1. Secondary Emission Monitor for very high radiation areas of LHC
Daniel Kramer for the BLM team

2. LHC Beam Loss Monitoring system
~ 3700 BLMI chambers installed along LHC
~ 280 SEM chambers installed in high radiation areas:
Collimation
Injection points
IPs
Beam Dumps
Aperture limits
Main SEM requirements
20 years lifetime (up to 70MGray/year)
Sensitivity ~7E4 lower than BLMI
D.Kramer BLM Audit

3. Secondary Emission Monitor working principle
Secondary electrons
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*
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)
D.Kramer BLM Audit

4. Secondary Emission Monitor working principle
Secondary electrons
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*
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)
D.Kramer BLM Audit

5. Secondary Emission Monitor working principle
Secondary electrons
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*
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)
D.Kramer BLM Audit

6. Secondary Emission Monitor working principle
Secondary electrons
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*
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)
D.Kramer BLM Audit

7. Secondary Emission Monitor working principle
Secondary electrons
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) Transit time 500ps
Signal created by e- drifting between the electrodes
Delta electrons do not contribute to signal due to symmetry*
Bias E field
Ti Signal electrode
Incoming particle
Ti HV electrodes
Incoming particle
Steel vessel (mass)
< 10-4 mbar
VHV necessary to keep ionization inside the detector negligible and avoid capture of electrons
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)
D.Kramer BLM Audit

8. SEM production assembly
All components chosen according to UHV standards
Steel/Ti parts vacuum fired
Detector contains 170 cm2 of NEG St707 to keep the vacuum < mbar during 20 years
Pinch off after vacuum bakeout and NEG activation (p<10-10mbar)
Ti electrodes partially activated (slow pumping observed during outgassing tests)
NEG St707 composed of Zr, Vn, Fe
Zr flamable -> insertion after the bottom is welded
Very high adsorbtion capacity of H2, CO, N2, O2
Not pumping CH4, Ar, He
D.Kramer BLM Audit

9. Vacuum bakeout and activation cycle for SEM and BLMI
NEG inside the SEM needs additional activation at 350°C
Activation means releasing adsorbed gases on the surface which have to be pumped
Pinchoff done during the cool down of the chamber
Resulting pressure below measurement threshold (<10-10mbar)
Manifold stays colder to limit the load to the pumping system
Activation temperature limited by the feedthroughs
NEG activation
Vacuum bakeout
He leak tests
pinchoff
Vacuum bakeout
Ion pump started
He leak tests
D.Kramer BLM Audit

10. Geant4 simulations of the SEM
Secondary Emission Yield is proportional to electronic dE/dx in the surface layer
LS = (0.23 Ng) 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
QGSP_BERT_HP as main physics model
Model calibration factor
Electronic energy loss
Penetration distance of SE
0° impact angle
Geant4 SEM Response function
Comparison to literature values => CF = 0.8
D.Kramer BLM Audit

11. SEM Calibration experiment in a mixed radiation field (CERF++ test)
Response of the SEM measured with 300GeV/c beam hitting 20cm copper target
Setup simulated in Geant4
Response of SEM filled by AIR measured and simulated as well
SEM Response expressed in absolute comparison to Air filled SEM
Response = Dose in AIR SEM / output charge of SEM
/ Gy/count
H4 Calibration setup with Cu target and a box with 16 SEMs on a movable table
D.Kramer BLM Audit

12. Calibration results
Only 2 chambers out of 250 had higher offset current
Not corrected for systematic position errors
Upper Limit on the SEM pressure: (equivalent to 3 of the histogram)
1bar(0.6 sigSEM / sigSEM AIR) = 0.26 mbar
Pressure inside SEMs smaller than this
Offset current without beam
D.Kramer BLM Audit

