DESIGN AND CONSTRUCTION OF TWIN LINAC PULSE RADIOLYSIS SYSTEM
H. Kobayashi, T. Ueda, T. Kobayashi andY. Tabata
Nuclear Engineering Research Laboratory
Faculty of Engineering University of Tokyo
Tokai-mura, Ibaraki-Prefecture 319-11, Japanare delayed step by stpe. The signals are repeated by Ntimes -typically 64 times- at every step to obtain goodsignal to noise (S/N) ratio. A portion of the Cerenkov
light pulse is absorbed in propotion to the concentrationof the transient species. The delay time of the Cerenkov
light pulses corresponds to the time axis of an absorption signal. So absorption signals can be obtained byusing two linacs.

ABSTRACTThe pulse radiolysis system which has a very hightime resolution has been developed by using two parallel
linacs. Each linac can accelerate single bunch beam(picosecond single beam) with a width of shorter than 10
picoseconds (ps). One linac of the two is used as anirradiation source, and the other is used as a Cerenkov
light source. A portion of Cerenkov light pulse is absorbed by the transient species produced in a sample
which is irradiated by a beam. Cerenkov light pulses
which are used as a probe of an absorbing product can bedelayed step by step and the time profile of the absorbing product can be obtained. The absorption singnals of several samples have been obtained with a time
resolution of 20 ps.

LINAC SYSTEM
The block diagram of the twin linac pulse radiolysis
system is shown in Fig. 2. Each linac can acceleratepicosecond single electron beams with a width of shorter
than 10 ps. Two linacs are installed at a distance of750 mm. One linac (ACC-I) of the two has a beam energyof 15 MeV and is used as an irradiation source. The
other one (ACC-II) has a beam energy of 20 MeV and isused as a Cerenkov light source. The beam energy of 20
MeV is lower than the threshold energy for Cerenkov light
generation in the air. So the Cerenkov radiator whichcontain xenon gas at 1 atm. is attached at the beam
window of the 1i nac. The .beam with a energy of 15 MeV
has enough penetration as the irradiation source. The
main parameters of these two linacs are shown in Table 1.

INTRODUCTIONThe bunched beam of a linac is one of the good
pulsed irradiation sources for pulse radiolysis systemwith a high time resolution 1 , because the pulse width of
the bunched beam is very short and the intensity of thebeam is comparatively high. Especially the single bunch
beams, that are also called picosecond single beam, arethe excellent irradiation source for pulse radiolysis
systems with a time resolution of better than 100 ps.2 •3•4
The purpose of the twin linac system is to obtain the 10
ps time resolution which has not yet achieved. by anyother systems. The twin linac pulse radiolysis system
is a kind of Stroboscopic Pulse Radiolysis Systems
developed by J. Hunt and his cowarkers. 1 The principleof the twin linac pulse radiolysis is briefly descibedusing Fig. 1.
The transient species which absorb the light are
produced in an irradiated sample and decay by reactions.
The purpose of the system is to obtain the time profileof the concentration of the transient species with avery high time resolution. The irradiation source andthe 1i ght source as a probe are necessary. One 1i nac of
the two is used as the irradiation sources and theCerenkov light pulses produced by the other linac areused as a probe. The measuring procedure is as follows.
The two linacs are operated repeatedly. Initially thelight pulses pass through the sample cell before thebeams irradiate the sample. The Cerenkov 1ight puls.es

BEAM

1st

2nd

3rd

4th

5th

1

l_

J_1_

J_

Irradiation source

Light source

Beam Energy

15 MeV

20 MeV

Output Charge

1.5 nC/pulse

0.1 nC/pulse

Rep. rate(max)200 Hz200 HzTable 1. Parameter of two linacs
TIME INTERVAL BETWEEN TWO BEAMS
The time interval between two beams which are accelerated by the ACC-I and by the ACC-II respectively canbe changed by delaying the beam of the ACC-II. The beamof the ACC-II can be delayed by three phase shifters
which are connected by a dotted line in Fig. 2. These
three phase shifters are drived simultaneously and are~ ~~11l~
INTENSITYofLIGHT

