Nuclear Instruments and Methods in Physics Research A 637 (2011) S183–S186

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Quasi-monochromatic hard X-ray source via laser Comptonscattering and its application

R. Kuroda a,n, H. Toyokawa a, M. Yasumoto a, H. Ikeura-Sekiguchi a, M. Koike a, K. Yamada a,T. Yanagida b, T. Nakajyo b, F Sakai b, K. Mori c

a Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba,

Ibaraki 305-8568, Japanb Sumitomo Heavy Industries, Ltd. (SHI), 2-1-1 Yatocho, Nishitokyo, Tokyo 188-8585, Japanc Ibaraki Prefectural University of Health Sciences, 4669-2, Ami, Inashiki, Ibaraki 300-0394, Japan

a r t i c l e i n f o

Available online 9 April 2010

Keywords:

Laser Compton scattering

Quasi-monochromatic X-ray source

In-line phase contrast imaging

K-edge imaging

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.04.001

esponding author. Tel.: +81 29 861 5104; fax

ail address: ryu-kuroda@aist.go.jp (R. Kuroda

a b s t r a c t

We have developed a quasi-monochromatic hard X-ray source via laser Compton scattering (LCS) based

on an S-band compact electron linac at AIST. The number of total photons and the maximum X-ray

energy was 107 photons/sec and about 40 keV, respectively, in 15-degree crossing angle between a

42 MeV electron beam and a 800 nm Ti:Sa laser. The biological observation of the human bone with

fractures has been successfully demonstrated using the LCS X-ray of 26.4 keV with the in-line phase

contrast scheme. Good contrast enhancement is clearly observed between the absorption and the phase

contrast images.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

A quasi-monochromatic, short-pulse X-ray source has beeninvestigated via laser Compton scattering (LCS) between a highquality electron beam and a high power laser for various researchfields [1–3]. Recently, many facilities have developed differenttypes of LCS X-ray sources such as linac based systems[4–6], storage ring based systems [7,8] and an energy recoverylinac (ERL) based system [9].

On the other hands, advanced quantum radiation sources suchas an LCS X-ray source and a high intense THz radiation sourcehave been developed based on an S-band compact electron linacat AIST [10,11]. All of system is built in one research room about10 m2 including an electron injector, an S-band electron linac,quadrupole magnets, bending magnets, an rf source of a S-band20 MW klystron system and a high power laser system forthe LCS.

2. Quasi-monochromatic LCS X-ray source

2.1. Apparatus

Fig. 1 shows the top view of the layout of the laser Comptonscattering (LCS) hard X-ray source at AIST. The injector consists of a

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: +81 29 861 5683.

).

laser photocathode rf gun which has the BNL type S-band 1.6 cellcavity with a Cs2Te photocathode load-lock system and a solenoidmagnet for emittance compensation [11]. The linac has two 1.5-m-long S-band accelerating tubes which have a 1/2 p mode standingwave structure with an alternative periodic structure (APS). Theelectron beam with charge of about 1 nC/bunch is generated fromthe Cs2Te photocathode and accelerated up to about 4 MeV withbunch length of about 3 ps (rms). The electron beam can beaccelerated up to about 42 MeV with S-band accelerating tubes. Itpasses through the achromatic arc section to be bended to 90-degreedirection in order to reduce backgrounds of bremsstrahlung due tothe dark current from the linac. The electron beam is focused to thecollision point using the Q-triplet and bended to the beam dumpafter colliding with the collision laser.

In our system, two mode-locked laser systems are operated in10 Hz, whose mode-lock frequencies are synchronized to 36thsub-harmonic frequency (79.3 MHz) of the linac acceleratingfrequency (S-band 2856 MHz). One is an all-solid state picosecondUV laser system (266 nm) which is used for the photocathode rfgun in order to generate the high quality electron beam. It has aNd:YVO4 oscillator, a pulse picker, a Nd:YAG multi-pass pre-amplifier, an AO modulator, a Nd:YAG multi-pass main-amplifierand frequency converters (SHG and FHG). It can arbitrary generateboth a single pulse and multi pulses (100 pulses) UV laser withenergy of about more than 10 mJ/pulse. The other laser system is afemtosecond terawatt (TW) Ti:Sapphire laser system (800 nm)with a chirped-pulse amplification (CPA), which is used as thecollision laser. It consists of a mode-locked oscillator (79.3 MHz),a pulse stretcher, a regenerative amplifier, a multi-pass

Fig. 1. Schematic layout of the laser Compton scattering hard X-ray source at AIST.

