FIELD OF THE INVENTION

The invention generally relates to a Flat Output Response Transmission X-ray Tube, wherein the output spectrum from a transmission type of x-ray tube measuring x-ray intensity at a some given x-ray energy does not change when the voltage applied to the tube increases.

BACKGROUND OF THE INVENTION

There is a growing need to use x-radiation to measure the concentrations of hazardous various materials simultaneously in a sample to provide high speed, low cost measurement, and highly accurate measurement. There is a universal problem for example in measuring the concentration of multiple materials without background x-radiation interfering with the fluorescent measurements.

Current x-ray tubes require multiple sample exposures at different tube voltages and different filtering schemes to provide such measurements, especially as in the Restriction of Hazardous Substances (ROHS) Directive requirement to measure cadmium along with other toxic materials simultaneously.

What is needed is an x-ray tube that can provide simultaneous high speed, high accuracy measurements of multiple elements with a single tube and filter configuration and a single measurement. SUMMARY OF THE INVENTION

The invention discloses a transmission x-ray tube. The transmission x-ray tube includes an evacuated housing; an end window anode disposed in said housing comprised of a target of at least one thin foil; a cathode disposed in said housing which emits electrons, wherein the electrons proceed along a beam path in said housing to strike said anode in a spot, generating a beam of x-rays which exits the housing through said end window; and a power supply connected to said cathode providing selected electron energies to produce a spectrum of x-rays of differing photon energies; wherein the total number of breamsstrahlung photons in at least one defined energy band does not increase appreciably with increasing voltage applied between said anode to cathode of several thousand volts.

It is preferred that the thickness of said thin foil is between 0.05 and 2 microns. It is preferred that a filter is optionally applied to alter said spectrum of x-rays by absorbing proportionally more x-ray photons at lower energies to produce an output spectrum from said tube.

It is preferred that said several thousand volts is 30 kVp.

It is preferred that said several thousand electron volts is 40 kVp. It is preferred that at least one defined energy band is 10 keV or less. It is preferred that at least one defined energy band is 20 keV or less.

It is preferred that said total number of bremsstrahlung photons in the said defined energy band does not vary more than 5% from the average number of counts in said increasing several thousand electron volts.

It is preferred that said total number of bremsstrahlung photons in the said energy band does not vary more than 10% from the average number in said energy band over said increasing several thousand electron volts.

It is preferred that said foil contains a metallic element chosen from one of scandium, titanium, chromium, iron, nickel, yttrium, molybdenum, rhodium, palladium, gadolinium, erbium, thulium, ytterbium, tantalum, tungsten, rhenium, platinum and gold.

It is preferred that the x-ray tube is used to produce x-rays used in x-ray fluorescent analysis.

It is preferred that the x-ray tube is used to produce x-rays used to detect the elements of interest in the RoHS directive.

It is preferred that a thin film of x-ray generating material is deposited on the atmospheric side of the end window producing low energy x-rays characteristic of said deposited material.

DESCRIPTIONS OF DRAWINGS

FIG. 1 is a representation of a typical transmission type x-ray tube.

FIG. 2 is a representation of a typical reflection type x-ray tube. FIG. 3 is a representation of typical x-ray spectrum from an x-ray tube showing the bremsstrahlung radiation between 20 keV and 50 keV of photon energy. The Fig. 3 is used to define the total counts in a given energy band. The shaded area shown as item 13 represents the total number of photons emitted by the tube between photon energies of 20 keV and 50 keV. There are 44,220 photon counts for this particular energy band and test condition.

