1. Field of the Invention
The invention relates to a method for determining a weight per unit area and/or a chemical composition of a conveyed material sample, and to a device for carrying out the method.
2. Discussion of the Background
For determining a weight per unit area (e.g., grams per square meter; g/m2) and/or a chemical composition of material samples or material webs in conveyor and production plants partially operated at high speeds, absorption measurements are carried out according to the state of the art, using ionized radiation, in particular x-ray, gamma or particulate radiation. In this case, the webs or samples to be tested are passed through between a source of radiation and a detector arrangement placed opposite the source of radiation, and, in a transmission measurement, the intensity of the radiation absorption is determined. This radiation absorption depends on the weight per unit area of the webs and samples so that the mentioned characteristics of the webs and samples may be measured in a contact-free manner by means of this known method and corresponding devices.
A further possibility of measuring the weight per unit area of a material web or a sheet material consists in detecting scattered radiation. NDC Infrared Engineering and Adaptive Technologies, for example, are companies who use this method in their products by detecting back-scattered photons originating from a gamma radiation source, e.g. photons of the 59 keV line of the Am 241 isotope.
A systematic measurement error occurs in this method when the distance between the place where the particles of the incident ray are scattered and the location of the detector varies. Such a distance variation may occur due to a random fluttering movement of the material web, and also always occurs when the thickness of the measured product changes. In this case, purely geometric reasons cause the measurement signal to be influenced, and these reasons, if not corrected by additional independent measurements, will lead to an erroneous reproduction of the value of the weight per unit area. This systematic effect, however, may be limited when a large distance between the measured product and the detector is selected. But the distance may not be so large, for the purpose of maximizing the detection efficiency of the scattered radiation and lowering the associated statistical error of the detector with using, at the same time, the least possible intensity of the source of the incident ray. It is therefore important to correct the inaccuracies caused by distance variations in order to attain minimal measurement error at an economic cost.
In all known methods for measuring weights per unit area in which ionized radiation is used, the radiation originates from the decomposition of radioactive isotopes. In this case, the operating company is subjected to the radiation-protection regulation requiring the operating company to observe rest times even during a standstill of the plant, as well as being subject to elaborate safety measures.
The economic availability and the natural properties of radioactive nuclides and the ionized radiation thereof, e.g. half-life, type and energy distribution, strongly restrict the number of industrially usable preparations. In addition, using a single radionuclide may allow only a restricted functional range to be covered. Thus, the beta radiation of the frequently used nuclide Kr 85, for example, having an end-point energy of 687.4 keV, due to its absorption behaviour is well suited for determining weights per unit area of up to about 1000 g/m2. Higher weights per unit area may no longer be detected with it in a satisfactory manner and are preferably measured with the nuclide Sr 90, in which in the consecutive decomposition of Y 90, a beta radiation of a higher energy and hence of a higher penetration occurs having an end-point energy of 2280.1 keV.