Characterization of Ge detectors using pulse shapes generated by alphas*

A method to accurately determine the charge carrier mobility in a HPGe detector is demonstrated experimentally, where a combination of a small thin-contact planar HPGe detector and an alpha source is utilized. The thin-contact eliminates the influence of surface dead layer. The planar configuration insures a simple electric field distribution. The small size guarantees a nearly constant impurity concentration. And alpha particles deposit energy right on the surface of the detector, which reduces the uncertainty of the starting point of the drift of charge carriers to the minimum. Electric pulses are simulated with input physics parameters and then fit to measured pulses to precisely determine the parameter values. The mobility of a home-made HPGe detector from a home-grown crystal at USD has been determined as 3.4x10⁴ cm²/Vs, and other important detector characteristics such as net impurity concentration have been measured using the same method and compared to other measurement techniques. This method can be used to determine some critical properties of crystals used to make HPGe detectors for scientific research, such as neutrinoless double beta decay search, etc., as long as a thin slice of wafer from the same crystal is made into a thin-contact planar detector.