The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Development and Application of Pulse Radiolysis Systems
in Time Region From Femtosecond To PicosecondSeiichi TAGAWA, Takahiro KOZAWA, Shu SEKI, and Yoichi YOSHIDAThe Institute of Scientific and Industrial Research, Osaka University
8-1 Mihogaoka, lbaraki, Osaka 567-0047, Japan

Abstract
Several types of pulse radiolysis systems in thetime region from femtosecond to picosecond have beendeveloped. New techniques, such as generation of ultra
fast electron pulse, synchronization of laser with electron
linac, and timing detection, are adopted. The
performance of the system and its application arereported.
1. lllltroduction

1992. The important factors are time resolution andwavelength region of analyzing light.
In the stroboscopic method, a sample is irradiatedby a picosecond electron pulse to occur aradiation-induced reaction. At the same time, apicosecond analyzing light is injected into the sample todetect the absorption of very short-lived intermediates.
Cherenkov radiation was used as the analyzing light inmany picosecond pulse radiolysis system [3,10 II]. We
used femtosecond laser as the analyzing light. The meritsof the laser pulse are as follows.
I) SIN of data is improved due to high intensity of laserpulse.
2) Wide wavelength is available by using the non-liner
effect of laser.
3) Whole system is simple.
Figure 2 shows concepts of the stroboscopic pulse
radiolysis system in the time region from femtosecond topicosecond.
In picosecond pulse radiolysis as shown fig.2a,electron beams from L-band linac [12] and analyzing
light pulse from femtosecond laser are used. Thesynchronization technique was used between the linac
and laser. The available wavelength was extended from
300 nm to 1000 nm by generation of conium light. The

Radiation application is spreading to wide files,
such as device fabrication, new material, and medicine. It
is important for the high technical application toinvestigate the primary processes in radiation chemistry[1,2], because the characteristic of the radiation-induced
reaction appears in short time region within picosecond
order.
Picosecond pulse radiolysis by using electronlinear accelerator has been developed since the firststroboscopic method by Hunt eta!. in 1979 [3]. However,
the time-resolution was kept about several tens
picoseconds for the 30 years. Recently, thetime-resolution is being broken into subpicosecond
time-scale by using several newtechniques. The new techniques are asfollows.
Fig. 1 Recent development of pulseI) Generation of subpicosecond andradiolysis at ISIR, Osaka, University
femtosecond electron pulse from linac[4,5]2) Synchronization of femtosecond laser
with electron linac [6-9]19973) Timing detection between electron
Subplcosecond
pulse and laser pulse [5]3poFemtosecond

100po
19A-nced
Timing detection
Femtosecond

....

r+
Figure I shows the development
history of pulse radiolysis at ISIR, Osaka
University. Several types of pulse
radiolysis
systems
forabsorption
spectroscopes have been developed since

I

Nanosecond

1979

3ps

-138-

I

Wide wawlength

Advanced

Picosecond

-

20po

Femtosecond

I

1992

Advance

1997

Picosecond
20po

JII

1998-

Subpicose<:ond

TlmlngO.tectlotl
1995

u-r

RFGun~

2. Developm ent of pulse radiolysis
system-100po

Wide wave/engtll

Nanosecond
300nm -1.5).llllIIJI

Advanced Nanosecond

200nm -2.51-lffi

1999

I

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

approximately 30 ps at the end of the accelerating
tube. In the magnetic pulse compressor, highenergy electrons in the early phase take a long
path and low energy electrons in the later phasetake a short path. By translating the energy
dispersion into the difference of the trajectory
length, the electron pulse is compressed at the endof the magnetic pulse compressor. This system
can compress the 30 ps electron single pulse tosubpicosecond [4]. The electron pulse was used in
subpicosecond pulse radiolysis (fig.2 b)
(a)

(b)

(c)

