Proceedings of IBIC2012, Tsukuba, Japan

TUPA08APPLICATION OF SINGLE CRYSTAL DIAMONDS (scCVD) AS BEAM
CONDITIONS MONITORS AT LHC
E. Castro, DESY, Zeuthen, Germany on behalf of the DESY and BRM CMS group.
Abstract

INTRODUCTION
The CMS experiment contains several systems within
BRM to measure independently the dose at several positions inside and near the detector, and the flux of ionisingparticles [1]. Three systems use diamond sensors. Two systems, using polycrystalline sensors [2], integrate the sensor
current over microsecond time intervalls and may induce
beam aborts when the current reaches preset limits.
The 8 BCM1F modules are particle counters to monitor the flux of particles with a time resolution of nanoseconds [3]. They consist of scCVD sensors, fast and radiation hard front-end ASICs and and a laser driven analog
signal transmission. The back-end electronics containing
fast ADCs, discriminators, scaler and TDCs and the dataacquisition are operated independently from the CMS dataacquisition.
To monitor the particle flux along the LHC 6 more modules, forming the BCM1F4LHC system, are installed atFigure 1: Location of BCM1F modules along the LHC ring.
several IPs, as shown in Figure 1. The back-end electronics of BCM1F4LHC is similar to the one of BCM1F andlocated near the LHC control room.

SINGLE CRYSTAL CVD DIAMONDSENSORSThe charge collections efficiency of scCVD sensors isnearly 100% and signals of sensors with a thickness ofabout 500µm are sufficiently large to count MIPs. Theyare operated as solid state ionisation chambers by applying
high voltage to thin metal plates on both sides of the sensorto generate an electric field in the bulk. Signals are created
due the drift of both electrons and holes with high mobility.
The scCVD sensors used in BCM1F have an area of
5×5mm2 and a thickness of 500µm. They have been manufactured by Element Six Ltd. as a result of the collaboration with the CERN RD42 project. A first applicationof an scCVD diamond sensor in a collider experiment wasdescribed in Ref. [4].

THE CMS BCM1F
The BCM1F consists of 2 planes of 4 modules each located at 1.8m on either side of the IP at a distance of 4.5cm
from the beam pipe. Each module contains a scCVD sensor, radiation hard front-end electronics and optical transmission of the signal, as shown in Figure 2. The peaking
time of the front-end ASIC is about 25 ns and the time resolution about 2 ns. The distance between the sensors andthe IP is optimal for the separation of incoming and outgoing particles and corresponds to a time-of-flight of ∼6 ns
for relativistic particles.

THE BCM1F4LHC BLM SYSTEM
Six scCVD diamonds were installed in the LHC tunnel
near the beam-pipe to monitor the beam halo and analysebeam losses. They are positioned next to collimators to besensitive to losses originating from protons hitting the wallsof the collimators, and next to ionisation profile monitors

ISBN 978-3-95450-119-9
Beam Loss Detection

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○The properties of single-crystal diamonds (scCVD): radiation hardness, low leakage current with negligible temperature dependence and fast signal response, make themattractive to be used as robust particle counters in areasof high radiation dose. The Beam Conditions and Radiation Monitoring system (BRM) of the CMS experiment
includes the monitor BCM1F (Fast Beam Conditions Monitor) consisting of 8 scCVD sensors. BCM1F monitors theflux of particles from beam induced backgrounds and fromcollisions. It protects the inner CMS detectors from highbackground and delivers feedback on the beam conditions
to the LHC.
Since the LHC start up in September 2008, BCM1F hasrevealed to be a valuable tool in the daily operation of theCMS experiment and the LHC. It was successfully used tokeep the background level low, to identify vacuum leaks
via the enhanced rate of halo particles from interactions ofbeam particles with residual gas atoms in the beam-pipe,
and to perform an on-line luminosity measurement. In order to cope in future with higher bunch density and luminosity an upgrade of the BCM1F front-end-electronics anddata acquisition is foreseen.
Due to the positive experience with the CMS BCM1F,
the LHC was equipped with currently 6 BCM1F modules
at several positions along the ring where beam losses will
be analysed. The modules deliver information about hit
rate and arrival time distributions projected on the time ofan orbit, allowing to count halo particles originating fromeach bunch. A characterisation of both BCM1F systems
using data collected during the LHC operation is presented.

TUPA08

Proceedings of IBIC2012, Tsukuba, Japan

Figure 4: Data acquisition se-tup for BCM1F at CMS andLHC.
Figure 2: Photograph of one of the CMS BCM1 planes
where the BCM1F modules are mounted around the beampipe.

PERFORMANCE OF THE CMS BCM1F
Beam Background Measurements

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○in the RF zone. Figure 3 shows an example of an installedsensor module.