Table 1Specification of our system.

Electron beam Energy �42 MeV

Bunch charge �1 nC

Energy spread 0.2%

Bunch length 3 ps (rms)

Beam size at collision point 43 mm�30 mm (rms)

Repetition rate 10 Hz

Ti:Sapphire laser Wavelength 800 nm

Pulse width 100 fs (FWHM)

Repetition rate 10 Hz

Pulse energy 140 mJ

Spot size 28 mm (rms)

X-ray Yield @ 15 deg 107 photons/s

Energy �40 keV

Pulse width 3 ps (rms)

Yield @ 90 deg 106 photons/s

Energy �20 keV

Pulse width 150 fs (rms)

R. Kuroda et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S183–S186S184

pre-amplifier, a multi-pass main amplifier and a pulse compressor.The compressed laser pulse has pulse width of 100 fs (FWHM) andenergy of about 140 mJ/pulse and it is guided to the collision point.The relative timing jitter between the master signal generator(2856 MHz) and the two lasers is controlled to be about less than10 fs using the timing stabilization system [12].

The X-ray is generated via the laser Compton scattering (LCS)between the electron bunch and the laser pulse at the collisionpoint. The pulse width of the LCS X-ray depends on beam sizes ofthe laser and the electron beam, the beam–beam crossing-angle,and pulse widths. The shortest X-ray pulse width is obtained at90-degree crossing-angle and it is estimated to be about 150 fs(rms). The maximum energy of the LCS X-rays can be tuned10–40 keV in a few energy spread by changing the electronenergy, and total photon yields are obtained to be about 107

photons/s at 15-degree crossing-angle. In this case, the pulsewidth of the X-ray is estimated to be about 3 ps (rms). Overallsystem specifications are summarized in Table 1.

2.2. Quasi-monochromaticity

The LCS photons from relativistic electrons produces fre-quency-upshifted photons in a narrow forward cone. When theelectron rest mass is m0, Lorentz factors are g, b and incidentphoton energy is E0, the energy of scattered X-ray (E) in thelaboratory frame is expressed by

E¼ð1þbcosfÞE0

1�bcosyþ 1þcosðyþfÞð ÞE0

gm0c2

ð1Þ

where y and f are the crossing angle and the scattered angle,respectively. From Eq. (1), the maximum energy of the scatteredX-ray is given with y¼0 (head-on collision) and f¼0. It indicatesthat the scattered photon energy of LCS can be upshifted to �4g 2E0.

The LCS X-ray around the beam center has quasi-monochroma-ticity and it can be applied to the biological and medical applicationssuch as the K-edge imaging like the angiography. In our system, theX-ray with energy above the iodine K-edge about 33.17 keV can begenerated using about the 39.5 MeV electron beam and the Ti:Sa laserwith the 15-degree crossing angle. The iodine is the typical contrastenhancement medium for the angiography. Fig. 2 shows the scattedangle (green line) and X-ray yield (red line) as a function of X-rayenergy using a numerical simulation. In Fig. 2, DE is about 364 eV and

the available photons for the iodine K-edge imaging is shown in theblue line area which is selected around the X-ray center within about4 mrad divergence. In this case, the X-ray energy distribution aroundX-ray center at about 2 m downstream from the collision point isshown in Fig. 3. Here, the red line is the mean X-ray energy at eachtransverse position and the blue area indicates the energy spread. Theenergy spreads are estimated to be about 4% and about 10% at theX-ray center and within the divergence angle of about 7.5 mrad,respectively. The available area for the iodine K-edge imaging isindicated in the sky blue area. The trial experiment of the iodineK-edge imaging was described in a previous article [13].

3. In-line phase contrast imaging

3.1. Imaging scheme

An in-line phase contrast scheme is one of the phase contrastimaging techniques [14]. The schematic explanation of thisscheme is given in Fig. 4. To take phase contrast images, X-raywith practical spatial coherence is required because the refractionangle due to the phase shift is on the order of tens of micro-radians. The LCS X-ray has not only quasi-monochromaticity but

Fig. 2. X-ray yield and scattered angle as a function of X-ray energy using

39.5 MeV electron beam and available photons for iodine K-edge imaging.

Fig. 3. X-ray energy distribution as a function of distance from X-ray center at

about 2 m downstream from collision point.