13. Table of SEM measurements and corresponding simulations
Test beam
Measured [e-/prim]
Geant4
Rel. Dif. [%]
PSI 63MeV
0.27 ± 0.014 ±
1.1
PSB 1.4GeV ± ± 19
TT20 400GeV
0.476 e-cm
0.608 e-cm 22
H4 target
3.40 ± 0.92
3.95 ± 0.19 14
LHC collimator in LSS5 of SPS
4.03 ± 0.25Gy
In progress
muons 160GeV
0.059 ± 0.016
0.08 ± 0.008 26
TIDV dump
Long term test -
D.Kramer BLM Audit

14. Thanks
D.Kramer BLM Audit

15. Backup slides Vacuum stand in IHEP for IC production
36 ICs in parallel baked out and filled by N2
For SEMs only 18 chambers in parallel
No N2 injection :o)
He leak detection done before and after bakeout (and after NEG activation for SEMs)
D.Kramer BLM Audit

16. Beam dumped on a Closed Jaw of LHC collimator in LSS5
Beam dumped on a Closed Jaw of LHC collimator in LSS5. SEM to BLMI comparison p+
BLMI A
SEM
Black line – signal not clipped 5*τ_filter = 350ms
D.Kramer BLM Audit

17. Xtalk clearly depends on the derivation
Cable crosstalks study – important crosstalks caused by long cables in the LSS
Ch 6..8 unconnected
Xtalk clearly depends on the derivation
Signal peak ratio 5e-2 (26dB) (worst case)
Integral ratio e-3 (47dB)
Similar behavior for system A
X-talks limited to 1 CFC card only!
D.Kramer BLM Audit

18. Standard BLMI ARC installation
HV Power Supply
HV ground cut here
BLMI
Small low pass filter in the CFC input stage
CFC is always close to the quadrupole
Up to 8 BLMs connected in parallel
D.Kramer BLM Audit

19. BLMI / SEM installation for collimation areas
6 HV capacitors in parallel
HV capacitor removed
8 chambers in 1 NG18 cable (up to 700m)
150k for current limitation
280pF = chamber’s capacity
~25pF = SEM’s capacity
SEM has not 150k protection!
D.Kramer BLM Audit

20. Limits the peak current on the chamber input to 1500 / 150k = 10mA
150kOhm Rp resistor for BLMI i/o current limitation between HV capacitor & IC)
Limits the peak current on the chamber input to / 150k = 10mA
Fast loss has only the Chamber charge available 280pF * 1500V = 0.4 uC
Corresponds to ~ 7 mGy total loss
Corresponds to ~ 180 Gy/s (PM limit = 22 Gy/s)
Slows down the signal collection
DC current limited to 1500V / 1Mohm = 1.5 mA
Corresponds to ~ 26 Gy/s (total in max 8 chambers)
D.Kramer BLM Audit

21. BLMI and SEM in the dump line IR6 on the MKB
D.Kramer BLM Audit

22. 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 in the wall
Integral of Simulation = e-mm
Integral of Scan2 = e-mm
Relative difference
22%
D.Kramer BLM Audit

23. Prototype tests with 63MeV cyclotron beam in Paul Scherer Institute
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 = 0.8%
PSI proton beam 62.9MeV BLMS prototypes F & C Type HV dependence of SEY
D.Kramer BLM Audit

24. Measurements in PS Booster Dump line with 1.4 GeV proton bunches
Older prototype measured Type C {Type F simulated}
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
D.Kramer BLM Audit

25. Example loss induced by the fast moving SPS scraper
Example loss induced by the fast moving SPS scraper. Measured in the collimation area by the LHC BLM system 4 different monitors (2006-old electronics)
D.Kramer BLM Audit

26. Example of beam losses in the SPS collimation area during a collimator movement of 10um (2006) Coasting beam
FFT spectrum
2006 data
CWG 19/3/07
D.Kramer BLM Audit

27. SPS Coasting beam 270GeV 200um Left jaw move and FFT spectra
D.Kramer BLM Audit

28. Complete FFT from the previous plot Horizontal Tune calculation from the BLM measurement -> oscillations in the beam not in the BLM system
D.Kramer BLM Audit