CMS detector
2.2 Beam & Radiation Monitoring for CMS
In relation to the protection and the requirement of stringent beam monitoring of CMS, it is worth mentioning the beam dump system located at point 6 of the LHC. The purpose of the beam dump system is to extract the two beams quickly and safely and is usually a planned action, initiated towards the end of a fill when the bunch intensities have been depleted. Occasionally, however, the control of the beams may be lost due to vacuum failures, triggering of magnet quench protection systems or a multitude of other reasons [35]. The beam dump system involves horizontally deflecting fast-pulsed magnets (‘kicker’ magnets) and vertically-deflecting septum magnets. A 3µs gap is incorporated into the bunch structures of both beams. These ‘abort gaps’ are created so that the kicker and septum magnets can turn on as the gaps pass the beam dump, directing the entire bunch train safely into absorber material outside of the LHC tunnel. This is known as a ‘synchronous beam dump’. There is, however, the possibility of a system failure resulting in an ‘asynchronous beam-dump’ where the kicker magnets turn on while the high energy proton beams are still passing, disturbing the momentum of the particles and causing large beam losses. In the event of such an occurrence, CMS is in danger of being showered with high energy, heavily ionising particles
The Compact Muon Solenoid 29 Table 2.5: A list of the BRM sub-detectors in CMS with their primary
func-tionality, location and time resolution.
Detector Location Purpose Sampling Time
Medipix z = 15 m, x = 12m Dose Monitoring & 1 minute Particle Identification
BCM1F Pixel Volume. Z = ±1.8 m Beam Monitoring ∼ns
BCM1L Pixel Volume. Z = ±1.8 m Beam Monitoring & abort 5µs BCM2 TOTEM T2. Z = ±14.4 m Beam Monitoring & abort 40µs BSC Z = ±10.9 m & Beam Monitoring & Triggers ∼ns
TOTEM T2, Z = ±14.4 m
BPTX Z =±175 m Beam Monitoring & Triggers ∼ps
which could damage the delicate silicon tracker (see section 2.1.2) and other sub-detectors [36]. It is extremely important that the monitoring of the beams and the radiation environment in CMS is done in such a way to protect the detector and allow for post-mortem analysis of dose and particle flux should an unplanned beam dump occur.
The CMS Beam & Radiation Monitoring (BRM) group have installed several subdetectors which are designed to detect and limit the damage due to sudden beam losses while allowing for quick post-mortem analysis should a large loss occur. Additionally, many of these subdetectors play further roles in CMS in the form of beam monitoring for normal running periods or triggering on minimum bias and zero bias events, as was the case with the BSC and the BPTX. Table 2.5 gives a list of the BRM sub-detectors in terms of their functionality, time resolution and location.
Medipix
The Medipix2 detectors are a collection of 256×256 pixel detectors measuring 1.4 cm2 and with pixel dimensions of 55µm2. The active 300µm silicon layer is covered by various conversion layers, assembled by the Czech Technical University in Prague. Figure 2.9 shows a photograph of the Medipix2 ASIC mounted on its readout board. Figure 2.10 shows an X-ray image from the Medipix chip taken through the various conversion layers. Table 2.6 lists the purpose of the various conversion layers.
Figure 2.9: A photograph of the Medipix2 ASIC on its readout board similar to those installed in
the CMS experimental cavern.
Figure 2.10: An x-ray image taken with the Medipix2 ASIC showing the various conversion
lay-ers.
Table 2.6: The various conversion layers glued to the silicon surface of the Medipix2 devices installed in CMS act to optimize small regions of the ASIC to certain particle fluxes and energies. The installation of these layers was done by a team lead by S. Posposil at the Czech Technical University in Prague.[37]
Layer Material Purpose
Al Aluminium Beam hardening. Removal of low energy
electrons.
PE Polyethylene Fast neutron conversion to protons.
LiF Lithium Fluoride Thermal neutrons to α particles.
Al + PE Aluminium + Polyethylene Remove low energy
electron signal from neutron signal.
Thick Al Aluminium More aggressive beam hardening.
Three Medipix devices were installed† on the walls of the CMS cavern and col-lected data throughout 2010 to compare particle fluence with those of Monte Carlo simulations[37].
2.2.1 BCM1F
The Beams Condition Monitor 1 - Fast (BCM1F)[38] is a single-crystal diamond based monitor located ±1.8 m from the nominal interaction point at a radius of only 4.5 cm. There are four diamonds (5 cm × 5 cm × 500µm) on each end positioned in the ±x and ±y locations and running in pulse counting mode. The
†Installed by A. J. Bell (2009). System improved and calibrated by D. Pfeiffer (2010).
The Compact Muon Solenoid 31 BCM1F is used to detect and diagnose problematic beam conditions which result in beam losses over a very short time period. Through 2010 - 2012, the BCM1F has been developed to measure the real-time luminosity and gate on colliding and non-colliding bunches to search for vacuum pressure spikes and beam gas events. This work was carried out by physicists from the Deutsches Elektronen-Synchrotron (DESY) in Zeuthen.
2.2.2 BCM1L
The Beams Condition Monitor 1 - Leakage Current (BCM1L) is a polycrystalline diamond system located alongside the BCM1F (|z| = 1.8 m, r = 4.5 cm). Each diamond is 10×10×0.4 mm3 and orientated parallel to the z direction. Unlike the BCM1F, which detects individual quanta of beam losses, the diamonds of the BCM1L run in leakage current mode in which the longer term beam losses cause a proportional current. A threshold value is applied to the integrated current measurement, over which, a hardware beam abort signal can be generated and transmitted to the LHC control via the Beam Interlock System. This causes the beams to be dumped within 3 orbits. Lower threshold levels can be set at which, should the leakage current cross, will send a hardware signal to the CMS sub-detectors to initiate a voltage ramp down.
2.2.3 BCM2
The BCM2 subsystem is virtually identical to the BCM1L subsystem in techni-calities. The BCM2 uses poly-crystal diamonds and is located at ±14.4 m from the I.P. It comprises of 12 diamonds per end; 4 inner diamonds (r =5 cm) which are in a direct line-of-sight to the I.P, and 8 outer diamonds (r = 29 cm). The readout system uses standard LHC Beam Loss Monitor electronics and data pro-cessing [39] running at a 40µs sampling time. Like the BCM1L, the BCM2 leakage current is proportional to the beam losses and thresholds are applied at which the system will trigger a hardware beam abort via the interlock system. This system was the responsibility of Karlsruhe Technical University, Germany and installed by Steffen Müller and the CMS BRM group.
2.2.4 BPTX
The Beam Position and Timing (BPTX) subdetector comprises of 2×2 electro-static detectors with picosecond timing resolution located at ±175 m from the CMS interaction point. These are the same devices used throughout the LHC for the beam position monitors. At ±175 m, the LHC beams are in separate beam pipes and the electrodes are positioned to pick-up the charge of only the incoming beams. The readout is achieved through a commercial 5 GigaSamples/second os-cilloscope. Comparisons of the relative timing from the electrodes allows the BRM group to display the collision offset with respect to the nominal z = 0 position with ∼200 ps precision (6 cm), ensuring that the data taken comes from collisions very close to the I.P. The coincidence of the BPTX signals acted as a zero bias trigger in many analyses, including the pp cross section analysis of Chapter 7. It also provides the non-colliding gating signals used in the BCM1F beam gas mon-itoring and Beam Halo Counter (BHC) system for triggering on bunches without collisions to allow a better measurement of the beam muon halo background.