Silicon counters

Silicon counters are sometimes called 'solid state wire chambers' because here in principle the same is happening like in a wire chamber. Silicon atoms are ionized along the track of a charged particle, and the freed electrons drift to the readout electrode. The ionized atoms don't drift, instead they receive an electron from their neighbouring atom, which against receives an electron from it's neighbour, a.s.o, so that a positive 'hole' drifts to the other electrode. The electrodes are on the surfaces of the silicon chip, so the field lines are orientated perpendicular to the chip.

sketch of a silicon counter

The silicon counters used in the COSY-11 experiment are called silicon pad detectors, because the electrodes on the readout side of the chips are four rectangular areas ('pads'). To form a large detectors these chips are staggered in three rows which overlap in order to get a complete geometrical coverage:

sketch of a section of the silicon detector

The pads are connected to AMPLEX-16 chips which contain 16 readout channels each consisting of a charge amplifier, a filter amplifier, and a sample-and-hold stage. These are followd by a multiplexer which switches the stored voltages sequentially to a single analog wire that is connected to an external ADC.
There are two detectors made of these silicon pads in the COSY-11 experiment: the small monitor detector (36 chips = 128 pads) for the measurement of elastically scattered protons and the longer (180 chips = 720 pads) one inside the dipole gap to detect reaction products with negative charge.
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Last updated: 27-November-1997 by T.Sefzick