For
the specific isoelectronic sequence measurements proposed here we shall
follow the procedures presented in Strohschein et al [Stro08a] used for the
measured C4+ spectra shown in Figs. 1 and 2. The same procedures
will be repeated for every single different ion species used until we cover
all the ions with atomic number between Lithium and Fluorine. These are described below:
8.1 Start up of negative ion source
- tuning the beam at the required energy for optimal transmission:
This
is the first part of every beam time and includes the startup of the ion source,
tuning the selected ion species through the accelerator with the required
energy, selecting the right charge state at the analyzing magnet, post
stripping if necessary prior to switching the beam to the right beam line by
setting the switching magnet. Depending on whether beam in the ground state
or mixed state is required, the tank gas stripper or the post-stripper is
activated. The selected beam is eventually tuned through the target gas cell
and the 00 spectrometer and optimized in the final Faraday cup.
Electron spectra are then accumulated and final beam tuning to minimize
background from slits etc. is performed.
8.2 Absolute energy calibration of
electron spectra
Before
projectile electron spectra can be taken an overall electron energy calibration
of the entire apparatus is required usually performed with known target Auger
lines as for example obtained in 3 MeV p + Ne/Ar [Zou97a]. Also required is
the accurate knowledge of the beam velocity which is usually obtained by
measuring the energy of the “cusp” electrons which are known to move at the
projectile speed. Finally, once this is known, projectile electron spectra
can be safely accumulated since the Auger energies in the lab frame are now
well specified and particular states/lines can be accurately identified.
8.3. Absolute calibration of
electron double differential cross sections (DDCS)
This
is an important calibration of the counts scale of an electron spectrum
related to the electron DDCS and the overall absolute efficiency of the
apparatus. It is typically performed by a procedure well established by
Zouros and his collaborators in Kansas.
It involves a measurement of the so called Binary Encounter peak electrons
using bare ions which correspond to elastically scattered electrons for which
the absolute DDCS are very well established from theory. This calibration is
usually performed with a strong completely stripped ion beam typically F9+,
O8+ or B5+ around 1 MeV/u [Zou97a].
8.4. Measurements of electron
spectra and conversion to DDCS
Once
the precise energy range of the spectrum to be recorded as well as the target
gas pressure are specified after a few standard tests requiring short runs of
spectra accumulation, the final high resolution spectra (under electron
pre-deceleration) are accumulated
consisting of (a) A spectrum with loaded gas cell, (b) A spectrum with no gas
(background spectrum). The subtraction of (b) from (a) will give the
necessary “clean” or background subtracted spectrum.
This
procedure is repeated for all required spectra for both types of He-like
beams, i.e. the “pure” ground state beams taken using the terminal gas
stripper and the mixed 1s2/1s2s 3S beams taken using a
foil post-stripper. Such spectra will be taken for both H2 and He
targets and possibly for other gas targets such as CH4, Ne or Ar,
having more than 2 electrons to be captured, as an extra test. Total high
statistics accumulation time for each spectrum can vary between 1 and 4 hours
depending on beam intensity.
8.5. Servicing of experimental
components after use
The
electron spectrometers with their detectors are normally kept in the
collision chamber under vacuum even when not used for measurements since this
preserves them in a clean environment and extends the life time of the MCP
plates and channeltron detectors. However, after some use the MCPs eventually
deteriorate and need to be replaced about once a year, depending on use.
Similarly, for channeltrons, which however being more rugged, need
replacement only once every 2 years. Spectrometers also sometimes, especially
if a vacuum accident occurs, need meticulous cleaning and even high voltage
conditioning. Graphite coating must also be refreshed every so often,
especially if the spectrometer needs to be taken apart for cleaning which is
a major operation. Both Prof. Benis and Zouros have done this procedure many
times.
8.6. Back-up of all data on special
storage media
In
a typical run many different spectra are collected. Especially the
two-dimensional images of the 2-D PSD require a lot more disk space on the
PC. The data of each run will be backed up onto a special hard disk, labeled
and logged so it can be readily accessed for future offline data analysis.
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