FIG. 4 is a representation showing the spectral output of a typical reflection type x-ray tube. This is a typical reflective type x-ray tube used in NDT imaging. As the voltage increases from 30 kVp to 60 kVp it can be seen that the peak bremsstrahlung radiation shift to a higher energy changing the spectrum considerably as the tube voltage in increased. Tube current is 50 microA (microamps or μΑ) and the spectrum is measured at 1 m using a 100 micron tungsten collimator and over a 3 minute test period. FIG. 5 is a representation of spectrum from a reflection type x-ray tube from 50 kVp to 150 kVp applied tube voltages. A micro-focused, reflective type tube as the voltage increases in increments of 20 kVp from 50 kVp to 150 kVp. Note that as tube voltage increases there is a shift in peak brem energy in the spectrum. FIG. 6 is a representation of the output spectrum of a transmission type x-ray tube with a target of 4 micron thick tantalum (4Ta) spectrum as voltage increases from 40 kVp to 80 kVp in 10 kVp increments with a tube current of 50 microA. Feature of transmission tubes peak brem energy in keV does not increase with increasing tube voltage. FIG. 7 is a representation of the output spectrum of a transmission type x-ray tube with a target of 2 micron thick tantalum (2Ta) from 90 kVp to 120 kVp in 10 kVp intervals. Note that the curves spectrum for 100, 110 and 120 kVp are substantially the same. Hence a 2Ta tube used at voltages above 100 kVp exhibits the flat output response. FIG. 8 is a representation of the output spectrum of a transmission type x-ray tube with a target of 1 micron thick tantalum. These spectra are for a ITa tube with tube voltages of 70 to 100 kVp in intervals of 10 kVp. Note that the curves start to superimpose at 80 kVp or 20 kVp less than for a 2Ta target. For thicker and thicker target materials the voltage at which the amount of total energy at a specific photon energy do not increase. As expected x-ray intensities decrease with decreasing target thickness.

FIG. 9 is a representation of the output spectrum of a transmission type x-ray tube with a target of 0.75 micron thick tantalum. (0.75 Ta from 40 kVp to 80 kVp in 10 kVp intervals at 50 microA tube current).

FIG. 10 is a representation of the output spectrum of a transmission type x-ray tube with a target of 0.3 micron thick tantalum. (0.3 Ta from 40 kVp to 80 kVp at 50 microA) Analysis must be done on a band width of x-ray energies.

FIG. 11 is a representation of the output spectrum of a transmission type x-ray tube with a target of 0.3 micron thick tantalum using two different copper filters. (0.3Ta with copper filters of 200 and 400 microns of copper, tube voltage 50 kVp tube current 50 microamps).

FIG. 12 is a representation of the output spectrum of a transmission type x-ray tube with a target of 0.75 micron thick tantalum using an aluminum filter 2 mm thick. (0.75 Ta at 120 kVp 50 microA with a filter of 2 mm of Aluminum). Generally flat response with increasing x-ray energies. Except for Ta K-alpha at 57.524 keV.

FIG. 13 is a representation of the output spectrum of a transmission type x-ray tube with a target of 0.75 micron thick tantalum to measure ROHS elements. (RoHS measurement with 0.75 Ta) FIG. 14 is a representation of the output spectrum of a transmission type x-ray tube with a target of 0.3 micron thick tantalum to measure ROHS elements. (RoHS measurement at 0.3 Ta at 50, 60 and 70 kVp) FIG. 15 is a graphical representation of the output energy spectrum of a transmission tube with a target foil of 1.5 micron thick silver.

FIG. 16 is a graphical representation of the output energy spectrum of a transmission tube with a target foil of 2.3 micron thick rhodium.

DESCRIPTION OF THE INVENTION

Open transmission tubes are typically used for imaging of electronic circuits, as well as other high-resolution applications. Closed tubes are sealed with a vacuum whereas open or "pumped down" tubes have a vacuum pump continuously attached drawing a vacuum as the tube is used usually to allow for frequent replacement of tube parts which tend to fail in operation. For purposes of this invention transmission tubes include both open and closed transmission type tubes, except as otherwise stated.

The present invention is a transmission x-ray tube that utilizes a very thin target, wherein the thin target combined with very high tube voltages to produce a flat output response. More specifically, the present invention is capable of producing bremsstrahlung photons in a defined energy band that does not vary with increased applied voltage.

FIG. 1 illustrates one embodiment of the present invention. FIG. 1 is a simple schematic representation of the function served by each of the elements in a transmission type tube of the present invention and is not intended to limit the specific implementation of any of those functions to the schematic drawing. Although FIG. 1 illustrates a sealed tube, alternative embodiments may utilize an open or pumped-down tube, wherein the vacuum inside the tube is continuously evacuated by a vacuum pump during operation of the tube.

