Minimum Bias Trigger Scintillator Counters (MBTS) for early ATLAS running

1. Minimum Bias Trigger Scintillator Counters (MBTS) for early ATLAS running M.Nessi ATLAS week, Freiburg

2. MBTS • Why: In the commissioning phase of ATLAS, we need to have an effective means to trigger on physics and to veto halo events. • What : scintillator counters with some segmentation in eta and phi, able to detect 1 minimum ionizing particle • Where : on the front face of the LAr endcap cryostat, covering the radial dimension of the Inner Detector • When : needed in 2007 for the first beam, 3-4 months at low luminosity (5*1032); it will be dismounted during the first shut down Marzio Nessi

3. MBTS readout • It will be to difficult to bring inside ATLAS new PMTs, cables,.. • The idea is to use the same optical path used for the existing crack scintillators. Optical cables will transmit the light signals from the counters to the existing connectors in place for the crack scintillators • Each PMT in the tile drawers, used for this purpose, will then be linked to the LVL1 trigger processor or equivalent • PHI segmentation of 8 should be enough. This can still be tested by simulation. • Eta segmentation of 2 is planned. Radius or eta of segmentation to be defined. Limit of Pixel detector? Marzio Nessi

4. MBTS Geometry • On each side a disk of scintillator Rmin=138 mm, Rmax=1200 mm Rmax can still be optimized. • Two options: • Each disk will either take the place of the JM moderator disk • or be mounted in front of it • Maximum thickness of the disk, including supports and eventual readout system DZ= 30 +- 1 mm Marzio Nessi

5. MBTS Segmentation Current design for MTBS counters Inner eta segment covers pixel detector Eta = 1.8 (1200 mm) Eta = 3.8 (1.58 mm) Eta = 2.8 (431 mm) Marzio Nessi

6. Possible mounting scheme Marzio Nessi

7. MBTS Optical readout • The requirement is to have enough light in the PMTs to easily detect minimum ionizing particles • The light budget should allow a degradation of the light output by a factor 2 because of radiation damage during its short lifetime • Standard technique is scintillator readout by wavelength-shifting (WLS) fibers • along edge of scintillator • can also try fibers embedded in groove in scintillator Marzio Nessi

8. Prototype construction • Prototype construction currently proceeding using 2 cm thick scintillator provided by Dubna • This is the same type of high-quality scintillator used in the CDF central pre-radiator upgrade currently underway • somewhat different design but there we get >12 pe/mip • WLS fibers for prototype will be along crack in middle of scintillator • fabrication is more efficient with scintillator in 2 pieces inner h outer h Marzio Nessi

9. Next steps for MBTS • Prototype counter will be completed this month • We will test prototype counter with cosmic rays at CERN using ATLAS readout • If design and light-yield proves adequate, then • by end of 2004, define final design and write a technical proposal • 2005 go for procurement • March ’06 install counters on the cryostats • April ’06 commissioning, with the LVL1 trigger electronics • This project will be funded with ATLAS Common Fund resources as well as by US ATLAS • Project is proceeding with participation by: • Michigan State University (J. Huston is project leader) • Dubna • CERN • University of Texas-Arlington Marzio Nessi