Electron Beam Sterilization
SystemSUSUMU URANO* 1
IKUO WAKAMOTO* 1
TAKASHI YAMAKAWA*2

Nowadays, electron beam (EB) irradiation method is attracting attentions as useful medical tools sterilization,
which has the advantages such as safety, zero emission and high speed processing. In this trend, in addition topresent main object (the low-density medical tools: ex. operation dress), other various objects are strongly desiredthat will be applied of EB sterilization method. Mitsubishi Heavy Industries, Ltd. (MHI) has succeeded in realization of high efficiency continuous EB sterilization processing for high-density medical tools, vessel surface, and soon. Further, compact EB systems are also developed, which can also be applicable for above mentioned objects.
system so as to replace the existing sterilization apparatus are required. This paper reports MHI's newtechnology and system of EB sterilization for such demands.

1. Introduction
Conventional sterilizing method of medical tools is
mainly the use of ethylene oxide gas (EOG), gamma ray,high-pressure steam etc. However, the sterilization industry is undergoing drastic changes because of the strict
regulations trend for EOG which is main method now,and the mounting demands for zero emission and highspeed treatment. Accordingly, recent attentions areturned to electron beam (EB) method which has the excellence for above mentioned subjects.
MHI started various research and development, andhas manufactured various electron beam sterilization
systems since 1996 (1) . They are designed to sterilize
mainly low-density disposable medical tools (such as operation dress), which are strongly requested at thebeginning. Subsequently, the requests for suitable system about various other objects (e.g. other medical tools)
have been increasing as the recognition of this means
spread.
In order to correspond to this trend, the efficient irradiation technology for each object and the downsizedElectron beam irradiation characteristics depend on
the absorbed dose and the acceleration energy.
2.1 Absorbed dose
As a result of the interaction between electrons andgenerated radicals, microorganisms are dead by a process of DNA chain cleavage. The amount of interaction
between electron beam and object is called the absorbed
dose, defined as the energy absorbed per unit mass ([J/kg] = [Gy]). The logarithmic value of survival fraction is
proportional to the absorbed dose. The absorbed dose
required to reduce the survival fraction to 1/10 is called
the D value, and a specific value is set for each microorganism. So, the required absorbed dose increases inproportion to the target reduction level.
2.2 Acceleration energyFig. 1 shows the depth dose curve. Relations between

4: 150 keV
: 300 keV
: 500 keV
: 750 keV
: 1 000 keV

6Absorbed dose (-)

Absorbed dose (-)

82. Electron beam irradiation characteristics

420123453210

Depth (mm)

: 1 MeV: 3 MeV: 5 MeV: 7.5 MeV
: 10 MeV

204060Depth (mm)Fig. 1 Depth dose curve
Depth is converted to specific gravity of 1. Relations between depth and dose depend on acceleration energy.

*1 Hiroshima Research & Development Center, Technical Headquarters*2 Hiroshima Machinery Works

1Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

Irradiation condition(absorbed dose)

100KGy30KGy
10KGy
3.0KGy1.0KGy0.3KGy0 Gy (control)MarkerSurvival fraction (-)

100

10-2

10-4
2 700
2 000
1500
1 000700
DNA500length400300(base)200
100 5010-60123Absorbed dose (kGy)(a) Survival curve(b) DNA chain shortening

Fig. 2 Correlation between survival fraction and DNA chain shortening
Correlation between survival fraction and DNA chain length on absorbed dose has been confirmed with E.coli irradiated by electron beam.

Electron beamAbsorbed doseWith rotation(a) Concept of dialyzer(b) Test system

Without rotation(c) Results

SmallLarge

Fig. 3 Evaluation result of absorbed dose in cylindrical container
Dose uniformity is confirmed by rotating the cylindrical container.

depth and dose depend greatly on the acceleration energy, and hence adequate setting according to theindividual object is necessary.
2.3 Necessity of optimum system
Surplus dose leads to the decrease of irradiation efficiency and of strength and the coloring of object. Excessenergy, e.g. high energy electron beam for sheets surface sterilization, leads to the decrease of irradiation
efficiency and the increase of size and cost. Besides, upper limitation of energy (up to 10 MeV) to avoid activationmust be considered.
So, construction of optimum irradiation system isgreatly important, which can apply a necessary dose
uniformly to individual items with adequate electron
beam energy.

