FIRST OPERATION OF A 72 K ELEMENT HYBRID SILICON MICROPATTERN PIXEL DETECTOR ARRAY

We have constructed and tested silicon pixel detector arrays of 96 x 378 (36288) sensor elements with 75 mu m x 500 mu m area. The low-noise signal processing circuit associated with each element occupies an identical area on a bump-bonded readout chip. The pixel cell response for ionizing particles is binary with an adjustable threshold between 4000 e(-) and 15000 e(-). Single chips, the array of 6 ladders and a double array have been characterized in particle test beams and in the Omega experiment WA97 at CERN. The two arrays together, staggered by similar to 4 mm cover hermetically a 53 mm x 55 mm area with 72576 pixels. The proportion of properly functioning pixels was 98% in the first 36 k pixel array and 80% in the second one. The similar to 1% ''always-on'' pixels could be masked electronically. After masking the rate of ''spurious noise hits'' was < 10(-8) of the identified particle hits while with beam off no hits at all were recorded With a beam trigger most events consisted of a single cluster with a single hit. At the 8000 e(-) threshold an efficiency > 99% was measured. Tracks were reconstructed with a precision of 22 mu m. The proportion of double hits (similar to 11%) depends only slightly on threshold and detector bias voltage, and for these double hits a precision of 10 mu m on the particle position was obtained.