The CMS BCM1F delivers to the LHC control room two
numbers from scalers: the total flux of particles in the inner
region of the pixel detector (BKGD 1) and the flux or particles due to beam halo (BKGD 2). For the calculation ofBKGD 2 only the rates of non-colliding bunches are taken.
These rates are also corrected for delayed hits originatingfrom the activation of the material surrounding the sensors.
The beam halo rates are a measure of proton scattering on
residual gas atoms in the vacuum. This is nicely seen inFigure 5 where the rate of halo particles is measured asa function of the vacuum pressure measured by the gauge
VGPB 183 1R5 during the ramp up after bad vacuum conditions on the right side of IP 5. The sensitivity is up to∼0.5×109 mbar.

Luminosity Measurement
Figure 3: Photo of the installation of the sensor in P4. The
yellow and white cables carry the sensor signals and theHV/LV respectively.
The amplified signals are transferred as analog opticalsignals through fibres to the CERN Prevessin site, wherethe data acquisition set-up is installed.

BCM1F will be used for the on-line luminosity measurement. Since it is independent of the central CMS DAQ it
measures the luminosity when the other monitors are notavailable. Currently the whole system, including special
logics and histogramming modules, is under commissioning. However the results are very promising to e.g. luminosity values for each bunch crossing.

Upgrade Plans for the CMS BCM1F
DATA ACQUISITION SET-UP
In the service room the analog optical signals are converted back to electrical signals. The analogue signals
are duplicated, fanned out, and monitored by either ananalogue-to-digital converter (ADC) with 500MS/s, or discriminated and fed into scalers and time-to-digital converters (TDCs). Relevant results are displayed in the control
rooms and raw data files are stored permanently on disk foroff-line analysis.

After 2014 LHC will be operated at higher energy andluminosity, with bunches crossing every 25 ns. Because of
the peaking time of the currently used FE ASIC of 25 ns,
the performance of the whole system will be reduced andparticularly the luminosity measurement difficult due to alarge dead-time. A new FE ASIC with a peaking time andsignal width below 10 ns and a fast baseline recovery iscurrently being developed. With a gain of 50 mV/fC and asignal-to-noise ratio of ∼10 it will discriminate MIPs fromnoise with better performance.

ISBN 978-3-95450-119-9
2Beam Loss DetectionProceedings of IBIC2012, Tsukuba, Japan

Figure 5: The rate of beam halo particles as a function ofthe vacuum pressure on the right side of IP 5.

TUPA08Figure 7: Sensor in IP8: zoom in one orbit of the distribution of hits with respect to the orbit trigger: lower peaks
are hits due Beam 1 travelling towards IP8, higher peaks
are the collision products overlapping with Beam 2 comingfrom the IP8.

PERFORMANCE OF THE LHC BCM1F

The discriminated sensor signals are fed to a scaler thatcounts the hit rate. Depending on the location of the sensors, the rates account for beam halo, beam losses, protoninteraction with residual gas atoms due to vacuum degradation or collision products coming from nearby interaction
points. Figure 6 shows an example of the rates for several
proton-proton fills with collisions in the experiments Aliceand LHCb (IP2, IP8 respectively).

CONCLUSIONSThe CMS BCM1F is a fully operational monitor thatprovides information to grant safe data of LHC and CMS.
Its sensitivity and reliability in the daily operation showsthat new monitoring capabilities can still be exploited. An
upgrade of the FEE will grant its performance after thefirst long LHC shutdown. The BCM1F4LHC delivers valuable information on the halo at several positions along thering and will allow diagnostics in case of beam losses withnanosecond time resolution. It demonstrates the potentialof the scCVD sensors to be used as BLMs.

ACKNOWLEDGEMENTS
Special thanks to Bernd Dehning and other members ofthe CERN BE department for support.

REFERENCES[1] A.J. Bell, “Beam and Radiation Monitoring for CMS”, 2008
IEEE Nuclear Science Symposium Conference Record, p.
2322.

Figure 6: OP Vistars display: Beam 1 and 2 currents (inblue and red respectively) are overlapped with the scalers
rates. Upper plot: P2 (black) and P8 (grey) rates.

[2] M. Guthoff, “Design and experiences with the beam condition monitor as a protection system in the CMS Experimentof the LHC”, Proceedings of DIPAC 2011, Hamburg, Germany.

Time Distribution of the Signals[3] A.J. Bell, “ Fast Beam Conditions Monitor BCM1F for
the CMS Experiment”, Nucl. Instrum. Meth. A614 (2010)433−438.

The TDC measures the time of a hit within an orbit. The
information is relevant ta assign hits to bunches. Interesting are also hits not synchronous with bunches or even hits
in the so called abort-gap, which should not contain beam
particles. Figure 7 is an example of a TDC plot for sensor
in IP8, revealing a clear bunch structure of the halo fromthe incoming bunch train and collision products.

[4] W. Lange, et al., “Investigation of a single crystal diamond sensor and its application in background measurements for HERA”, talk given at the NoRHDia Workshop 2006, GSI Darmstadt, http://www-norhdia.gsi.
de/index.php?mgid=6

ISBN 978-3-95450-119-9
Beam Loss Detection

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Total Flux