Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, Japan

PASJ2018 THP025

MICROTRON CT SYSTEM ACHIEVING 250 μm SPACE RESOLUTION
WHICH IS UNABLE BY LINAC
T. Yamada, K. Ozaki, S. Maeo, D. Hasegawa, and H. Yamada†,
Photon Production Laboratory Ltd. and Mirrorcle Analysis Center, [525-0012]Kusatsu, Shiga
Abstract
More than one MeV X-ray energy is necessary to visualize inside of iron object thicker than 8 cm. Particularly it
is necessary when 3D metal printing technology is infashion, hollow detailed big metal object must be tested tofind inside defects.
We will show how X-ray CT image using Microtron(MIC) demonstrates space resolution as high as 250 μm.
In addition very good contrast visualize density distribution at the density 0.1 g/cm3. For instance Tantalum andTantalum Oxide can be identified by MIC-CT. Measurement of density distribution is carried out by neutron, butwhen focal point is an order of 100 μm, it is possible byCompton scattering with X-ray energy higher than MeV,
since Compton scattering cross section is proportional tothe electron density, instead of atomic number, while thephoto electron scattering is proportional to the atomic
number Z4.

In this paper we discuss how our MIC achieve 250 μm
resolution [8] while LINAC achieve a few mm, and how
high beam current 300 mA peak current is achievable,
when LINAC can achieve around 100 mA.

MIC-CT SYSTEM AND OBTAINED 3D
IMAGES
Fig. 2 shows X-ray CT manipulator and flat panel 2D
Detector (FP) seen in the 1 m thick concrete shieldingroom. Maximum 70 kg object is acceptable on the manipulator. FP size is 40 x 40 cm2 with 200 μm pixel size.
Electron accelerator is the 6 MeV microtron MIC6 asshown in Fig. 3.

INTRODUCTION
Photon Production Laboratory Ltd. (PPL) is a 20-year
old venture company (see Fig. 1). One of the world's
leading company holding particle accelerator technology.
PPL is the only company who produce tabletop synchrotron light source MIRRORCLE [1] and compact Microtron MIC [2-8] accelerator. PPL provides fine resolution& high energy X-ray CT system which might be an onlytool for testing 3D printing metal products. MIC-CTachieves 250 μm and MIRRORCLE-CT achieves 20 μm
space resolution. Group company MIRRORCLE analysis
center provides analysis service using these instruments
mentioned.

Figure 2: X-ray CT manipulator and flat panel 2D Detector (FP) are seen in the 1m thick concrete shielding room.
Maximum 70 kg object is acceptable on the manipulator.
FP size is 40 x 40 cm2 with 200 μm pixel.

Figure 3: MIC6 is used for advanced X-ray CT nondestructive testing. Electron focal spot size 0.25 mm isachieved for X-ray imaging.
Figure 1: New PPL laboratory is established in the springof 2017. Analysis service is provided by MIRRORCLE
Analysis Center Co. Ltd. using MIC-CT and MIRRORCLE synchrotron light source beam lines.
___________________________________________† hironari@photon-production.co.jp

We limit beam current to around 50 mA peak, pulse
width 1 μs, and repetition 500 Hz to avoid over exposure.
We scan the manipulator at every half degree over 360
deg. Sample is usually set at SO = 1 m from the source
point, and detector at SD = 2 m from the source to see the0.25 mm resolving power. Some time that is SO: SD = 0.5
m: 2 m to achieve higher space resolution. Manipulator isscanned every half degree. Each exposure takes 1 to 2 sec.

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August 7-10, 2018, Nagaoka, JapanPASJ2018 THP025

So we can complete whole CT scan in 30 to 60 min. For
large objects we apply helical scan as well as offset scan.
Typical reconstructed results are shown in Fig. 4. Top
picture is a whole body of motor engine. You will see theprecise structure and differences in materials. Body ismade of Al cylinder is made of stainless steel. Bottom is a30 cm wide trans. You will see 1 mm wide coil in a 10 cm
thick iron yoke. You will agree how coil image is sharp.
The space resolution is 0.25 mm.

Figure 4: Top is cross sections of a motor engine. Bottom
is a 30cm wide trans.

SPACE RESOLUTION POWER OF THESE
FINE CT IMAGES
Focal spot size [8]Beam spot size of MIC at focal point strictly defines thespecial resolution of X-ray image. MIC beam is focusedon the X-ray converter through set of FD quadrupole
magnets.
Beam size is measured by 1mm diameter pin hole madeof lead or by the knife edge profile as seen Fig. 5 (top) for
6 MeV MIC as well as Fig. 5 (bottom) for 0.95 MeV MIC.

Since Fig. 5 (top) is the 3 times magnified the real spot
size is 0.27 mm W and 0.36 H for MIC1, and Fig. 5 (bottom) the 2 times magnified one, the real spot size is 0.23mm W and 0.26 mm H for MIC6.
The resulting fine image is inspected by MTF methodby CT scanning of standard phantom indicated in Fig. 6.
As a result 10 % MTF 2.39 LP/mm is obtained which isequivalent to 209 μm resolution.

Figure 5: MIC beam spot size is measured by pinhole(top) for MIC1 and by knife edge (bottom) for MIC6.

Density resolving power [5]When high energy X-ray is used we are able to measure
the density distribution, because the scattering mechanism
is Compton scattering and the scattering cross section islinearly proportional to the electron density. When however the X-ray energy is lower than 100 keV, the scattering mechanism is photo-electron effects and cross section
is proportional to the atomic number Z in factor 4.
The density resolving power of the X-ray energy higher
than 1MeV was unclear before, but it is now clear because the space resolution is extremely improved withMIC. When the space resolution is low, the density contrast is smeared off.

