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

Abstract

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 and simulations, especially in single-crystalline (sCVD) diamonds, which have a low initial concentration of defects. After an integrated luminosity of a few fb −1 corresponding to a few weeks of LHC operation, the CCE of the sCVD diamonds dropped by a factor of five or more and quickly approached the poor CCE of pCVD diamonds. The reason why in real experiments the CCE is much worse than in laboratory experiments is related to the ionization rate.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 recombination and hence reduces the CCE in a strongly non-linear way. A diamond irradiation campaign was started to investigate the rate dependent electrical field deformation with respect to the radiation damage. Besides the electrical field measurements via the Transient Current Technique (TCT), the CCE was measured. The experimental results were used to create an effective deep trap model that takes the radiation damage into account. Using this trap model the rate dependent electrical field deformation and the CCE were simulated with the software SILVACO TCAD. The simulation, tuned to rate dependent measurements from a strong radioactive source, was able to predict the nonlinear decrease of the CCE in the harsh environment of the LHC, where the particle rate was a factor 30 higher.

Irradiated Single Crystal Chemical Vapor Deposition Diamond Characterized with Various Ionizing Particles

“…A similar lifetime for both charge carriers is observed in most studies . For the sensors in this study, lifetimes ns and ns have been measured with a transient current technique at fluence 10 14 p cm −2 .…”

“…To mitigate the interface trapping, the electrodes have been (re)processed after irradiation. The experiences with diamond detectors in high energy physics experiments differ significantly from these laboratory experiments. It is possible that the electrodes suffer from irradiation alongside the crystal.…”

Description of Radiation Damage in Diamond Sensors Using an Effective Defect Model

“…The basic properties of this effective defect model like energy level and charge carrier cross sections for electrons and holes are based on Ref. .…”

“…More detailed information to the diamond samples, the TCT and CCE measurement setup can be found in Ref. .…”

“…After a total fluence of the CCD of the pCVD diamonds used at the CMS detector dropped about twice as fast compared to the expectations based on laboratory measurements. This discrepancy in CCD between the real application in a particle detector and laboratory experiments can be explained by the rate dependent polarization of the diamond detector, as was deduced from detailed laboratory measurements and simulations. The rate dependent diamond polarization describes the asymmetrical build‐up of space charge in the diamond bulk, which leads to a locally reduced electric field configuration.…”