EDMS 534575
ORGANISATION EUROPEENNE POUR LA RECHERCHE NUCLEAIRE
EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
Laboratoire Européen pour la Physique des Particules
European Laboratory for Particle Physics
Safety Commission
Technical Note
CERN-SC-2004-084-RP-TN
Measurement of beam losses in ECX4 during the LHC high-intensity
beam extraction into TT40
Helmut Vincke and Doris Forkel-Wirth
Abstract
The CNGS/LHC extraction facility, which is housed in ECX4 leads the 400/450 GeV/c
proton beam from the SPS into the TT40 tunnel. From there the beam is routed towards
the TI8 tunnel (direction LHC) or the TT41 tunnel (CNGS). The ECX4 area is separated
by a ~5 m concrete shielding wall from the accessible area ECA4. This shielding wall
separating the two areas shows weaknesses. As a result of that strongly elevated dose rate
levels have to be expected in ECA4 during extraction operation at high intensities (e.g.
nominal CNGS operation). The final dose rate in ECA4 will depend on the beam loss
level at the extraction unit in ECX4. Calculations for the full beam loss were performed,
however, up to now no clear figure concerning real beam losses during high-intensity
extraction operation were available. During the high-intensity extraction test in
November 2004 we performed radiation measurements, which provided us with
information concerning the loss rate in ECX4. This paper discusses the outcome of this
measurement campaign and concludes on the beam loss levels for the nominal LHC 25 ns
beam.
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Introductions
For the LHC and CNGS operation the SPS beam is extracted into the TT40 tunnel. From
there it is guided into the TI8 tunnel (direction LHC) or the TT41 tunnel (CNGS). This
extraction operation causes losses at the extraction facility located in the ECX4 area.
Figure 1 presents the geometrical details of the beam extraction area ECX4 as it was
implemented in FLUKA simulations used to calculate the dose rate per impacting beam
particle in the adjacent ECA4 area. The ECX4 area is separated from the ECA4 area by a
shielding wall which shows weaknesses in various points (see Figure 2). Since the real
operational beam loss is not known, this value was calculated for the total beam loss in
[Vin02a, Vin02b]. However, the dose rate during CNGS and LHC operation strongly
depends on the real beam loss values in the extraction unit. In order to get a clearer
image of this value, measurements with a radiation monitor on the weak part of the
shielding between ECX4 and ECA4 were performed during the beam extraction test in
November 2004.
Cable tubes
ECA4
Chicane entrance
to ECA4
Absorber
TPSG
Extraction
magnets
Chicane
to ECA4
SPS tunnel
Figure 1: Geometry of ECX4, housing the extraction facility used for routing the beam
towards the transfer tunnel TT40.
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ECX4
SPS tunnel
Access gallery
Shielding
of corners
Weak
shielding
slit
Cable tubes
Chicane entrance
to SPS
Shielding wall
Radiation
detector
Figure 2: FLUKA geometry of the ECA4 area, which shows also the ECX4, the SPS and
the access gallery to TT40. The barracks located at the end of ECA4 are not displayed.
Furthermore, a cross section through the shielding wall is presented which shows the
weak parts of the shielding (red circle).
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Radiation measurement procedure
A hydrogen ionization chamber was placed in the middle of the top of the shielding (see
indication in Figure 2). During the high-intensity extraction period, measurements of the
radiation levels at this weak part of the shielding were performed. The parameters of this
experiment are as follows:
Radiation monitor parameters
Monitor type:
Pressure:
Active volume:
Calibration:
IG5, hydrogen filled ionization chamber, Ref no.: 112546
20 bar
5.4 l
calibrated with an PuBe source, 1Count = 6.95 nSv
Beam loss parameters
Beam momentum:
Assumed loss point:
450 GeV/c
front face of absorber TPSG, which is foreseen to capture
misguided beam particles (see Figure 1)
up to 3.2×1013 proton
extraction intensity as a function of time is presented in
Figure 3.
nominal LHC 25 ns beam (SPS LHC)
Intensity per extraction:
Extraction time structure:
Beam type:
beam intensity in TT40
3.50E+13
3.00E+13
protons per cycle in TT40
2.50E+13
2.00E+13
1.50E+13
1.00E+13
5.00E+12
02:50
02:40
02:30
02:20
02:10
02:00
01:50
01:40
01:30
01:20
01:10
01:00
00:50
00:40
00:30
00:20
00:10
00:00
23:50
23:40
23:30
0.00E+00
time
Figure 3: Proton intensity extracted into TT40 on November 8th/9th between 23:30 and
2:40.
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Results
The simulated dose rate in the ECX4/ECA4 area per lost particles in the protection unit is
presented in Figure 4. This color contour plot shows the situation as it appears in the
middle of the ECA4 area perpendicular to the beam axis. The arrow indicates the position
of the hydrogen monitor during the experiment.
2.3E-13
1.1E-13
5.1E-14
2.3E-14
1.1E-14
5.1E-15
2.3E-15
1.1E-15
5.1E-16
2.3E-16
1.1E-16
5.1E-17
2.3E-17
1.1E-17
5.1E-18
2.3E-18
1.1E-18
5.1E-19
2.3E-19
1.1E-19
5.1E-20
2.3E-20
Monitor
position
Figure 4: Radiation levels caused by a loss of one particle in the TPSG absorber, shown
in a cross section through the middle of the ECA4 area (see also Figure 2) perpendicular
to the beam line. The unit of the scale is given in Sv per particle impacting in the
protection unit.
