Application of Laser Plasma X-rays to Time-resolved
Debye-Sherrer Diffraction
Y.Kanegae, K.Kinoshita, T.Hosokai, T.Ohkubo, K.Yoshii, T.Ueda, T.Watanabe,
A.Zhidkov, and M.Uesaka
Nuclear Engineering Research Laboratory, University of Tokyo,
2-22 Shirakata-Sirane, Toukai-mura, Naka-gun, Ibaraki-ken, 319-1188 Japan
Abstract. We have studied Laser Plasma X-ray(LPX) in order to apply to time-resolved protein crystallography. We
consider that our works will contribute for application of X-ray pulse and breed short pulse handling techniques. LPX
pulse duration is femto~pico-second. So, we expect that the laser plasma X-ray system has the potential to satisfy the
requirement of time-scale to resolve the early period of protein’s structural change. We need about more than
1012photns/shot X-ray to get a diffraction image of organics. We have reinforced our LPX system to get a diffraction
image. Now, we try laser pre-pulse effect by experiments and calculations. As the first step of our aim, we will obtain
the Debey-Sherrer diffraction image of a biological sample.
INTRODUCTION
Since the appearance of the third generation
synchrotron radiation sources, a variety of application
researches (for example protein structure analysis)
have been accelerated[1-4]. Time-resolved X-ray
diffraction to investigate atomic dynamics is the major
trend there and contributed to the understanding
ultrafast phenomena. At the Nuclear Engineering
Research Laboratory (NERL), University of Tokyo,
we have constructed the time-resolved laser plasma Xray(LPX) diffraction system utilizing 12-TW-50-fs
laser pulses, and observed ultrafast transient changes
in a laser-irradiated GaAs mono-crystal from its Bragg
diffraction patterns[5]. As the next stage, we extend
our work forward to time-resolved DebeySherrer/Laue diffraction analysis of proteins by the
laser plasma X-ray system. In this report we show
LPX intensity measurement experiments and discuss
the possibility of the Debey-Sherrer diffraction
analysis by the LPX source.
Isomerization of retinal
Time scale: ps ~ sub-ps[6]
Total structural change
Time scale: μ s~ms[3]
Fig1. bR and time scale of structural change
When a bR is irradiated by light, the retinal isomerizes
within the time-scale of pico- to subpico-seconds. The
isomerization of a retinal in bR can be observed, with
time resolution is 5fs, by pulse laser photolysis[6].
And this isomerization induces the total structural
change within the time-scale of micro- to milliseconds. The total structural change of bR is observed,
with time resolution is 244ms, by X-ray diffraction
analysis[3]. Therefore, we expect that LPX system has
the potential to satisfy the time-resolution of the early
period of protein’s structural change. It is conceivable
For example bacteriorhodpsin(bR, see Fig.1) is a
famous photoactive protein, which exists in membrane
cell of halophilic bacteria and has a function of proton
pump. bR includes a retinal as a light acceptor.
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© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
840
TABLE 1. Expected X-ray photons on the sample and expected time to get each diffraction images by the present
LPX system.
Bragg diffraction
Laue Diffraction
Debey-Sherrer
diffraction
~106
~1011
~1011
Necessary X-ray photons
photons/image
photons/image
photons/image
on the sampl to get each
diffraction images
800hours
800hours
Expected time by the
～30sec
present LPX system
that if we use a biological sample, we need long time
to obtain a diffraction image. Because in generally, a
biological sample consists of light atoms and dose not
diffract X-ray so much. Table.1 shows necessary Xray photon number to get three diffraction images and
our present status. We estimate this table1 based on
experiments at the SPring-8. And we assume that LPX
source intensity is 105photons/sec/mm2, distance from
X-ray source to a sample is 30cm, and that a biological
sample is used in Laue and Debey-Sherrer diffraction.
Too much time is needed to obtain the diffraction
image of a biological sample at the present status for
LPX. To increase the LPX intensity is one of the
important hurdles for LPX. We aim to increase the
LPX intensity up to 1012(phtons/shot/4πsr) to get a
Debey-Sherrer diffraction image of a biological
sample within 2 hours.
