RACETRACK MICROTRON FOR NONDESTRUCTIVE NUCLEAR
MATERIAL DETECTION SYSTEM*Toshitada Hori#, A), Ryota Kinjo A), Toshiteru KiiA), Hideaki OhgakiA), Heishun ZenA), Mohamed OmerA),
Izuru DaitoB), Hideyuki KotakiB), Masaki KandoB), Ryouichi HajimaC), Takehito HayakawaC), Fumio SakaiD)A)
IAE, Kyoto University, Gokasyo, Uji, Kyoto 611-0011
B)
JAEA, Kansai, Kizugawa, Kyoto 619-0215
C)
JAEA, Tokai, Ibaraki 319-1195
D)
Sumitomo Heavy Ind. Ltd., Nishitokyo, Tokyo, 188-8585

Abstract
A nuclear material detection system (NMDS) based on
neutron / -ray hybrid approach has been proposed for thecontainer inspection at sea ports [1]. While neutron is to
be used for a fast pre-screening, quasi-monochromatic ray beam from the laser Compton scattering (LCS) source
will be used for an isotope identification on the precise
inspection of the cargoes. Nuclear resonance fluorescencemethod is going to be employed for the isotope
identification because of its superiority in high selectivity
and in high penetration capability through the shieldingobjects. In the system a high energy electron beam ofgood quality is required for LCS. Racetrack microtron(RTM) is one of the most promising candidates as anelectron source for such the practical use. Some adequate
combinations of basic design parameters are investigated
for the RTM over 200 MeV acceleration.

much difference between old and new designs.
Another important component is a linac placed between
two main magnets, on the center portion of the first orbit(see Fig. 1). In the 150 MeV RTM, S-band linac wasadopted, and this system will be kept on for the new
design because of the popularity of this frequency 2856
MHz. Later shown are some exception on this design.
When considering further high energy RTM, 300 MeV forinstance, L-band (1300 MHz) linac is another superior
candidate. First of all frequency is fixed, however, thecondition of acceleration (energy gain per turn) will beremained as one of the flexible parameters. It is decided
to 6 MeV/turn on the base design of 150 MeV RTM.

INTRODUCTION
At present four 150 MeV RTMs are in operation in
Japan [2]. While three of them are for the injector of acompact electron storage ring, the fourth is for variousresearches including LCS at JAEA. On the contrary tothree injector RTMs which have a thermionic gun as theelectron source, the fourth has an RF gun as the source.
Therefore in principle the fourth accelerates a single
bunch at a time. Higher energy RTMs over 200 MeV for
NDMS has been designed on the basis of this establishedmachine settings [3, 4].
The configuration of the practical 150 MeV RTM [2] isshown in Fig. 1 and 2. It is shown that about the size ofthe main body is approximately W4m  L1.5m  H2m ,excluding the injection line where the gun is located (notappeared in Fig. 1). It shoud be pointed out that one of theunique features of this type of RTM lies in the design ofthe main (180° bending) magnets which are the biggest
components in the RTM. In contrast with its big body,they have a narrow gap, only 10 mm in height between
the upper and lower poles. This method continues on thedesign of high energy RTMs. Assuming 220 MeV
required for the new RTM [3], it would increase at about
20 ~ 30 % on its total size. The magnetic field of the main(bending) magnets has been set to around 1.2 Tesla, not soFigure 1: Cross sections of conventional 150 MeV RTM.

DESIGN CRITERIAThere are some constrants on a design of RTM [5]. The
basic relationship shown below must be strictly fulfilled.

E(MeV) ____________________________________________*Work supported by Strategic Funds for Promotion of Science andTechnology (Grant No. 066)
#tosihori@iae.kyoto-u.ac.jp- 1308 -   (cm )2.096

B(Tesla ) .

(1)Where E: energy gain per turn, : wave length offrequency, B: magnetic field strength of the main magnets,
and  (integer) is a characteristic parameter of RTM
which indicates how much the circulating path elongatedfrom the previous lap (Ln) to the next (Ln-1), normalizedby , thus  = (Ln  Ln-1)  . Violation of Eq.1 results inthe lost of synchronization between accelerated electrons
and accelerating RF fields in the linac. As alreadymentioned, typical parameters of the 150 MeV RTM are;
E = 6.0 MeV, B = 1.2 Tesla,  = 10.5 cm, and  = 1.
Energy 150 MeV is obtained after 25 times ofacceleration. The simulation and experimental results of
the 150 MeV output beam qualities were already reported
in detail [5]. There are two directions towards thedesigning of higher energy RTMs, one is to increase theenergy gain per turn (E), and the other to increase thenumber of circulation with E little changed.
The -value is normally set to the minimum digit (=1),
and it is rare to use 2 since we lose phase acceptancestep by step upon the increasing of -value. The next Eq.
2 shows the relationship between -value and the width ofthe obtainable stable phase region, in other words phase
acceptance.

0  tan  s 2.

(2)

Fig. 3). The phase drift occurs continuously from the first.
The simulation results of 150 MeV RTM tell us that theinitial phase stable region 32° is decreased to ~20° after
25 turns of acceleration. Suppose electrons be accelerated
further, the phase stable region should be decreasing more,
and some electrons would inevitably be dropping out one
after another.

Figure 2: Transmission efficiency in the case of 300 MeV
acceleration with 6 MeV/turn.

