The target online-diagnosis at SHIP


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 1: Set up for target diagnosis. (Image source: GSI Helmholtzzentrum für Schwerionenforschung, R.Mann)


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.


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.

Picture 4: Topview of a 3D visualization generated by automated target scanning with a PC based control system. A curium target (red, length: 35mm, width: 8mm, not true to scale) rests on a titanium surface (orange). A scratch (violett) indicates a damage of target and titanium backing. (Image source: GSI Helmholtzzentrum für Schwerionenforschung, J.Maurer)
Picture 5: Top- and perspective view of an undamaged target. Its thickness (deep blue to red) is about 350nm. The resolution is (upper picture): horizontal 31µm, vertical 50µm, thickness 0,55nm. (Image source: GSI Helmholtzzentrum für Schwerionenforschung, J.Maurer)