CERN Accelerating science

Beam Monitoring

The fundamental role of monitoring beams

Experiments at accelerators require high quality beams, which are guarantee, among others, with devices like Beam Conditions and Beam Loss Monitors. The detection and analysis of beam losses allow to trace their origin and to take prompt actions to avoid them. Near and inside the experiments the measurement of the beam halo is mandatory to protect the detectors from adverse beam conditions and to reduce the background during data taking.

Beam Monitors are used in fundamental Physics for luminosity measurements at colliders, intensity measurements at synchrotron facilities,  beam profile measurements at heavy ion facilities while, in the social context, they are exploited as radiation monitoring in medicine and neutron monitors at nuclear fusion facilities.

Challenges for beam monitoring detectors

Beam conditions and radiation monitoring exploit detectors with very stringent requirements: extreme radiation hardness and robustness, fast response to resolve the time structure or to discriminate between beam halo and collision products, good energy resolution especially important for measurements at  heavy ion accelerators.

Currently only Chemical Vapor Deposition (CVD) diamond sensors match these requirements. They are characterized by high electron and hole mobilities and low leakage current over a wide temperature range. Polycrystalline (pcCVD) sensors, available in size up to several cm2, reach charge collection efficiencies up to 60%, while the single crystal (scCVD) sensors achieve full efficiencies but their size is considerably smaller. Fortunately novel growth technologies like ‘diamond on iridium’ are promising to get larger sensor areas for a lower price. 

Other sensor materials, like artificial sapphire, are so far characterized by very low charge collection efficiency; however the sensor size might be much larger and the price lower. Pursuing R&D with industrial partners might overcome this limiting feature leading to a material comparable to diamond sensors in performance.

Diamond sensors generate intrinsically very fast signals that can be exploited only with the use of an optimized front-end technologies to be developed in order improve the time resolution maintaining a reasonable signal-to-noise ratio.

The radiation damage mechanisms in diamond sensors are complex and need to be further investigated. More studies may lead to new detector layouts, e.g. 3D diamond sensors or to new materials and operation regimes that would allow larger radiation tolerances.

A dedicated research program in these fields would significantly improve beam monitoring in future and potentially widen the applications at even harsher conditions at accelerators and in new fields.

Exploitation of beam monitoring detectors

Sensors made of scCVD and pcCVD diamonds are installed at several critical positions around collider rings and are widely used for beam monitoring, beam-loss analysis, luminosity measurements, to separate collision products from beam-halo particles by measuring the arrival time with respect to the bunch-crossing and to understand unforeseen events like the UFO: beam losses due to falling dust particles inside the beam pipe.

Recently diamond sensors are used as pixel sensors. In heavy ion facilities diamond strip and pad sensors are used for beam profile measurements and are foreseen to trigger interactions. At synchrotron facilities diamond pad sensors are applied for beam position and intensity measurements. Sensors made of scCVD diamonds are a very interesting option for neutron counters at fusion reactors.