1. Measurement of background rate in ATLAS cavern with micromegas Work in progress J. Wotschack

2. The issue • The measurement of the background rate in the ATLAS cavern, using the micromegas detectors, was suggested by L. Pontecorvo • The idea is to compare the measured rates with the predictions and the measured background rates in the corresponding MDTs • Expected and measured (by MDT) rates are ≈10 Hz/cm2 J. Wotschack

3. Measured rate in Hz/cm2 in the MDT Plot kindly made available by Geraldine Conti EOL2/3 data from run 180400 <L> = 5.5 1032 MMs J. Wotschack

4. The setup • Since beginning of 2011 four small (9 x 9 cm2 active area) micromegas detectors are installed on the HO structure, 6th floor, side A • The setup is located 1.5 m behind EOL2A7, in the range of the outer 15 tubes and close to EOS2A6 • Two detectors (R11 and R12) are used as trigger and two detectors (R13 and R16) are read out • R13 and R16 have readout strips with a pitch of 0.25 mm, R13 has 360 vertical (x) strips, R16 has 360 vertical (x) and 360 horizontal (y) strips • The setup is operated stand-alone and has recorded so far several 10k events J. Wotschack

5. Cavern background measurement • We recorded events taken with a random trigger, with a rate of 156 Hz, during Fill 2000 and 2009, for about 20 hours and 11 hours • Total number of triggers: 11.4 M + 6.2 M • For each trigger the detector activity was measured for 28 time bins of 25 ns, i.e. 700ns. • Events were accepted in a time window from 5 to 25 time bins, i.e. over 500 ns. • Total time covered: ≈6 s, total area: 2 x 81 cm2 J. Wotschack

6. Event characteristics • 475 + 312 events with more than 150 ADC counts in a single strip were recorded • In 5 +13 events some activity, arriving at the same time, was recorded in R13 and R16; these events have been removed, They are likely to have come from an interaction. • 323 + 172 triggers were retained with activity in the detector in the time window between 5 and 25 time bins of 25 ns • 149 + 84 events were recorded in R13 and 175 + 88 in R16 • Comparing to events from interactions, a few % of the events may be lost by requiring a strip signal >150 ADC counts J. Wotschack

7. Two types of typical events Total charge: 1700 ADC counts Total charge: >10000 ADC counts Photon ? Neutron ? induced nuclear break-up J. Wotschack

8. The result: R≈2.7±0.2 Hz/cm2 at L=1034 J. Wotschack

9. Conclusions • Background events have been recorded with a random trigger in R13 and R16 for several long LHC fills • We observe two types of events: photon-like and neutron-like • We see a background rate consistent between runs and the two chambers of ≈2.7 Hz/cm2 • The observed rate is about a factor 3 lower than what is seen in the nearby EOL2 MDT chambers J. Wotschack