very high radiation detector for the lhc bl m system based on s econdary e lectron emission

2.11.20072.11.2007 IEEE NSS 2007 D.KramerIEEE NSS 2007 D.Kramer 11

Very High Radiation Detector for the LHC BLM System based onSecondary Electron Emission

Daniel KramerDaniel Kramer, Eva Barbara Holzer, Bernd Dehning, , Eva Barbara Holzer, Bernd Dehning, Gianfranco FerioliGianfranco Ferioli

CERN AB-BICERN AB-BI


LHC Beam Loss Monitoring LHC Beam Loss Monitoring systemsystem

~ 3700 ~ 3700 BLMIBLMI chambers chambers installed along LHCinstalled along LHC

~ ~ 280 280 SEMSEM chambers chambers required for high radiation required for high radiation areas:areas:

– CollimationCollimation

– Injection pointsInjection points

– IPsIPs

– Beam DumpsBeam Dumps

– Aperture limitsAperture limits

Main SEM requirementsMain SEM requirements

– 20 years lifetime (up to 20 years lifetime (up to 70MGray/year70MGray/year))

– Sensitivity ~Sensitivity ~3E43E4 lower than lower than BLMIBLMI


Secondary Emission Monitor Secondary Emission Monitor working principleworking principle

Secondary electronsBias E fieldTi Signal electrodeTi HV electrodes

Steel vessel (mass)

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*

< 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)No direct contact between Signal and Bias (guard ring)




Steel vessel (mass)






< 10-4 mbar

Incoming

particle







Steel vessel (mass)






< 10-4 mbar

Incoming

particle




Incoming

particle


SEM production assemblySEM 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°C vacuum bakeout and NEG activation (p<10-

10mbar)

Ti electrodes partially activated (slow pumping observed)


Simulations in Geant4 Simulations in Geant4

Detailed Geometry of SEM F type implemented Detailed Geometry of SEM F type implemented

– Signal electrode covered by thin layer of TiOSignal electrode covered by thin layer of TiO2 2

PPhoto-hoto-AAbsorption-bsorption-IIonization module used for onization module used for production of delta electronsproduction of delta electrons

QGSP_BERT_HP used as main physics listQGSP_BERT_HP used as main physics list

Signal generation done bySignal generation done by

– calculating calculating charge balancecharge balance on signal electrode on signal electrode

– recording “recording “True SETrue SE” produced by custom generator” produced by custom generator

Production threshold for eProduction threshold for e++/e/e-- set to 9 set to 9mm


Semi empirical approach using Semi empirical approach using simplified Sternglass formulasimplified Sternglass formula

Secondary Emission Yield is Secondary Emission Yield is proportional to electronic proportional to electronic dE/dx in the surface layerdE/dx in the surface layer

– LLSS = (0.23 N = (0.23 Ngg))-1-1

gg = 1.6 Z = 1.6 Z1/31/31010-16-16cmcm22

““TrueSEY” of each particle TrueSEY” of each particle crossing the surface boundary crossing the surface boundary calculated and SE recorded calculated and SE recorded with this probabilitywith this probability

Correction for impact angle Correction for impact angle included in simulationincluded in simulation

Fast Fast electrons considered as electrons considered as other primariesother primaries

Model calibration factor

Penetration distance of SE

Electronic energy loss

Comparison => CF = 0.8


Simulated response curves for Simulated response curves for different particle typesdifferent particle types

Geant4 version Geant4 version 8.1.p018.1.p01

30k primaries 30k primaries for each for each energy pointenergy point

Longitudinal Longitudinal impactimpact

Gaussian Gaussian beam beam

rr = 2mm = 2mm

e- absorbed in electrode


Longitudinal Longitudinal impact of proton impact of proton beambeam

rr = 2mm = 2mm

Chamber tilted Chamber tilted by ~1by ~1°°

Simulation sensitive Simulation sensitive to beam angle and to beam angle and divergencedivergence

Negative signal due Negative signal due to low energy e- to low energy e- from secondary from secondary shower shower

