Coaxial Detectors

A coaxial detector crystal is a cylindrical crystal with a center hole that may be closed on one end. The outer and inner walls comprise the two electrical contacts, which are substantially coaxial. High-purity germanium (HPGe), also called intrinsic germanium (IG), is designated as either p-type or n-type, based on the type of impurity. To fabricate a detector, a thin P+ layer is formed on one surface, usually by boron ion implantation or gold metallization. The thickness of this layer is about 0.3 µm. The other surface is a thicker N+ layer (~0.5 mm) formed by lithium diffusion. When a bias voltage is applied to the crystal, incident radiation creates charge carriers that are swept toward the oppositely charged contacts. For p-type HPGe coaxial detectors, the applied voltage is positive, and holes are the primary carriers.

For detectors with a P+ layer on the face, a negative bias is applied. In this case, electrons become the main charge carriers. Such crystals are less sensitive to neutron damage.

Coaxial detectors, available in a range of sizes and efficiencies, are used at energies up to 10 MeV.

Coaxial Detectors

P-Type HPGe Detectors

N-Type HPGe Detectors

Well Detectors