EXPERIMENT
In order to estimate the necessary X-ray intensity to
acquire a powder diffraction image (Debey-Sherrer
ring), we tried to get diffraction images of Siliconpowder and a Silicon-monocrystal by a conventional
X-ray source(Mac Science, MXC3HKF). Figure1 is
the diffraction image of the two diffraction
images(Debey-Sherrer and Bragg). It takes more than
5000 times longer time to get the Debey-Sherrer
diffraction image than to get the Bragg one. We find
that we will take more than 40hours to get DebeySherrer diffraction image by laser plasma X-ray
system. It is necessary to reinforce the LPX system.
(a)
(b)
LASER PLASMA X-RAY
When the ultrashort and high intense laser pulses
focused in a solid target, overdense plasma is
generated and X-rays are generated from there. Thses
X-rays can have short pulse durations(pico- to
subpico-seconds) because of the short interaction time
between the laser and the plasmas, and rapid cooling
of the plasma. Recently, it has been founded that the
ASE (amplified spontaneous emission) plays one of
the important roles to the generation of LPX[7]. First,
ASE make overdense pre-plasma, and the main-pulse
is absorbed resonantly in the place of the critical
density, and generate hot electrons in plasma. These
hot electrons penetrate inside the solid target, and
characteristic X-rays and breamsstrahlung X-rays are
produced. Therefore, it is important that to control
ASE in order to control pre-plasma distribution. In
other word, it is conceivable that as ASE becomes
small, the main-pulse are absorbed at nearer solid
target efficiently. Finally, the produced X-rays are
increased due to the many hot electrons.
Fig2. Diffraction images of (a) powder Silicon and (b)
Silicon mono-crystal on imaging plate.(a)Cu(8.0keV),
20kV,20mA,90min.(b)Cu(8.0keV),20kV,10mA,1sec.
We estimated the generated LPX intensity as
follows. Figure2 shows the schematic view of the LPX
experimental setup. The Ti:Sappfire laser pulse is
introduced into the vacuum chamber and focused on
the cooper target with an off-axis parabolic mirror in
order to generate the X-ray pulse. The characteristic
X-ray(CuKα 1:8.04778keV, CuKα 2:8.02779keV) is
obtained by the Bragg diffraction with a LiF(200) and
detected with the photodiode(IRD,AXUV) outside the
chamber through the Be window. The LiF crystal are
surrounded by the lead plates for shielding of the
background X-rays scattered from the circumference.
841
XPD signal(mV
40
30
20
Cu
10
0
3.5
4
4.5
5
5.5
6
laser pow er(W )
XPD signal(mV)
Fig5. Laser power dependence on X-ray intensity
25
20
Cu
15
10
4.0×109photons /shot/4πsr
5
0
0
100
200
300
400
500
posotion(μm ）
Fig6. The laser focal power density dependence on
generated X-ray intensity. The horizontal axis represented
the Cu target position.
Cu & Al
XPD signal(mV)
Fig3. Schematic view of the LPX experimental setup for Xray intensity measurement with photodiode.
Figure4 shows one of the laser-spot-sizes measured
by a CCD camera(HAMAMATSU,C4880) before the
setting of Cu target. We obtained the spot of 7.8μｍ
×8.0μｍ(1/e2) at the minimum. As for the difference
with the diffraction limit(3.6μm), the quality of the
laser, optics and miss-alignment are conceivable. The
power density of the laser is around 1018W/cm2 in this
experiment. We can find the laser power dependence
on generated X-ray intensity in fig5. The vertical axis
represents the signal height of the photodiode. We
estimated the number of photons of generated X-rays
4.0×109(photons/shot/4πsr) at the maximum.
40
30
20
Cu
Al
10
0
0
100
200
300
400
500
position
Fig.7
The laser focal power density dependence on
generated X-ray intensity evaporated ~100nm thick gold film.
Cu & Au
XPD signal(mV)
40
30
20
Cu
Au
10
0
0
100
200
position
300
400
Fig.8
The laser focal power density dependence on
generated X-ray intensity evaporated ~100nm thick
aluminum film.
