The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

DEVELOPMEN T OF A PULSE RADIOLYSIS SYSTEM BY USING PICOSECONDELECTRON PULSES AND FEMTOSECOND LASER PULSES
Yasuhiro MIZUTANI, Katsunori HORI, Tamotsu YAMAMOTO, Miyako MIKI, Yoshinobu IZUMI**,Shu SEKI, Takahiro KOZAWA, Kiminori USHIDA*, Yoichi YOSHIDA, Seiichi TAGAWA
The Institute of Scientific and Industrial Research, Osaka University
8-1 Mihogaoka, Ibaraki, Osaka 567, Japan*Institute of Physical and Chemical Research (RIKEN)
2-1 Hirosawa, Wako, Saitama 351-01, Japan**Department of Nuclear Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka 565, Japan

Abstract
A new picosecond pulse radiolysis system has
been developed at the Institute of Scientific andIndustrial Research, Osaka University. The system
is composed of a 38 MeV linear accelerator for airradiation source and a Ti-sapphire laser for ananalyzing light source. The electron pulses (FWHM= 20ps) and the laser pulses (FWHM = 60fs) are
synchronized with a radio frequency of 27 MHz. The
wavelength region of the analyzing light is from 300
nm to 1500 nm by using a fundamental light and awhite light continuum by using a regenerative
amplifier. The present time resolution of the system
is about several ten picoseconds.
1. Introduction
The pulse radiolysis is a powerful method toinvestigate the time-dependent behavior of the short
lived intermediates produced by a radiation. The
first picosecond pulse radiolysis, so called thestroboscopic method, was planed [1] and done by J.
W. Hunt at Toronto university in 1969 [2]. The
irradiation source was an electron pulse and theanalyzing light source was the Cherenkov lightpulse produced by an electron pulse. The time
profile was obtained by changing the time difference
between an electron pulse and a Cherenkov light.
The time resolution was decided by the pulse widthsof the irradiation and analyzing light source.
However, the observable time-scale was limited
within 350 ps which corresponded to the interval of
the electron pulse train of the S-hand linac.
To solve such a problem, the two different
picosecond pulse radiolysis system by using a single
electron pulse was developed at Argonne nationallab.[2] and Univ. of Tokyo[3]. In the Argonne, theanalyzing light pulse could irradiate the sample
before the electron pulse by using a 270° magnet.
On the other hand, the two linacs were used in theUniv. of Tokyo. One was used for a irradiation source
and the another was used for a Cherenkov lightsource. However, in both methods the wavelength islimited in the visible region, because the intensity of
the Cherenkov light in the infrared region is weak.

LightD election
SystemSynchro·n ized
CircuitFig.l

Block diagram of a new picosecond pulse radiolysis system-148-The 11th Symposium on Accelerator Science arul Technology, Harima Seience Garden City, 1997

Later in order to measure in the infrared region, theLL-twin system[5] by using a diode laser wasdeveloped at Univ. of Tokyo. However, thewavelength was fixed.
To solve such two problems (the time-scale andthe detectable wavelength), a new picosecond pulse
radiolysis system by using an electron for anirradiation source and a Ti-sapphire laser for ananalyzing light has been developed. Although thefundamental wavelength of the Ti-sapphire laser isfrom 750 nm to 850 nm, the wavelength is extendedfrom 300 nm to 1500 nm by using the white lightcontinuum.
Fig.2 Trigger system for the generation of the white
2. Outline of new picosecond pulse radiolysis systemlight continuum.
An schematic of a new pulse radiolysis system isshown in Fig.l. The system is mainly composed of
the ISIR 38MeV L-band linac[6], a femtosecond Ti- repetition of the single electron pulse was 60Hz. It
sapphire laser system, a rf system, a synchronized is necessary to pick up a single laser pulse which iscircuit, a light detection system and a personal synchronized with a single electron pulse. The pulse
selector (the acoustic-optic modulator) was inser.tedcomputer.
A sample was irradiated by a 20 ps electron pulse after the Ti-sapphire laser. The timing trigger to thefrom the linac. At the same time, the femtosecond pulse selector was provided from the synchronized
analyzing light (FWHM=60fs) from the laser system circuit.
The time difference between an electron pulse
passed through the sample cell. The analyzing
light was detected by the light detection system and and a laser pulse should be changed continuously tothe time-dependent absorption was calculated by obtain a time profile of the absorption. In Toronto
the computer. Also, all equipment was controlled by and Argonne, the time difference was changed by a
optical delay line system. In this system, two electric
the computer.
Usually, it is difficult to synchronize a delay lines (phase shifters) were used in stead of thefe.mtosecond laser pulse with a picosecond electron optical delay line to make an alignment easily in thepulse within the timing jitter of several picoseconds. radiation field: The phase shifters were installedIn the new system, the timing jitter was controlled into the 81 MHz line for the Ti-sapphire laser andby the RF synchronization method. The frequency the trigger line for the pulse selector.
The fundamental wavelength region was fromof the master oscillator of the linac was 108 MHz
which was multiplied by 12 to drive the klystrons at 750 to 850 nm. The wavelength region was extended
1296 MHz. On the other hand, the Ti-sapphire laser by using of the nonlinear optics such as SHG(secondis operated at 81 MHz. Therefore, the rf of 27 MHz harmonics generation), THG(third harmonics
which was the greatest common divisor was used as generation), and OPO(optical parametric oscillation)
the standard radio frequency. The rfs of 81 MHz (27 as shown in Fig. 1. However, these techniques can
MHz multiplied by 3) and 108 MHz (27 MHz not cover the all wavelength region from 300 to 1500
multiplied by 4) were provided to the laser system nm. The system of the white light continuum will
and the linac, respectively.
mention in the next section.
Although the laser timing was controlled by therf of 81 MHz, there still remains the timing jitter 3. Generation of the white light continuum
The system for the generation of the white light
due to the change of the laser cavity length. To solve
the problem, the compensation system of the cavity continuum is shown in Fig.2. The white lightlength (so called the lock-to-clock-system) worked on continuum was generated by focusing a single highvery well. The system keeps the cavity length to be intensity laser into an appropriate material, such as
constant by the PZT. The laser pulse could be · H 20, D20 and quartz block. The high intensity laser
was produced by the Ti-sapphire regenerativesynchronized with the electron pulse completely.
Output laser from the Ti-sapphire laser is a pulse amplifier excited by the YAG laser.
The trigger system was changed to control thetrain (81 MHz). On the other hand, the maximum-149-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

