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Quartz Cherenkov Beam Halo Monitor for LHC

This monitor uses quartz Cherenkov counters to measure the halo distribution, position, and flux of protons in the LHC beam. It can detect individual protons with high resolution and can be used for beam diagnostics and controlling upstream backgrounds for other detectors.

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Quartz Cherenkov Beam Halo Monitor for LHC

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  1. A Beam Halo Monitor for the LHC based on Quartz Cherenkov Counters Mike Albrow (Fermilab) Features: Variable distance from beam. Full azimuthal halo distribution measured. Incoming and outgoing bunches monitored separately (when in same pipe). Centroid of halo measureable (beam position monitor, if beam center = halo center) Bunch x bunch, and inter-bunch capability. Distribution in z also within a bunch may be possible. Basis: Small quartz (fused silica) bars, typically* ~2 x 2 x 5mm long, parallel to beam. Inside the LHC vacuum chamber, at a radial distance R from the beam center. High energy protons generate Cherenkov light in the bar, detected by a small SiPM (Silicon photomultiplier) at the back end. The light (UV-optical) is directional, emitted at about 48o to the proton, and is totally internally reflected on the sides. One option is to have a SiPM at each end to measure and distinguish beam halo in opposite directions, when both beams are in the same pipe. SiPMs require only a pair of small wires carrying HV (typically ~40V) and the signal; the feedthrough’s from inside the vacuum pipe should be straightforward. * Dimensions are indicative only, not optimized.

  2. These small counters can detect individual protons and give their time with a resolution of order 20-30 ps, to compare with the bunch length of about 150 ps. They can be gated to record only during a bunch (~ 1 ns gate), or with a time-displaced gate to measure between bunches. If the halo fluxes are such that many halo protons are inside the bar per bunch, the overall pulse height can be measured in an ADC, the analog signal being proportional to the number of protons. The geometry, and all dimensions, are to be optimized, but I give one option on the next slide. One can have (e.g.) 8 of these detectors spaced by 45o around the beam to measure the azimuthal halo distribution. They can be in a mechanical support that could optionally be displaceable as a whole in x,y to equalize opposite signals and thus find the centroid of the halo. Rotations in 45o steps can inter-calibrate the eight detectors. One finds the shape (“circularity” or “ellipticity”) of the halo from the eight measurements. The mechanical support can have another degree of freedom: the radius of the ring. A suggested way of doing this is given next slide. Thus we can measure: Halo fn(R,φ,bunch #, z), and read it out using (LHC) standard HPTDC DAQ.

  3. Schematic of one detector: Wires (~40V & signal) SiPM Quartz bar ~ 3 mm proton photons (SiPM at both ends if two opposite beams) ~ 5 mm Beam pipe (vac) proton Possible support scheme:: x Vary spacing of ring supports Varies radius of detector ring HINGES BEAM Feed- through halo proton BEAM Transverse view Detector ring may have x-y position control May rotate to cross-calibrate detectors Ring supports

  4. Example of SiPM (Mamamatsu MPPC). Other sources exist We tested 3mm x 3mm SiPMs

  5. Beam tests in Fermilab MTest, Anatoly Ronzhin et al. Report to AEM Apr 12th 2010 http://www.fnal.gov/directorate/program_planning/ all_experimenters_meetings/special_reports/Ronzhin_T979_04_12_10.ppt

  6. See also A.Ronzhin et al, NIM A 616 (2010) p.38

  7. Some questions, and studies that can be done: What are expected fluxes f(R) /bunch at various beam line positions? (Nikolai Mokhov?)  bar sizes? 1mm x 1mm ? 3 mm x 3 mm? Bars or fibers? Fiber-bundle? Radiation hardness of SiPM, is it sufficient? 1014 p/cm2 prob OK. More rad tests can be done.(Freeman) Evaluate different SiPM’s, manufacturers (they are cheap, <~ $100 each) Rad hardness of fused Silica, prob OK. Is this the best Cherenkov radiator? Optimize bar length. We have simulations which need to be to be tuned. Precision mechanics, calibrations of motions etc, and vacuum issues. At what z-locations would these be practical and most useful? CMS (Pt 5) +/- ~ 200m looks good. Electronics: Preferable to use some existing LHC standard. E.g. HPTDC-chip based read-out is used in CMS and ALICE (at least) and can measure pulse-heights and time on a /25ns cycle. Have 8 channels on a board with 25 ps least count. May be developed to have higher resolution if effort is provided or supported. >>> Apart from its generic use as a beam diagnostic, if and when we install HPS (High Precision Spectrometers) at z = 240 m, 420m, it will be very valuable to control upstream backgrounds for those detectors. I would expect these could be developed, built and tested in time for an installation in the 2012 Shutdown. Close collaboration between Fermilab (Manfred Wendt +) and CERN-LHC instrumentation. Anatoly Ronzhin, Andriy Zatserklyaniy and Erik Ramberg have stimulated the development of this idea and will probably continue to develop the detector aspects.

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