The transmission tube of the present invention comprises an evacuated housing 6, an anode 1, an x-ray target foil 2, a cathode 3, a focusing cup 5, and a power supply 36. The anode 1 has an inner surface and an outer surface and is disposed at one the end of the evacuated housing 6. The outer surface of the anode 1 is exposed to atmosphere. The anode 1 also functions as an end-window anode in the present invention. In a preferred embodiment, the end-window is made of material with a low atomic number (Z), such as beryllium, copper or aluminum.

The x-ray target foil 2 is deposited on the inner surface of the anode 1. In a preferred embodiment of the current invention, the thickness of the x-ray target foil 2 is chosen to be less than about 2 microns thick.

The cathode 3 is disposed in the evacuated housing 6 opposite the anode 1. The preferred cathode 3 is electrically heated, but alternative types of cathodes may be used. The cathode 3 emits a plurality of electrons to form an electron beam. The electron beam is accelerated along the electron beam path 4 and strike the x-ray target foil 2 to produce a beam of x-rays 8. The positive terminal of power supply 36 is connected to the anode 1 and negative terminal of power supply 36 is connected to the cathode 3. The electrical connection of the power supply 36 provides the accelerating force for the electron beam. The preferred cathode 3 is capable of providing selected electron energies to produce a spectrum of x-rays of differing photon energies. X-rays 8 exit the transmission tube through the end-window 1 with a defined cone angle.

The focusing cup 5 focuses the electron beam onto a spot on the x-ray target foil 2. In alternate embodiments, the focusing cup 5 is not utilized. The focusing cup 5 may be electrically biased negatively or positively or may be neutral depending on the degree of focusing required. The largest dimension of the spot is referred to as the focal spot size or spot size. The output x-rays 8 contain both bremsstrahlung (or braking radiation) and characteristic line radiation unique to the target material. .

FIG. 2 is provided for reference and illustrates a reflection type x-ray tube. A reflection type x-ray tube comprises a housing containing a reflection anode 7, a reflection cathode 9, and a side window 11. The power source 36 is connected between the reflection cathode 9 and the reflection anode 7 to provide an electric field which accelerates the electrons from the reflection cathode. The electrons travel along a reflection election beam path 10 and strike the reflection anode 7 generating a beam of x-rays 8 which then exit the tube through the side window 11. The reflection tube harvests produced x-rays 8 from the same side of the target that the electron beam impinges, as well as from x-rays generated within the x-ray target. For purposes of clarification, the term "energy band" is defined as an arbitrarily determined continuous band in keV of the x-ray energies produced from an x-ray tube. The term "total number of bremsstrahlung photons in a given energy band" is defined as the total number of photons which impinge an x-ray photon sensor such as a CdTe or ZCdTe or similar sensor in a given energy band containing no characteristic x-radiation, as calculated by integrating the area under the curve of intensity in photon counts versus the x-ray energy in keV emitted by an x-ray tube.

Unless otherwise specified, x-ray tube spectral data presented in FIGS. 3-15 was obtained with an Amptek Model XR-100 with a CdTe sensor 1 mm thick and 10 mils of Be filter. The sensor was placed at a distance of 1 meter from the x-ray tube and a tungsten collimator, with a collimator hole of 100 μηι diameter, placed in front of the sensor. Tube current was fixed at 50 microamperes. The data was collected over a test period of 3 minutes.

A shaded area 13 under the curve in FIG. 3 provides a graphical illustration of the total number of bremsstrahlung photons in an energy band from 20 keV to 50 keV from a transmission tube of the current invention with a tantalum foil anode target, 0.75 microns thick, and an applied accelerating voltage of 100 kVp. There are a total of 44,220 photon counts for this particular energy band. There is no significant characteristic line x-radiation produced by tantalum in this energy band. Although this illustration uses tantalum as the x-ray target foil 2, the target foil material could be any of a number of different materials suitable for use as x-ray targets in a transmission, including but not limited to Sc, Ti, Cr, Fe, Cu, Ni, Y, Mo, Rh, Ag, Pd, Gd, Er, Tm, Yb, Ta, W, Re, Pt, or Au.

