Secondary Emission Monitor
for very high radiation areas of LHC
Daniel Kramer for the BLM team
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
10.6.2008
D.Kramer
BLM Audit
2
Secondary Emission Monitor
working principle








Secondary
electrons
Secondary Electron Emission is a surface
phenomenon
Bias E field
Energy of SE (below ~ 50 eV, dominant
for signal) is independent on primary
energy
Ti Signal
electrode
Ti HV
electrodes
SE are pulled away by HV bias field
(1.5kV)
Signal created by e- drifting between
the electrodes
<
10-4 mbar
Steel vessel
(mass)
Delta electrons do not contribute to
signal due to symmetry*
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)
10.6.2008
D.Kramer
BLM Audit
3
Secondary Emission Monitor
working principle








Secondary
electrons
Secondary Electron Emission is a surface
phenomenon
Bias E field
Energy of SE (below ~ 50 eV, dominant
for signal) is independent on primary
energy
Ti Signal
electrode
Ti HV
electrodes
SE are pulled away by HV bias field
(1.5kV)
Signal created by e- drifting between
the electrodes
<
10-4 mbar
Steel vessel
(mass)
Delta electrons do not contribute to
signal due to symmetry*
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)
10.6.2008
D.Kramer
BLM Audit
4
Secondary Emission Monitor
working principle








Secondary
electrons
Secondary Electron Emission is a surface
phenomenon
Bias E field
Energy of SE (below ~ 50 eV, dominant
for signal) is independent on primary
energy
Ti Signal
electrode
Ti HV
electrodes
SE are pulled away by HV bias field
(1.5kV)
Signal created by e- drifting between
the electrodes
<
10-4 mbar
Steel vessel
(mass)
Delta electrons do not contribute to
signal due to symmetry*
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)
10.6.2008
D.Kramer
BLM Audit
5
Secondary Emission Monitor
working principle








Secondary
electrons
Secondary Electron Emission is a surface
phenomenon
Bias E field
Energy of SE (below ~ 50 eV, dominant
for signal) is independent on primary
energy
Ti Signal
electrode
Ti HV
electrodes
SE are pulled away by HV bias field
(1.5kV)
Signal created by e- drifting between
the electrodes
<
10-4 mbar
Steel vessel
(mass)
Delta electrons do not contribute to
signal due to symmetry*
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)
10.6.2008
D.Kramer
BLM Audit
6
Secondary Emission Monitor
working principle








Secondary
electrons
Secondary Electron Emission is a surface
phenomenon
Bias E field
Energy of SE (below ~ 50 eV, dominant
for signal) is independent on primary
energy
Ti Signal
electrode
Ti HV
electrodes
SE are pulled away by HV bias field
(1.5kV) Transit time 500ps
Signal created by e- drifting between
the electrodes
< 10-4 mbar
Steel vessel
(mass)
Delta electrons do not contribute to
signal due to symmetry*
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)
10.6.2008
D.Kramer
BLM Audit
7
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 < 10-4
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
10.6.2008
D.Kramer


Very high adsorbtion capacity of
H2, CO, N2, O2
Not pumping CH4, Ar, He
BLM Audit
8
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)
400
1.E+04
Bakeout cycle for IHEP IC
350
Temperature [°C]

36xIC
300
Pressure
He leak
tests
250
200
1.E+02
350
1.E+00
300
1.E-02
1.E-04
150
1.E-06
Vacuum
bakeout
100
50
1.E-08
0
1.E-10
0
5
10.6.2008
10
15
20
25
30
35
40
45
Time [h]
D.Kramer
Manifold stays colder to
limit the load to the
pumping system
Activation temperature
limited by the feedthroughs
Bakeout cycle for IHEP SEM
400
Temperature [°C]

18xSEM
NEG
activation
Vacuum
bakeout
250
Manifold
pinchoff
200
150
100
Ion pump
started
He leak
tests
50
0
0
BLM Audit
5
10
15
20
25
30
35
40
45
Time [h]
9
Geant4 simulations of the SEM

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

QGSP_BERT_HP as main physics model
Model calibration factor
Electronic
Penetration
energy loss
distance of SE
0°
impact
angle
Geant4 SEM
Response function
Comparison to literature
values => CF = 0.8
10.6.2008
D.Kramer
BLM Audit
10
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