The accurate value of the calculated dose rate per lost particle at the position of the
radiation monitor is 1.5×10-16 Sv (20 % uncertainty). These values are based on the
calculation results around the detector position. With this simulation value and the
measured dose rate at the detector position one can calculate the number of particles lost
on the ECX4 side of the installation.
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The analysis concerning the beam loss is performed for two cases:
1. Highest observed loss caused by a single extraction: the highest dose rate at the
detector position was measured at 0:00, when 2×1013 particles were extracted.
The background corrected dose caused by this extraction was 3.2 Sv, leading to
a dose of 1.6×10-19 Sv per particle extracted into TT40.
2. Average loss during 18 high-intensity (mainly  3×1013 particles) extractions: the
background corrected dose measured during the 18 high-intensity extractions
between 0:00 and 2:40 was found to be 15.1 Sv. The integral intensity of the 18
shots extracted into TT40 was 52.4×1013. This leads to a dose of 2.88×10-20 Sv
per particle extracted into TT40.
The final ratio of the beam lost in the protection unit can be calculated for both cases by
equation 1:
Loss ratio 
measured dose per extracted particle
calculated dose per lost particle
(1)
For the two cases the beam loss rate results are as follows:
Case 1) Highest observed loss caused by a single extraction: beam loss rate = 0.11%
Case 2) Average loss during 18 high-intensity (mainly  3×1013 particles) extractions:
beam loss rate = 0.019%.
Note: the beam extracted during the experiment was the one which will be extracted into
the LHC. The CNGS beam might have a higher loss rate due to its bigger size and its
different extraction pattern (double extraction spaced by 50 ms).
Two other IG5 hydrogen chambers (older models) were located 4 m further downstream
and 4 m further upstream of our detector position. The IG5 chamber upstream (detector
no.: 7379) showed already problems during installation. It did not react at all to the
presence of a radioactive source. The malfunctioning of this chamber was also confirmed
by the readout results (way too low) during the test campaign. The chamber located 4 m
further downstream (detector number: 7419) confirmed the result concerning the average
loss within an uncertainty of 15 %. The result based on the single loss was confirmed
within 40 %. The two different disagreements can possibly be explained by a different
beam loss position during the various extractions.
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Uncertainty of the beam loss assessments
The main uncertainty of the calculation can be found in the following points:
1. Uncertainty caused by detector calibration with a PuBe source instead the
real particle field occurring at ECA4: The mixed particle field occurring at
the detector position shows an energy spectrum ranging from fractions of
eV up to several GeV. Although the main component of this field is
composed of neutrons, other particle sorts do contribute to the final
counting rate. During the PuBe calibration campaign the detector is
exposed to a neutron field with an energy ranging up to 11 MeV. By using
the PuBe-response of the detector, the response to other particle and
energy components than those occurring in the PuBe field might be
considered wrongly.
2. Since in this irradiation situation hadrons pose the main component in
terms of H*(10), the electromagnetic cascade was omitted in FLUKA.
This procedure saves CPU time and leads therefore to simulation results
within reasonable time.
Both effects together are assessed to contribute to the uncertainty with less than a factor
of 2. This assessment is based on experience from benchmark studies concerning the
hydrogen IG5 chambers performed in the CERF field at CERN.
Discussion of the result
The simulations and therefore the beam loss results are based on the assumption that the
beam loss is located at the front face of the TPSG absorber. The simulated radiation
values per lost beam particle on the ECA4 side become higher if the losses occur in one
of the six extraction magnets (MSE). If one assumes that the losses during this test
occurred in the magnets and not in the TPSG absorber, the final calculated beam loss rate
would drop (see Equation 1). The TPSG absorber is build to absorb misguided particles
before hitting and damaging the magnet. Therefore, this assumption does not only result
in the highest possible beam loss value but it reflects also a realistic scenario.
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Conclusion
The analysis of the dose measurement at the weak area of the ECX4/ECA4 shielding
during the LHC extraction operation on November 8th/9th was performed for two cases of
the high-intensity extraction. The first is based on a single extraction with an intensity of
2×1013 particles. This particular extraction showed the highest losses during the test
period. The calculated beam loss rate based on this scenario was 0.11%. The second
scenario analyses 18 high-intensity extractions (~3×1013 per extraction) in order to
evaluate the average beam loss rate at the protection unit in ECX4. The resulting average
beam loss ratio in ECX4 was 0.024%. These values are valid for the SPS beam, which is
extracted into the LHC. The uncertainty of these values is assumed to be within a factor
of two. The CNGS beam might show higher loss rates due to its bigger size and its
different extraction pattern.
Acknowledgements
We would like to express many thanks to NorbertAguilar, Nadine Conan, Sylvain Gatard,
Angelito Herve and Thimothee Schmittler for their help in preparing the measurement
set-up.
References
Vin02a: Helmut Vincke, Graham R. Stevenson, Doris Forkel-Wirth, Radiation in ECA4
caused by beam losses in the dummy protection unit installed in ECX4, CERN-TIS2002-026-RP-TN, EDMS Id 350451
Vin02b: Helmut Vincke, Graham R. Stevenson, Doris Forkel-Wirth, Radiation levels in
ECA4 caused by beam losses in the septum magnets to be installed in ECX4, CERN-TIS2002-025-RP-TN, EDMS Id 350427
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