Pulse radiolysis using double-decker electron beam
K. Kan #, J. Yang, A. Ogata, T. Kondoh, T. Toigawa, K. Norizawa, H. Kobayashi, Y. Yoshida,
Institute of Scientific and Industrial Research, Osaka University, Osaka, Japan

Abstract
Double-decker pulse radiolysis (DDPR), which utilizes double-decker electron beams, was investigated to develop anew pulse radiolysis with a high time resolution. The double-decker electron beams were generated by injecting two UV
pulses into a photocathode radio-frequency gun. In the pulse radiolysis, one electron beam was used as a pump beam,and the other was converted to a probe pulse. Finally, as its first application, the DDPR was successfully used forobserving solvated electrons in water.

ダブルデッカー電子ビームを用いたパルスラジオリシスの研究
1.

Introduction

Pulse radiolysis [1,2], which utilizes a pump electron
beam and a probe pulse, is a powerful tool that can beused for the observation of ultrafast radiation-induced
phenomena involving the mechanical motion of electrons
and atomic nuclei in reaction mechanisms that are studiedin physics, chemistry [3,4,5], and biology [6]. In the1960s, the first picosecond pulse radiolysis [1] wasdeveloped at the University of Toronto using an electron
bunch train produced by an S-band accelerator. In 2000, asub-picosecond pulse radiolysis was developed by a groupat Osaka University using an electron beam from an Lband accelerator and a probe pulse from a synchronized
femtosecond laser [7]. Pulse radiolysis with a high time
resolution, which will be developed in the future, would
give a more comprehensive understanding of ultrafast
phenomena not only for monitoring but also forcontrolling radiation-induced phenomena. However, thetime resolution of pulse radiolysis is limited by the pulse
widths of the electron beam (pump source) and the probe
pulse, the difference in their velocities due to therefractive index of the sample, and the synchronized time
jitter between them [5].
At Osaka University, a photocathode-based linear
accelerator (linac) and a magnetic bunch compressor wereconstructed for femtosecond pulse radiolysis based on afemtosecond electron bunch [ 8,9,10]. In the linac, apicosecond electron beam was generated using aphotocathode radio-frequency (RF) gun and a Nd:YLF
picosecond UV laser as a driving laser. The picosecond
electron beam was accelerated up to 32 MeV by a booster
linac with an optimal energy-phase correlation forcompression. Finally, the electron bunch was successfullycompressed into femtoseconds, e.g., 98 fs in rms at 0.2 nC[8]. As a result, the combination of a femtosecond
electron beam and a femtosecond laser realized pulse
radiolysis with femtosecond time resolution, e.g., ~230 fs,
due to the femtosecond pulse widths of both [5]. For
improving the synchronized time jitter between a pump___________________________________________#koichi81@sanken.osaka-u.ac.jp

beam and probe pulse, double-decker electron beams wereused as the pump beam and probe pulse; the doubledecker beams were generated using two UV pulses fromthe driving laser [11,12]. Double-decker pulse radiolysis(DDPR) based on these beams would improve thesynchronized time jitter because it would preclude RFsynchronization between a pump electron beam and aprobe laser.
In this paper, DDPR, which utilizes double-decker
electron beams, was investigated for developing a newpulse radiolysis with a high time resolution. The doubledecker electron beams were generated by two UV pulses
and a photocathode RF gun for use as the pump beam andprobe pulse in the pulse radiolysis measurement.

2.

