How to Choose the Right Photon Detector for Your Application

Choosing the correct detector for your specific application involves many criteria. No single table or graph can uniquely specify a detector for a particular measurement. The recommendations that follow are intended to guide a user to the detector likely to be best suited for the application. (If you have questions about whether a particular detector’s performance will meet your needs, we suggest that you contact your local sales representative or the ORTEC facility in Oak Ridge, Tennessee, USA.)

Germanium (HPGe) Detector Types

Let us briefly review the various types of Germanium (HPGe) detector available in order to make what follows easier to understand.

All Germanium (HPGe) radiation detectors are just large, reverse-biased diodes. The germanium material can be either "n-type" or "ptype". The type depends on the concentration of donor or acceptor atoms in the crystal. To connect the diode to an electrical circuit to amplify the signal, we need to put contacts on the crystal. These electrical contacts on the crystal are a thick, lithium contact, which is the N+ contact, and a thin, ionimplanted contact, which is the P+ contact. The lithium contact is thick because the lithium is diffused into the germanium.

Figure 1. P-Type HPGe Crystal Geometries

Figure 2. N-Type HPGe Crystal Geometries

The crystal can be cut or ground to any shape. However, the electrical field inside the crystal (diode) is very important. This limits the useful shapes to a disk or a cylinder with a hollow core. The cylinders are closed at one end and called coaxial; the disks are called planar. These are shown in Fig. 1.

Depending on the type of material used (N or P), the contacts are applied differently. For P-type material, the thick, lithium contact is on the outer surface and the thin, ion-implanted contact is on the inside. The ORTEC name is GEM. For N-type material the contacts are reversed. The ORTEC name is GMX. Very short coaxial detectors are called LO-AX detectors. Figure 2 shows the two N-types detectors: GMX and LO-AX. Coaxial construction allows larger (deeper) detectors to be fabricated, but the large size means a higher capacitance. The higher capacitance increases the resolution, which means there is a tradeoff to be made between having the best lowenergy resolution and having the highest efficiency at higher energies. The ORTEC LO-AX geometry is a semi-planar (or maybe a semi-coaxial) geometry, with a good low energy resolution and better high energy efficiency (deeper) than a planar.

The P-type coaxial (GEM) is the most commonly used Germanium (HPGe )in counting laboratories. The N-type coaxial (GMX) has extended low-energy efficiency because of the thin contact and has slightly worse resolution specifications at higher energies than a GEM. The largest GEM detectors are about 75% higher than the largest GMX. LO-AX and planar (GLP) detectors have excellent low-energy resolution, but reduced high-energy efficiency and resolution.