Rad-hard Luminosity Monitoring for the LHC Page: 1 of 3
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RAD-HARD LUMINOSITY MONITORING FOR THE LHC*
J. F. Beche, J. M. Byrd, K. Chow, P. Denes, W. Ghiorso, H. S. Matis, M. Monroy, A. Ratti,
S. de Santis, W. C. Turner, LBNL, Berkeley CA, USA
E. Bravin, CERN, Geneva, Switzerland
P. F. Manfredi, W. Vandelli, Univ. di Pavia, Pavia Italy
Luminosity measurements at the high luminosity points
of the LHC are very challenging due to the extremely
high radiation levels in the order of 180 MGy/yr. We have
designed an ionization chamber that uses a flowing
inorganic gas mixture and a combination of metals and
ceramics. With such a choice, an additional challenge is
achieving the necessary speed to be able to resolve bunch-
by-bunch luminosity data. We present the design, analysis
and experimental results of the early demonstration tests
of this device.
The LHC beam luminosity monitor is a gas ionization
chamber that observes minimum ionization particles
(MIPs) near the shower maximum in the zero degree
neutral particle absorbers (TAN) of IPs 1 and 5. The
shower energy, measured by suitable detectors in the
absorbers is proportional to the energy flux of neutral
particle collision fragments from the IPs and, therefore, to
the luminosity. The principle lends itself to a luminosity
measurement on a bunch-by-bunch basis. While simple in
principle, the system must comply with extremely
stringent requirements. On one side, its speed of operation
must match the 40 MHz bunch repetition rate of LHC. On
the other, the detector must stand extremely high radiation
In order to cope with these requirements, we have
designed a multi-gap, flowing gas ionization chamber,
where no active and only few passive components will be
installed on the main path of the shower. The gas
ionization chamber with an Ar plus N2 mixture was
chosen because of its radiation resistance.
To preserve the speed of the signal, we need to install
the front end analog amplifiers in the LHC tunnel close to
the ionization chamber. A low noise active termination
properly prevents signal reflections and is integrated in
the front end amplifier, together with a driver to transmit
the analog signal back to the counting building, where the
signal is shaped and digitized.
CERN has defined the system requirements for
luminosity monitors, which are referred to as Beam Rate
of Neutrals (BRAN) . The highlights of these
*This was supported by the Director, Office of Energy Research,
Office of High Energy and Nuclear Physics, High Energy Physics
Division, U. S. Department of Energy, under Contract No. DE-
specifications are a relative luminosity signal stable to
1%, bunch-by-bunch measurement capability, crossing
angle measurements and 'reasonable integration times',
where several tens of seconds is considered acceptable.
The extreme radiation levels seen by the detector
determine the technical approach of using a gas ionization
chamber. In this case, we are only allowing ceramics,
metals and passive components to be in the path of the
hadronic shower. Flowing the gas further ensures
The number of gaps is chosen by trading off between
signal strength, proportional to the number of parallel
gaps, and speed, which is negatively affected by the larger
capacitance shown by the parallel gaps. The final
compromise, supported by MAGBOLZ modeling, is to
use 6 gaps of 1mm each. Table 1 summarizes the baseline
parameters of the device.
To allow for crossing angle calculations, the LUMI
detector is segmented into four electrically isolated
chambers, so that a rough position measurement of
transverse position of the IP can be made. As the
ionization chamber is designed to operate with the
pressure in the range from one to ten atmospheres, a
pressure vessel encloses the full detector.
Table 1 - Ionization chamber main parameters
Quadrant area,mm2 1600
Gap between plates,mm 1.0
No. of gaps in parallel 6
Gas pressure, atmos abs 6
Ioniz pairs/mip-mm 58.32
E/p, V/mm-atm 200
Gap voltage, V 1200
Electron drift velocity, mm/sec 45.0
Amplifier/Shaper gain, V/e- 0.16
rms noise,mV 1.24
The gas mixture must feature an adequately high
electron drift velocity, without using organic molecules
that may polymerize under the effect of radiation. We
opted for a mixture of Argon with a few percent of
Nitrogen. The charge yield in the shower detection can be
raised to the desired value by acting on the pressure of the
filling gas. The nominal gas mixture is 94% Ar + 6% N2
at a pressure of 6 atmospheres. At this pressure, the HV
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Beche, J.F.; Byrd, J.M.; Chow, K.; Denes, P.; Ghiorso, W.; Matis,H.S. et al. Rad-hard Luminosity Monitoring for the LHC, article, June 24, 2007; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc901784/m1/1/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.