Development of 950 kV X-ray source with small focal spot using a linear accelerator

Abstract A 950 kV X-ray source with small focal spot using a linear accelerator (LINAC) has been developed. By installing a focusing mechanism for electron beam in this system, a small focal spot (less than 0.3 mm in FWHM of Line Spread Function) is successfully achieved at 950 kV with the target electron beam current greater than 30 mA. Several comparison data of conventional focal spot are also acquired and clearer images are obtained with the small focal spot. We are convinced that this new X-ray source is useful for industrial non-destructive inspection of large structures because of high transmission ability and spatial resolution.


Introduction
X-ray source has been progressing as an important tool for performing non-destructive inspection.After X-ray CT technology had been developed in the 1970's, high-energy industrial CT imaging using X-ray source around 1 MeV has been used for nondestructive inspection for large structures made of iron and aluminium [1].In such systems, LINAC is often used as the highenergy X-ray source because higher acceleration of electrons is easily obtained without concern about discharge.For example, it is used for non-destructive inspection of automobile parts and shape evaluation of combustion chambers of rocket engines [1,2].In such situations, it is certain that high-energy X-ray imaging with higher spatial resolution is required in the future.However, it is difficult to achieve a small X-ray spot or a small X-ray beam diameter less than one millimeter in the case using a conventional LINAC.This time, by installing a focusing mechanism for electron beam, we have developed 950 keV LINAC with a small focal spot (less than 0.3 mm in FWHM of Line Spread Function).In addition, experimental transmission images have been obtained for several industrial samples.It is demonstrated that the resolution is improved compared with the conventional one.

Development Result and Improvement of Spatial Resolution
X-ray-source using 950 keV LINAC was developed.X-band (9.3 GHz) radio frequency was used.Figure 1 shows a photograph of the developed 950 kV X-ray source with size and weight of W800 mm×D600 mm×H500 mm and 230 kg, respectively.This X-ray source is very versatile for non-destructive inspection of large structures because of its compact size and light weight.It is also possible to load it on a robot arm for positioning.Achieved specification is described in Table 1.

Item Specification
Electron Acceleration LINAC, 9.The X-ray source has a focusing mechanism for electron beam and X-ray spot size is designed to be made small by activating it.At conventional setup, wherein the focusing mechanism is not activated, the target electrical current is 130 mA.At small spot setup, wherein the focusing mechanism is activated, the target electrical current of 30 mA.Reducing electrical current is necessary to avoid melting the target material.Those two setups are switchable.Figure 2(a) shows a schematic figure of the measurement system of X-ray spot distribution (more precisely, Line Spread Function).This measurement system has a metal slit installed on a one dimensional stage that can be controlled automatically.X-ray spot distribution can be obtained by measuring the X-ray intensity with CsI scintillator and a PIN photodiode (PIN-PD) while moving a metal slit of 0.01 mm width.In order to reduce the influence of blur effect at the slit, distance between X-ray spot and metal slit is adjusted to be as short as possible.Figure 2(b) shows measurement results of X-ray spot distributions, where the blue line corresponds to an Xray spot distribution with conventional setup.On the other hand, the red line corresponds to an X-ray spot distribution with small spot setup.FWHM values of X-ray spot distributions are about 1.14 mm and 0.26 mm, respectively.Spatial resolution of X-ray image depends on X-ray spot size [3], therefore, this result suggests that the spatial resolution of X-ray image can be improved by about 4.4 times by activating the focusing mechanism.The resolution has also been evaluated using an X-ray test chart (KYOKKO X-ray test chart JIS M-3Pb100).Figure 3 shows transmission images of an X-ray test chart, where (a) corresponding to small spot setup (FWHM 0.26mm), (b) corresponding to a conventional setup (FWHM 1.14mm).From these results, it is demonstrated that 1.75LP/mm can be resolved by the small spot setup (corresponding linewidth resolution is about 0.3mm).On the other hand, it cannot be resolved by the conventional setup.

Experimental Results
Several samples were observed using the developed 950kV X-ray source.The imager Perkin Elmer 0822 Flat Panel Detector (200 m x 200  m pixel size, 1024x1024, 16 bits ADC) was used.Exposure time was 1/16 second for both small spot and conventional spot setups.

