The ATLAS beam conditions monitor

The ATLAS beam conditions monitor (BCM) was developed as a stand-alone device allowing separation of LHC p-p collisions from background events induced on beam gas or by beam accidents. This separation can be achieved by timing coincidences between two detector rings placed symmetrically around the interaction point at z=plusmn184 cm and r=55 mm (eta~4.2). The expected LHC lifetime dose at this position exceeds 0.5 MGy and 10¹⁵ particles/cm². Each ring is composed of four detector modules each having two pCVD diamond sensors read out in parallel by fast current amplifiers yielding 3 ns wide pulses with 1 ns rise time and baseline restoration in 10 ns. The device is aimed at single particle counting, with sub-bunch-crossing timing and pulse height information, providing a flexible and versatile tool to monitor beam conditions as well as to provide a coarse measurement of bunch-by-bunch luminosities in ATLAS. Eleven detector modules have been fully tested and assembled. Tests performed range from full characterization of diamond sensors to full module tests with electron sources and pion test-beams. Recent test-beam results in the CERN SPS show a module median signal to noise of 11:1 for minimum ionizing particles incident at 45deg, using the envisaged digitization electronics. The best eight modules have now been installed on the ATLAS pixel support frame. One of the modules was irradiated in steps to 1 and 3 times 10¹⁴ p/cm² at the CERN PS and its performance evaluated in test-beams this fall, followed by a further irradiation to 10¹⁵ p/cm² next year.