For reference purposes, FIGS. 4 and 5 show a graphical representation of the x-rays produced by a commercially available reflection type x-ray tube as the accelerating voltage is progressively increased. FIG. 4 was obtained using a reflective type x-ray tube typically used in x-ray imaging for non-destructive testing of circuit boards. The target material of this tube is tungsten. The intensity of x-rays produced in counts from a photon sensor as a function of the energy of the produced x-rays in keV is represented in FIG. 4 as spectral lines 14, 15, 16 and 17. Spectral line 14 represents a tube voltage of 30 kVp. Spectral line 15 represents a tube voltage of 40 kVp. Spectral line 16 represents a tube voltage of 50 kVp, and spectral line 17 represents a tube voltage of 60 kVp. FIG. 4 illustrates that the peak bremsstrahlung radiation shifts to a higher energy, thereby changing the spectrum considerably as the tube voltage in increased. Spectral lines 14, 15, 16 and 17 illustrate measurements made at each of a number of increasing tube voltages.

FIG. 5 shows a graphical representation of the output of a typical commercially available micro-focused reflective type x-ray tube as the voltage increases in increments of 20 kVp from 50 kVp to 150 kVp. Spectral line 26 represents a tube voltage of 50 kVp; spectral line 27 represents a tube voltage of 70 kVp; spectral line 28 represents a tube voltage of 90 kVp, spectral line 29 represents a tube voltage of 1 10 kVp; spectral line 30 represents a tube voltage of 130 kVP and spectral line 31 represents a tube voltage of 150 kVp. It is noted that as tube voltage increases there is a shift in peak brem energy in the spectrum. At 130 and 150 kVp there is considerable distortion of sensor readings above 60 keV from x-rays passing through the tungsten collimator of the instrument used to collect the spectral data.

In contrast, FIG. 6 is a graphical representation of the x-rays produced by a preferred embodiment of the present invention with at 4 micron thick tantalum target foil used at low applied tube voltages from 40 kVp to 80 kVp. It is a novel feature of the transmission tubes of the present invention that the peak brem energy in keV does not increase with increasing tube voltage. In FIG. 6, the peak occurs approximately midway between 11.98 keV and 23.65 keV, or about 18 keV, and does not increase as tube voltage is increased from 40 kVp to 80 kVp. Commercially available reflective type x-ray tubes exhibit two features that clearly separate such tubes from x-ray tubes of the current invention. In conventional reflective type x-ray tubes, the bremsstrahlung radiation peak shifts to higher and higher energy as applied tube voltage increases. Furthermore, the total number of bremsstrahlung photons increases across the entire spectrum of output x-ray energies as applied voltage increases. These features limit the use of such x-ray tubes to solve problems in x-ray fluorescence and x-ray imaging markets.

The problems found in commercially available reflective tubes are not present in the disclosed invention. In one preferred embodiment of the current invention, when a very thin target foil is used, the total number of bremsstrhalung photons in a specifically defined band will not increase substantially, even though the applied accelerating voltage of the tube increases by a large percentage. FIGS. 7, 8, 9, and 10 illustrate this unique and novel aspect of the transmission tube of the current invention.

FIG. 7 is the output spectrum from a preferred embodiment of the present invention with a foil target 2 μηι thick and tube voltages varying from 90 kVp to 120 kVp in 10 kVp intervals. Spectral line 37 represents tube voltage of 90 kVp. Spectral lines for tube voltages of 100 to 120 kVp were undistinguishable and are represented as spectral line 38. FIG. 7 clearly shows that the curves spectra for 100, 110 and 120 kVp are substantially the same at lower x-ray energies of about 12 to 50 keV. This embodiment used at voltages of 100, 110 and 120 kVp and exhibits the flat output response-novelty of the current invention. Although tantalum was chosen as the target material for this embodiment, any metallic material which can be deposited in very thin films on the end-window can also be used. This includes pure metals, as well as alloys.

FIG. 8 represents the output spectrum of another embodiment of the current invention with a target foil 1 μιη thick of tantalum and tube voltages of 70 to 100 kVp in intervals of 10 kVp. Spectral line 39 represents tube voltage of 70 kVp. Spectral lines for tube voltages of 80 to 100 kVp were undistinguishable and are represented as spectral line 40. As the voltage is increased from 80kVp to 100 kVp there is very little change in the total number of bremsstrahlung photons in the energy band of about 12 keV to 40 keV, further demonstrating the novelty of the transmission tube of the current invention.