Fig. 4. Schematic explanation of in-line phase contrast imaging using X-ray with

practical spatial coherence.

Fig. 5. Schematic drawing of in-line phase contrast imaging using LCS X-ray

source.

Fig. 6. X-ray images of human bone with fracture: (upper figure) absorption

image at 0 cm distance (lower figure) phase contrast image at 20 cm between the

specimen and IP.

R. Kuroda et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S183–S186 S185

also partial spatial coherence due to the small source size. Wehave applied such characteristics to the high-contrast imaging ofbiological specimens using the in-line phase contrast schemes. Asthe X-ray with spatial coherence passes through the specimenincluding high- and low-density regions, it is refracted around theregion boundary due to some phase shift caused by deferentrefractive indices of each density region. The images with contrastenhancement is observed at an appropriate distance between thespecimen and the X-ray detector. The trial experiment of the in-line phase contrast imaging using our system with the lumbarvertebra of rat was described in previous article [15].

3.2. Imaging experiment with human bone

We have successfully demonstrated the X-ray in-line phasecontrast imaging of a human one with fractures using LCS X-raywith energy of 26.4 keV. The experimental setup is schematicallyshown in Fig. 5. The LCS X-ray was extracted from a Beryllium(Be) window (30 mmf�500 mmt) located at about 2.0 mdownstream from the X-ray source position which correspondsto the collision point between the electron bunch and the laserpulse. The electron beam is dumped to the beam dump aftercolliding with the laser. The specimen was located at about2.75 m downstream from the source point. An imaging plate (IP)is placed as an X-ray detector behind the specimen. The distancebetween the specimen and the IP is changed from 0 to 20 cm. Themeasurement time to obtain 1 image is about 30 min whichcorresponds to 18,000 pulses at 10 Hz operation. It means theactual exposure time with the LCS X-ray is only about 100 ns (2s)due to its pulse width of 3 ps (rms). Fig. 6 shows imaging resultsof the human bone in both cases of 0 and 20 cm distances, whichcorrespond to the absorption image (upper figure) and the phase

Fig. 7. Line profiles of the bone fracture vertically integrated over selected areas

marked by white rectangles in Fig. 6.

Fig. 8. Line profiles of the bone trabeculae horizontally integrated over selected

areas marked by red rectangles in Fig. 6.

R. Kuroda et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S183–S186S186

contrast image (lower figure), respectively. The enlarged area inFig. 6 shows the detail of a part of the fracture. Fig. 7 indicates lineprofiles of it vertically integrated over selected areas marked by

white rectangles in both cases. As a result, it is found that about20% contrast enhancement is observed for the micro-fractureusing the in-line phase contrast scheme. A bone trabeculae is alsoclearly observed in Fig. 6, Fig. 8 shows line profiles of the bonetrabeculae horizontally integrated over selected areas marked byred rectangles in both cases in Fig. 6. As a result, it is found thatthe contrast of the bone trabeculae image is also enhanced by thisscheme.

4. Summary

We have developed a quasi-monochromatic hard X-ray sourcevia the laser Compton scattering (LCS) based on the S-bandcompact electron linac at AIST. The number of total photons andthe maximum X-ray energy were 107 photons/pulse and about40 keV, respectively, in 15-degree crossing angle. The biologicalobservation of the human bone with fractures has been success-fully demonstrated using the LCS X-ray of 26.4 keV with thein-line phase contrast scheme. In this work, the contrastenhancement was clearly observed between the absorption andthe phase contrast images in both cases of the bone fracture andthe bone trabeculae. On the other hand, the upgrade plan toincrease the LCS X-ray yields several orders of magnitude hasbeen executed with multi-pulse LCS scheme, that is to generate atrain of X-ray pulses using laser and electron pulse-trains [16,17].Although further improvement is necessary, it is verified that thequasi-monochromatic LCS hard X-ray source has the possibility ofa high-contrast imaging for the biological observation like a largesynchrotron radiation ring.

Acknowledgments

Authors would like to express sincere thanks to Prof. Urakawa,Dr. Terunuma at KEK-ATF, Prof. Washio at Waseda Universityand other contributing members for the development of theCs2Te photocathode load-lock system. This work was partiallysupported by Grant-in-Aid for Scientific Research (B) no. 20360043from MEXT Japan.

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