400 GeV Beam scan in TT20 SPS 400 GeV Beam scan in TT20 SPS lineline


Longitudinal Longitudinal impact of proton impact of proton beambeam

rr = 2mm = 2mm

Chamber tilted Chamber tilted by ~1by ~1°°

Simulation sensitive Simulation sensitive to beam angle and to beam angle and divergencedivergence

Negative signal due Negative signal due to low energy e- to low energy e- from secondary from secondary shower shower

400 GeV Beam scan in TT20 SPS 400 GeV Beam scan in TT20 SPS lineline

chamber diameter


Prototype tests with 63MeV Prototype tests with 63MeV cyclotron beam in Paul Scherer cyclotron beam in Paul Scherer InstituteInstitute

Prototype C -> more Prototype C -> more ceramics inside (no guard ceramics inside (no guard ring) ring)

Prototype F -> close to Prototype F -> close to production versionproduction version

Current measured with Current measured with electrometer Keithley 6517Aelectrometer Keithley 6517A

HV power supply FUG HLC14HV power supply FUG HLC14

Pattern not yet fully Pattern not yet fully understoodunderstood

– Not reproduced by Not reproduced by simulationsimulation

High SE response if High SE response if U_bias > 2VU_bias > 2V

Geant4.9.0 simulated SEY = Geant4.9.0 simulated SEY = 25.525.50.8%0.8%

PSI proton beam 62.9MeV BLMS prototypes F & C Type HV dependence of SEY

10-3

10-2

10-1

100

0.2

0.25

0.3

0.35

0.4

HV [kV]

De

tect

or

resp

on

se [c

ha

rge

s/p+

]

C typeF typeGeant4


Measurements in PS Booster Measurements in PS Booster Dump line with 1.4 GeV proton Dump line with 1.4 GeV proton bunchesbunches Older prototype used - Older prototype used -

Type CType C

Profiles integrated with Profiles integrated with digital oscilloscopedigital oscilloscope

– 1.5kV bias voltage1.5kV bias voltage

– 80m cable length80m cable length

– 50 50 termination termination

– Single bunch passageSingle bunch passage

SEY measurementSEY measurement

– 4.9 4.9 0.2% 0.2%

Geant4.9.0 simulationGeant4.9.0 simulation

– 4.2 4.2 0.5% 0.5%

Normalized response

0 0.5 1 1.5 2 2.5

x 1012

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Beam intensity [p+/bunch]

Det

ecto

r re

spon

se [c

harg

es/p

+]

DataGeant4.9fit Data


BLMS compared to BLMS compared to reference radiation reference radiation monitor ACEM monitor ACEM (Aluminum Cathode (Aluminum Cathode Electron Multiplier Electron Multiplier tube)tube)

ACEM not directly in ACEM not directly in the beamthe beam

Rise/fall time < 50 nsRise/fall time < 50 ns

– Dominated by Dominated by unknown intensity unknown intensity distributiondistribution

Normalized intensity Normalized intensity 1.3 101.3 101919pp++/s/s

SEM response to single proton bunch of SEM response to single proton bunch of 2.16 102.16 101313 protons with 160ns length protons with 160ns length

Measurements in PS Booster Measurements in PS Booster Dump line with 1.4 GeV proton Dump line with 1.4 GeV proton bunchesbunches

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-20

0

20

40

60

80

100

120

140

160

Cur

rent

SE

M [m

A]

time [s]-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0

1

2

3

4

5

Cur

rent

AC

EM

[mA

]


ConclusionsConclusions The BLMS detector was successfully tested in The BLMS detector was successfully tested in

different proton beamsdifferent proton beams

Geant4 simulations are in good agreement with Geant4 simulations are in good agreement with these experimentsthese experiments

– => chosen model is validated=> chosen model is validated

Sign change of output current possible Sign change of output current possible under very specific circumstancesunder very specific circumstances

Verification measurements in mixed radiation Verification measurements in mixed radiation field of LHC test collimation area in SPS are field of LHC test collimation area in SPS are ongoingongoing

360 BLMS Detectors were produced in IHEP 360 BLMS Detectors were produced in IHEP Protvino and will be tested soonProtvino and will be tested soon

very high radiation detector for the lhc bl m system based on s econdary e lectron emission

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