FIG. 9 further illustrates an x-ray tube of the current invention using a target foil of 0.75 μπι thick tantalum. Tube voltages were varied from 40 kVp to 80 kVp in 10 kVp intervals. Spectral line 41 represents tube voltage of 40 kVp; spectral line 42 represents tube voltage of 50 kVp; spectral line 43 represents tube voltage of 60 kVp; spectral line 44 represents tube voltage of 70 kVp; and spectral line 45 represents tube voltage of 80 kVp.

FIG. 10 illustrates an x-ray tube of the current invention using a target foil of 0.3 μηι thick tantalum with tube voltages varying from 40 kVp to 70 kVp in 10 kVp intervals. Spectral line 46 represents tube voltage of 40 kVp; spectral line 47 represents tube voltage of 50 kVp; spectral line 48 represents tube voltage of 60 kVp and spectral line 49 represents tube voltage of 70 kVp. FIGS. 9 and 10 clearly demonstrate the flat output response of the current invention.

Although data from a 4 micron thick tantalum target foil was not included, it is obvious that by increasing the tube voltage to the range of about 110 to 160 kVp, the total number of bremsstrahlung photons in an energy band from about 12 to 40 keV will not increase appreciably.

Table 1 is a summary of measurements made by four different x-ray tubes of the present invention using tantalum target foils 0.3, 0.75, 1 and 2 μηι thick. The data shows the general salient features of the current invention. Experimental error and normal variations in x-ray tube construction and precision must all be considered in interpreting the data. The data were taken with an Amptek Model XR-100 with a CdTe sensor 1 mm thick and 10 mils of Be filter. The sensor was placed at a distance of 1 meter from the x-ray tube and a tungsten collimator with an collimator hole of 100 μιη diameter placed in front of the sensor. Tube current was held constant at 50 microamperes and tube voltage was varied as shown in Table 1. Spectrum measurements were made and the area under the curve from 13kev to 27 keV was measured and recorded. The Energy Band 13-27 keV was arbitrarily chosen to generally illustrate the output from a tube of the current invention. A smaller or larger energy band could have been chosen with varying results. Generally, as target foil thickness increases, the tube voltages which demonstrate that the total number of bremsstrahlung photons do not increase appreciably with increasing tube voltage generally shift to higher voltages.

From Table 1, the average number of photons for 0.3 μιτι tantalum target tube between 40 and 70 kVp is 32,851 with the maximum variation from average of 1,559 counts or a maximum variation of + 4.7%. The average number of photons for the 0.75 μπι tantalum target tube between 40 and 80 kVp is 34,412 with the maximum variation from average of 578 counts or a maximum variation of +1.6%. The average number of photons for the 1 μιη tantalum target tube between 80 and 100 kVp is 74,943 counts with the maximum variation from average of 4, 163 counts or a maximum variation from average of 5.5%. The average number of photons for the 2 μηι tantalum target tube between 90 kVp and 120 kVp was 109,784 with a maximum variation from average of 1,631 or 1.4%. TABLE 1

Table 2 presents measurements of total bremsstrahlung radiation from two commercially available tubes not of the current invention. An NDT reflective type x-ray tube was measured using a tube amperage of 375 microamperes, as compared to measurements of Table 1 above made with 50 microamperes of tube current. The second tube is a transmission type of x-ray tube with a target foil thickness of 4 μηι of tantalum.

TABLE 2

Comparison of Tables 1 and 2 provides a clear difference in the way tubes of the current invention provide relatively unchanging total bremsstrahlung radiation over a given x-ray energy band from 13 to 27 keV. Furthermore, the commercially available reflective type tubes show no indication that there is an energy band comparable to transmission tubes of the current invention. This difference is attributed to the inherent differences between reflective tubes and transmission tubes. Reflective tubes are not capable of producing a flat output response. For a reflective tube, as the voltages increases, the output in any given band continues to increase.

