GB2110365A - X-ray fluorescence analyser for determining concentration of compound elements in materials - Google Patents

Abstract

An X-ray fluorescence analyser for determining the concentration of compound elements in materials, comprises a radiation source (1) for irradiating a sample (6) to be analysed, sensing means (2) for energy-selectively detecting radiation emitted by the sample (6) within narrow value ranges, and a measuring unit (3) connected to the sensitive means (1). For widening the scope of use of the analyser and for increasing its accuracy, the sensing means includes a balanced pair of filters (4) and an ionization sensing means (5) arranged in series. The measuring unit (3) is suitable for providing a measurement of the current density or the amount of charge. <IMAGE>

Description

SPECIFICATION X-ray fluorescence analyser for determining concentration of compound elements in materials The invention relates to an X-ray fluorescence analyser for determining concentration of compound elements in materials, which comprises a radiation source for irradiation the material to be analysed, energy-selective sensing means for detecting radiation emitted by the material and a measuring unit connected to the sensing means. The X-ray fluorescence analyser according to the invention ensures high accuracy of determining the concentration of compound elements in different objects and samples to be analysed.

The X-ray fluorescence analysis is a very important method among the various methods of chemical analysis. The essence of this method is that the inner electron shell of the atoms will be ionised under the influence of radiation of corresponding energy, and the ionisated atom results in emitting X-ray fluorescence radiation the energy of which is characteristic for the emitting atom. By determining the intensity of Xradiation of different energies-generally after plotting a graph of calibration-it becomes possible to determine the relative quantitative values for the compound elements contained in the material.

In the method of X-ray fluorescence analysis the excitation of the material or sample is carried out generally by gamma-radiation or X-ray radiation, and the source of radiation can be a radioactive isotope (for example the isotope of americium with mass number 241) or a X-ray tube. In case of special investigations it may be advantageous to use a beam of charged particles (electrons, protons, etc.) and a source for generating it.

The X-ray fluorescence analysers can be put into three groups when considering the method of differing characteristic X-ray radiation of compound elements.

1. The spectrometers on the basis of diffraction at a crystal lattice give high resolution but so is their power consumption. They operate on the basis of detecting under the Bragg-angle characterising the direction of the characteristic X-radiation. In the corresponding places the radiation can be detected by ionisation chamber, coincidence counter or by decatron. The high power demand requires performing the method in laboratories.

2. The energy selective X-ray spectrometers are based on generating a signal proportionately to the energy of the X-radiation. For this objective there are known, for example, semiconductor spectrometers which render the use of low power radioactive sources or X-ray tubes possible. The resolution of the energy selective X-ray spectrometers is low, not enough to differentiate adjacent elements, or the devices demand extremely rigorous conditions for operating which can be assured only in laboratories (for example, the resolution of the semi-conductor spectrometers is high enough only at the temperature of liquid nitrogen or below it).

3. In an X-ray fluorescence analyser constructed with a balanced pair of filters the determination of energy is carried out by a pair of filters having two filter members of the absorption-edge-type. The filter members consist of two different materials for energy selective sensing. For determining the intensity of radiation to be analysed the material of the filters should be chosen in such a way that one of the filters shall not absorb the characteristic X-radiation of the element to be determined, and the other filter shall do so; however, the characteristic energy of cutting (corresponding to the absorption edge) shall have near values for the members (differential pair of filters). The thickness of the members of the pair of filters is chosen advantageously so that their absorption is nearly the same in the value domain to be not analysed (balanced pair of filters).

For analysing radiation passing through the filters there are known detectors with low energy resolution, this means scintillation counter or decatron or semiconductor counters being usable in chamber temperature.

As compared to the analysers of the first two groups the last mentioned are cheaper and more simple. Therefore they are used in mining, in processing raw materials, for quality control in the machine industry and for selecting materials.

Their use is advantageous in conditions while the decision should be taken up on the basis of the amount of one or a low number of compound elements. Their accuracy, however, is low. For example with devices of this art the alloying components of steel or iron can be determined with accuracy up to 0.2; .0.3% range. This accuracy is good enough in many kinds of technology, but it is not sufficient when the different sorts of steel should be selected on the basis of the Mn-content.

The aim of the invention is to create an X-ray fluorescence analyser which can be constructed in portable form and ensures higher accuracy of measurements than the known devices.

