Research of a new type of semiconductor detector of ionization radiation is presented. This work describes the basic principles of semiconductor detectors and summarizes the current application of such devices. Fundamental information about fullerene-like materials, the potential reasons for the use of fullerit and our first results are given.

The basic principle of detection of ionization radiation by semiconductor detectors is based on collecting the charge resulting from ionization. This collection is possible only in the case, when the detector material is able to conduct the charge carriers (electrons and also holes). However, this detector material must have such a little amount of native charge carriers so that the ionized charge can be measured. The collection is enabled by an electric field created between electrodes situated on the crystal surface. The electrodes have three roles. First, to provide the collection field. Second, namely in the case of Schottky and P-N junction, to reduce the current streaming through the detector bulk. And third, to provide a connection with the electronics, which reads the collected charge. Consequently, the requirement for contact and material homogeneity, as well as for the little amount of native carriers, is crucial for the suitable performance of semiconductor detectors.

Silicon and germanium are semiconductors the most frequently used materials in radiation detector manufacture. Both materials can be produced with good homogeneity and with 100% charge collection efficiency. Other semiconductors, such as InP, GaAs, CdTe, CZT, etc, are continuously investigated, however without significant definite results.

Silicon surface barrier detectors are appropriate for detection of heavy charged particles (e.g. protons, alpha-particles, etc). With a thin film of a neutron sensitive material on their front side these devices can serve as neutron detector. Silicon lithium-drifted detectors are suitable for the precise X-ray and low-energy gamma spectroscopy. However, the use of there SiLi-detectors requires cooling down to liquid nitrogen temperatures.

High Purity Germanium (HPGe) detectors are suitable for precise gamma spectroscopy. Their disadvantage is, as for SiLi-detectors, the need for cooling to liquid nitrogen temperatures. Therefore, composed semiconductors (InP, GaAs, etc) are being developed as detectors which can be operated at room temperatures. However, these materials exhibit many defects, namely anti-site defects, causing very short life time of holes. This reality degrades their spectroscopic resolution. This problem can be solved by the discovery of a new material.

The fullerene is a macromolecule consisting of 60 or 70 carbon atoms arranged to form a sphere. The term fullerene is used in this work to denote the molecules which are composed of 60 atoms of carbon. Fullerene molecules crystallize in the face centered cubic lattice. This crystal is called fullerit. The bonds between the molecules are mediated only by Van der Waals forces. Therefore, the fullerit is soft and brittle. The fullerit exhibits a low sublimation temperature which can be exploited to obtain millimeter-size single crystals by the vapor growth technique. Moreover, the fullerit is a semiconductor with a band-gap of 1.5 eV which is greater than the band-gap of other semiconductors such as Si (1.124 eV), InP (1.344 eV), and GaAs (1.429 eV).

The large band-gap of fullerit brings about a negligible amount of native carriers into the conduction band, and therefore, fullerit detectors would be able to operate under room temperature. Furthermore, a fullerit consists of identical molecules – fullerenes. This fact renders anti-site defects impossible, and consequently, an improved spectroscopic resolution is expected. Detector manufacturing from the fullerenes which are stuffed by atoms or molecules sensitive to indirect ionization radiation would permit production of detectors which are sensitive to any arbitrary radiation. If there ideas prove correct, the fullerit, as a new detection material from nanotechnology, would open new incalculable frontiers. The main aim of our project is to learn how to prepare macro-crystals out of fullerit, to verify if these crystals are suitable as radiation detector, to find out a way of metallization, and to prepare an application project submitted to an external grant agency.

Acquiring a fullerit macro-crystal is the first result of our project. The crystal was produced by the vapor growth technique by the following way: The fullerit pieces, no greater than 500 μm, were placed into a quartz tube with vacuum of 4x10^-4 Pa. Before sealing the tube, the fullerit pieces were heated up in order that fullerit impurities are desorbed. After sealing, the 20 cm long tube was inserted into a horizontal furnace. One end of the tube, containing the fullerit pieces, was kept at 700°C. In the opposite end of the tube the macrocrystal of the fullerene was grown, the temperature kept at 660°C for several days. The macro-crystal produced has diameter 1 cm and length also about 1 cm. Further evaluation of results is underway.