وضعیت فعالیت های R & D در ردیاب های ذرات الماس

Chemical Vapor Deposited (CVD) polycrystalline diamond has been proposed as a radiation-hard alternative to silicon in the extreme radiation levels occurring close to the interaction region of the Large Hadron Collider. Due to an intense research effort, reliable high-quality polycrystalline CVD diamond detectors, with up to View the MathML source charge collection distance and good spatial uniformity, are now available. The most recent progress on the diamond quality, on the development of diamond trackers and on radiation hardness studies are presented and discussed.

A large effort is presently spent by the scientific community in the development of semiconductor-based particle detectors to be operative in extremely severe radiation environments. The study is mainly focussed on microstrip and pixel position-sensitive detectors to be applied in the forward tracker region of high-energy physics experiments at the Large Hadron Collider (LHC), where a luminosity of View the MathML source will give rise to hadron fluences up to View the MathML source after 10 years operation [1]. A possible further increase of the luminosity up to View the MathML source (SuperLHC) [2] would raise the hadron fluence after 5 years operation up to View the MathML source at the innermost radius. The development of novel, radiation-hard, position-sensitive detectors which can operate in such a hostile radiation environment is mandatory, as standard silicon detectors, due to their limited radiation hardness [3], would not survive. The exceptionally high radiation hardness of diamond makes this material a potential alternative to silicon for particle detection. A low read-out noise, due to the low leakage current and a low dielectric constant, a fast signal collection time, due to high electron and hole mobilities and high saturation velocity and the possibility of room temperature operation are other significant strengths of diamond for this application. For these reasons, the CERN RD42 Collaboration [4] started a research program to investigate the efficiency of position-sensitive diamond particle detectors: due to the potential low-cost and large-scale production of synthetic material, this research activity has been mainly focussed on Chemical Vapor Deposited (CVD) polycrystalline diamond. CVD diamond films are commercially available with typical diameters up to 5–6′′ and thickness in the range View the MathML source–View the MathML source; this material mainly consists of diamond micro-crystals (typical linear size 1–View the MathML source) growing columnarly from the substrate. The performance of CVD polycrystalline diamond devices is strongly dependent on the microscopic quality of the films, and it is usually degraded by the presence of native defects (like nitrogen-related complexes, graphitic remnants, dislocations, etc.) incorporated in the material bulk during the growth process, mainly at grain boundaries. Native defects can significantly reduce the mobility weighted charge carrier lifetime τ through trapping and recombination: this effect can lead to a charge collection distance d=μEτ smaller than the sensor thickness (with μ the summed electron and hole mobility and E the electric field). This can result in significantly lower pulse heights than in silicon detectors: the charge collection distance is therefore the most important figure of merit for CVD diamond sensors. In order to significantly improve the charge collection distance of these devices, the RD42 Collaboration has recently undergone a research program with the main CVD diamond manufacturers to produce a detector-grade material with optimized charge collection properties. Recent advances in this research program are discussed in this paper.

As a result of an intense research program carried out in close collaboration with the main diamond manufacturer, the RD42 Collaboration has been able to manufacture reliable high-quality polycrystalline CVD diamond detectors, with up to View the MathML source charge collection distance and good spatial uniformity (99% of charge distribution above 3000e). The optimized material bulk is now available in 5–6″ wafers which can be delivered in a large-scale industrial production. To test the tracking performance of detectors produced with such material, a seven diamond plane telescope with View the MathML source pitch microstrip detectors has been built for high efficiency and tracking precision measurement, a rad-hard SCTA128 electronics (DMILL ) has been produced and beam test is in progress. Radiation hardness studies have been carried out with View the MathML source protons up to View the MathML source and View the MathML source pions up to View the MathML source. The main results are: a slight dark current decrease up to the highest fluence, a loss of the S/N with the fluence (most probable signal decreases from 41 to 35 after the highest proton fluence) and an improved resolution (from View the MathML source to View the MathML source after the highest pion fluence). A qualitative picture of the correlation between microscopic defects and charge collection properties has been obtained as a result of the detailed investigation of the native defects in as-grown and irradiated medium- to-high quality polycrystalline CVD diamond samples. Further investigations will be focussed on: further improving the charge collection distance of polycrystalline CVD diamond to possibly achieve the value of View the MathML source; continue the radiation hardness studies up to the fluence of View the MathML source with protons, neutrons and pions; carry out beam tests with microstrip and pixels modules with the SCTA128 electronics, test the first batch of single-crystal CVD diamond detectors to assess the feasibility of new diamond particle detectors with exceptional charge collection properties. The correlation between the native and radiation-induced defects and the device performance will be further investigated on polycrystalline and single-crystal CVD diamond detectors.