We considered the quantum efficiency of the
photodiode, the attenuation of the X-rays, decrease in
air, signal efficiency and gain, the distance between
Fig4. Laser spot size in vacuum measured by CCD
camera and its transverse profile.
842
the detector and the target. The energy conversion
efficiency from the laser pulse to the X-ray pulse
becomes 3.4×10-5. Figure.6 shows the laser focused
power density dependence on generated X-ray
intensity. We adjust the power density by moving the
target stage toward the focal depth axis. The horizontal
axis represents target positions. The Cu target is
mounted on a automatic positioner in order to set the
Cu target appropriate focus position. We find this
technique is effective to increase X-rays. In order to
check the influence of the pre-plasma, we tried the Cu
target with ~100nm thick gold(high Z) and
aluminum(low Z) formed by vacuum evaporation on
the Cu target. The results for Au and Al are shown in
fig7and 8. However, we could not find significant
difference of X-ray intensity. It seems that X-ray
intensity a little decrease in the case of the Au
evaporation target. We mention this result at following
chapter.
Electron Density [/cm
1E+24
Au
Cu
(a)
(b)
(c)
(d)
1E+23
1E+22
1E+21
1E+20
-10
0
10
20
30
D istance from T arget S urface [μm ]
Fig.9 HYADES outputs of the plasma density distributions
with the ASE pre-pulse power density of 1013 W/cm2 (black
dotted line), 1012 (gray dotted line) and 1011 (light gray
dotted line) together with the inputs for the PIC simulation.
The pre-pulse with the intensity of 1011 may give appropriate
distribution.
We estimated LPX intensity as 4.0×109 (photons/shot/4π
sr) at maximum. We becomable to monitor X-ray intensity
by photodiode and to adjust Cu target position appropriately
using automatic positioner. To see the laser pre-pulse effect,
we tried coated target with aluminum and gold. We are
going to calculate LPX generation process in order to control
the pre-plasma distribution. We think that more than
1012photons-/shot/4πsr is needed to achieve 1011 photons on
a biological sample within 2 hours, distance 30cm X-ray
source to a sample, i.e. to get a Debey-Sherrer diffraction
pattern. We have to complete the simulation of LPX
generation process, control the ASE for our aim
SIMULATION
We are going to calculate the LPX generation
process to estimate the appropriate laser parameter. In
simulation, we consider the LPX generation process
make divided three steps. First, laser ASE make preplasma on the surface of a solid target. We use the
HYADES code[8] to calculate the plasma distribution
formed by ASE. Second, we use a collisional prticlein-cell code[9] to simulate the electron dynamics due
to the resonance absorption of the main pulse energy.
Finaly, we calculate X-ray generation from electron
distribution by ITS-3.0 code. Fig.9 shows HYADES
output of the plasma density distribution. We can find
that plasma of Au is more expanded than that of Cu.
So, we think that the X-ray intensity decrease when we
apply to Au evaporated target(fig.7).
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1. V.Srajer, et al., SCIENCE 274 (1996) 1726.
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3. T.Oka, et al., PANS 97 (2000) 14278
SUMMARY AND SUBJECTS
4. B.Perman, et al., SCIENCE 279 (1998) 1946
We have studied application of the LPX for the
time-resolved Debey-Sherre diffraction. We find the
relationship of the diffracted X-ray intensity between
Bragg diffraction and Debey-Sherrer diffraction from
experiment. Debye-Sherrer diffraction intensity is
about 1/5000 than that for the Bragg. Therefore, we
take more than 40 hour to get Silicon powder
diffraction image by LPX. It is conceivable that if we
use a biological sample, we take more as table1.
5. K. Kinoshita et al., Laser and Particle Beams 19 (2001)
pp.125-131
6. T.Kobayashi et al., NATURE Vol 414. 29 November
2001 531-534
7. A.Zhidkov et al., Phys. Rev. E 62 (2000) 7232
8. J.Tlarsen, et al., J.Quant.Spectros. Radiat. Transf. 51
(1994) 174
9. A.Zhidkov et al., Phys. Rev. E 59 (1999) 7085
843