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The target online-diagnosis at SHIP
Picture 1: Set up for target diagnosis.
(Image source: GSI Helmholtzzentrum für Schwerionenforschung, R.Mann)
For the experiments at the SHIP-apparatus 8 sectors of banana shaped
targets are mounted on a wheel rotating with a velocity of ~20 m/s and ~
19 rps (figure 1). Target changes and damages due to irradiation by
heavy ions are controlled using an electron beam from an electron gun.
The electron beam penetrates the targets at the opposite of the ion beam
path and is received by a slit shaped Faraday cup (figure 1). The
electron signal is attenuated due to angular scattering and absorption
of electrons in the material corresponding to the thickness of the
target. Current fluctuations of the gun are compensated by normalizing
the Faraday cup signal with the signal from a reference grid passed by
the electrons in front of the target. A focusing magnetic lens defines
the resolution of presently ~ 0.3 mm at the target. The target area is
scanned in radial direction with a precision of < 0.1 mm by using a
magnetic deflector. Scans along the circumference of the target wheel
with a position resolution of < 0.1 mm results from the rotation
where a trigger pulse starts the time base of an oscilloscope. The
display of the oscilloscope is stored in a personal computer for
handling the measured spectra. Thereby individual targets and areas at
the rotating wheel can be displayed by defined delays switched to the
trigger pulse. This set up allows a fast inspection of all targets
within 60 ms without disturbing a running experiment.
 Picture 2: Examples of target diagnosis by a 20 keV electron beam. The upper part exhibit one of eight UF4 targets covered by two thin carbon layers before it was irradiated by heavy ions. Two spokes of 4mm width at the target boarder are seen from the blocked electron signal. Signal fluctuations along the target are due to wavelike structures resulting to different effective target thicknesses. The bottom picture indicate the destruction of the target after 130 hour irradiation by a Cr10+ beam with 40μA current and 4.9 AMeV energy. The strong increase of signal is due to loss of UF4 matter as well as due to changes in the distribution of UF4 composition in the target. It would correspond to 34% of the original target thickness assuming the structure and matter distribution has not noticeable changed. The onset and offset of beam at the target is clearly seen.
(Image source: GSI Helmholtzzentrum für Schwerionenforschung, R.Mann)
 Picture 3: Example of an PbS target sector carrying an impurity spot.
(Image source: GSI Helmholtzzentrum für Schwerionenforschung, R.Mann)
The method allows not only a permanent watching of changes induced by ion beam impacts but also contaminations, pin holes and variation of thickness from the production process itself can be detected. This is of relevance for nuclear reactions products which are favourably produced in a small energy window requiring a homogeneous energy loss in the target in order to receive the best efficiency through the SHIP filter. Also background reaction products can be strongly produced by impurities on the target (grains, dust particles,...) which can be made visible by the electron beam diagnosis.
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