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Time [ps]Time [ps]Fig.3 Typical signals obtained in the new picosecond
pulse radiolysis. Signal A, B and C are detected by a lightdetection system. D is detected by a beam current
monitor. (see in the content)

Fig.4 The time-dependent behavior of n-dodecane cation
radical obtained in the new pulse radiolysis of liquid ndodecane, monitored at 820 nm.

regenerative amplifier. A single amplified laser
pulse (12Hz) was picked up by two Q-switches (thepockels cells) installed in the regenerative amplifierin stead of the pulse selector. One controlled theinjection timing of the seed pulse (81 MHz) from theTi-sapphire laser and the other controlled the output
timing of the amplified laser pulse. To control thetiming of the two Q-switches, the triggers from thesynchronized circuit are supplied to the two Qswitches.
To change the time difference between a single
electron pulse and a single amplified laser pulse,
three phase sifters were installed into the triggerlines of the two Q-switches and the YAG laser. The
detectable wavelength region was from 300 nm to1500 nm.

5. Example of the measurement

4. Light detection system

The light detection system was consists of a
monochromator, a photodiode (Si or InGaAs), and anoscilloscope. Fig 3 shows an example of the detectedsignals. The four types of the signals were measured.
Signal A is the intensity of the analyzing light
containing the absorption and the background(Cherenkov light and the emission from samples, if
there is the emission from the samples). Signal B is
the intensity of the analyzing light. Signal C is theintensity of the background. Signal D is the beamintensity which is detected by the beam current
monitor. The optical density (O.D.) was calculated asfollows.
O.D. = log {(B-C) I (A-C)}· D 0 I D
where D0 is a initial beam intensity.

Fig. 4 shows the typical time-dependent absorbance ofthe n-dodecane cation radicals obtained in the new
picosecond pulse radiolysis of liquid n-dodecane monitored
at 820 nm. Mter the ionization, an electron and aradical cation of n-dodecane, so called the geminate
ion pair, are produced. The cation radicals formed
within the time resolution of the system. The decay
part shows the geminate ion recombination[?] of thecation radical and the electron which is important
reaction in primary processes.
6. Development in the future
The present time-resolution is about several ten
picoseconds which depends mainly on the pulse
width of the electron pulse ( ~ 20 ps). Recently, aplane for the generation of the femtosecond electron
pulse has started at Osaka university. At present, afew picosecond electron pulse is obtained by using
the magnetic pulse compression. In the future, thesystem will be developed into a femtosecond pulse
radiolysis system.
References[1] M.J. Bronskill and J.W. Hunt, J. Phys. Chern. 72(1968) 3762
[2] M.J. Bronskill, W.B. Taylor, R.K. Wilff and J.W. Hunt,Rev. Sci. Instr. 41 {1970) 333.
[3] C.D. Jonah, Rev. Sci. Instr. 46 {1975) 62.
[4] H. Kobayashi, T. Ueda, T. Kobayashi, S. Tagawa andY.
Tabata, Nucl. Instr. and Meth. 179 (1981) 223[5] Y. Yoshida, T. Ueda, T. Kobayashi, H. Shibata and S.
Tagawa, Nucl. Instr. Meth. A327, 41{1993).
[6] S. Takeda, K. Tsumori, N. Kimura, T. Ya,amoto, T.
Hori, T. Sawai, J. Ohkuma, S. Takamuku, T. Okada, K.
Hayashi, M. Kawanishi, IEEE Trans. Nucl. Sci., NS32, 3219 {1985).
[7] Y. Yoshida, S. Tagawa andY. Tabata, Pulse Radiolysis,(Edited by Y. Tabata), CRC Press, 343 {1991).

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