Symbiotic Fungi: Principles and Practice (Soil Biology)

234 K. Vogel-Mikusˇ et al.
STIM detector
& Faraday cup
(on rotational
feedthrough)
High energy
HpGe X-Ray
Detector
(100 µm polyimide
absorber)
Low energy
SiLi X-Ray detector
Scanning
coils
Collimator
slits
Quadrupole
Triplet OM Rotating chopper:
dose normalization
Fig. 14.4 Schematic diagram of the nuclear microprobe at JSI
Object
slits 3 MeV P +
beam
microscopy (STIM), which is used for the determination of specimen thickness
(Vogel-Mikusˇ et al. 2008b).
The detection of X-ray energies from 1 keV up to 25 keV is provided by a pair
of X-ray detectors. These include a high-purity germanium X-ray detector with an
active area of 95 mm 2 , a 25-mm-thick beryllium window and a 100-mm-thick
polyimide absorber positioned at an angle of 135 with respect to the beam direction.
Simultaneously, a Si(Li) detector with an area of 10 mm 2 and an 8-mm-thick
Be window is installed at the angle of 125 with respect to the beam direction, for
the detection of low-energy X-rays, in the energy range from 0.8 to 4 keV. At the
JSI, the samples are sprayed with low-energy electrons from a hot tungsten filament
during the measurements, to avoid sample charging, thus efficiently avoiding timeconsuming
specimen carbon coating.
Precise proton-dose determination is required for quantitative micro-PIXE analysis.
For this reason, an in-beam chopping device is positioned in the beam line
after the last collimation of the beam before it hits the sample. The rotating chopper
is of gold-plated graphite, and periodically intersects the beam with a frequency
of ca. 10 Hz, which makes the method insensitive to beam-intensity fluctuations.
The spectrum of back-scattered protons from the chopper is recorded in parallel
with the PIXE spectra in the list mode. The high-energy part of the spectrum
consists of protons scattered from the Au layer, and it appears as a separate peak,
the area of which is proportional to the proton flux. During the off-line data
processing, the proton dose corresponding to an arbitrary scanning area selection
can be extracted from the list-mode results simultaneously with the PIXE spectra
(Vogel-Mikusˇ et al. 2007, 2008b).
The regions of interest on the samples are preselected by short PIXE mapping in
high-current mode. After the final sample positioning and scan size selection, the
object slits are closed and the STIM maps are measured in the list mode. A partially
depleted planar silicon detector is positioned directly in the beam to obtain the best
contrast for the STIM. This is followed by high-current mode, during which the
PIXE maps of the same region are measured in list mode over a longer period of