1. FIELD OF THE INVENTION
The present invention relates to a system for analysing material such as coal, ore, or similar substances (hereinafter referred to as "coal", by bombardment of the material with neutrons and the detection of gamma rays emitted. It also relates to a gamma ray detector suitable for (but not exclusively for) use in such a system.
2. DESCRIPTION OF THE PRIOR ART
In recent years systems have been developed which use neutron bombardment of coal to analyse the composition of that coal, and so determine its economic value. The general principle of such a system is that a sample of coal is bombarded with neutrons, and these neutrons interact with atoms in the coal to generate gamma rays whose spectrum depends on the atoms with which the neutrons have interacted. If the energy of each gamma ray emitted from the coal can be detected, then the spectrum can be developed and so the elements within the coal identified. The problem, however, with such systems is that it is difficult to detect accurately the gamma ray energies.
It should be noted at this point that the present invention is concerned with detection of gamma rays emitted very quickly (times less than e.g. 10.sup.-12 s) after the neutron interaction. This is commonly referred to as "prompt" neutron activation analysis, to distinguish it from another form of neutron activation analysis in which the gamma rays investigated are those from .beta. decay in which the times involved are very much longer. Neutron activation analysis involving .beta. decay is unsuitable because not all elements are activated in this way, so that it is not possible to get an accurate analysis of the contents of the coal. Furthermore, different elements have different .beta. decay half lives, and so the activity of each element will change in a different way, making accurate analysis very difficult.
Therefore, the present invention is concerned with prompt analysis as this enables coal to be analysed continuously, for example analysis of coal passing continuously down a conveyor. However, the present invention is also applicable to analysis of bulk coal samples, but again with the intention that results are obtained rapidly.
Before discussing the present invention, it is necessary to understand the different types of gamma ray detectors currently used. The first type is the scintillator detector, consisting of a block of material which generates light when a particle such as an electron moving within it is slowed down. Such detectors are commonly made of sodium iodide, usually doped with thallium, and are referred to as NaI(Tl) scintillators. The use of such scintillators is well known for the detection of gamma rays. When a gamma ray interacts with such a NaI(Tl) scintillator there are two possible reactions depending on the energy of the gamma ray. Firstly, the gamma ray may react with an electron of an atom of the scintillator, knocking that electron out of its position in the lattice. This is known as Compton scattering. As the electron moves within the lattice it is slowed by interactions with other electrons and in doing so, generates light. By detecting the total amount of light generated (using a light detector such as a photomultiplier) it is possible to determine the energy of the electron. If all the energy of the gamma ray is absorbed by the electron, then this gives a measure of the gamma ray energy. However, in some cases, only a part of the gamma ray's energy is transmitted, and a gamma ray of different energy continues to move through the lattice. If this gamma ray then reacts with another electron within the scintillator, then all or part of its energy may be transmitted to that electron, generating more light. Thus, the light output from the scintillator will be directly related to the energy of the gamma ray, provided the gamma ray reacts with one or more electrons so that its energy is totally absorbed within the scintillator. However, in many cases only a part of the gamma ray's energy is transmitted to the electrons, and a part is lost, when a gamma ray emerges from the scintillator. Thus, the light spectrum for a gamma ray of a particular energy will have a peak corresponding to the energy of that gamma ray, but a large background level due to other possible reactions, where not all the energy is absorbed within the scintillator.
The second possible interaction, known as pair production, applies when the gamma ray has an energy of at least 1.022 MeV. Such a gamma ray may interact with a nucleus of an atom of the scintillator to generate an electron-positron pair. The electron of this pair passes through the scintillator, and loses energy as light radiation which may again be detected by a photomultiplier. The positron, on the other hand, will react with the first electron it encounters, and they mutually annihilate to generate a pair of gamma rays each having an energy of 511 keV, the two gamma rays travelling in opposite directions. These two gamma rays may then react with electrons in the scintillator by Compton scattering, knocking the electrons out of position and causing them to generate light in the way described above, or may pass directly out of the scintillator. Thus, the spectrum produced by pair production will have three peaks, one corresponding to the energy of the initial gamma ray, one corresponding to the case where one of the 511 keV gamma rays is totally absorbed whilst the other passes directly out of the scintillator, which peak will have an energy 511 keV less than the first, and a third peak corresponding to the case where both 511 keV gamma rays pass out of the scintillator, which peak will be 1.022 MeV less than the first. Furthermore, there will be a high background level corresponding to the other reactions possible.
For any element in the coal, the neutrons will generate a spectrum of gamma rays having several lines. Each of these lines may generate a corresponding peak in the light from the detector, if they are less than 1.022 MeV, or three peaks if they are greater than that energy. If the peaks were very sharp (i.e. had a very low energy spread), it would be possible to analyse the light output from the scintillator and thereby generate the full spectrum of incident gamma rays, enabling the elements to be identified. However, in practice, each peak generated by the scintillator has an energy spread of about 7 to 10% of its value, so that if there are a large number of peaks, they tend to blend together. Furthermore, the background noise of one line in the incident gamma ray spectrum, may be greater than the maximum peak height for another line, so that lines may be completely lost within the background noise. The result is that the output from a single scintillator is impossible to analyse because all the peaks are blurred together.
The second type of detector used is a solid-state detector based on a single crystal of germanium (Ge). Again, an incident gamma ray may react by Compton scattering or by pair production, but instead of generating electrons which generate light as they move through the scintillator, the effect of interaction within a solid-state detector is to generate electron-hole pairs which move through the crystal as a current which may be detected by applying a voltage across the semiconductor. This current will have energy peaks corresponding to the energy peaks generated by the gamma ray interactions. The advantage of solid-state detectors is that they have very sharp peaks (with a spread often 0.1% or less of the energy of the peak, so that it would be possible to use the energy peaks from a solid-state detector to obtain a complete analysis of the gamma ray spectrum and hence of the elements present in the coal. However, the problem with solid-state detectors is that they have a low efficiency, in that they produce a much lower output than a scintillator for the same gamma ray flux. Therefore, a longer analysis time is needed when using a solid-state detector to obtain accurate results, and in practice, this time is far too long to permit continuous, or relatively short-term, analysis of the coal. Therefore, despite its high accuracy, a solid-state detector is unsuitable for analysis of coal in the way proposed by the present invention.