The design and construction of an experimental MgO cold cathode X-ray tube for use in XRF spectrometry.

A detailed description of the output characteristics of the X-ray tube power supply is also given, which has a direct effect on the design requirements and therefore the performance of the X-ray tube. The configuration of the XRF spectrometer in which the tube was tested is discussed. completed X-ray, in which X-ray current and voltage measurement techniques are described.

Introduction

The central concept of the cold cathode X-ray tube is the magnesium oxide cold cathode used as the electron emitter. In Chapter 5 the spectrometer setup for the evaluation of the X-ray tube is discussed.

The high potential across the tube and the tube current are parameters that affect the intensity of the X-rays produced by the X-ray tube. The primary beam consists of the continuum and the spectral lines of the target and other elements contained in the X-ray tube components.

Fig. 2.1: The electromagnetic spectrum [25].

The Design of the X-ray Tube and the MgO Cold Cathode

FOCUSING OF THE X-RAY TUBE ELECTRON BEAM

In this way, it is possible to generate X-ray radiation with optimal intensity on the X-ray tube target. Figure 3.1.13 shows the relative positions of the focusing electrode, anode and cathode relative to each other [56].

Fig. 3.1.8: The direction of an electron travelling' from the cathode to the anode of the X- X-ray tube

DESIGN OF THE CATHODE-WINDOW STRUCTURE

In Figure 3.1.15 and Figure Alc (refer to Appendix 1) the final design of the X-ray window cold cathode structure is illustrated. DESIGN OF THE ANODE AND THE EFFECT OF THE TARGET GEOMETRY ON THE ELECTRON AND THE X-RAY.

Table 3.1.1: Transmission ratios of a 0.1 mm and a 1 mm aluminium X-ray window.

DESIGN OF THE ANODE AND THE EFFECT OF THE TARGET GEOMETRY ON THE ELECTRON AND THE X-RAY BEAM

The remainder of the collimated X-ray beam will impact the cathode and cause photoemission of additional electrons from its surface. Only the central beam with a diameter of 5 mm (beam A) comes out of the window, while the rest of the beam acts on the cold cathode.

Fig. 3.1.16: Target position relative to the electron and central X-ray beams in a side window tube

THE RATING OF THE X-RAY TUBE

Tcu = the temperature of the surface of the copper plate, which is kept at room temperature (24°C) by the coolant;. The design of the copper cooling block attached to the X-ray tube is shown in Figure Ale in Appendix 1.

Fig. 3.1.20: Temperatures, that develop at the tungsten surface, the tungsten-copper interface and the copper surface, which is kept at 24°C room temperature by the cooling liquid

SUMMARY

Another important aspect to consider when designing the X-ray tube is determining the maximum allowable load on the X-ray tube. The maximum allowable load is a quantity that gives an indication of the maximum power input that can be dissipated across the pipe without causing damage to the pipe, mainly in the form of cracks in the ceramic liner and the metal-to-metal connection. . ceramic compounds.

INTRODUCTION

FIELD EMISSION OF ELECTRONS BY SOLIDS

ELECTRON EMISSION DUE TO THE SCHOTTKY EFFECT

For the following calculation it will therefore be assumed that the MgO film of the cold cathode will have an emission constant of 1x104 A m-2. It can be seen that the above expression is independent of temperature, which is characteristic of the field effect.

Table 3.2.2: Absolute currents (rnA) at different temperatures emitted by a MgO film of 1.256 cm 2 surface area assuming the entire cathode structure has a minimum work function of 3.1 eV

COLD CATHODE ELECTRON EMISSION DUE TO CHANGING ELECTRIC FIELDS So far it has been established that emission from a cold MgO cathode is merely due to high electric

At the same time, V x B in equation 3.2.16 shows that a circular magnetic field arises around the current present in the dielectric medium as a result of the electric field gradient 8Etat. The total current ifield released as a result of the electric field in the MgO layer in the vacuum of the X-ray tube through the cathode can be calculated by multiplying the total surface area A of the MgO cathode by j. In Figure 3.2.11 we can see how the cathode is positioned in relation to the target plane of the tube.

