Experiments with a synchrotron x-ray source and conventional, ECR, and storage-ring ion sources Page: 3 of 13
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National Laborat ry Conceptual Design Report for a National Atomic
Physics Facility and in the proceedings of a Workshop on Photon-Ion
Interactions4 held to study directions for ion physics experiments at
the European Synchrotron Radiation Facility at Grenoble, France. The
ideas that are listed can be grouped in three different categories:
fundamental issues, poorly understood areas, and applications. The
applications include the use of atomic physics in astronomy, plasmas,
and Earth and planetary atmospheres.
One example of the type of experiment that might be done would
be a measurement of the photoionization cross sections for a single
element as a function of the ion charge state. This type of
measurement has already been employed to study the importance of
many-body correlations on atomic structure, but the sco e was
restricted because of the lack of a multi-purpose ion source.
The conclusions drawn from this look at the scientific needs is
that the ion source to be used in these experiments must be able to
deliver beams of all elements for charge states that correspond to
neutral to fully-stripped atoms. The needs for ion intensity for the
performance of the experiments and associated questions of backgrounds
will be considered below.
The usefulness of a particular type of ion source for a
colliding- or merged-beam experiment is conveniently assessed in terms
of the luminosity. The luminosity is defined as the number of
interactions taking place per second per unit cross section. The
luminosity can be evaluated for both the signal rate and for the
background rate. Some, but not all, details of a particular
experimental arrangement are contained within the luminosity value.
For instance, the form factor that is used to define the overlap of
the two beams is considered, but the detector efficiency and solid
angle are not.
Adequate signal rates demand both high photon and high ion-beam
intensities. The background rate is generally determined by
interactions of the ion beam with residual gas molecules in the
photon-ion interaction region. The signal-to-background ratio is then
improved by increases in the photon flux or a reduction in the
operating pressure. An increase in the ion current helps to improve
the data acquisition rate, but will not improve the signal-to-
For photoionization or excitation measurements of ion beams it
is interesting to compare the values of ion densities produced by the
various types of sources. This is done in Tables I-IV for the three
types of ion sources that are under consideration here. For sake of
comparison the density for a gas target is also included. It can be
seen that the ion densities are always low compared to the gas target
so that the crossed beam experiments are not easy to do.
The ion beams that are produced by the three sources are at
radically different energies. The singly charged sources can be used
at energies of a few keV. The ECRIS typically operates at a few tens
of keV, while uhe operating energies of the heavy ion storage ring are
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Jones, K.W.; Johnson, B.M. & Meron, M. Experiments with a synchrotron x-ray source and conventional, ECR, and storage-ring ion sources, article, January 1, 1988; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc1107459/m1/3/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.