Fig. 2. The block diagram of
Twin Linac Pulse Radiolysis System

Fig. 1. The principle of stroboscopic
pulse radiolysis (Twin LinacPulse Radiolysis)163

used to delay the grid triggers for the electron gun,the microwave for the subharmonic prebunching cavity(SHB) and the microwave for the accelerating structurein the ACC-II system. The maximum delay time is 2 ns,
although it is easy to expand the longer time limitation.
The accuracy of time axis is± 0.7 ps. The time axis
has 1024 channels each of which corresponds to 2 ps.
MICROWAVE SYSTEM
The block diagram of microwave system for the twin
linac pulse radiolysis system is shown in Fig. 3. The
frequency of the master oscillator is 476 MHz. Themicrowave for the klystrons is obtained by multiplying
the master microwave by 6. The microwave with a frequency of 476 MHz is amplified by solid state amplifiers
and is divided into two aprts. Each of them is amplified by amplifiers up to 2 kW and is fed to the SHB.
The microwave with a frequency of 2856 MHz is also amplified by a solid state amplifiers and is divided into
two parts. Each of them is amplified by the amplifiers
composed of plannar triodes up to 100 Wand is fed tothe klystron. The outputs of the klystron I and of theklystron II are fed to the accelerator I and to theaccelerator II respectively.

emit the electron beam from the electron gun with a time
jitter smaller than 20 ps. But the time jitter of injected beam is reduced through bunching procedure.
Electrons on the acceptance angle are bunched in thenarrow phase. Suppose that the two linacs which havethe same bunching section are prepared. The time jitter
between two beams accelerated by the parallel linacs
corresponds to the phase jitter between two high power
microwave sources which are for two accelerating structures. So the timing of output beams does not dependon the timing of injector but on the phase of microwave.
The phase modulation of a klystron is mainly due to thefluctuation of anode voltage for the klystron. The time
jitter between two beams due to the fluctuation of anodevoltage of the klystron which is used in the system canbe calculated and is about 3 ps/0.5 %. The very stablehigh power pulser for the klystron is needed to obtainhigh time resolution.
BEAM SWITCHING SYSTEM
To calculate the absorption of an analyzing light,
the background light produced in a sample by an i rrad i ation source should be subtracted. So the three different kinds of operational modes are necessary. These areas follows.
(A) The only analyzing light is produced. The accelerator for the Cerenkov light is operated.
(B) Both the electron beam and the Cerenkov light areproduced. Both the accelerator for the electron
beam and the accelerator for the Cerenkov light areoperated.
(C) The only electron beam is produced. The accelerator
for the electron beam is operated.
The fraction of absorption can be calculated by using
these three operational modes. These three operationalmodes are controlled by the grid triggers of the grid
modulators for electron guns. The block diagram of thebeam switching system is shown in Fig. 4. The output
triggers of the synchronization circuit are gated by alogic IC. The gate for the ACC-I is opened at the modesof (B) and (C). The gate for the ACC-II is opened at
the modes of (A) and (B). The system is operated incontinuous sequence ABCABCABC .....

Fig. 3. The block diagram uf microwave system
ELECTRON GUN SYSTEM
476MHzFor the twin linac pulse radiolysis system, twoelectron guns are used but only one high voltage power
supply for them is used and is enough. To produce apicosecond single beam, the very short electron beam isnecessary. For example, to obtain the output charge of
1.5 nC, the input charge of about 3 nC with a pulse
width of 0.8 ns is necessary. The beam energy of thegun emission is considered to be supplied by the stored
energy in the gun structure. The energy of beam whichis extracted from the gun is about 2.7 x 10- 4 J, andthe stored energy in the gun structure is estimated to
be about 4 x 10- 2 J.
The grid modulators for the guns are composed ofavalanche transistors and installed at the high voltage
deck of the high voltage power supply. The triggers forthe grid modulators are applied through glass fibers to
insulate the high voltage of 90 kV.

CLOCKFig. 4. The sequence of the switching system.
The output triggers of the synchronization
circuit are switched by an emitter coupledlogic IC. The gate for the ACC-I is openedat the mo.des of (B) and (C). The gate for
the ACC-II is opened at the modes of (A)and (B).

