PRACTICAL RADIATION-CONTROL AT KEK
Takenori Suzuki, Radiation Science Centre & J-PARC Project Division
High-energy Accelerator Research Organization (KEK),Tsukuba, Ibaraki, 305-0801, Japan
Abstract
At a high-energy proton accelerator facility,
radioactivities are induced in the accelerator components
due to the loss of primary beams; these radioactivities
produce an area of high radiation. From experience at the12-GeV proton accelerator of KEK, due to beam loss atthe septum magnets, a high radiation level was observed.
For hands-on maintenance, these magnets must be cooled
down for a few weeks or months in order to reach theworking exposure radiation level employed at KEK. The
major exposure of radiation workers at a high-energy
proton accelerator facility is due to maintenance work inthe beam-line tunnel after the beam has stopped.

1 INTRODUCTION
The radiation-control scheme during operation at highenergy proton accelerator facilities is different from thatduring non-operation. During operation, a loss of the
primary beam causes interactions with the accelerator
components (magnet, beam duct, shielding andcollimator) inside the beam-line tunnel, and, then,secondary particles, such as neutrons, are produced. Theradiation level outside of the accelerator facilities
increases due to these neutrons. The radiation-shielding
design aims to reduce the radiation level to below thestandard determined by the Government.
The radiation level inside laboratories, where radiation
workers and experimentalists work during operation, must
be reasonably lower than the standard. The radiation levelat the boundaries of the radiation-control areas for nonradiation workers on the site and outside the site boundary
for the public must be designed to be much lower than thestandard determined by law, since, in Japan, the influenceof radiation on the environment for the public must be
controlled to be quite low.
On the other hand, during the non-operation period, theradiation-control scheme to restrict maintenance inside
the beam-line is important. The radiation exposure caused
by maintenance work inside the beam-line is the majorpart at high-energy proton accelerator facilities.
Radiation emitted from induced radioactivities due tobeam loss in the accelerator components is the major
cause of exposure. If the radiation level is quite high afterstopping the beam, a long cooling period must benecessary, since hands-on maintenance can not be done,
because of the high radiation level.
Although it is important to reduce the beam loss during
beam acceleration by developing beam-control
technology, any unnecessary parts of the beam must bescraped off in order to improve the beam quality. It is also
necessary to shape the size of the beam so as to meet thepurposes of the experimental users. In such a case,activation of the scraper and collimator becomes
extremely high. Moreover, since magnets installed along
the entire beam-line are used to tune and control the beam,a small portion of the primary beam is always expected tohit the magnets. Hence, a small amount of activation ofthese magnets can not be avoided. In order to conducthands-on maintenance, the beam-control technology must
be improved so as to reduce the radiation level of theaccelerator components. It is also important to localize
activation in the limited space, or limited materials, bydesigning shielding and the beam optics. A maintenancescenario of the highly activated components must beconsidered in detail before constructing the accelerator.
This paper gives an outline of the radiation-control at
KEK and activation of the accelerator components, whichcauses trouble for maintenance inside the beam-linetunnel.