Fig. 3.2.5: Graphic representation of X-ray tube dielectric layers and the directions of the electric field E and the current density j across them

DEFINITION OF PRIMARY, SECONDARY AND TERTIARY FLUORESCENCE Primary fluorescence results from the direct effect of the incident beam or primary radiation on the

The intensities calculated in this way will then be related to the total current of the tube i, which arises from the combined effect of the X-radiation of the nickel substrate of the cathode and the high electric fields present in the dielectric layers of the tube ie. In section 3.2.11 of this chapter an idealized case is presented, in which it is assumed that all electrons excited and removed from their atomic shell will be accelerated through the high electric fields present in the vacuum space of the tube towards the anode. than they undergo recombination processes. DEFINITION OF PRIMARY, SECONDARY AND TERTIARY FLUORESCENCE Primary fluorescence results from the direct effect of the incident beam or primary radiation on.

CALCULATION OF PRIMARY FLUORESCENCE

The fraction of primary radiation that is absorbed in the nickel bulk element is. In the case of the end window cold cathode X-ray tube (see Section 3.1) no collimators are used. It can therefore be concluded that equation 3.2.38 is certainly applicable to the calculation of the primary fluorescence of nickel PNi.

CALCULATION OF RELATIVE CHARACTERISTIC LINE AND CONTINUOUS RADIATION INTENSITIES OF NICKEL EXCITED BY X-RAYS FROM A

The sum of the photocurrent i char due to the action of tungsten characteristic X-rays and i cont. Additionally, additional electrons will be emitted as a result of the tungsten continuum acting on the cathode. Therefore, the current emitted due to the action of continuous radiation on the cathode becomes i.

Fig. 3.2.14: Accelerating potential curve for the X-ray tube with times at which the different critical excitation potentials of Ware passed

RADIATION BY MOVING CHARGES

The total charge passing through the tube during each pulse can therefore be equated to the product of the current and the duration of one pulse. After determining the net charge dQ flowing through an area, the number of charges or electrons moving from the cathode to the anode of the tube per applied pulse can be found by dividing dQ by e = 1.6xlO-19 C, which is the electric charge. of an electron. However, this does suggest that X-rays, no matter how low the intensity, will actually be produced by the interaction of the radiation from the accelerated electrons with the target before the electrons themselves reach the anode of the X-ray tube. .

SUMMARY

The Construction of the X-ray Tube

INTRODUCTION
ACTIVE ALLOY BRAZING
BRAZING AND TESTING OF EXPERIMENTAL METAL-TO-CERAMIC SEALS
ASSEMBLY OF THE X-RAY TUBE
OUTGASSING
LEAD SHIELDING AND HIGH VOLTAGE INSULATION OF THE X-RAY TUBE After successfully assembling, brazing and pumping the X-ray tube it is necessary to insulate the
SUMMARY

According to Hensley [34] a cooling rate of 23°C/min consistently produced mechanical failure of the ceramic in the form of cracks forming in the ceramic sleeve adjacent to the metal-ceramic seal. After the tube was attached using the temperature curve in Figure 4.3, the metal-ceramic seals thus formed were allowed to cool to room temperature. After completing the metal-to-metal bonds as well as the metal-ceramic seals, the X-ray tube was virtually completely assembled and only the X-ray window remained to be added to the tube.

Table 4.1: Composition of CH active solder alloys by Degussa [42].

The Spectrometer Set-up for the Evaluation of the X-ray Tube

INTRODUCTION

In the case of the cold MgO cathode of the X-ray tube not all photons arriving at the cathode are absorbed. The photoefficiency of the X-ray tube relative to the diode can be expressed as the ratio of the average photocurrent of the X-ray tube iave• photo,. To determine the absolute intensity of the primary X radiation from the X-ray tube, the total intensity had to be measured at the window of the tube.

Fig. 5.1: The X-ray fluorescence spectrometer set-up, in which the cold cathode X-ray tube was evaluated

Ray Energy (keV)

SUMMARY

Since the main concern of this study is the performance of the X-ray tube for XRF spectrometry, the spectrometer was mainly used for the characterization of the X-ray tube. After presenting a method for measuring the characteristic X-ray tube currents and voltages, a method for determining the photoefficiency of the X-ray tube cold cathode was illustrated in Section 5.3. In measuring the photoefficiency of the X-ray tube, the average diode current was assumed to be representative of the field-emitted current in the X-ray tube.

