(558c) Pulse Shape Study On Large Point Contact HPGe Detector

Pulse shape study on large point contact HPGe detector

Introduction

In order to better understand the process of charge collection process inside a semiconductor, a study approach is developed and described in this work. First, a Monte Carlo module is created based on manufacturing details available only at the factory. This module is then benchmarked against a set of well controlled measurements through Canberra ISOCS characterization process. Using this module, position and value of each energy deposition step inside the crystal can be simulated by Monte Carlo method. The electric field distribution can be calculated using commercially available analysis software, utilizing our knowledge of the geometric dimensions and material properties. A customized code package then integrates all the information and predicts the pulse shape of the output signal. This approach can be used to study the effect of impurity level, operating voltage and other parameters. Hopefully, it will help to provide guidance for development of new detectors as well as understand behavior of existing designs.

In this work, we will show each step of the study approach. We also plan to show comparison between simulation and measurement results.

Detector Module

The detectors module studied here is Canberra's largest point contact planar HPGe detector (BEGe - Broad Energy Germanium detector) by now. The detector crystal has a diameter of 90 mm and a length of 30 mm. The detector efficiency is measured to be 58%. The energy resolution at 1.33 MeV Co-60 peak is measured to be 1.6 keV on this particular detector as shown in Figure 1.

Monte Carlo Simulation

Being the manufacturer, we have access to all the manufacturing details of our HPGe detectors. Through Canberra's ISOCS characterization, an accurate Monte Carlo model has been created for this large BEGe detector. An example of such modules is shown in Figure 2. The model is then further verified by comparison between predicted and measured efficiencies. The model reflects the best knowledge of dimensions and construction materials of this HPGe detector. The module is then used to simulate the energy deposition inside the crystal by gamma rays using GEANT4. For each photon event, the position and value of each energy deposition are recorded during the simulation.

Electric Field Calculation

A commercial software QuickField is used to calculate the field distribution inside the germanium crystal. QuickField is a very efficient finite element analysis package for electromagnetic, thermal, and stress design simulation. The DC magnetic package is used here. We first carefully model our detector with specific geometric dimensions and material properties include impurity distribution. Then, the operating electric field and weighting potential field are calculated and exported on a fine grid that covers the entire crystal, as shown in Figure 3-5.

Pulse Shape Simulation and Measurement

A customized code package is used to integrate all the information obtained through previous steps.  The code reads in the position and value of each energy deposition step. Electron and hole pairs are generated and tracked through the charge collection process. Output signal is simulated based on the changes of weighting potential along the collection track. The pre-amp signal is then passed through a digital shaper same as the one used in Canberra's Lynx MCA.

Spectroscopic Measurement

The energy spectra are measured using Canberra's digital spectroscopic system. The peak detection efficiency and spectra shape are studied during a scan over the detector crystal using collimated sources. The measurement results will be benchmarked against the prediction of our model.

Conclusions

An approach is shown here to study pulse shape of the output signal from a large point contact planar HPGe detector. Energy deposition positions and values are recorded during Monte Carlo simulation using a benchmarked module. Electric field distribution is calculated within the crystal. Pulse shape of the output signal is predicted based on calculation of charge collection process. Some measurements will be shown in comparison with simulation results.

Figure 6. Charge collection tracks, pre-amp signal and shaping amplifier output