Severe signal loss in diamond beam loss monitors in high particle rate environments by charge trapping in radiation-induced defects

摘要

The beam condition monitoring leakage (BCML) system is a beam monitoringdevice in the compact muon solenoid (CMS) experiment at the large hadroncollider (LHC). As detectors 32 poly-crystalline (pCVD) diamond sensors arepositioned in rings around the beam pipe. Here, high particle rates occur fromthe colliding beams scattering particles outside the beam pipe. These particlescause defects, which act as traps for the ionization, thus reducing the chargecollection efficiency (CCE). However, the loss in CCE was much more severe thanexpected from low rate laboratory measurements and simulations, especially insingle-crystalline (sCVD) diamonds, which have a low initial concentration ofdefects. The reason why in real experiments the CCE is much worse than inlaboratory experiments is related to the ionization rate. At high particlerates the trapping rate of the ionization is so high compared with thedetrapping rate, that space charge builds up. This space charge reduces locallythe internal electric field, which in turn increases the trapping rate andrecombination and hence reduces the CCE in a strongly non-linear way. A diamondirradiation campaign was started to investigate the rate dependent electricalfield deformation with respect to the radiation damage. Besides the electricalfield measurements via the Transient Current Technique (TCT), the CCE wasmeasured. The experimental results were used to create an effective deep trapmodel that takes the radiation damage into account. Using this trap model therate dependent electrical field deformation and the CCE were simulated with thesoftware SILVACO TCAD. The simulation, tuned to rate dependent measurementsfrom a strong radioactive source, was able to predict the non-linear decreaseof the CCE in the harsh environment of the LHC, where the particle rate was afactor 30 higher.