Experimental Results

THE X-RAY TUBE CURRENT AND VOLTAGE CHARACTERISTICS

Fig.6.2: The voltage drop across resistor R2 of the proportional voltage divider network, where 1 V corresponds to 40 kV discharged across the tube. As in Figure 6.1, the waveform in Figure 6.4 was obtained by measuring the voltage drop across the 10 n measuring resistor R1, which, as in the previous case of the X-ray tube (see Section 5.2, Chapter 5), was placed in series. with the diode. The photoefficiency of the X-ray tube versus the diode was finally determined by substituting the measured values for iAVE.X-RAY TUBE and i,./VE.

TUBE

SPECTRA ANALYSIS USING THE COLD CATHODE X-RAY TUBE

SPECTRA ANALYSIS WITH THE XR-100-CZT CADMIUM ZINC TELLURIDE X-RAY DETECTOR

To determine the channels, in which the K-shell excitation lines of the elements to be measured occur, it was necessary to calibrate the multichannel analyzer. Figure 6.7 shows the measured channel values of the K lines of molybdenum, silver, lanthanum, neodymium and gadolinium versus their respective photon energies. These values were plotted against the photon excitation energies (see Table A5b and Table A5c, Appendix 5) of the respective elements as shown in Figure 6.8.

Fig. 6.5: Linear plot of the measured channel number versus the photon energy for the K excitation lines of elements Mo, Ag and La

Discussion of Experimental Results

MEASUREMENT OF THE PRIMARY RADIATION INTENSITY DISTRIBUTION OF THE X-RAY TUBE
SPECTRA ANALYSIS USING THE COLD CATHODE X-RAY TUBE

In Section 6.2 of Chapter 6, the photoefficiency of the cold cathode of the X-ray tube was determined. In Section 6.3 (see Chapter 6) the radiation distribution of the X-ray tube was recorded from an angle of 90° with the target plane up to an angle of -45°. According to equation 2.5 (see Section 2.6, Chapter 2), the term Eo, which represents the acceleration potential applied across the X-ray tube, expressed in kV, is equal to the maximum energy Emax, expressed in ke V, of the X -radiation produced by the tube.

Conclusion

As a result, it was thus possible to measure the entire energy range of the continuous spectrum of the X-ray tube from the aforementioned samples. The maximum energy of the continuous spectrum from a secondary tungsten target irradiated by the X-ray tube was found to be about 84.87 keY, which confirmed the fact that the maximum voltage developed across the X-ray tube was equal to 85 keY (see Figure 6.2 and Figure 6.3, Chapter 6). Along with the Ka lines of the elements chromium, iron, nickel, copper, zinc, lead, and molybdenum, which were used as calibration samples for the multichannel analyzer, the positions of the channels in which the Ka lines of the elements cobalt, nickel, and bromine were recorded are still shown once that the channel-excitation energy relationship shown by the 'MCA 8000A' multichannel analyzer is linear.

Computer Aided Design Drawings of the Cold Cathode X-ray Tube

Alc: Schematic diagram of the cathode wind, flow structure of the cold cathode X-ray tube.

Fig. Alb: Schematic diagram of the anode structure of the cold cathode X-ray tube.

TOP VIEW

Relative Intensities of the Tungsten Emission Lines produced by the Cold Cathode X-ray Tube

Wavelengths of the principal Emission Lines and Absorption Edges

Mass Attenuation Coefficients of the Chemical Elements

Excitation Energies of the Characteristic Excitation Lines of Elements of the Periodic Table

Fluorescent Yields of Elements of the Periodic Table

Absorption Jump factors of the Elements of the Periodic Table

Wavelengths of the Absorption Edges

X-ray Spectra Measurements

A9c: Characteristic spectra for gadolinium (Gd) obtained with the AMPTEK XR-IOOT-CZT cadmium zinc telluride detector. A9d: Characteristic spectra of the elements chromium (Cr), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), lead (Pb) and molybdenum (Mo) obtained with the AMPTEK XR-IOOT silicon detector. A9f: Characteristic spectra of nickel (Ni) obtained with the AMPTEK XR-IOOT silicon detector. Energy [keV).