WO1987007021A1

WO1987007021A1 - Method for stabilisation of a square weight detector

Info

Publication number: WO1987007021A1
Application number: PCT/FI1986/000047
Authority: WO
Inventors: Pertti Puumalainen
Original assignee: Puumalaisen Tutkimuslaitos Oy
Priority date: 1984-12-04
Filing date: 1986-05-08
Publication date: 1987-11-19

Abstract

A method, whereby it is possible to stabilize a beta square weight detector continuously even during the measuring process. The method is based on the use of fast detecting elements (4) and amplifiers (5), whereat the energy of each beta particle is analyzed and calculated on the measuring window separately differentiated from the electronic background noise. The system of analysis is formed by a separate pulse calculation at least three pulse height windows (8, 9, 10) made of a comparator chain, out of which windows with one (10) the amount of betas is counted and by at least two with a suitable window width possessing counters the beta spectrum is stabilized by supporting on the relative form of the beta spectrum. On the ground of the in such a way obtained number-of-piece counting of the beta particles the absorption of the body placed in the way of the beta emission to be examined can be determined, wherefrom further is obtained the square weight or thickness of the body, when the result is compared with a calibration made out with known samples.

Description

Method for stabilisation of a square weight detector.

The object of the invention is a method for continuous stabil ization of the square weight detector based on beta emission.

At the measuring of the square weight of thin materials or of the thickness of homogeneous substances particularly from mobile material webs very commonly a method based on absorp¬ tion of beta emission is applied. The measuring proceeding is often the following: On the one side of the planelike material to be measured a radio isotope source of beta ac- tivity is placed and on the other side an ionization chamber, from which a current signal is obtained. The current signal is proportional to the amount of beta emission absorbed into the chamber. The decrescence of the current signal is thus comparable with the thickness of the quantity of material between the detecting element and the beta emission source. When this comparability is calibrated with the help of known material thicknesses, the material thickness of the unknown sample can be determined by measuring. The advantage of the beta measurings is its very low debendability of the dif- ferent elements or in many cases the square weight can be determined directly, although the consistency of the elements in the material to be measured varies. Another advantage in the beta measurings is, that without touching the material to be measured the square weight can determined with suf- ficient accuracy quickly, the setting times being even below one second. The disadvantage in the measurings is the poor observation effeiciency of the commonly used gas-filled ionization chambers, which often is 5 - 20 per cent. Because the accuracy of the measuring event is comparable with the square root of the amount of observed betas, divided by the amount of observed betas, the poor observation efficiency thus abates the accuracy of the measuring or the quantity of activity in the emission source must be increased even to the twentyfold, if the same accuracy in the same measuring time is wished. The second disadvantage is the changing of the responses in as well the ionization chamber as in the current amplifier as a function of time, which to its great- est part is caused by variations in the temperature. Due to this variation the measuring instrument must be calibrated frequently.

The aim of the invention is to bring forth a method for a con tinuous stabilization of the square weight detector, with the help of which nearly all the beta emission coming into the detecting element is observed and the meter is calibrated continuously also during the measuring process.

The aim of the invention is reached by the method, which main ly is characterized in, what is presented in the claims.

The invention is explained by referring to the attached draw¬ ing, in which figure 1 presents diagrammatically the energy spectrum of the beta particles present in the beta emission and figure 2 presents one application of the appara¬ tus in order to apply the method in accordance with the in- vention partly in cross-profile seen from the side.

The principle of the measuring method is examined with the help of figure 1. The figure presents diagrammatically the energy dispersions or the energy spectra of the beta par- tides present in the beta emission. Into the axis of coor¬ dinates intensity (I) - energy (E) is firstly drawn the theor etical spectrum of the beta emission with the broken line 13 or the spectrum, when no absorption into the intermediate agent has occurred yet. When absorption to a certain degree has occurred, the beta spectrum is changed in accordance with the delineator 14. In the method a quick detecting element 4 in accordance with figure 2 is applied, which also is in pos¬ session of the ability to differentiate energy. One such de¬ tecting element is constructed on scintillation plastic, into which the beta particles are stopped and in which each par¬ ticle is causing a flash of light comparable with its energy. The flashes of light are preamplified with a fast multiplier tube 7, at the rear end of which is further a fast amplifier 5. With this detecting element each beta observation is ob- served in form of a pulse of a 30 ns in length, the height of which indicates the energy of the observed beta particle and shows as a voltage in the figure 1 below on the U-axis.