then uniform irradiation is difficult by previous EB
irradiation technique that irradiated from one or bothdirection.
Noticing that dialyser is rotation symmetry, MHI
developed a uniform irradiation technology inside itwith the combination of EB irradiation from peripheral direction and the rotation them around the axis
under conveyance. To confirm the validity of thismethod, an acrylic cylindrical container modeling dialyzer is irradiated by EB which was filled with agar
containing a sensitive pigment which color changes
depending on the absorbed dose, and then the degreeof discoloration on the cross-section was evaluated.
Fig. 3 shows that the evaluated result of absorbed
dose in the cylindrical container is uniform. The validity of this method has been also verified by theevaluation of absolute values of absorbed doses by filmdosimeter, and of the fluidity of liquid in a cylindricalcontainer.
Henceforth, MHI intends to apply this for high-density medical tools sterilization system with our various
physical distribution technologies.
(2) Inner surface of vessel
To sterilize the inner surface of vessel, such as
storage containers of medicine or the like, the electron beam trajectory must be controlled suitably for

3. Example of optimum irradiation technology
MHI has studied the electron beam sterilization characteristic, e.g. the correlation between survival fraction
Fig. 2and DNA chain shortening (Fig.
2). Examples of developed optimum irradiation technology, for highly
requested objects, are shown below.
(1) High density medical tools
Most dialyzer contains physiological saline withhollow fibers inside. Therefore, the density is higher
than present main objects for EB sterilization, andMitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

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Incident ofelectron beam

Ti membrane

Incident ofelectron beam

Ti membrane
Atmosphere

Applieddirection ofmagnetic field

Magnetic field applied
Magnetic field not applied(a) Electron beam trajectory in airElectron beam

Electron beam

0.6 (-)

1.0 (-)

Applieddirection ofmagnetic field

1.0 (-)
Magnetic field not applied

1.0 (-)
Magnetic field applied(b) Absorbed dose ratio on container inner surface
Fig. 4 Magnetic field deflecting effect of electron beam in air
Uniformity of absorbed dose at inner surface of vessel can be improved by controlling electron
beam trajectory with applied magnetic field.
Table 1 Line-up of microwave acceleration type electron beam system
Item

OthersPerformance

Concept

Small modelMedium model

Large model

Compact

On-lineHigh power

Energy(MeV)1 - 10

1 - 1010

2-4

3-6

14 - 31

Power(kW)

Scanning width(cm)30 - 50

30 - 50

30 - 80

Estimated throughout(kg/h)

130 - 230

200 - 350

1 000 - 1 800

C-band standing
wave typeS-band standing
wave typeS-band travelingwave type

50 - 300

70 - 350

550

Acceleration system
Required area(m2)Note: Estimated throughout is converted to 25 kGy application in low density medical tools.

each object.
Seeing that the trajectory of an electron beam in
air can be controlled by a magnetic field, MHI developed a technology for applying a magnetic field
simultaneously with irradiation.
Fig. 4 shows the deflection effect of a magnetic
field on an electron beam in air. This Monte Carlosimulation results show that the spread of the electron beam trajectory in air caused by scattering withgas can be controlled by magnetic field. Actual ratioof measured absorbed dose in inner surface of vessel
also shows the uniform irradiation. The outer surface can also be irradiated uniformly by thistechnology.
In Future, MHI will be developing sterilization system for various vessels of medicines, foods andbeverages along with appropriate objects handlingtechnology above mentioned. Also, electron beam handling technology will be further developed for gas

treatment such as air sterilization, exhaust gas treatment (e.g. VOC decomposition), and so on.