Experimental arrangement

Figure 1 shows the DDPR setup, including the laser
injection system, the photocathode RF gun linac, and thepulse radiolysis system from upstream. In the laser
injection system, a Nd:YLF picosecond laser was used todrive the photocathode. The output of the picosecond
laser was 200 μJ/pulse of UV pulse (262 nm) at 10 Hz.
The FWHM pulse width of the UV light pulse wasmeasured to be 5 ps using a femtosecond streak camera.
The UV pulse was separated by a beam splitter (BS) togenerate the double-decker electron beams. The temporalseparation of the two UV pulses was adjusted by an
optical delay (OD1). The interval of the two pulses wasset to 4.2 ns, which corresponded to 12 periods of S-band
RF (2.856 GHz, 1 period = 0.35 ns) because the doubledecker electron beams should be accelerated by a similarelectric field. Furthermore, the spatial separation was also
adjusted by mirrors with actuators. The two UV pulses
were directed to the photocathode RF gun linac. The linac
system consisted of a 1.6-cell S-band RF gun with acopper cathode, a 2-m-long traveling-wave linac, and amagnetic bunch compressor; the details of the linac arediscussed in references [8,9,10]. In order to generate thedouble-decker electron beams, the two UV pulses were
injected into the photocathode. The incident angle of the- 1301 -UV pulses was ~68° with respect to the direction of theelectron beam, and this angle was chosen to increase thebeam charge. The incident angle enhanced the bunchcharge due to a polarization effect on the copper cathode[13]. In this RF gun, the enhancement factor of the bunchcharge due to the polarization effect at 68° incident angle
was found to be ~5 compared with that at 2° incident
angle. The UV pulses on the cathode were arranged
almost horizontally because the double-decker electron
beams were rotated azimuthally due to a longitudinalmagnetic field of a solenoid magnet at the gun exit. The
double-decker electron beams were accelerated in the gun
and the linac using a 35-MW klystron. The beam energy
at the gun exit was ~4 MeV at a laser injection phase of
30° according to a measurement using a dipole section
and screen. In the linac, the electron beams wereaccelerated up to 32.7 MeV. A total beam charge of 1.4
nC was generated by a UV pulse of 200 μJ/pulse at a laserinjection phase of 30°. In the experiment, the linac phase
was set to 85° to minimize energy spread of the beams
from the viewpoint of transportation. The beams traveledthrough the magnetic bunch compressor. The rms pulse
widths of the double-decker beams were estimated to be~1 ps according to the linac phase condition [8]. Finally,
the double-decker electron beams were used in the pulse
radiolysis system. The front electron beam of the doubledecker beams was converted into a probe pulse by
Cherenkov radiation in the air. The path length for theCherenkov radiation was set to 280 mm to obtainsufficient light intensity, which depends on the path length[14]. The probe pulse traveled through an optical delay(OD2) in order to measure time-resolved transientabsorption in the sample. The back electron beam, which
was used as a pump electron beam, reached the sample
with a delay of 4.2 ns with respect to the front beam. Theprobe pulse was injected into the sample after it had
traveled a long path. The spatial distance between thepaths for Cherenkov radiation and the pump beam was set
to ~5 mm in the vertical direction to avoid the beam usedas the probe pulse from traveling through mediums, e.g.,
the mirrors and sample, as shown in Fig. 1. The intensityof the probe pulse, which decreased due to transientabsorption in the sample caused by the back beam, wasmonitored by a silicon avalanche photodiode (APD)(S2382, Hamamatsu Photonics K. K.) through an opticalfiber. The APD was driven by a bias voltage of 60 V. The
waveforms from the APD were measured by an
oscilloscope (6100A, LeCroy Co.). Spectral informationof the absorption was selected by a band-pass filter (BPF)(FKB-VIS-40, Thorlabs). Shutters (S1/S2) controlled thegeneration of the front/back electron beams for theprobe/pump sources, respectively. Figure 1(b) and 1(c)show pictures of the laser injection system and pulse
radiolysis system, respectively.

Figure 1: (a) Laser injection system (top) and pulse
radiolysis system (bottom) in DDPR. BS denotes abeam splitter; S, a shutter; OD, an optical delay. (b)Picture of the laser injection system. Laser paths forthe pump electron beam (solid line) and the beam forthe probe light (dashed line) are shown. (c) Picture of
the pulse radiolysis system. Paths for the pumpelectron beam (solid line) and the probe light with
Cherenkov radiation (dashed line) are shown.

3.

RESULTS AND DISCUSSION

3.1

Generation of double-decker electron beams

Figure 2 shows the spatial and temporal separations of
the double-decker electron beams. Figure 2(a) shows thespatial separation at the sample position measured by ascreen. The distance between the beams was adjusted to~5 mm because of the geometry of the sample andconversion to the probe pulse with Cherenkov radiation.
The spatial distances between the double-decker electron- 1302 -beams at the gun exit and at the linac exit were 7.8 and 6.6mm, respectively. The distance was not constant, because
the beams traveled through magnets and RF cavities,
which focused or defocused the beams. The back(top)/front (bottom) electron beams were used for thepump/probe sources, respectively. Figure 2(b) shows thetemporal separation measured by a current transformer(CT) at the gun exit. To adjust the temporal separation,
the energies of the beams at the linac exit were set to thesame level by the optical delay (OD1) in the laser
injection system. The beam energies were measured by abending magnet and screen. The interval of the doubledecker electron beams was measured to be 4.2 ns. The
electron bunch charges were 0.74 (front) and 0.62 (back)nC according to the CT sensitivity of 0.35 nC/V peak-topeak.

