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        OTPC -- Optical Time Projection chamber


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The OTPC detector consists of a set of parallel 20 cm 20 cm electrodes forming several electric field regions. Active volume of the detector has a length of 30 cm. A whole volume of the OTPC was filled with a gas mixture of 95% of He and 5% of N2 at atmospheric pressure. Such gas mixture was chosen to have long enough tracks of low energy decay products of 8He.

Schematic view of the Optical Time Projection Chamber.

Incoming ions as well as their charged decay products produced ionization electrons along their trajectories in the gas filling the chamber. These primary ionization electrons were transported with a drift velocity of Vd = 0.5 cm/mks towards two stage charge amplification structures. First of them was formed by three gas electron multiplier (GEM) foils, second one by two closely spaced wire-mesh electrodes. At the final amplification stage ultraviolet and visible photons were emitted. A visible part of this light spectrum was recorded by a 2/3” 1M pixel CCD camera with light amplification and a 5” photomultiplier (PMT). The data acquisition was triggered by a coincidence of a TOF signal and a PMT signal which provided a signature of a 8He ion entering the OTPC detector. At the arrival of the trigger signal, the beam from the separator was switched off and the exposure of the camera was initiated for a period of 1–1.2 s. At the end of this time interval the camera image, the digitized PMT signal, and the dE – TOF information of the triggering ion were saved on a hard disk. While the camera registered the projection of a particle’s track on the electrode plane, the shape (width) of the time distribution of a PMT signal provided information on the track’s projection in the direction normal to the image plane (dLz = Vd*dt). The combination of information contained in the camera image and in the recorded drift time profile allows a complete reconstruction of particle’s momentum. In the applied procedure the energy and particle emission angle were determined by fitting the projected theoretical ionization density distributions simulated by using SRIM code to the light intensity distributions measured by the CCD and PMT. Fit parameters were a normalization factor, the energy and the emission angle. The experimentally determined response function of the OTPC detector was taken into account when comparing the calculated and measured profiles.