It suggests that the widest phase stable region 0<s<32° isobtained with =1, and decreasing to 0<s<18° with =2.
Even the widest phase acceptance at =1, it is rather
narrow when compared with the linac’s. Fortunately thisunique characteristics reflect on the good beam quality
which is inevitable to LCS, and also is well matched to
the RF gun which generates short bunches 10 ps. This
gives us a occasion to design a high energy RTM under
the condition =2, as actually shown later. On thecontrary the noticeable demerit is obvious, namely RTM
is inadequate to produce high current beams.

THE WAY TO HIGHER ENERGY
Required electron energy for NMDS is expected atabout 220 MeV, when detecting Uranium. It is preferable,
however, to have the capability to accelerate electrons
further, 250 MeV for instance, when considering thepossibility to investigate other nuclei.

Upgrading the 150-MeV Version
It is already reported that what will happen when wecontinue accelerating electrons over 25 turns by theexisting 150 MeV RTM [5]. The result is clear in thefollowing survival plot up to 50 turns (see Fig. 2).
After 25 turns, a part of the beam gradually dropped out
until 30 turns. However, after that the survival rate keeps
in constant till ~40 turns. Thus three fourths of the beam
get more energy than 220 MeV at 37 turns of acceleration.
The reason of this 25% beam loss between 25~30 turns
can explain by the phase drift of the circulating beam (seeFigure 3: Phase drift of survival electrons until 50 turns(300 MeV) of acceleration with 6 MeV/turn.
It is well known that the field gradient in the bending
magnets, which is necessary to produce the vertical beamfocusing force, causes this phase drift. There are no QD
magnets in this design, therefore one cannot remove then-value from the main magnets. However the strength ofn-value, fixed to 0.14 Tesla/m at present, might beadjustable.
When increasing E from 6.0→7.2 MeV and B from
1.2→1.5 Tesla, one can obtain 220 MeV beam after 30
times circulation. Simulations under the initial condition
Ein = 8.3 MeV with 5000 particles distributed in the phasespace of normalized x,y(rms) = 150 mmmrad were- 1309 -executed. The survival rate is 714/5000, and distributionsof these particles are shown in Fig. 4, where rms (x, y) =(0.20, 0.16) mmmrad and (E, ) = (0.3 MeV, 5.3deg). These values are equivalent to the acceptance of
this RTM, which are 5~10 times larger than the 150-MeV
conventional RTM’s.

sufficient for the acceleration of single bunch from the RFgun. One example of the design parameter is alreadyreported [3]. The phase acceptance is checked by thesimulation using a zero transverse emittance beam on thelinac axis. The obtained phase stable region is ~6° after
18 turns of acceleration at 12 MeV/turn. Less than 10° of
this phase stable region might be insufficient.
Alternatively the combination of much more enrgy gain,
15 MeV/turn, with reducing turn number to 15 is anothercandidate. In this case 9° of phase acceptance is reserved
under the assumption B=1.5 Tesla for the main magnets
and S-band linac. The magnet gap is narrow only 10 mm,
so no difficulty is expected for realizing 1.5 Tesla.
However, so far as RTM adopts the weak-focusing
system in the bending magnets, it is found by the recent
simulations that the parameter =2 is in general notsuitable to the purpose of increasing beam energy,
because the influence of this phase drift is relatively toolarge compared with the case =1. The parameter =2
might be only allowable for the circulation less than 15,
which is equivalent to four times of the synchrotron
oscillation in the RTM.

Acceleration by L-band (1.3 GHz) Linac
There was a design report on the 300 MeV RTM, 14.3
MeV×21 turn, using L-band (=23 cm) linac [6]. The
simulation results have not so much difference from theRTM’s with S-band linac. Emittances of the 300 MeV
beam, rms (x, y) = (0.5, 0.2) mmmrad, are about oneorder larger than those of 150 MeV beam. Main reason
will be derived from the large aperture of the linac bore,
10 mm  S-band vs. 22 mm  L-band. When a small
emittance of RF gun is assumed, the emittance of output
beam might become somewhat smaller. The effect of nvalue is also noticeable, only 10° phase acceptance isobtained for the case =1. These parameters are worth toconsider in NMDS, suppose we might take into accountthe compensation of heavy beam loadings.

REFERENCES

Figure 4: Distribution of survaival particles at 220 MeV.

High Energy Gain per Turn (=2)
Assuming that RF gun would be employed, there arises
another possibility to adopt =2 on the sacrifice of thephase acceptance becoming narrow. The stable phaseregion 18° of S-band is equivalent to ~18 ps, which seems[1] H. Ohgaki et al., “Conceptual Design of a NMDS Based onthe Neutron/Gamma-ray Hybrid Approach”, Proc. 2010
IEEE Int. Conf. on Technologies for Homeland Security,Waltham, MA, USA, Nov. 2010 p.525.
[2] T. Hori et al., “Improvement of 150 MeV Racetrack
Microtron”, PAC’91, San Francisco, May 1991, p.2877.
[3] R. Hajima et al., “Compact Gamma-ray Source for Nondestructive Detection of Nuclear Material in Cargo”,
IPAC11, San Sebastian, Sept. 2011, THPS098, p.3663.
[4] T. Hori et al., “Racetrack Microtron for Nondestructive
Nuclear Material Detection System”, THPPR066, IPAC12,New Orleans, May20-25 2012.
[5] T. Hori, “R&D of Injector Microtron for Compact SRRing”, SOKENDAI Doctoral Thesis (in Japanese), March,
2002.
[6] T. Hori et al., “Report on High Energy 300 MeV Microtron
as an Injector for GeV-class SR Ring”, Internal Report ofSumitomo Heavy Ind., Ltd., March 1997.

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