SU1325336A1 - Method of x-ray radiometric measurement of thickness - Google Patents

Abstract

The invention relates to a measurement technique and can be used to control thickness or surface density. The aim of the invention is to improve the accuracy of the measurement by eliminating the influence of the overlap of the instrumental peaks of the reference and reflected radiation, which is especially important if the peak from the radiation of the primary source is taken as the reference peak. The measured thickness is judged by the number of pulses in the reflected radiation channel, accumulated during the time of filling the counter in the reference radiation channel. To achieve the goal, the frequency of the pulses in the reference radiation channel is adjusted so that the admixture of reflected radiation is completely excluded from it. For this reason, this frequency is reduced by an amount proportional to the average pulse frequency of the reflected radiation channel. The coefficient of proportionality is found from the results of preliminary measurements of the loads in the reference and reflected channels during consecutive irradiation of two controlled samples of different thickness and is chosen equal to the ratio of the load difference in the radiation canap to the difference in load in the reflected radiation channel. To simplify the implementation of the method, the width of the reflected radiation channel and / or the width of the reference radiation channel is chosen so that the inverse of the proportionality coefficient is an integer. 1 hp f-ly, 1 silt, 1 tab. (L with 00 O)

Description

The invention relates to a measuring technique and can be used in devices for non-destructive Conrol process parameters, e.g.

The purpose of the invention is to improve the measurement accuracy by eliminating the effect of overlapping of peaks of reference and reflected radiation, as well as simplifying the implementation of the method.

The figure shows schematically the amplitude distribution of pulses from a radiation detector, consisting of two (in this example) partially overlapping peaks: a reference 1 and a reflected 2, as well as a reflected peak 3 of a different intensity.

The invention is as follows.

A controlled sample is irradiated with the source radiation, the reflected and reference from the

Then, using differential discriminators (amplitude selectors), pulses that fall into the reflected radiation channel (width 1) of the AH channel) and into the channel of the reference radiation (width of the DM channel) are separated from the total flow, and the pulse of the reference channel the radiation and the pulse of the reflected radiation channel during the time of filling the pulse counter of the reference radiation channel according to the result of the pulse counting of the channel of the reflected radiation judge the thickness of the uncontrolled sample. When this part of the reflected radiation falls into the reference channel (the area of the DECM figure). As the thickness of the test sample decreases, the intensity of the negative radiation decreases (the peak changes to peak 3). At the same time, the contribution of the pulses of reflected radiation to the reference channel decreases (pl

radiation. The peaks of the reflected and reference 5 spheres of the DE figure,), which leads to the emissions partially overlap. If, as a reference radiation, we use radiation of the primary and change in the pulse counting time and measurement errors of the thickness of the radiation being monitored. Since the shape (the amplitude distribution of pulses from the detector from monoenergy radiation) does not depend on the intense peak, but is determined by the energy resolution of the detector, at the points of amplitude distribution the intensity changes

Therefore, using the well-known Kempton ratio, it is not difficult to show that in the most frequently used energy range (up to 150–200 keV) the energy of quanta of scattered radiation differs from the energy of quanta of primary radiation by no more than 20%. In the X-ray fluorescence method, in order to increase the excitation efficiency, the primary radiation energy is chosen as close as possible to the corresponding excitation threshold, as a result of which the energy of fluorescent radiation quanta differs little from the energy of the primary quantum (re-proportional to. The relative contribution of reflected radiation to the perpendicular channel remains unchanged ( The area of the DECM figure to the area Q of the ABCG figure is equal to the ratio of the area of the figure to the area of AB, C, D figures for any intensities of the reflected 2 1 and reference peaks arbitrary (but nnnyh constant) width values Kamov Ka-50

radiation). The energy resolution of most of the AG and DM spectrometry, respectively, which detectors, such as scintillators, can be carried out before carrying out and proportional counters, in this energy range is 15–25%, which is not enough for a qualitative separation of the reference and reflected radiation. Due to the partial overlap of the instrumental peaks of the reference and reflected (scattered or fluorescent) radiation, a part of the reflected radiation, the intensity of which depends on the thickness of the sample being monitored, enters the reference channel and causes a corresponding change in the pulse count time55

the counting and ipyl of the reference channel automatically correct the average frequency of their following by decreasing it by an amount proportional to the average frequency of the pulse of the reflected radiation channel. The sought proportionality coefficient can be determined from the results of measuring the average pulse frequency in both channels during the sequential irradiation of two control samples of various totsins. Since the half-life of the isotope source

This leads to additional measurement error.

Then, using differential discriminators (amplitude selectors), pulses that fall into the reflected radiation channel (width 1) of the AH channel) and the reference radiation channel (channel width DM) are extracted from the common flow, and the reference radiation channel pulses are simultaneously counted the reflected radiation during the time of filling the pulse counter of the channel of the reference radiation and judging by the result of the counting of the pulses of the channel of the reflected radiation, the thickness of the test sample is judged. In this case, part of the reflected radiation falls into the reference channel (area of the DECM figure). As the thickness of the test sample decreases, the intensity of the reflected radiation decreases (peak 2 turns into peak 3). At the same time, the contribution of the reflected radiation pulses to the reference channel (the area of the DE figure,) decreases, which leads to

spouse figure de,), which leads to

a change in the pulse counting time and measurement errors of the thickness of the monitored radiation. Since the shape (amplitude distribution of pulses from the detector from monoenergy radiation) does not depend on the intense peak, but is determined by the energy resolution of the detector, all points of the amplitude distribution change with intensity.

in proportion. At the same time, the relative contribution of reflected radiation to the perpendicular channel remains unchanged (the ratio of the area of the DECM figure to the area of the ABC figure is equal to the ratio of the area of the figure to the area of AB, C, D) for any intensities of the reflected 2 and reference 1 peaks and arbitrary (but constant) widths of the AG and DM channels, respectively, which allows