In the methods based on low power radiation sources, for example X-ray tubes there are used pulse counting sensitive means. The lower the number of the pulses the lower is the amount of the element to be analysed and therefore the background may be intensive. In these conditions the measurement should last long as to balance the influence of the high background level.

Therefore the ionization chamber detector, generally used as a pulse counter, has been considered as unsuited for X-ray fluorescence analysis.

By using X-ray tubes of power up to 10 Wa high number of pulses can be detected, which leads to decreasing the statistical error of the measured value and in this way a higher accuracy can be achieved and/or the time interval of the measurements can be shortened. A perception leading to the creating this invention lies in that the mentioned power values render using ionization chamber detectors possible however, as indicated for example, by an article of G. Carr Brion in "X-ray Spectrometry" (9, 184, 1980) it is considered to be less suitable to such measurements. The consideration is based on the fact that when using as pulse counter the ionization chamber detector cannot detect effectively the X-radiation.These detectors give low signal level in measuring low energy Xradiation, the signals can be processed only at low temperature especially if a high level of reliability required. A further perception is that instead of pulse counting the ionization chamber detector can be used for measuring current or amount of accummulated charge in a predetermined time-period and in this way for providing X-ray fluorescence analysis. The ionization chamber detector used in this way is suitable for detecting high intensity and is characterized by lack of dead time.

A further perception is that the ionization chamber detector is suitable for this way of operating independently of its construction. As for the invention as ionization chamber detector are considered all devices wherein the ionization current comes at the electrodes into being in consequence of moving the pairs of charge carriers, generated by X-radiation to be detected.

Should the ionization current be measured and not the number of pulses, the selection of the radiation according to the energy cannot be carried out on the basis of the measured values, therefore it should be taken out before detecting.

For this aim a balanced pairs of filters are suitable.

Supposing measurements of current or current intensity immediately or indirectly on the basis of charge accumulated in a predetermined time period a high speed of measurements is ensured.

The objective of the invention is to utilize the described perceptions and to create an X-ray fluorescence analyser that renders measurements with high accuracy and short response time possible.

To reach this objective an X-ray fluorescence analyser has been created, mainly for measuring concentration of elements in a material to be analysed, comprising a radiation source for irradiating material to be analysed, energyselective sensing means for detecting radiation emitted by the material and a measuring unit, wherein the energy-selective sensing means are equipped with a balanced pair of filters, coupled to an ionization sensing means and the output of the ionization sensing means is connected to the measuring unit constructed, for example, as an ammeter or coulometer.

When basing on the proposed construction analysers can be created portable and stationary equipment, as weli. The portable device is equipped with an X-ray tube of power up to 10 W as radiation source, and ensures high accuracy of measured values. It gives the measured values practically without dead time. It is also advantageous that dependently on the demands the device can be constructed with different radiation sources, as high power X-ray tubes, gamma-source, for example with isotope of americium. or as a source suitable to emit current of charged particles, e.g. of electrons.

The device according to the invention can be built up relatively cheaply if equipped with ionisation sensitive means comprising a single detector, wherein only one member of the balanced pair of filters is arranged before the detector, e.g. exchangeably.

The measurements can be carried out quickly and in a simple way when the ionisation sensing means comprises two detectors, and before the detector are arranged the corresponding members of the balanced pair of filters respectively; however, this solution involves higher costs than the last mentioned one.

The two detectors can be connected advantageously in a compensation circuit, wherein the independent electrodes of the ionisation sensing means are connected to poles of opposite polarity of supply voltage. In this way the common electrode forwards the difference current of the two ionisation sensing means. By this solution the measured data can be processed in a simple way; however, two different supply voltage of two different values would be required.

When using the device according to the invention during the same measurement time as in known solutions the accuracy of determination can be increased by an order of magnitude at least; thus the limit of determining concentrations is similarly lowered. This is very important when considering the possibility of using the equipment. By advantageous construction as proposed the sensitivity of the equipment can be improved to a very high level.

The invention will be further described, purely by way of example, with reference to preferred embodiments illustrated in the accompanying drawings, wherein: Figure 1 is a block diagram of the X-ray fluorescence analyser according to the invention, Figure 2 is a block diagram of the X-ray fluorescence analyser according to the invention in an embodiment equipped with one detector, Figure 3 is a block diagram of the X-ray fluorescence analyser according to the invention in an embodiment equipped with two detectors, and Figure 4 is a block diagram of the X-ray fluorescence analyser according to the invention in an embodiment equipped with two detectors connected to a compensation circuit with one another.