Picosecond
Pulse Rdiolysis

Femtosecond
Pulse Rdiolysis

Femtosecond
Pulse Rdiolysis

4. Synchronization of femtosecond laser

In the picosecond and subpicosecond pulse
radiolysis (fig. 2a, b), mode locked Ti:Sapphire
Fig. 2 Cross section of laser photocathod RF electron
laser was synchronized to the L-band linac. Thegun.
frequency of the laser was 81 MHz. On the otherhand, the basic oscillator of the L-band linac wasprimary process of radiation chemistry has been studied
108 MHz. The frequency of 27 MHz, which is theby this system.
greatest common divisor, was used as a common master
By generating subpicosecond or femtosecond
oscillator.
electron pulse with the pulse compression technique, theIn the laser photocathod RF electron gun, (fig. 2c),time resolution can be improved, as shown fig. 2b. To
the frequency of the picosecond YAG laser for the drive
obtained high time resolution, the timing detectionof the gun was l19MHz. To synchronize with the s-band
technique was used. The first subpicosecond data werelinac, the basic frequency of 119 MHz was used.
obtained in 1998.
The timing jitter between the electron pulse andAnother method for femtosecond pulse radiolysis
the laser pulse was kept in the picosecond order by ais shown fig.2c. The linac is equipped with a laser
timing stabilizer system equipped in the laser system.
photocathod RF electron gun [13] to produce highquality electron beam from which ultra short pulse can
5. Timing detection system
be generated easily. Two type lasers are used for theanalyzing light and for the driver of the gun. TheThe jitter between the laser pulse and the electron
synchronization and timing detection techniques are also
pulse was several picoseconds. Therefore, it is difficult toused. Preliminary experiment by using .the s-band linac
obtain high time resolution in subpicosecond andhas been started since 1998.
femtosecond pulse radiolysis only by using theThe details of the techniques for pulse radiolysis
will be mentioned below.
45°Bending Magnet
TriPletf- ~QOmll! I
c::=:-==--==-=~~==:j

3. Generation of ultra short electron pulse

Screen J.
Mon i tor'ry

Beam SI it

The ultra short pulse was generated by magnetic
pulse compression. The magnetic pulse compressor
which consists of two 45° sector magnets, four
quadrupole magnets and a vertical beam slit wasinstalled in the L-band linac [12], as shown in Fig. 3.
The longitudinal energy distribution of theelectron pulse was modulated so that the energy ofelectrons in the early phase of the pulse was higher thanthat in the later phase of the pulse. The phase ofaccelerating electric field was 70? . The peak energy ofaccelerated pulse was 26.5 MeV. The energy spread afterthe modulation was 9.4 %. The pulse length was-139-' .•.• •~on\~l=~Gate Valve!

I•·~Gate:\Beam Slit

ACCIl Valve
0__EEL-~~-~~~~-----'7___,1

7

Swi tchyard Room\___iiCCilliEJFGun I

llagnetic Pulse
C011Pressor

ISIR L-band Linac

Fig.3 Magnetic pulse compression system

Linac RoomIj

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

The geminate ion recombination [9,14,15] is theone of the most important reaction in the primary process.
A geminate pair of cation radical and electron producedby high energy electron recombines quickly in non-polarliquid. The time-dependent behavior of geminate
recombination process was observed directly. The data
was analyzed based on the diffusion theory [16].
The formation process of solvated ·electron[ 17-19] in polar liquid was also studied. The formation
time of solvated electron is picoseconds order. Especially,that of hydrated electron in water is several hundred
femtoseconds.
Other studies on primary process in many fields,such as organic chemistry, polymer chemistry andbiology, will be started.

,Cherenkov:
Radiation Mirror Half Mirror

1/········A:'" --+-- Laser
PhotodiodeI [_ }~lelMeasurement of :Laser Intensity•~-·------1~

MirrorBeam~~~--------~

Streak Camera
Measurement ofTime Interval

Fig. 4 Timing detection system

synchronizing technique. To break the time resolution
below picosecond, the timing detection system wasdeveloped.
Figure 4 shows the timing detection system. Thetime interval between the electron pulse (Cherenkovlight) and the laser pulse was measured by the streak
camera. The Cherenkov radiation was emitted by theelectron pulse in air at the end of the beam line. The laser
pulse was separated from the analyzing light by a half
mirror. The precise time interval could be obtained by theanalysis of the streak image.
6. Time-resolution

The time-resolution are decided mainly by the pulse
width both electron and laser in picosecond pulse
radiolysis. However, in subpicosecond and femtosecond
pulse radiolysis, the time resolution is limited by thedifference between the velocity of the light and that of
the electron pulse in a sample. The passing time of theelectron pulse through a sample is given by V(f3c), wherel is an optical length of a sample, f3 the ratio of thevelocity of electron to that of light in the vacuum and cthe velocity of light in the vacuum. On the other hand, inthe case of light, the passing time is given by ln/c, where
n is a refractive index of a sample. Therefore, the time
resolution is limited by the thickness of a sample. Forexample, the time resolution is limited to >1.8 ps(10-90% rise time) at n = 1.33 an41 = 2 mm.
7. Pulse radiolysis application

The primary process of radiation chemistry has
been studied by using the picosecond and subpicosecondpulse radiolysis.

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