The x-ray target foil 2 of the present invention may be chosen from any of a number of possible elements. In FIGS. 7, 8, 9 and 10 the target material was chosen to be tantalum. Tantalum has a high atomic number producing proportionally higher x-ray intensities, as is well know by those skilled in the art. Yet at x-ray energies of 57.524 and 65.21 keV, there are noticeable spikes in the output caused by the -a and Κ-β characteristic lines of tantalum. In most applications such spikes in an otherwise flat intensity response with photon energy are not a limiting issue in the use of the current invention. However, in some applications it is advantageous to eliminate or limit such characteristic energy spikes. In one preferred embodiment of the current invention, the target material and the filtering parameters are chosen so that there is no characteristic energy spike in the flat output response of the tube. When the x-ray producing foil in the anode target contains elements of low atomic number, for example Scandium, chromium, nickel, iron and copper, as well as others, the characteristic -lines are below about 10 keV, eliminating such -line spikes from the output spectrum. Usually this is done at the expense of x-ray output intensity.

Although a single target material was chosen to demonstrate the novel phenomenon of the current invention, layered target materials and target materials containing more than a single element may be used. In one alternate embodiment of the current invention, multiple target materials may be used on a single target with an electron beam that may be moved to strike the desired the target section of the current invention. A variety of target materials may be used in the current invention. A partial list of possible metallic elements present in the target foil of the current invention may be chosen from those of scandium, copper, silver, chromium, titanium, iron, nickel, yttrium, molybdenum, rhodium, palladium, gadolinium, erbium, ytterbium, thulium, tantalum, tungsten, rhenium, platinum and gold.

In one preferred embodiment of the current invention soft x-rays below about 10 keV may be produced, which is particularly useful for x-ray fluorescence applications. Because the end-window anode 1 must be sufficiently thick to seal the evacuated housing 6, the end-window filters out most of the wanted low energy x-rays. Hence when low energy characteristic x-rays are required, a material generating such low energy characteristic lines can be deposited on the outer surface of end-window anode 1 by any of a number of known methods, including sputtering. Typical thickness of such material can be as low as 0.05 microns up to about 2 or 3 microns.

High energy X-rays above about lOkeV generated in the target pass through the end-window exciting fluorescent characteristic radiation from the material deposited outer surface of end-window anode 1. Because the low energy generating material can be located very close to the spot generating x-rays, a highly efficient transfer of energy to low energy characteristic x-rays is effected. The thickness of the end-window can be any thickness, from as low as about 50 microns to many millimeters thick, since little high-energy x-radiation is attenuated by the end-window. Although beryllium is usually the preferred end-window material, but any number of low Z number element such as aluminum, copper, titanium, or alloys thereof, may alternatively be used. By varying the thickness of the deposited material and the accelerating voltage and current of the electrons impinging the target, the brightness of the low-energy characteristic lines can be selected.

In one alternate embodiment of the current invention filters are used to absorb low energy radiation from a transmission tube of the current invention. The filters are placed in the path of the x-ray beam on atmospheric side of the end window. FIG. 11 is graphical representation of the output of an embodiment of the current invention with a target foil of 0.3 μιη of tantalum. Spectral line 19 is the output spectrum with no filtering. A spectrum using copper filters 200 μιη thick is shown as spectral line 20 and 400 microns of copper as spectral line 21. Tube voltage is 50 kVp and tube current is 50 microamps.

In general fluorescent applications, the background x-radiation noise generated by the x-ray tube for elements to be detected is particularly important. Using an appropriate filtering scheme, the background noise at the K-alpha energy to be measured can be reduced to a minimum. Using the filtered output, spectral line 21, to measure elements with characteristic K-alpha line emissions in the range of about 20 to 23 keV, the background noise is determined by the portion of the energy band 5 to 10 keV above the line emission energy. The tube of the current invention provides a spectrum in that band which is essentially constant over that band as the tube voltage is increased. Increasing the tube voltage adds x-ray photons of higher energy which can increase the k-alpha response of the element to be detected without adding to the noise level. This feature makes the tube of the current invention particularly useful in the field of x-ray fluorescent analysis.

Various filtering schemes may be used with the transmission tube of the current invention to improve the usefulness of the tube in the market. In one preferred embodiment of the current invention represented in FIG. 12, the output x-radiation from a tube with 0.75 μιη thick tantalum target foil operated at 120kVp and 50 microampere tube current and an external 2 mm thick aluminum filter produces an output spectrum over an energy band from about 25 keV to about 90 keV which is essentially constant except for the characteristic K-lines, spectral line 18, for the Tantalum foil target at 57.52 keV. The graphical representation of the output spectrum is distorted by the well documented decreasing response of the 1 mm thick CdTe sensor with energies above 60 keV, spurious radiation from the tungsten collimator, tailing effects of the sensor and other experimental deficiencies. Although 2 mm of aluminum filter is used in this example, any of a number of different output filter schemes well known to those skilled in the art may be used to provide an essentially flat output tube. This output is particularly useful when spectral data is used for x-ray imaging. The x-ray input to the object being examined is basically a step function between the energies of 25 and 90 keV.