4. Electron beam sterilization system
MHI is promoting development of a compact EB system to meet the space-saving demands of users.
4.1 Microwave acceleration system
Table 1 is a line-up of EB products for using LINAC
type acceleration guide and microwave electric field.
Development in the high-energy region (1 to 10 MeV)
has already been completed, and the line-up will be further expanded over a wide energy range depending ondemands. A high-power model has already been reported (2) , so more recent models are briefly describedbelow.
(1) On-line typeThe on-line type electron beam irradiation system
is intended for use in irradiating intermediate packaged products in mid process on the productions line.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

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Final package irradiation

On-line irradiation

Production
manufacturing

Individual
Inner package
Outer package
product package
Fig. 5 Electron beam irradiation systems for use at various stages of production
Suitable irradiation systems can be selected depending on the type of product and stage of manufacture.
Movableshielding door

Electron beam system

Power source

Smallconveyorsystem

Fig. 6 On-line type electron beam irradiation system
Electron beam is irradiated in shielding to individual items which istransferred by conveyor system directly connected to productions line.

Electronbeam system

Fig. 7 Wide-range electron beam irradiation system
Compact structure containing all components in shielding
is realized.

OthersPerform-ance

Table 2 Line-up of electrostatic acceleration type electron beam system
Item

Ultra-low energy

Low energy

Concept

Surface layer treatment

Surface treatment

80 - 120

120 - 300

Energy(keV)

Output(kW)1536 or less

Irradiation width(cm)200 or less

200 or less(m/min)40 or less(at 100 keV)68
(at 300 keV)Electrostatic type

Electrostatic type8

15

Processing speed
Accelerating system
Required area(m2)Note: Processing speed is converted to 25 kGy application.

Fig. 5 shows methods of irradiation at various stagesof production. This system is intended to treat in minimum packaged state as required by end users.
Therefore, this system requires relatively less acceleration energy and less power compared with previous
methods which treat final package.
Fig. 6 shows the system configuration. Significantly
small required area is realized with newly developed
item, the standing-wave type acceleration guide(suited for less power) and the low curvature conveyor.
Further, all the components, including the shielding
walls, are incorporated in a combined structure, so it
is easy to adapts the modification of the productionlines. This system will be supplied mainly to users
manufacturing sterilized products, as a substitute for
their existing facility.

(2) Ultra-small C-band accelerator
By applying the super-precision MHI processing
technology, the world's first commercial systems havebeen developed using C-band (5.7GHz) acceleration
guide, which are much smaller than the existing Sband ones(3). In this system, maximum 10 MeV EB isrealized in about 60 cm length acceleration guide. In
future, in addition to application in EB sterilization
for smaller scale users, it is planned to other applications, e.g. radiation therapeutic equipment, by taking
advantages of small size and light-weight.
4.2 Electrostatic acceleration system
Table 2 is a line-up of EB products using an electrostatic field(4).
In this system, wide electron exposure unit has already been developed. Fig. 7 shows the product image.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

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The feature of this system is as follows that miniaturization is realized by installing all components in its own
shielding, long-life is attained with our original doublewindow structure and conservativeness is improvedusing cartridge type maintenance parts.
In future, it is planned to apply this system in gas
disposal and surface modification, in addition to the sterilization of packaging and others.

References(1) Ando, K., An Introduction to Radiation Facilities and Equipment for Their Own Company in Japan, Bokin Bobai Vol.30
No.9 (2002) p.609(2) Hisanaga, N. et al., Developments of High Power Electron
Beam Sterilizer (10MeV 25 kW), Mitsubishi Juko Giho Vol.33
No.6 (1996) p.388(3) Miura, S. et al., Developments of Small C-BAND standingwave accelerator structure, LAM25 13P-09 (2000) p.305(4) Yonago, A. et al., Ultra Low Energy Wide Electron Exposure
Unit, Mitsubishi Juko Giho Vol.38 No.3 (2001) p.174

5. Conclusions
Efficient uniform irradiation technology has been established with the optimum object handling technology
and/or suitable EB trajectory control technology, which
are applicable to so-called upcoming objects, e.g. highdensity medical tools, pharmaceutical and foodcontainers and gases.
Further, compact EB sterilization systems are developed, which can also be applicable for above- mentionedtechnology. So, MHI line-up of EB sterilization system
is fully realized for various demands, now.
In future, applications of this technology will be further promoted in the field of crosslinking, environmental,medical engineering, etc. in addition to sterilization, so
as to contribute to widespread technical innovation.

Susumu Urano

Ikuo Wakamoto

Takashi Yamakawa

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.5 (Oct. 2003)

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