Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Patent
1996-12-02
1998-04-28
Wong, Don
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
250256, 250262, G01T 116
Patent
active
057448043
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a system for determining a composition of radionuclides in, for instance, a mineral-containing material by detecting gamma and/or X-ray radiation emitted by the nuclides, comprising a detector unit which supplies an electrical signal containing information about the intensity and energy of the emitted radiation and a signal processing system by which these electrical signals are further processed for determining the composition referred to.
Such systems are known per se and are employed inter alia for the exploration and exploitation of oil and gas. This method is based on the presence of radionuclides in minerals. These nuclides emit radiation which is characteristic of the nuclide in question. By detecting this radiation, information can be obtained about the composition of nuclides in minerals.
The nuclides of interest to be detected are nuclides having a long life (half-time in excess of 10.sup.9 years), such as e.g. .sup.40 K, .sup.232 Th, .sup.235 U and .sup.238 U and their decay products. During the period in which minerals were formed, these nuclides were incorporated into them. The concentration of these nuclides in the minerals is highly dependent on the type of mineral. Accordingly, these concentrations are often characteristic of the mineral type and site.
The radionuclides mentioned typically decay by way of a long sequence of intermediate products to a stable lead isotope. During this decay, inter alia alpha, beta, gamma and x-ray radiation are released. The present invention, however, only relates to the detection and interpretation of emitted gamma and x-ray radiation.
Although the gamma radiation is not emitted by the nuclides .sup.232 Th, .sup.235 U and .sup.238 U but by nuclides further down the decay sequence, for convenience the terms Th and U activity are used in this specification.
In the known systems, the radiation to be detected is for instance allowed to fall on a crystal, whereby a light flash can be generated. In case of gamma and x-ray radiation, such a photon causes such a light flash. These light flashes are applied to a photomultiplier which converts the light flashes into electrical pulses. The magnitude of the electrical pulse is a measure for the energy of the photon incident on the crystal.
By counting the number of pulses for a period of, for instance, twenty seconds, and sorting them according to pulse magnitude, an energy spectrum can be composed, i.e. the number of photons recorded per unit time as a function of their energy.
Such a spectrum will contain peaks or lines which are respectively caused at least substantially by the radionuclides mentioned. A peak or a set of peaks can then be ascribed to a nuclide. In addition, however, contributions from other physical phenomena are present in the spectrum, such as for instance the Compton effect.
In the known systems the spectrum is analyzed per window. Within a window a peak a present. That is to say that the content of generally three windows is determined. On the basis of the content of these windows, it can be determined which nuclides have been detected. The information about the content of the windows is then processed in combination in order to determine the concentrations of the nuclides in the material in question, since the concentrations of the nuclides appear to comprise a practically linear relation with the intensity of the radiation, i.e. with the number of photons emitted.
These systems often utilize NaI(Tl) crystals; such a crystal has a moderate energy resolution and further is fairly insensitive to highly energetic radiation, so that a long integration time, for instance more than 20 seconds, is required to obtain a useful spectrum.
The object of the invention is to overcome the above-mentioned disadvantage of low sensitivity, on the one hand by preferably using a crystal of higher sensitivity and, on the other, by involving as good as the total energy spectrum in the analysis. The object of the invention is moreover to provide a system which, if so desired, also enables cor
REFERENCES:
patent: 4717825 (1988-01-01), Smith, Jr. et al.
Meijer Robert Johan
Stapel Cornelis
Rijksuniversiteit te Groningen
Wong Don
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