0

AH and DM, respectively, which allows for the implementation of 5

the counting and ipyl of the reference channel automatically correct the average frequency of their following by decreasing it by an amount proportional to the average frequency of the pulse of the reflected radiation channel. The sought proportionality coefficient can be determined from the results of measuring the average pulse frequency in both channels during the sequential irradiation of two control samples of various totsins. Since the half-life of the isotope source

313

radiation significantly exceeds the time required to adjust the device, the intensity of the reference peak 1 can be considered constant. Then the desired proportionality coefficient is equal to the ratio of the difference between the average frequency of the pulse following in the reference radiation channel (equal to

the areas of the DECM figure and the figure, M)) d and the scattered peaks partially overlap the difference in the average frequencies of pulses in the reflected radiation channel (equal to the difference in the areas of the ABCG figure and AB CG figure).

Possible embodiments of the method.

If, after detection and separation of pulses, a digital-to-analog conversion of the signal is performed, then any necessary fraction of the signal of the reflected radiation channel is extracted and subtracted from the signal of the channel of the reference radiation before the reference signal is applied to the time setting element, for example, to an integrating capacitor. However, in a number of cases it is advisable to maintain the counting mode of the device. At the same time, to simplify the implementation of the method, it is desirable that the return value of the proportionality coefficient, i.e. coefficient and the average frequency of the im. The pulses of the reflected radiation channel were equal to an integer number, which can be achieved by appropriately choosing the width (or position relative to the peak) of any of the channels.

The implementation of the method by the device operating in the counting mode is also not difficult, since a large number of counters with an arbitrary conversion factor and circuits are known that can subtract another from one sequence of pulses.

The laboratory test of the method was carried out using a GIK-57 serial radiation source based on the Cobalt-57 radioactive isotope. The reference line was the main line in the spectrum of a source with a quanta energy of 122 keV. The reflected peak was the peak of radiation scattered from aluminum samples, while the collimation system was tuned and fixed so that the average energy of the scattered radiation quanta was 100 keV. The intensity of the scattered radiation peak was varied by varying the thickness of the controlled aluminum samples to 10 mm. A standard BDEG4-31-02A scintillation block was used as a detector; its energy resolution in the region of 100 keV was 20%, which resulted in the instrumental reference spectra

tossed Amplitude selectors were built on the basis of the 521CA4 comparators, the K155TM2 flip-flops, and the K555LE1 AND-N circuits implementing logical

5 selection of pulses that fall into the corresponding differential channel. To correct the average pulse frequency of the reference radiation channel, a synchronous counting was built.

0 parallel transfer with a division factor of 30, whose input is connected to the output of the selector channel of the reflected radiation, and the output to the R input of the control RS flip-flop,

5 S-input of which is connected to the output of the selector channel. The non-inverting output of the control trigger is connected to the first input of the AND circuit, the second input of which is connected to the output of the reference channel selector.

Thus, at the output of the circuit, And the average pulse frequency is less than the average pulse frequency of the reference radiation channel by 1/30 (which is equal to the proportionality coefficient measured by the method) from the average pulse frequency of the scattered radiation channel. The pulse frequency at the indicated points was measured using instruments of the PS02-4 type. The measurement results are shown in the table.

0

five

0

Since the activity of the radiation source for the measurement period can be considered constant (the half-life of the Cobalt 57 isotope is 280 days), the measurement error with correction due to overlapping of the peaks, according to the proposed method (-0.7%) is much less than without correction (15 %).

The proposed X-ray radiometric thickness measurement method makes it possible to almost completely eliminate the influence of the overlap of the reference and reflected radiation peaks and thereby increase the accuracy of measuring the thickness of the products, weaken the energy resolution of radiation detectors, expand the class of isotopic radiation sources used in radiation thickness and thickness gauges themselves.

Claims (2)

1. The method of X-ray radiometric measurement of the thickness, which consists in irradiating a test sample, the radiation source, register the reflected and reference radiation by the detector, separate the total pulse current from the detector to the pulse flux of the reflected radiation channel and pulse flux of the reference radiation , the pulses of the channel of the reference radiation and the pulses of the channel of the reflected radiation are simultaneously counted during the time of filling the pulse counter of the channel of the reference radiation and according to the result of counting the pulse a channel reflected radiation judged thickness of the test sample, characterized in that, in order povsheni measuring accuracy by eliminating the effect of overlapping of the peaks of the reflected and reference 28.1 49, -3 70.5 91.5 112.0 149.0 195.0 before the pulse counting, the average pulse frequency in the reference and reflected radiation channels is measured, the average pulse frequency of the reference radiation channel is reduced by an amount proportional to the average pulse frequency of the reflected radiation channel, and to determine the proportionality coefficient, the average frequencies are preliminarily measured following pulses in the channel of the reference radiation and in the channel of the reflected radiation during sequential irradiation of two Controlled samples of different thickness and the value of the coefficient of proportionality is chosen equal to the ratio of the difference between the average pulse frequency in the 20th channel of the reference radiation and the difference between the average frequency of the pulse following in the reflected radiation channel. 2. The method according to claim 1, wherein the width of the reflected radiation and the width of the reference radiation channel are chosen so that the reciprocal of the proportionality coefficient is equal to the whole number. 14.7 14.7 14.6 14.6 14.7 14.7 1.7 Editor A. Kozoriz Compiled by M. Danilov Tehred L. Serdyukova Order 3042/37 Circulation 776 - Subscription VNIIPI USSR State Committee for inventions and discoveries 113035, Moscow, Zh-35, Raushsk nab., d. A / 5 Printing and printing company, Uzhgorod, st. Design, Proofreader E.Roshko