1. M. Jones, A. Boston, M. Chartier, B. McGuirk, P. Nolan, R.D. Page, P. Pusa, D. Seddon, J. Thornhill, D. Wells and the ALPHA collaboration Department of Physics, Oliver Lodge Laboratory, The University of Liverpool, Liverpool L69 7ZE, United Kingdom ALPHA – ANTIHYDROGEN LASER PHYSICS APPARATUS. SILICON TRACKING DETECTOR FOR ANTIHYDROGEN ANNIHILATION DETECTION Introduction The aim of the ALPHA experiment at CERN is to trap cold atomic antihydrogen and study its properties. The final goal is to test CPT-symmetry by comparing the atomic 1s- 2s transition of hydrogen and antihydrogen with an accuracy of 1 part per 10 18. As the trapped antihydrogen annihilates, either in the trap walls or with residual gas, pions are emitted. A silicon tracking device, consisting of 120 double sided 128x256 strip detectors, is being constructed at the University of Liverpool to surround the antihydrogen trap. By reconstruction of the pion tracks the antihydrogen gas can be monitored and imaged during the experiment. Antiprotons 5.3 MeV positrons Principles of the Experiment Antiprotons are injected to the ALPHA-apparatus by an antiproton decelerator (seen in the figure above) at 5.3 MeV. They are further slowed down by degrader foil and cooled down by synchrotron radiation with electrons. Positrons enter the apparatus from the other end. The cooled plasmas are blended in the centre of the trap. The neutralized antihydrogen gas is held with an octupole magnetic field at temperature of a few mK. By reconstruction of the pion tracks the antihydrogen gas can be monitored and imaged during the experiment giving essential information of the neutralised gas and the antiproton plasma under the octupole magnetic field used to trap the gas. The Detector Configuration The silicon tracking detector is constructed around the centre of the ALPHA-trap and operates in air. The detector consists of 60 hybrids, each having two Si detectors and integrated ASICs. The silicon detectors are double sided with 128 vertical and 256 horizontal strips. The detectors each have an active area of 58x112 mm and thickness of 300 μm. The p-side is DC coupled and external AC-coupling is applied to the n-side. The detectors have strip bias resistors of 5MΩ and they operate at 70V bias. The detector collects data for the pion interaction points in the silicon layers. The co-ordinates of an annihilation are determined by track reconstructions from these data. To the right is a completed Hybrid in a handling jig with detail of a bonded corner of the n-side of the detector (128 strips). The board has four charge sensitive 128 channel readout circuits integrated. Below this are schematic representations of the detector setup. Silicon testing The Silicon testing procedures include life testing, IV-characteristics and individual strip scans for both voltage and current. During the life test the detector is biased at 100V for typically three days to check for any current fluctuations. The IV-characteristics ensure the detector has diode properties without breakdown behaviour and to check there is no damage. The quality of individual strips is monitored for high currents, short circuitry and bad resistor values. Screen shots of these tests scans are shown on the left hand side. When testing is finished it is then deemed whether the piece of Silicon is good enough for the final experiment. The final hybrid is expected to be completed in June 2008 and the main run at CERN is to begin in the summer of 2008