Fundamentals of nuclear radiation detectors:
An x-ray, gamma-ray detector is a device that senses radiations and then converts the radiation energy into an electrical or optical signal that can be used either for imaging purpose (imaging detectors) or simply detecting and identifying the presence of the nuclear radiation (spectrometers). Detectors that convert incoming radiation directly into the electrical signal is called direct conversion type detector, where the incident radiation interacts with the detecting medium and creates a bunch of positive and negative charge which are then collected at the electrodes. Semiconductor detectors and gas filled detectors (Geiger-Muller counters) belong to this group. On the other hand, the incoming radiation is sensed in a scintillator material in an indirect conversion type detector, where the radiation energy is first converted into the visible light. The visible light is then either captured on a film emulsive layer or a photomultiplier tube or photodiode or CCD converts the light into an electric signal.
Geiger-Muller counters and sodium iodide scintillators are widely used today to detect nuclear radiation. These devices conveniently work at room temperature but have either non or very poor energy resolutions, which means they can tell the presence of radiation, but cannot identify the types or source of radiations. Likewise, a-Se photoconductive layer and a-Si TFT array or scintillator detectors coupled with photomultiplier tubes are currently used for the imaging detectors, however, they have lower quantum efficiency the efficiency, i.e. output signal is poor in comparison to the input radiation, leading to a poor imaging quality. Germanium based spectrometers or imaging detectors give high-energy resolutions and efficiency, but must be cooled to cryogenic temperatures (about -190 oC), which makes using them in the field difficult. Their bulky cryogenic system increases the instrument weight, consumes a lot of energy, and needs almost daily maintenances. Hence, there has been continuous research activity on the development of compact and portable semiconductor detectors that offer high-energy resolutions and efficiency, and can work at room temperature. CdTe, CdZnTe, HgI2, GaAs, TlBr are the leading semiconductor materials for this purpose because they provide a high stoppage power for the striking radiations thus increasing the detection efficiency. Their wide bandgap makes them suitable for the room temperature operation. Besides, they have moderately good charge transport properties so that photon generated carriers can be easily collected at the electrodes as an electric signal, thus increasing the overall efficiency. These semiconductor detectors are not only suitable for spectrometers working at room temperatures, but also offer a real time imaging with better quality image (high energy and spatial resolutions) when developed for imaging detectors, and are fast in response over the conventional indirect conversion type detectors. Research on single element spectroscopic detectors based on these wide bandgap semiconductors has already reached to an advanced stage and some of these spectrometers are now available commercially. However, there is a continuous effort in making a large imaging array based on these materials for the imaging applications in medicine or astrophysics.
What we do?
1.Construction of a large area imaging array based on thick epitaxial layers for medicine:
We are investigating an epitaxial growth of thick CdTe layers (thickness about 500 um) on GaAs and on Si substrates in a metalorganic vapor phase epitaxy (MOVPE) system for their applications in fabricating a large area imaging array for nuclear medicine applications.
As described earlier, the well-balanced material characteristics, like wide bandgap, high average atomic number, good charge transport properties, make CdTe an attractive detector material for nuclear radiation detectors. Large area, thick and high quality crystals are necessary for the nuclear imaging detectors in order to cover a large imaging area and to achieve the uniform response and the good efficiency. Despite considerable progress made in the material development, uniform and large area bulk crystal growth is still very difficult. Large area bulk crystals generally incorporate macroscopic and microscopic crystal defects, which severely affects the uniformity of the crystal and hence the detector performance. The only way to achieve a large area imaging array at present is mounting thousands of small area bulk crystals in an array, after examining the performance of each small detectors separately, which make the process very cumbersome and prohibitively expensive.
In order to overcome those drawbacks and to construct a large imaging array, we are studying an epitaxial growth of thick CdTe layers on GaAs and Si substrates using an MOVPE growth system. Nuclear imaging detector for medical applications working in the photon energy up to 140 keV range requires detector thickness about 100 to 500 um for the efficient of nuclear radiation and hence to provide a good detector efficiency. However, achieving an epitaxial layer of such a large thickness is a challenging job. The quality of the epitaxial layer and the thermal stress on them change when sufficiently thick layers are grown, leading the poor quality of the layer and even results peeling off of the grown layers. We are studying such thick layer growths by strictly controlling the growth conditions, and high quality epitaxial layers of thickness up to 200 um with smooth surface morphology and high structural properties have already achieved. Further investigations are being made for going higher thickness.
Metalorganic Vapor Phase Epitaxy (MOVPE) growth is a versatile growth technique for producing highly uniform, high quality films (layers) of semiconductor materials. MOVPE relies on volatile metalorganic compounds that act as source materials for the film growth. The process consists of transporting the metalorganic compounds to the growth chamber using a carrier gas (usually hydrogen), where these compounds are dissociated or reacted with each other chemically by the thermal energy applied in the growth chamber to produce the semiconductor films. This growth system can produce a large area growth with a high growth rate. Besides it also offers a precise control of impurity doing of the semiconductor films, thus provides flexibility in device fabrications.
2. Fabrication of detectors for hard x-ray gamma-ray spectroscopy using bulk CdTe crystals:
As stated above, detectors based on thick epitaxial layers of thickness up to 500 um can be used for detection of nuclear radiation up to 140 keV or less. However, for higher energy of a few hundred keV to several hundreds of keV, detectors based on thick epitaxial layers will not be sufficient, most of the radiation passes out from the detectors without being detected. In such case we need thicker detectors of thickness from 1 mm to 1 cm, and currently only bulk crystal growth technique can yield of detectors of such thickness.
The usual way of fabrication detectors based on bulk crystals is to place metallic electrodes on both sides, and applying an electric field on the detector to collect the charges (electrons and holes) generated by the incoming radiation. However, as discussed earlier, in spite of inability of large area growth bulk crystals have many imperfections and defects leading to a low carrier transport, mainly for holes which degrades the detector performance significantly because of trapping in the defects inside the crystal before getting collected the electrode as a useful signal. And this problem gets worse on thick detectors as holes have to travel a longer distance in the detector crystals before collecting at the electrode, that means greater chance to be trapped at the defects. This problem can be overcome to some extent by applying a high electric field on the detector. However, there is an adverse effect on applying higher filed because of the increased leakage current from the detector, which adds additional noise on the collected signal. In worst case, such detectors only produce noise, but no useful signals.
We have been studying different ways to overcome this problem. One is by forming a diode type junction on the bulk crystal by growing a thin epitaxial layer on the bulk crystal and constructing a p-n junction. The leakage current of the detector can be suppressed significantly by operating it in a reverse bias mode, so that we can applying higher electric fields. This method works well for detectors 0.5 to 1 mm thick. For higher thickness, the effect of hole trapping is observed.
In an another approach, we are studying modified electrode designs so that the effect of the slow holes and its trapping can be neglected on the collected spectrum. The main concept here is to alter the electric filed inside the detector by the modified electode design so that the detector works as a single carrier (electron only) device. Further works on both types of detector fabrications are under progress.
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