1. Variable distance from beam.
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 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

6. Beam tests in Fermilab MTest, Anatoly Ronzhin et al
Beam tests in Fermilab MTest, Anatoly Ronzhin et al. Report to AEM Apr 12th 2010
all_experimenters_meetings/special_reports/Ronzhin_T979_04_12_10.ppt

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

8. 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.