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Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, JapanPASJ2018 THP025

Density at the ROI in 3D volume is proportional to thegray value at the ROI in the reconstructed image. Normalization factor is obtained by carrying out with the density
known materials. Example is shown in Fig. 7.
The density resolution obtained in such method was 0.1
g/cm3. This method is applicable to the light materials
shown in Fig. 7 as well as heavy materials shown in Fig.
8.

Figure 6: MTF inspection of CT resolution taken bystandard phantom 61.99 mm wide iron rod.

Figure 8: Our density measurement method is applied toheavy metals. Top is X-ray CT image taken for various
metals. Bottom shows that the density ratio and obtainedgray value ratio are well correlated.

Principle of MIC accelerator

Figure 7: (top) X-ray CT image is taken for various materials. (middle) It is seen that the obtained gray values anddensities are well correlated. (bottom) It is well shownthat the density and gray values are linearly correlated.

MIC is a microtron accelerator that accelerates electrons under a magnetic field, and electrons are extracted
from LaB6 emitter directory by the 40 MV/m RF field set
in a wall of RF cavity. Because of internal emitter MIC isvery compact and gives high performance compared withLINAC.
Microtron was proposed by Vladimir Veksler in 1944
during the former Soviet era and developed mainly in theeast side [2]. Because it accelerates electrons in the magnetic field, it has energy sorting function, it is small but
low dispersion high energy electron accelerator. Since theenergy dispersion is low, the focal size of the submillimetre can be easily achieved. Linear accelerator
LINAC is invented in the United States has been used in
the industry, but energy dispersion is large because low
energy electrons which are not sufficiently accelerated aretaken out together. Therefore, the focal size is several mm,and high-precision X-ray inspection can not be performed.
The focal size of the MIC to the other is 0.25 mm.
Furthermore, we invented and demonstrated the principle of generating a peak beam current of 300 mA by making the magnetic field of MIC uniform [3, 4]. As a result,by attaching the X-ray target directly to the MIC, it realized a very small focal size of 0.25 mm with high brightness, making high-precision X-ray CT inspection possible.

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August 7-10, 2018, Nagaoka, Japan

PASJ2018 THP025

under magnetic field has no back bombardment to theelectron gun, so stable beam extraction is possible.

CHARACTERISTICS OF MIC ISCOMPARED WITH LINAC
Differences between MIC and LINAC are describedbelow.

Differences in acceleration mechanism
Figure 9 shows that differences in acceleration mechanism between MIC and LINAC from our experience
with the same energy and frequency. MIC accelerating

Difference in hard wear
Differences in hard ware between MIC and LINAC areshown in Fig. 10. In our MIC, an RF gun is built in thesingle acceleration cavity, so we do not need a high voltage floating power supply such as a grid pulsar.

Figure 9: Differences in acceleration mechanism are shown between MIC (left) and LINAC (right).

Figure 10: Differences in hard ware are shown between MIC (left) and LINAC (right).

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Proceedings of the 15th Annual Meeting of Particle Accelerator Society of Japan
August 7-10, 2018, Nagaoka, JapanPASJ2018 THP025

Difference in beam nature
As mentioned above, since MIC circulates and accelerates electrons under magnetic field, the energy spread ofthe electron beam extracted is less than 1%. While, withthe linear accelerator LINAC accelerates all electrons,
thus the energy dispersion is larger. Therefore, as shownin Table 1, the X-ray focal point of MIC achieves 0.25
mm due to the small energy dispersion.

Difference in imaging
X-ray images of a gas valve is taken with both MIC andLINAC are shown in Fig. 11. Smaller X-ray focal spot
size demonstrate more details.

Table 1: Compare Differences in Beam Nature between MIC and LINAC

SampleMIC

LINAC

Figure 11: Difference in imaging between MIC and LINAC are shown. Because the focal spot size is smaller with MIC,
the image of MIC has less blur than with LINAC, and image is clearer.

CONCLUDING REMARKS
We have presented how MIC microtron is advanced
compared with LINAC. The beam focal point size is assmall as 0.25 mm while LINAC is a few mm. Beam energy spread is 2 % while LINAC is more than 5 %. The
beam peak current is 300 mA at maximum while LINAC
is 100 mA. Size is only 30 cm wide for 4-MeV machine,
while LINAC is 50 cm long. Cathode is grounded with
MIC while LINAC is floating. It is obvious that MIC is
more powerful as an electron gun, high power electron
source used for sterilization, and as an injector of storage
ring.

REFERENCES[1] H. Yamada et al., “Tabletop synchrotron light source”, inbook Comprehensive Biomedical Physics, vol. 8, p. 43-65,
2014.

[2] S. P. Kapitza et al., “The Microtron: Development andApplications”, The Physics and Technology of Particle andPhoton Beams, Vol. 10, 1978.
[3] D. Hasegawa et al., “Portable Microtron Achieved 300 mA
Beam Current”, in Proc. 8th Particle Accelerator Society ofJapan, Tsukuba, Aug. 2011.
[4] D. Hasegawa et al., in Proc. 10th Particle Accelerator
Society of Japan, Nagoya, Aug. 2013.
[5] S. Maeo et al., Inspection Engineering, vol. 20, No. 5, p. 6972, 2015.
[6] D. Hasegawa et al., Inspection Engineering, vol. 20, No. 3,p. 55-59, 2015.
[7] D. Hasegawa et al., in Proc. 12th Particle Accelerator
Society of Japan, Tsuruga, Aug. 2015.
[8] H. Yamada, “High Power Microtron for X-ray CT, Sterilization and as the Injector of the Tabletop Synchrotron LightSource”, IAEA Technical Meeting 2015, IAEA HQ Vienna,Austria, Sept. 7-11, 2015.

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