Figure 2: (a) Spatial separation of the double-decker
electron beams at the sample position. The back(top)/front (bottom) electron beams were used for thepump/probe sources, respectively. (b) Temporalseparation at the gun exit with only the offsets
adjusted for comparison. Waveforms for cases using
two UV pulses (solid line) and using one UV pulse forthe back beam (dotted line) are shown.
3.2

First double-decker pulse radiolysis

Figure 3 shows the results of the first DDPR
measurement. A water sample with a thickness of 10 mm
was used. Figure 3(a) shows the photodiode outputs(peak-to-peak) as a function of the optical delay (OD2) inthe pulse radiolysis system with a BPF of 800 ±20 nm.
The BPF was set upstream from the optical fiber in order
to select spectral information. Data for L and B denote theoutputs for usage of only “Light” generated by Cherenkov
radiation and only “Beam” from the pump source,

respectively. Data for LB denote the output for thecombination of L and B. Each data set corresponds to theconditions of the shutters in the laser injection system inFig. 1. Briefly, LB, L, and B denote the shutter conditions(S1/S2) for front/back electron beams of Opened/Opened,Opened/Closed, and Closed/Opened, respectively. Eachdata set corresponds to the averaged value using 40
sweeps of the oscilloscope. Only when the probe pulse
went through the sample following the pump beam waslight absorbed due to the solvated electrons generated by
the pump beam. As a result, the plot for LB decreased in atime of ~0 ps because of the light absorption. The
intensity for B, which corresponded to Cherenkov
radiation from the pump beam in the sample, could not beignored, because of the intensity for L. In evaluation ofthe transient absorption according to the Beer-Lambertlaw, the optical density was estimated asO.D.  LogI 0 I   LogL LB  B  , where I0 denotes theintensity of the probe pulse without the pump beam and I
denotes the intensity with the pump beam. The terms (LB,L, and B) denote the photodiode outputs for the conditions
mentioned above. Figure 3(b) shows the transientabsorption due to the solvated electrons according to theequation. The data in Fig. 3(a) were used to calculate theO.D. Expectedly, the O.D. increased in a time of ~0 ps.
Thus, the first DDPR was demonstrated with an O.D.
change of 0.043, although the 10-to-90% rise time wasobtained as 8.6 ps, which depended mostly on the sample
thickness [5].
To improve the time resolution in DDPR, optimizationsof the double-decker electron beams and pulse radiolysis
system are intended to be carried out. In the optimizationof the electron beams, generation of femtosecond doubledecker electron beams [11] is intended to be applied to thegeneration of a femtosecond probe pulse. As regards theoptimization of the pulse radiolysis system, a gas cell for
Cherenkov radiator would increase the intensity of theprobe pulse according to references [2,14], resulting in an
improvement of the S/N ratio and time resolution whenusing reference measurement of the probe pulse [15] and athin sample cell. The overlap between the pump electron
beam and probe pulse should also be optimized according
to the optical density reported in reference [5]. At thesame time, for improving the time resolution, which islimited by the difference in the velocities of the electron
beam and the probe pulse, equivalent velocity
spectroscopy (EVS) can be carried out. The use of anoblique electron beam in EVS has been shown to improve
the time resolution of the pulse radiolysis compared with acase using a normal electron beam [16,17]. For improving
the synchronized time jitter between a pump electron
beam and a probe pulse, DDPR based on EVS would havethe advantage of precise synchronization without RF
synchronization, from which jitter is estimated to be ~60
fs [5]. Optical systems used in DDPR would be morereliable for the jitter in terms of the precision of pump-- 1303 -probe measurements conducted using lasers. Because
DDPR involves the use of a probe light based on anelectron beam, it could be especially useful in cases where
an electron beam of less than a few tens of femtoseconds[10] is generated. Application of the proposed pulse
radiolysis in the terahertz (THz) range is also expected for
detecting transient quasi-free electrons [18] because ofTHz-wave generation based on an electron beam [14,19].

e.g., the Cherenkov radiator, reference measurement, andEVS. Besides improving the time resolution, anotherapplication to a pulse radiolysis in THz-range would beexpected.

Acknowledgement
We thank the staff of the Radiation Laboratory at theInstitute of Scientific and Industrial Research (ISIR),
Osaka University for the linac operation. This work was
supported by KAKENHI (21226022).

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Figure 3: (a) Photodiode outputs as a function of the
optical delay in the pulse radiolysis system with a
BPF of 800 nm. LB, L, and B denotes the shutters(S1/S2) conditions for front/back electron beams ofOpened/Opened, Opened/Closed, and Closed/Opened,respectively. (b) Transient absorption due to thesolvated electrons. Solid line denotes the simulation
result using least-squares fitting.

4.

CONCLUSIONS

Double-decker pulse radiolysis (DDPR) using doubledecker electron beams was demonstrated, with a 10-to90% rise time of 8.6 ps. DDPR was also applied tospectrum measurement. In the future, the time resolution
in DDPR is intended to be improved by generating
femtosecond double-decker electron beams andoptimizing various aspects of the pulse radiolysis system,- 1304 -