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Very High Radiation Detector for the LHC BL M System based on S econdary E lectron Emission

Very High Radiation Detector for the LHC BL M System based on S econdary E lectron Emission. Daniel Kramer , Eva Barbara Holzer, Bernd Dehning, Gianfranco Ferioli CERN AB-BI. LHC Beam Loss Monitoring system. ~ 3700 BLMI chambers installed along LHC

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Very High Radiation Detector for the LHC BL M System based on S econdary E lectron Emission

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  1. Very High Radiation Detector for the LHC BLM System based onSecondary Electron Emission Daniel Kramer, Eva Barbara Holzer, Bernd Dehning, Gianfranco Ferioli CERN AB-BI IEEE NSS 2007 D.Kramer

  2. LHC Beam Loss Monitoring system • ~ 3700 BLMI chambers installed along LHC • ~ 280 SEM chambers required for high radiation areas: • Collimation • Injection points • IPs • Beam Dumps • Aperture limits • Main SEM requirements • 20 years lifetime (up to 70MGray/year) • Sensitivity ~3E4 lower than BLMI IEEE NSS 2007 D.Kramer

  3. LHC Beam Loss Monitoring system • ~ 3700 BLMI chambers installed along LHC • ~ 280 SEM chambers required for high radiation areas: • Collimation • Injection points • IPs • Beam Dumps • Aperture limits • Main SEM requirements • 20 years lifetime (up to 70MGray/year) • Sensitivity ~3E4 lower than BLMI IEEE NSS 2007 D.Kramer

  4. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Ti HV electrodes Steel vessel (mass) < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) IEEE NSS 2007 D.Kramer

  5. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Incoming particle Ti HV electrodes Steel vessel (mass) < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) IEEE NSS 2007 D.Kramer

  6. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Incoming particle Ti HV electrodes Steel vessel (mass) < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) IEEE NSS 2007 D.Kramer

  7. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Incoming particle Ti HV electrodes Steel vessel (mass) < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) IEEE NSS 2007 D.Kramer

  8. Secondary Emission Monitor working principle • Secondary Electron Emission is a surface phenomenon • Energy of SE (below ~ 50 eV, dominant for signal) is independent on primary energy • SE are pulled away by HV bias field (1.5kV) • Signal created by e- drifting between the electrodes • Delta electrons do not contribute to signal due to symmetry* Secondary electrons Bias E field Ti Signal electrode Incoming particle Ti HV electrodes Steel vessel (mass) Incoming particle < 10-4 mbar • VHV necessary to keep ionization inside the detector negligible • Very careful insulation and shielding of signal path to eliminate ionization in air (otherwise nonlinear response) • No direct contact between Signal and Bias (guard ring) IEEE NSS 2007 D.Kramer

  9. SEM production assembly • All components chosen according to UHV standards • Steel parts vacuum fired • Detector contains 170 cm2 of NEG St707 to keep the vacuum < 10-4 mbar during 20 years • Pinch off after 350°Cvacuum bakeout and NEG activation (p<10-10mbar) • Ti electrodes partially activated (slow pumping observed) IEEE NSS 2007 D.Kramer

  10. Simulations in Geant4 • Detailed Geometry of SEM F type implemented • Signal electrode covered by thin layer of TiO2 • Photo-Absorption-Ionization module used for production of delta electrons • QGSP_BERT_HP used as main physics list • Signal generation done by • calculating charge balance on signal electrode • recording “True SE” produced by custom generator • Production threshold for e+/e- set to 9m IEEE NSS 2007 D.Kramer

  11. Semi empirical approach using simplified Sternglass formula • Secondary Emission Yield is proportional to electronic dE/dx in the surface layer • LS = (0.23 Ng)-1 • g = 1.6 Z1/310-16cm2 • “TrueSEY” of each particle crossing the surface boundary calculated and SE recorded with this probability • Correction for impact angle included in simulation • Fast  electrons considered as other primaries Electronic energy loss Model calibration factor Penetration distance of SE Comparison => CF = 0.8 IEEE NSS 2007 D.Kramer

  12. Simulated response curves for different particle types • Geant4 version 8.1.p01 • 30k primaries for each energy point • Longitudinal impact • Gaussian beam • r = 2mm e- absorbed in electrode IEEE NSS 2007 D.Kramer

  13. 400 GeV Beam scan in TT20 SPS line • Longitudinal impact of proton beam • r = 2mm • Chamber tilted by ~1° • Simulation sensitive to beam angle and divergence • Negative signal due to low energy e- from secondary shower IEEE NSS 2007 D.Kramer

  14. 400 GeV Beam scan in TT20 SPS line • Longitudinal impact of proton beam • r = 2mm • Chamber tilted by ~1° • Simulation sensitive to beam angle and divergence • Negative signal due to low energy e- from secondary shower chamber diameter IEEE NSS 2007 D.Kramer

  15. Prototype tests with 63MeV cyclotron beam in Paul Scherer Institute PSI proton beam 62.9MeV BLMS prototypes F & C Type HV dependence of SEY • Prototype C -> more ceramics inside (no guard ring) • Prototype F -> close to production version • Current measured with electrometer Keithley 6517A • HV power supply FUG HLC14 • Pattern not yet fully understood • Not reproduced by simulation • High SE response if U_bias > 2V • Geant4.9.0 simulated SEY = 25.50.8% IEEE NSS 2007 D.Kramer

  16. Measurements in PS Booster Dump line with 1.4 GeV proton bunches • Older prototype used - Type C • Profiles integrated with digital oscilloscope • 1.5kV bias voltage • 80m cable length • 50  termination • Single bunch passage • SEY measurement • 4.9  0.2% • Geant4.9.0 simulation • 4.2  0.5% Normalized response IEEE NSS 2007 D.Kramer

  17. Measurements in PS Booster Dump line with 1.4 GeV proton bunches SEM response to single proton bunch of 2.16 1013 protons with 160ns length • BLMS compared to reference radiation monitor ACEM (Aluminum Cathode Electron Multiplier tube) • ACEM not directly in the beam • Rise/fall time < 50 ns • Dominated by unknown intensity distribution • Normalized intensity 1.3 1019p+/s IEEE NSS 2007 D.Kramer

  18. Conclusions • The BLMS detector was successfully tested in different proton beams • Geant4 simulations are in good agreement with these experiments • => chosen model is validated • Sign change of output current possible under very specific circumstances • Verification measurements in mixed radiation field of LHC test collimation area in SPS are ongoing • 360 BLMS Detectors were produced in IHEP Protvino and will be tested soon IEEE NSS 2007 D.Kramer

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