Industrial Samples
We prepared industrial samples, such as a turbine blade, an engine block and a turbo molecular pump, to confirm higher transmission ability and resolution at 950kV.

Other Samples
In order to compare the difference in resolution, we observed other samples which contain fine structures even though it is not necessary to use 950 kV X-ray source for those samples.Figure 7 represents images of a needle, where (a) a photograph, (b) a transmission image with small spot setup, and (c) a transmission image with conventional spot setup.By comparing (b) and (c) in Figure 7, we can say that the resolution in the small spot setup is much improved.In particular, (b) shows that the cavity of the needle can be verified because of the spatial resolution with small spot setup is less than 0.5 mm diameter of a needle.

Summary
A 950 kV LINAC with X-ray spot size of about FWHM 0.3 mm has been developed using a focusing mechanism of electron beam.By using an X-ray test chart, we demonstrated high resolution with small spot setup.In addition, several industrial samples were also observed using developed X-ray source, and it is demonstrated that the resolution is improved compared with conventional one.It is convincing that this new X-ray source is useful for industrial non-destructive inspection of large structures because of high transmission ability and spatial resolution.CT image reconstruction analysis is expected as the next research step.

Figure 2 :
Figure 2: (a) Schematic figure of the measurement system of X-ray spot distribution.(b) Measurement results in conventional setup (FWHM 1.14 mm, blue line) and small spot setup (FWHM 0.26 mm, red line).

Figure 3 :
Figure 3: Transmission images of an X-ray test chart (KYOKKO X-ray test chart JIS M-3Pb100) at 950 kV and magnification of about 2.0, where (a) transmission image with small spot setup, (b) transmission image with conventional spot setup.

Figure 4
shows images of a turbine blade, where (a) a photograph, (b) a magnified photograph of the slit like structures, (c) a transmission image with small spot setup, and (d) a transmission image with conventional spot setup.By comparing (c) and (d) in Figure4, we can say that the resolution in the small spot setup is much improved.In particular, because the size of the slitlike structures included in dashed yellow circle in (b) is about 1 mm, we can verify that the image of the slit-like structures with small spot setup is clearer than conventional one.

Figure 4 :Figure 5
Figure 4: (a) Photograph of a turbine blade.(b) Magnified photograph of the slit like structures.(c) Transmission image with small spot setup at 950 kV and magnification of about 2.0.(d) Transmission image with conventional spot setup at 950 kV and magnification of about 2.0.

Figure 5 :
Figure 5: (a) Photograph of an engine block.(b) Transmission image with small spot setup at 950 kV and magnification of about 2.0.(c) Transmission image in conventional setup at 950 kV and magnification of about 2.0.

Figure 6
Figure 6 are images of a turbo molecular pump (TMP), where (a) a photograph (X-ray beam direction is indicated by the fellow arrow), (b) (c) are case of small and conventional spot setups, respectively.By comparing small and conventional spot setups in Figure 7, the improvement of the resolution in the small spot setup is clearly observed around an internal screw structure of TMP included in dashed yellow circles in (b) and (c).

Figure 6 :
Figure 6: (a) Photograph of a TMP.(b) Transmission image with small spot setup at 950 kV and magnification of about 2.0.(c) Transmission image with conventional setup at 950 kV and magnification of about 2.0.

Figure 7 :
Figure 7: (a) Photograph of a needle with a diameter of 0.5 mm.(b) Transmission image with small spot setup at 950 kV and magnification of about 7.0.(c) Transmission image with conventional spot setup at 950 kV and magnification of about 7.0.

Figure 8 Figure 8 :
Figure8shows images of an alarm clock, where (a) a transmission image with small spot setup, (b) a magnified transmission image with small spot setup, and (c) a magnified transmission image with conventional spot setup.By comparing (b) and (c) in Figure8, we can say that the resolution with the small spot setup is much improved.In particular, the improvement of the resolution in the small spot setup is clearly observed in dashed circles in (b) and (c).

Table 1 :
Achieved specification of developed X-ray source.