First operation of a 72 k element hybrid silicon micropattern pixel detector array

We have constructed and tested silicon pixel detector arrays of 96 × 378 (36 288) sensor elements with 75 μm × 500 μm 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 15 000 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 ∼ 4 mm cover hermetically a 53 mm × 55 mm area with 72 576 pixels. The proportion of properly functioning pixels was 98% in the first 36 k pixel array and 80% in the second one. The ∼ 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 μm. The proportion of double hits (∼ 11%) depends only slightly on threshold and detector bias voltage, and for these double hits a precision of 10 μm on the particle position was obtained.