Work Package 8: Measurements of electron spectra using ions from the TANDEM

Work Package

8

Start:

Month 19

Finish:

Month 48

Cost:

Title:

WP8: Measurements of electron spectra using ions from the TANDEM

Description:

Research Teams:

RT1, RT2, RT3, RT4

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.

Deliverables:

2 Reports, part of BS, MS or PhD thesis