Production and properties of a charging-up "Free" THGEM with DLC coating

THGEM detectors usually exhibit time-evolution of gain, caused by avalanche charges accumulating on the open insulator surfaces (charging-up effect). Coating resistive layer on the insulator surfaces may help to release the charges to overcome the charging-up effect. Diamond-like carbon (DLC), as a class of amorphous carbon material, can be used as a resistive layer. In this study, thin DLC layers with stable and controllable resistivity were coated on a series of THGEMs by magnetron sputtering technique. A batch of single THGEM detectors was fabricated with these DLC-THGEM films. The gas gain of DLC coated THGEMs (DLC-THGEM) have been tested in detail and compared with normal THGEM. DLC-THGEM with suitable resistivity show very low gain variation versus time and literally no charging-up effect. However, a DLC-THGEM detector shows a gain decrease with increasing irradiation rate. A qualitative theoretical model was built to understand the gain decrease. And the simulation results fit the experimental results well.