exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

The Pegase project, a new solid surface probe :
focused massive cluster ion beams.
IPNO Participation: Della-Negra S., Arianer J., Depauw J. , Blache P., Dozon F., Kaminski M., Pereira
M., Pochon O., Amorri T. and Yaniche J.F.
Collaboration: Verkhoturov S.V. and Schweikert E.A. , Department of Chemistry, Texas A&M University,
College Station, Texas 77842-3012 USA
Les résultats obtenus au Tandem d’Orsay ont montré que le rendement d’émission ionique secondaire d’une
surface, bombardée par un faisceau d’agrégats, augmente de façon importante avec le nombre de
constituants de l’agrégat et aussi avec son énergie. D’où l’intérêt d’utiliser des agrégats massifs d’or
(plusieurs centaines à plus d’un millier d’atomes) ou de bismuth, à une énergie bien choisie, pour une analyse
fine de surfaces métalliques et organiques, en particulier biologiques. Un autre paramètre décisif est la
dimension du faisceau d’ions poly-atomiques, le but étant d’obtenir une mesure localisée d’un diamètre de
1 à 10 μm. Pour remplir ces conditions, la collaboration entre l’IPN et Texas A&M University a mis sur pied
un projet comportant une plate-forme isolée à 130 kV qui supporte la colonne ionique et une chambre de
diagnostic, suivie par un gap accélérateur, un spectromètre de masse par temps de vol de haute résolution
et un microscope à émission électronique.
Ce projet Pégase, accepté par la National Science Foundation (USA) en février 2008 a été réalisé et testé
à l’IPN et expédié fin novembre 2009 au Texas où les expériences de physique ont pu commencer en février
2010.
Polyatomic ion beams, for an efficient surface
analysis, were first used at the Institut de Physique
Nucléaire in Orsay. The projectiles were then
made of a few atoms and this type of beam is now
commercially available for mass spectrometry. But
fundamental studies of the ion-solid interaction
show that increasing the particles mass leads to an
important enhancement of the ionic emission rates
and, consequently, to a more accurate analysis.
This is particularly interesting for the complex and
molecular ions, such as biological molecules or
tissues for example. Through a continuous R&D on
the liquid metal ion sources (LMIS), beams of massive
multi-charged clusters containing hundreds to
thousands gold atoms were obtained; the ions are
extracted from a melted down metal and their size
is in the nanometer scale, from which the gold
nano-droplets name [1] (fig 1)
Figure 1: n/q (n and q being the atom number and
the charge of the ions) spectrum obtained with the
Pegase Wien filter for a Gold LMISource through
a 150 µm collimator.
An advantage of these beams was proved, as
soon as they were available, with ionic emission
rates 1000 times those of atomic ions. In collaboration
with the Centre of Chemical Characterization
& Analysis of Texas A&M University, directed by
Pr. E.A. Schweikert, we explore the exceptional
properties of these new beams for surface analysis.[2-5]
The main surface analysis object is, at present, to
produce the most focused possible beam. The new
projectiles are perfect candidates for this kind of
application and a micro-beam project was proposed
in 2006 to obtain massive cluster beams
with a diameter of 1-10 μm. To reach this object a
100 kV acceleration at least is necessary. A feasibility
study was undertaken at the TANDEM accelerator
in order to first confirm the possibility to accelerate
these particles while keeping them intact,
and secondly to verify the energy value choice.
This study, performed in 2007, was conclusive for
the first point and showed that the choice of an
energy between 100 and 200 keV was justified as
the ionic emission rates increase strongly between
20 keV per charge and 200 keV per charge and
then follow a quasi linear growth with energy
above 200 keV per charge up to 4 MeV per
charge. The 100-200 keV per charge energy region
is most interesting because the ionic emission
rates per impact lie far beyond unity which permits
to get a mass spectrum [6]and to analyse 10 nm
diameter areas with a single impact. The IPN Orsay
team received for this collaboration a grant, in
the frame of the CNRS-USA collaborations, which
was renewed up to 2008.
Following the preliminary encouraging results the
collaboration built a project aiming at producing
and using gold (or bismuth) nano-droplets for surface
analysis on a nanometer scale. This project,
called the PEGASE project, is made of an insulated
130 kV platform which supports the ionic col-
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