In another preferred embodiment of the current invention, FIGS. 13 and 14 provide a graphical representation of energy spectrums taken using two transmission tubes of the current invention with standard BCR-680 RoHS sample measured at a distance of 5 cm from the end of the tube. Because a transmission tube provides a very broad cone, the fluorescent sample can be placed considerably closer to the x-ray tube than it can with a reflection type of x-ray tube. The 1 mm CdTe sensor is placed on a line perpendicular to a line between the spot of the x-ray tube and the center of the BCR-680 sample adjacent to the outside of the BCR-680 sample container. The European RoHS specification requires simultaneous fluorescent measurement of five elements to include cadmium, lead, mercury, bromine and chromium. FIG. 13 illustrates a transmission tube of the present invention with a foil target 0.3 μιη thick tantalum and a 0.4 mm copper filter. Spectral lines 19, 20 and 21 represent the x-ray spectrum of said tube at 50 kVp, 60 kVp and 70 kVp tube voltages, respectively. FIG. 14 was obtained using a foil target 0.75 μιτι thick tantalum and the same filter. Spectral line 22 represents the characteristic x-radiation produced by cadmium. Tube voltages of 50, 60 and 70 kVp are shown as spectral lines 23, 24 and 25, respectively. Although the spectrum show an increase of background noise at the k-alpha energy of cadmium, that noise is a sensor noise generated by the tailing effect of the CdTe sensor used and the background noise is considerably lower than represented. As tube voltage is increased the signal is increased considerably. This allows the significant advantage of using higher tube voltage and low tube current to improve speed and accuracy of fluorescent measurements. Reducing tube current to 20 microamperes in this particular example allows for reduced tube heating, decreased spot size and longer filament life, which are all major commercial advantages of using the x-ray tube of this invention for fluorescent analysis. Since the x-ray intensity output is flat with increasing tube voltage, the intensity of the x-ray tube output adjacent to the K-lines of cadmium does not increase with increasing applied tube voltage insuring low background radiation. The filter thickness and material is chosen from any of a number of possible filter materials, including but not limited to, aluminum and copper so that the Compton Scattering from the sample being examined does not appreciably interfere with the measurement of the characteristic line of interest generated by the sample.

FIGS. 15 and 16 are graphical representations of output from transmission tubes currently used for x-ray fluorescent measurements and thickness measurements using x-ray fluorescence of surface materials as is well known by those skilled in the art. One industry where surface material thickness and analysis is important is the silicon wafer processing industry. The composition percentages must be held within certain limits and the thickness of the measured layer deposited onto the silicon wafer is similarly must be closely controlled. A second is the RHOS measurements required for exporting product to Europe to assure that unsafe materials are not used.

FIG. 15 demonstrates the output from a silver target 1.5 microns thick of a transmission x-ray tube operated at a tube voltage of 40 kVp, which is typically used in the x-ray fluorescence market. Although this is useful in measuring elements whose k-lines are below about 20 keV, it cannot provide k-line data for elements whose k-lines are more than about 25 keV.

Similarly FIG. 16 demonstrates the output from a rhodium target 2.3 microns thick of a transmission tube operated at 45 kVp, which is typically used in the x-ray fluorescent market. By increasing the voltage applied between the anode and cathode to variably higher and higher settings, up to as high as about 180 kV, the same tube can provide k-line information about composition percentages for all elements whose atomic numbers are above 50. Since the x-ray output for each photon energy at a given tube current does not change as the applied tube voltage increases, there is no need to recalibrate the x-ray fluorescence output as the tube voltage increases. The higher voltages applied will provide a flat output response for target thicknesses up to 5 microns thick. This is a major advantage of the x-ray tube of the current invention. For target materials, including but not limited to, molybdenum, rhodium, palladium, and silver, a single target material can be used to provide fluorescence information for elements with Z numbers above about 50 in addition to the elements they currently sense.