallic titanium cathodes in the gas cathode SNICSII using
ammonia gas to produce TiH beams and H beams with no noticeable ioniser
poisoning. We have also run TiH3 cathodes without evidence of poisoning.
Perhaps since we only run low proton intensities, the rate of titanium
deposit is compatible with the diffusion into the tantalum.

Regards,

David

********************************************
D.C. Weisser
Nuclear Physics Department
Research School of Physical Sciences and Engineering
Australian National University
Canberra A.C.T. 0200
AUSTRALIA

Tel:    + 61 (0)2 6125 2080
Fax:    + 61 (0)2 6125 0748
Mobile: + 61 (0)414 249 209

david.weisser at anu.edu.au

  -----Original Message-----
  From: sneap-bounces at tunl.duke.edu [mailto:sneap-bounces at tunl.duke.edu]On
Behalf Of Colin Maden
  Sent: Thursday, 6 March 2008 8:13 PM
  To: Symposium of Northeastern Accelerator Personnel
  Subject: Re: [Sneap] Caesium Sputter Source and possible contamination
from Ti hydride

  Dear Chris,

  During my time at the SUERC AMS Lab we also saw negative effects that we
believe were caused by titanium, but as in Tom Browns's case hard evidence
is lacking.

  The observations we made were mainly during 10Be analysis on the MC-SNICS
134 cathode source. Samples with considerably more residue after sample
preparation, instead of a small amount of white BeO, would not only produce
less ion current during measurement, but seemed to "poison" the source in
the sense that samples running with good current before the analysis of such
a "bad" sample would yield less current after the analysis of the bad
sample. An analysis of this large residue showed that titanium was the only
contamination that potentially could be the cause of the problem. The sample
chemistry was adapted to get rid of the Ti and the situation improved.

  The state of the "poisoned" ion source could be relieved during the run by
heating the ioniser to a high temperature for an hour or two and then
reducing the temperature again to continue with the measurements. This would
go towards confirming Tom's suspicion that the problem is caused by a Ti
coating of the ioniser.
  Because we cleaned the source after almost every Be run we didn't see any
effect on C14 measurements.

  After asking a few people in the AMS community it seems as if this problem
is source specific and not all ion sources are affected by it. Other AMS
labs have run tests and seen no effect on their ion source due to titanium.

  It should also be mentioned that at SUERC we measured C14 from CO2 gas
samples on the MC-SNICS 39 cathode source modified for gas samples, where C
is sputtered off a titanium insert in the cathode, onto which the CO2 is
gettered. We never saw any decrease in source performance during these runs
other than the normal ageing effect of a source getting dirty. But the
source was run as hot as reasonably possible during these runs, which
possibly prevented the deposition of Ti on the ioniser.

  Cheers,
  Colin

  Thomas A Brown wrote:
Chris,

   We have ongoing 3H AMS projects and run Ti-hydride targets in our
sources for those projects.  We have found that running Ti-hydride in
the ion source does (is likely to) have an undesirable impact on
subsequent efforts to run 14C targets in the source.  We routinely
consider just cleaning the source after Ti-hydride runs before
beginning further 14C runs.  Our suspicions (which have included many
theories - some conflicting but coming from the same person) tend to
center on possible problems caused by the deposition of Ti (sputtered
neutral ions?) all over the surface of the ionizer - but hard
evidence is lacking.

Hope this helps.
Tom.

Hi Guys
Our SNICS Caesium sputter ion source is mainly used for C-14 Measurements.

However, occasionally we use Ti-hydride targets for Proton beams.
 When we then go immediately back to measuring C-14 targets ( without
opening the source for cleaning etc) and we appear to have stability
problems etc.
Are there issues with possible contamination of the Caesium or other
reasons that people have experienced?
i.e. is there evidence against using Ti-hydride on a sputter source used
for C-14 AMS mainly ?

Thanks
Chris

Notice: This email and any attachments are confidential. If received
in error please destroy and immediately notify us. Do not copy or
disclose the contents.

_______________________________________________
Sneap mailing list
Sneap at daytona.tunl.duke.edu
http://daytona.tunl.duke.edu/mailman/listinfo/sneap

  --
  ----------------------------------------------------------------------
  ETH Zurich
  Dr. Colin Maden
  Institute of Isotope Geochemistry and Mineral Resources (IGMR)
  NW C 83.1
  Clausiusstrasse 25
  8092 Zurich
  SWITZERLAND

  maden at erdw.ethz.ch
  www.igmr.ethz.ch/madenc

  Tel.: +41 44 632 78 39
  Fax: +41 44 632 11 79
  ----------------------------------------------------------------------

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://daytona.tunl.duke.edu/pipermail/sneap/attachments/20080307/5d1efff3/attachment.html 