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Measurement of background rate in ATLAS cavern with micromegas

Measurement of background rate in ATLAS cavern with micromegas. Work in progress. The issue. The measurement of the background rate in the ATLAS cavern, using the micromegas detectors, was suggested by L. Pontecorvo

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Measurement of background rate in ATLAS cavern with micromegas

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  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

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