The goal of this project is to develop new materials for high resolution, room temperature gamma radiation detection. We propose to develop high Z, high resistivity, amorphous, semiconductors to be used as solid-state detectors at near ambient temperatures.  The principle of operation will be analogous to single crystal semi-conducting detectors (e.g. Ge or CdZnTe) which are designed to exploit the electron generation aspects of gamma ray interactions with matter (e.g. photo-electric effect, Compton scattering, and pair production).

These new materials must have two important properties: 1) maximize the number of electrons produced from gamma events and 2) efficiently collect those electrons as the measurable response signal.  The first property requires materials with high attenuation coefficients for gamma interaction.  This can be roughly correlated to the atomic number, and thus is strongly compositionally dependant.  The second property for efficient collection of electrons produced from gamma interactions is strongly correlated to the electronic structure of the material.  This can be achieved by using highly biased, high resistivity, semiconductors, such that any electrons produced from a gamma event are rapidly swept across the band gap into the conduction band and collected.

Polarized light micrograph of CdGeAs2 (courtesy of PNNL)

Since successful production of large single crystals of CdZnTe has been elusive, this project is targeting the use of amorphous semiconductors.  Successful application of amorphous semiconductors will require simultaneously maximizing the linear attenuation coefficient for gamma radiation while minimizing the trapping length of electrons in the material.  Compared to single crystals, amorphous semiconductors have poorer electronic properties, but have advantages of rapid cost effective bulk fabrication, compositional flexibility, and greater electronic property control through composition.

Experiments conducted at the University of Illinois during 2006 have focused on developing ohmic contacts to and characterization of single-phase amorphous and polycrystalline disks of CdGeAs2.  Results have included testing of a variety of materials as contact metallizations, temperature-dependent Hall effect measurements of the resistivity of the material, optical transmission, and others.

   Quartz tube furnace (PNNL)                    CdGeAs2 sample in Hall machine