TIME JITTER
The time jitter between two beams which are accelerated by two parallel linacs is one of the most importrant problems in the twin linac pulse radiolysis system.
The bunching procedure in an accelerating structure
should be considered. At the bunching section, the injected electrons oscillate and are gradually bunched at
the stable phase. When the buncher section of a linac
is properly designed, a comparatively wide phase acceptance can be obtained. At present, it is difficult to164

in Fig. 6. The following important results have beenobtained by using the twin linac pulse radiolysis system.
l. The absorption around 330 nm is composed of at least
two parts; very short-lived and long-lived ones.
2. The formation processes of the absorption around 480
nm is composed of at least two parts; the faster(within the limitation of time resolution of thesystem) and the slower ones.

OPERATION OF LINAC SYSTEM
The linac system was operated and the performanceof that was measured. Each of two linacs was confirmed
to produce picosecond single beams by a streak camera.
The output charge of each linac is shown in Table l.
The sequence of beam switching was also confirmed
to be the same as described in the section of beamswitching.
The delay of the beams of the ACC-II was also confirmed by an oscilloscope. The oscilloscope was triggered by the beam of the ACC-I and the beams of ACC-II
were monitored by the oscilloscope. The beams weredelaying step by step and were shown in Fig. 5.

CCI 4 at 320 nm

CCI 4 at 480 nm

O.D.

230ps/div

230ps/divo. o -I--'. -/'0.0TimeFig. 6.

Fig: 5. An oscilloscope trace of the beams of ACC-II.
The beams of the ACC-I were used as triggers
for the oscilloscope. The shutter of a camera
was continuously opened during the beams ofthe ACC-II were delaying.

TimeThe absorption signals of liquid carbon
tetrachloride (CC1 4 ) at 330 nm and at 480 nm
are shown on the left and on the right respectively. These signals are obtained by using
the twin linac pulse radiolysis system.
ACKNOWLEDGEMENT·The authers wish to thank Prof. S. Tagawa, Dr. Y.
Katsumura and Dr. M. Washio for their helpful advice.

DATA ACQUISITIONREFERENCES

The intensity of the light is detected by a photomultiplier. As described in the section of beam switching system, when the mode (A) is operated, the intensityof analyzing light which is not absorbed is detected.
When the mode (B) is operated, the analyzing light whichcontain absorption signal with the background light suchas emission from the sample is detected. When the mode(C) is operated, only the background light is detected.
The absorption signal can be obtained by addition andsubtraction of these three signals, ie., absorption
signal =A- (B- C). The intensity of light is converted to digital form by an A/D converter. A minicomputer is used to calculate and to store the absorption signals. To obtain good S/N, the set of (A), (B)and (C) is repeated by N times. And then the stepping
motors for phase shifters are driven. Then the next
data acquisition starts.

1. M.J. Bronskill, W.B. Taylor, R.K. Wolff and J.W. Hunt,
Rev. Sci. Instrum. 41 (1970) 333
2. (a) G. Mavrogens; W. Ramler, W. Wesolowski, K. Johnson
and G. Cl ifft, IEEE Trans. Nucl. Sci. NS-20 (1973)
919(b) C.D. Johnah, Rev. Sci. Instrum., 46 (1975) 62
3. (a) H. Kobayashi, T. Ueda, T. Kobayashi, S. Tagawa
andY. Tabata, Nucl. Instrum. Meth. 179 (1981)
223(b) H. Kobayashi, T. Ueda, T. Kobayashi, M. Washio,
S. Tagawa and Y. Tabata, Radiat. Phys. Chern. 21(1983) 13(c) H. Kobayashi, T. Ueda, T. Kobayashi, S. Tagawa,Y. Yoshida andY. Tabata, Radiat. Phys. Chern. 23(1984) 393
4. S. Takeda, M. Kawanishi, K. Hayashi and H. Sakurai,
Nucl. Instrum. Meth. 188 (1981) lRESULTThe system has been applied to absorption spectroscopy of several samples. The time resolution of thesystem was checked by absorption signal of hydrated
electron in water at 700 nm. The rise time of the signal was about 25 ps. The time resolution of the system
was mainly due to the desynchronization of the electron
pulses with the slower analyzing light pulses as they
pass through the 2 em detection cell. An appreciableimprovement in time resolution would be obtained byusing the thin detection cell. The time resolution ofbetter than 10 ps is expected by using 5 mm detection
cell.
As examples of signals obtained by using the twin
linac pulse radiolysis system, the absorption signals of
liquid carbon tetrachloride (CCl,) at 330 nm and at 480
nm are shown on the left and on the right respectively
165