The X-ray fluorescence analyser as shown (Fig.

1), comprises a radiation source 1, energyselective sensing means 2 and a measuring unit 3. The radiation source 1 may be an X-ray tube, a gamma-radiation source or a source emitting beams of charged particles. The radiation source 1 irradiates a sample 6, the radiation thereof, consisting essentially of secondary X-radiation emitted by the material of the sample falls on a balanced pair 4 of filters in the energy-selective sensing means 2, comprising ionization sensing means 5 at the output of the balanced pair 4 of filters. For detecting the characteristic X-radiation to be measured the balanced pair 4 has to contain an element which does not absorb this radiation and another element which absorb fully the radiation; however, the difference of energy of cutting of these members should be low. The balanced pair 4 of filters consists of a low-pass filter 9 and a high-pass filter 8.The absorption of these filters is advantageously the same in value domain, wherein no measurement should be provided. The balanced pair 4 of filters as mentioned is arranged at the input of the ionization sensing means 5.

It is advantageous to equip the ionisation sensing means 5 of the energy-selective sensing means 2 with a single detector 7 (Figure 2). In this case one member of the balanced pair 4 of filters is arranged between the sample 6 and the detector 7, for example the high-pass filter 8.

After finishing the measurement with this filter, in its place the low-pass filter 9 should be inserted and the measurement has to be repeated. In this way only one ionization detector may be sufficient to carry out the measurements, but a mechanical construction is required for changing the filters.

As shown in Figures 3 and 4, it is advantageous also to equip the ionization sensing means 5 of the energy-selective sensing means 2 with two detectors 11 and 12. In this arrangement the high-pass filter 8 is in front of the detector 11, and the low-pass filter 9 in front of the detector 12. The output terminals of the detectors 11 and 12 are connected to the input of measuring unit 3, which is e.g. an ammeter. In this device the data measured are obtained in only one step and changing the members of the balanced pair 4 of filters is not required.

As shown in Figure 4, it is advantageous to connect the detectors 11 and 12 in a compensation circuit. In this case the detectors 11 and 1 2 are equipped with respective independent electrodes connected to opposite poles of two voltage sources. According to this solution the common electrode provides the difference current of the two detectors and this current is flows to the input of the measuring unit 3, in this case of an ammeter. In this embodiment the electronic circuits can be built up in a simple mode, however, two different supply voltages are required.

The X-ray fluorescence analyser according to the present invention can be carried out as a portable device having an X-ray tube of power range 1 to 10 W as radiation source. The measuring unit 3 can be e.g. an ammeter or a coulometer, i.e. a unit for direct or indirect measurement of current.

The X-ray fluorescence analyser as invented can be used as other similar devices, i.e. before the measurement calibration data should be determined. The device has a simple construction, low consumption of energy, is usable in all conditions. Due to these advantages it may be used in industrial process control systems, in quality supervision of parts and objects of large size. The determination of the data can be carried out quickly and with high accuracy.

Claims (8)

Claims

1. A X-ray fluorescence analyser for determining the concentration of compound elements in materials, comprising a radiation source for irradiating material, energy-selective sensing means for detecting radiation emitted by the material within narrow value ranges and measuring means connected to said energyselective sensing means, wherein said energyselective sensing means includes at least one balanced pair of filters and ionization sensing means connected to the output of said pair of filters.

2. An X-ray fluorescence analyser according to claim 1, wherein said measuring unit is adapted for measuring current.

3. An X-ray fluorescence analyser according to claim 1, wherein said measuring unit is adapted for measuring charge.

4. An X-ray fluorescence analyser according to any preceding claim, wherein said energyselective sensing means comprises a single detector, and before the detector a member of said balanced pair of filters is exchangeably arranged.

5. An X-ray fluorescence analyser according to claim 1, 2 or 3, wherein said ionization sensing means inciudes two detectors, arranged after a corresponding member of said balanced pair of filters.

6. An X-ray fluorescence analyser according to claim 5, wherein said detectors are connected in a current compensation circuit.

7. An X-ray fluorescence analyser according to any preceding claim, wherein said balanced pair of filters consists of a low-pass filter and a highpass filter.

8. An X-ray fluorescence analyser substantially as herein described with reference to and as shown in any of the Figures of the accompanying drawings.