After the amplifier outlet in the figure 1 are presented the adjustable comparator windows 8, 9, 10. To these comparator windows pulse counters of their own are connected. The count¬ ing of the beta rays at a certain measuring period occurs in the following way: All comparators are in the same chain, the limits of which are set on the voltage scale with the voltage U. This voltage of the comparator chain is adjusted on ground of the amount of pulses hitting the windows 8 and 9. At the beginning, when the measuring is started, there is a great voltage or the windows 8 and 9 are above the spectrum. After this the voltage is decreased until the window 8 starts counting observations. Now between the windows 8 and 9 the condition is set: In both windows the incoming amount of pulses is counted to for instance 10000 and it is monitored, the amount of pulses of which of the windows is fulfilled first. If to the window 8 first comes the mentioned amount of pulses, the windows are lowered e.g. 1 per mille downwards or correspondingly, if to the window 9 first comes the men¬ tioned amount of pulses, the windows are moved by 1 per mille upwards. This can be done, because the window 8 is set nar- rower than the window 9, and as the beta spectrum always bend to the horizontal position after a steep ascent, when look¬ ing at the spectrum from the direction of the higher energy towards the lower energy direction.

With the help of these windows 8, 9 the beta spectrum thus is stabilized and from the factual measuring window 10 in the same comparator chain the amount of beta observations is ob¬ tained. In practice this means it, that if for instance the amount of observations in the factual measuring window is 80 x 10 pcs/s of betas, during this measuring second the

3 spectrum is calibrated about 10 times and in the accuracy of the measuring results is a theoretical dispersion of about 0.1 per mille.

This kind of a measuring method is based on that natural phenomenon, which it measures or with the form examination of the beta spectrum it is possible to count the beta par¬ ticles always electronically separated from the background noise correctly. The measuring method allows the migration of the amplifiers and the detecting element, because it sets the lower limit of the measuring window 10 always relatively on the same spot. With the measuring method nearly all beta emission coming into the detecting element is observed and the meter is calibrated continuously also during the measur- ing.

In the figure 2 the structure of the measure detector is presented. In the casing 11 on the one side of the material web 1 is the detecting element 4 and on the other side is the source casing 2, in which the radio isotope 3 is placed. The beta emission emanates from the radioactive preparation 3 and is directed through the material to be measured and ab¬ sorbed into the scintillation plastic 4, the flashes of light of which are amplified by means of the light multiplier tube 7 and the amplifier 5. After this the pulses are trans¬ ferred for analyzing and counting to the analyzer 6. In practice, in case there because of the surrounding exists the danger of the windows 12 getting dirty or dusty, the measur¬ ing apparatus can inbetween be zeroed without an analysis sample. In practice this is done for instance in the square weight meters of the paper machines by running the probe from time to time to the calibration station on the side of the paper track.

The invention is not limited to the presented application, but it can vary within the frames of the patent claims.

Claims

PATENT CLAIMS

1. Method for continuous stabilization of square weight de¬ tector based on beta emission, c h a r a c t e r i z e d in, that the beta emission eradiated from a radio isotope source (3) placed in a source casing (2) located on the one side of the material to be measured (1), is directed through the material to an on the opposite side placed detecting element (4), the signals of the detecting element are ampli¬ fied with the amplifier (5) and the pulses of each observed beta ray comparable to their pulse height with the energy are conducted to a pulse height analyzer (6), consisting of a chain of comparators, with several pulse height analyzing windows, out of which by means of at least two windows dif¬ ferent to their width the form of the beta spectrum is ex¬ amined and with the help of this form fixation the relative position of the windows is adjusted in such a way, that the lower limit of the factual analyzing window is always in the relative spectrum above the noise limit and that the upper limit of the analysing window delimits the wished measuring part of the beta spectrum, out of the thus obtained amount of pulses from the measuring window per time unit is counted the damping of the beta emission caused by the material lo¬ cated between the detecting element and the beta source compared with the amount of pulses, which is measured with¬ out the material to be analyzed, whereby with the calibra- tion of the damping by means of samples known to their square weight or thickness, the square weight or the thickness of the material to be analyzed is obtained.

2. Method in accordance with the patent claim 1, c h a r a c t e r i z e d in, that as the detecting element (4) scin¬ tillation plastic is used, whereafter the flashes of light caused by the beta absorption are amplified with a fast light multiplier tube (7) and amplifier (5), whereby for each beta particle a pulse is obtained, the height of which is compar- able with the beta emission energy, the thus obtained pulses are directed to a comparator-analyzer chain, in which are two the voltage of the comparator chain regulating windows, out of which the lower (8) is narrower than the upper (9), and on ground of the into these coming amounts of pulses the voltage of the comparator chain is adjusted in such a way, that if to the lower measuring window more pulses are com¬ ing, the voltage of the comparator chain is changed lower and if to the upper measuring window comes more pulses the voltage of the comparator chain is increased, and this ad¬ justment is performed continuously during the actual measur¬ ing period, whereby pulses are collected to the wide factual measuring window 10, which is arranged to reach from immedi- ately above the noise level over the whole energy spectrum, on ground of the in such a way to the measuring window ob¬ tained beta observations the damping effect of the material located between the source of emission and the detecting element on the beta emission is calculated, from which the square weight of the material is evaluated.