Development of a Reconfigurable Monolithic Active Pixel Sensor in Radiation-hard Technology for Outer Tracking and Digital Electromagnetic Calorimetry

As particle physics progresses, the demands in terms of detector technologies always push toward needing fine segmentation, faster read-out and greater radiation tolerance; all at lower cost. To find particles too massive to have been seen already, particle colliders have to continuously push towards higher energies. However, as collision energies increase, the rates for events containing potential evidence for such new physics decreases as a fraction of the total, making it important at high energies to also have very high interaction rates. Also, as energies increase, the numbers of particles produced per collision increase and the mean energies of the produced particles of interest increase. The latter means that charged particles become harder to bend in the detector magnetic field and so better spatial precision (hence smaller sense elements) is needed leading to the need for much greater granularity (number of pixels per unit area). At higher energies, particle showers tend to also be much more collimated, making track separation also harder without many more read-out cells per unit area. Finally, at hadron colliders, high rates require many collisions per beam crossing so the interesting interactions are typically buried in 200 uninteresting ones produced at the same time. Unpicking the tracks from the interesting collision also requires very precise measurements. Finally, billions of collisions per second lead to radiation levels over years of operation that go way beyond the capabilities of normal electronics requiring specialist designs to work after the expected doses.

Particle physics sensors are not able to exploit the development of ultra-cheap mega-pixel cameras in many commercial devices because these neither have the speed or radiation hardness needed but they easily meet the requirements in terms of granularity. Therefore, if fast, radiation-hard versions could be designed, these would be a game-changer in terms of sensor affordability. Recent work has shown that there are commercial CMOS technologies available that can support radiation-hard designs and this proposal seeks to exploit these to demonstrate devices that could allow very large areas of experiments to be equipped with such detectors affordably. Aspects of the design that makes the sensors radiation hard also make them faster which is an added advantage for these applications. Nevertheless, proving small sensors with the required properties is a first step towards final arrays requiring thousands of square metres of sensors and much development time is needed before a very large array could be envisaged. Therefore, even though the possible applications are quite a long way off, the amount of time to develop new technologies to the required maturity to be exploited in such large arrays means that this R&D is very timely.

The UK has a lead in a number of the technology aspects for this but there are many other groups internationally pursuing the same goals and so this proposal is also important if current leadership, much of it developed through work for ATLAS and CMS upgrades and for the International Linear Colldier, is not to be lost.

The proposal includes discussion of an immediate benefit from the developments proposed here in the context of activities linking Birmingham Medical Physics oncology experts with Birmingham Particle Physics on the Wellcome Trust funded Proton Radiotherapy Verification and Dosimetry Applications (PRaVDA) programme. This project targets the need in hadron therapy to improve the beam diagnostics, monitor proton/ion distributions during treatment and provide proton computed tomography imaging capability (pCT). Improvements are required for the full benefits of hadron therapy in terms of the high specificity of the delivered dose to translate into even more accurate localisation of the radiation field within the patient. The aim being to minimise dose to healthy tissue using the characteristic energy loss distribution of hadrons in material compared with x-rays or gamma-rays. If future cancers are to be avoided this is particularly important for paediatrics and such localisation can also be critical for tumour located close to vital organs.

There are at least 40 operational centres worldwide offering hadron therapy (either protons or carbon ions) with many more in the planning or construction phase. In the UK there is only currently one (60 MeV) centre at Clatterbridge treating eye tumours but two more are due to open in 2017 in London and Manchester. Several privately funded centres are also in discussion or planning stages at a number of locations including London, Oxford and Newport.

The PRaVDA proposal uses radiation-hard 10cm*10cm p-type strip detectors developed for the HL-LHC with Micron Semiconductors (UK) Ltd for tracking and a range telescope based on a stack 10cm*10cm large format MAPS (CMOS) sensors interspersed with thin absorber layers. However, the strip tracker, while fast, is limited in the number of tracks that can be unambiguously reconstructed from their projections in 3 directions at 60 degrees (x-u-v) while the MAPS technology is neither fast or radiation-hard enough for operation at very high beam currents. This means that while beam diagnostics and pCT can be successfully demonstrated at modest beam currents, the detector system is the limiting factor in the time required and is not suitable for operation at the much higher currents associated with monitoring the actual treatment.

A fast, radiation-hard pixelated detector system could serve both for the tracking and range telescope (calorimetry) aspects using a common detector throughout and benefit from the much lower expected costs with large scale CMOS technology. It would offer tracking capability at much higher proton currents and provide a range telescope able to cope with much higher doses and rates.

Other areas of application would include areas where fast imaging is required in high radiation environments. MAPS technology in small format devices is ubiquitous in a very wide range of commercial imaging devices. However, these devices collect charge through diffusion and so can be too slow and radiation soft for some applications. Imaging in harsh radiation environments in general, and not necessarily just for imaging ionising radiation, would benefit from developments of radiation-hard MAPS technologies which are designed to finally achieve very large area coverage affordably. Such areas could include applications in aspects of the nuclear industry (possibly using coatings to improve sensitivity for different particle types), in transmission electron microscopy or at intense X-ray sources (needing good radiation tolerance and very fast signal registration), as well as across a range of other areas in nuclear medicine. The potential for low cost bulk production and for thin devices offer a range of uses where current CMOS and CCD devices suffer limitations in terms of radiation survival and signal collection speed.