The Rate Dependent Radiation Induced Signal Degradation of Diamond Detectors
The Rate Dependent Radiation Induced Signal Degradation of Diamond Detectors
Presenter: Florian KASSEL - CERN EP-CMX
The Beam Condition Monitoring Leakage (BCML) system is a beam monitoring device in the CMS experiment at the LHC. As detectors 32 poly-crystalline (pCVD) diamond sensors are positioned in rings around the beam pipe. Here high particle rates occur from the colliding beams scattering particles outside the beam pipe. These particles cause defects, which act as traps for the ionization, thus reducing the charge collection efficiency (CCE). However, the loss in CCE was much more severe than expected from low rate laboratory measurements see Fig. 1. The reason why in real experiments the CCE is so much worse than in laboratory experiments is related to the rate of incident particles. At high particle rates the trapping rate of the ionization is so high compared with the detrapping rate, that space charge builds up. This space charge reduces locally the internal electric field, which in turn increases the trapping rate and hence reduces the CCE even further.
In order to connect these macroscopic measurements with the microscopic defects acting as traps for the ionization charge the TCAD simulation program SILVACO was used. It allows to introduce the defects as effective donator and acceptor levels and can calculate the TCT signals and CCE as function of the effective trap properties, like density, energy level and trapping cross section. After each irradiation step these properties were fitted to the data. Surprisingly, only two effective acceptor and donor levels were needed to fit the data after each step. It turned out that the energy levels and cross sections could be kept constant and the trap density was proportional to the cumulative fluence of the irradiation steps.
Using this effective defect model the highly non-linear rate dependent diamond polarization as function of the particle rate environment and the resulting signal loss can be simulated. Furthermore, the CCD as function of the electrical field at which the diamond is operated can be calculated.