0.259 +/- 0.016 Gy/count
10.6.2008
D.Kramer
H4 Calibration setup with Cu target and a box
with 16 SEMs on a movable table
BLM Audit
11
Calibration results
Only 2 chambers out of 250
had higher offset current
Not corrected
for systematic
position errors
SEM calibration in H4, expect from Geant4 3.95+/-0.17
-
ratio QSEM/Qbeam [e /pot]
8
6
4
2
0
0
2
4
6
8
10
monitor position
12
14
16
300
350
12
SEM calibration in H4 week 45
1bar(0.6 sigSEM / sigSEM AIR) = 0.26 mbar
Pressure inside SEMs smaller than this
10.6.2008
D.Kramer
offset current [pA]
Upper Limit on the SEM pressure:
(equivalent to 3 of the histogram)
2
Offset current
without beam
1.5
1
0.5
0
-0.5
0
50
BLM Audit
100
150
200
250
SEM ID number
400
Table of SEM measurements and
corresponding simulations
Test beam
Measured [e-/prim]
Geant4
Rel. Dif. [%]
PSI 63MeV
0.27 ± 0.014
0.2665 ± 0.0043
1.1
PSB 1.4GeV
0.0495 ± 0.0006
0.0416 ± 0.0046
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
TIDV dump
Long term test
-
10.6.2008
D.Kramer
BLM Audit
26
13
Thanks
10.6.2008
D.Kramer
BLM Audit
14
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)
10.6.2008
D.Kramer
BLM Audit
15
Beam dumped on a Closed Jaw
of LHC collimator in LSS5.
SEM to BLMI comparison 1.3 1013p+
BLMI A
SEM
Black line – signal not clipped
5*τ_filter = 350ms
10.6.2008
D.Kramer
BLM Audit
16
Cable crosstalks study – important crosstalks
caused by long cables in the LSS
Xtalks CD3 - B
15
CH6
CH7
CH8

counts
10

5
0

0
0.1
0.2
0.3
0.4
time [ms]
0.5
0.6
0.7

Differential signals of CD3 - B
differences [counts]
60
CH1
CH2
CH3
40

Ch 6..8 unconnected
Xtalk clearly depends on
the derivation
Signal peak ratio 5e-2
(26dB) (worst case)
Integral ratio
(47dB)
4.4e-3
Similar behavior for
system A
20
0
-20
0
10.6.2008

0.1
0.2
0.3
0.4
time [ms]
0.5
0.6
D.Kramer
0.7
BLM Audit
X-talks limited to 1 CFC
card only!
17
Standard BLMI ARC installation
HV Power
Supply
HV ground
cut here
Small low pass filter in
the CFC input stage
BLMI
CFC
BLMI
1.5kV
CBH50
280pF
BJBHT
1M
CBH50
integrator
10M
COAX < 30m
470
2k2
4n7
0.47u
8X
Up to 8 BLMs
connected in
parallel
10.6.2008
D.Kramer
CFC is always
close to the
quadrupole
BLM Audit
18
BLMI / SEM installation for
collimation areas
6 HV
capacitors
in parallel
8 chambers in 1
NG18 cable
(up to 700m)
HV
capacitor
removed
CFC
BLMI
BJBHT
1.5kV
CBH50
1M
280pF
BJBHT
2u8
CBH50
COAX < 30m
150k
B
J
B
A
P
integrator
NG18 < 700m
470
2k2
4n7
8X
150k for
current
limitation
10.6.2008
D.Kramer
280pF =
chamber’s
capacity
BLM Audit
~25pF =
SEM’s
capacity
SEM has
not 150k
protection!
19
150kOhm Rp resistor for BLMI i/o current
limitation between HV capacitor & IC)


Limits the peak current on the chamber input to
1500 / 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)
10.6.2008
D.Kramer
BLM Audit
20
BLMI and SEM in the dump line IR6 on the MKB
10.6.2008
D.Kramer
BLM Audit
21
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 efrom secondary
shower in the wall Integral of Simulation = 0.608 e-mm
Integral of Scan2 = 0.476 e-mm
10.6.2008
D.Kramer
BLM Audit
Relative difference
22%
22
Prototype tests with 63MeV cyclotron
beam in Paul Scherer Institute



0.4
Prototype F -> close to
production version
C type
F type
Geant4
Current measured with
electrometer Keithley 6517A
0.35
+

Prototype C -> more ceramics PSI proton beam 62.9MeV BLMS prototypes F & C Type HV
dependence of SEY
inside (no guard ring)
HV power supply FUG HLC14
Pattern not yet fully
understood
– Not reproduced by simulation


High SE response if
U_bias > 2V
Detector response [charges/p ]

0.25
0.2
Geant4.9.0 simulated SEY =
25.50.8%
10.6.2008
0.3
10
D.Kramer
-3
BLM Audit
10
-2
10
HV [kV]
-1
10
0
23
Measurements in PS Booster Dump
line with 1.4 GeV proton bunches


Older prototype measured Type C {Type F simulated}
Normalized response
0.07
Profiles integrated with
digital oscilloscope
Data
Geant4.9
fit Data
0.06
Detector response [charges/p+]
– 1.5kV bias voltage
– 80m cable length
– 50  termination
– Single bunch passage

SEY measurement
–

4.9  0.2%
0.05
0.04
0.03
0.02
0.01
Geant4.9.0 simulation
–
0
0
4.2  0.5%
10.6.2008
D.Kramer
BLM Audit
0.5
1
1.5
2
Beam intensity [p+/bunch]
2.5
12
x 10
24
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)
10.6.2008
D.Kramer
BLM Audit
25
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
10.6.2008
D.Kramer
BLM Audit
26
SPS Coasting beam 270GeV
200um Left jaw move and FFT spectra
10.6.2008
D.Kramer
BLM Audit
27
Complete FFT from the previous plot
Horizontal Tune calculation from the BLM measurement
-> oscillations in the beam not in the BLM system
10.6.2008
D.Kramer
BLM Audit
28

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