2 OUTLINE OF RADIATION-CONTROLAT KEK
Radiation-control at high-energy proton accelerator
facilities can be summarized as follows: (1) radiation
dose-rate control, (2) access control, (3) radioactivesubstance control, and (4) radiation-exposure control.
These control plans are different during and after
operation, and can be outlined as follows:(1) The radiation dose rate is continuously monitoredaccording to the standard of the dose rate given in Tables
1 and 2; many monitors are placed at the boundaries andradiation-controlled areas. The radiation-control centre
conducts a detailed survey of the radiation-controlledareas once in each cycle. During operation, the majorradiation which influences exposure is high-energy
neutrons, which penetrate thick concrete shielding. Theenergy of neutrons is distributed from a low energy toTable 1: Dose Rate for Various Areas (Japanese Law)
Radiation-controlled Area
1.3mSv/3m
Working Area within Radiation1mSv/w
Controlled Area
Within Site Boundary
250µSv/3mNote: The dose rate per one hour can be calculatedusing 500h/3m and 40h/w.
Table 2: Dose Rate for Various Areas (KEK)
Radiation-controlled Forbidden>100mSv/hArea
Restricted> 20µSv/h
GeneralWarning Area
Non-Controlled Area within Site
At the Site Boundary> 1.5µSv/h> 0.2µSv/h≤ 0.2µSv/h≤ 50µSv/ha high energy of about 100 MeV. Besides, muons areobserved downstream of the beam dump and target, if
they are not injected towards the earth, and causes
exposure. It is important to confine these radiations inside
the shielding and earth.
(2) Access to the primary beam-line tunnel must berestricted at high-energy proton accelerator facilities.
Before maintenance work, an application form must besubmitted in advance. On the form, the details must bewritten, such as the name of the person in charge, thework place, the contents of the work and prediction of theworking hours. When radiation workers enter the beamline tunnel, they must carry a personal key assigned toeach individual and an alarm dosimeter. The personal key
notifies the accelerator control centre about who enters
the beam-line tunnel and what kind of work is being
done; also, the key prohibits operation of the accelerator.
During operation, access to the beam-line tunnel isprohibited, and any change in the status of the entrance
door or the personal key terminates operation of theaccelerator by an interlock system.
(3) In the control of radioactive substances, attention
must be paid to articles like tools, accelerator components,and shielding, which are left inside the beam-line tunnel
during operation. Any radioactive articles must not be
brought out from the beam-line tunnel. When thesearticles are needed to be removed, a gate monitor is used
to measure the activities automatically. If the gate monitor
can not be used because of the size or other reason, theradiation-control centre must be notified to survey thearticles.
(4) Exposure control is conducted in accordance withthe standards (Table 4) of KEK, which are designed to belower than that of government law (Table 3). Furthermore,
the working levels for a day, a week, and a year areemployed for practical radiation-exposure control (Table5), which are lower than the KEK standard; this isintended to prevent large exposure within a short period.
In most cases, exposure problems occur during
maintenance work inside the beam-line tunnel after
stopping the beam. Therefore, before maintenance work,
the status of the radiation level around the beam-line ismeasured, and a radiation map is posted in front of theentrance gate to notify radiation workers of the dose map
in advance. Also, warning signs are posted at places ofhigh radiation levels along the beam-line. If it isunnecessary to come close, signs notify workers to keepaway from the place. On some occasions, it is necessary
to do maintenance work on the highly activatedcomponents, on the order of several tens of mSv/h. In
such cases, in order to reduce the exposures and to avoid
large doses to specific individuals, a working plan mustbe submitted in advance to deal with the situation.
In the next chapter, the radiation levels of theaccelerator components observed in the 12-GeV-PS andthe related facilities are introduced.

Table 3: Dose Limits for Radiation Workers(Japanese Law)
Radiation Worker(1) 100mSv/5y and(2) 50mSv/yWoman
5mSv/3m+(1)&(2) above,2mSv/ gestation
Table 4: Dose Limits Employed at KEK
Radiation Worker
20mSv/yWoman
6mSv/y, 2mSv/3m
General Public
150µSv/3m
Table 5: Working Levels for Daily Control
ManWoman
0.5mSv/d
1.0mSv/w
7.0mSv/y

0.3mSv/d
0.5mSv/w
2.0mSv/y

3 RADTION LEVELS DUE TO INDUCEDRADIOACTIVITIES AT THE 12-GEV-PS
AND RELATED FACILITIESThe KEK12-GeV-PS has two modes: fast extraction
and slow extraction, because of user requests. For
neutrino experiments, the fast-extraction mode is used totransport 3×1012pps of 12-GeV protons to an Al target(65cm×2cmφ). When this beam is extracted from the PS,
about 10% of the beam loss occurs at the septum magnets.
Since the slow-extraction mode has a much larger beamloss than the fast-extraction mode, less of the beam thanin the fast-extraction mode is extracted to keep the same
beam loss as that for the fast-extraction mode. It isreported that extraction loss induces radioactivities and, inorder to conduct hands-on maintenance, the loss for any
mode of extraction must be kept lower than about 400W
[1,2]. Proton numbers equivalent to the 400 W beam loss
are about 0.2×1012pps (32nA), and the maximum dose
rate around the loss point is about 30-40mSv/h, with anaverage of 10mSv/h [1,2].
The recent record (Dec. 2002) of the dose rate due to aloss of about 580W after a half-day cooling showedabout 80mSv/h at the highest point, 30~40mSv/h at 30 cm
away from the point, and then 10~20mSv/h around thesehigh-radiation areas (Fig. 1). After 3 weeks of cooling,
the dose rate decreased down to 15mSv/h at the highest
point and 1mSv/h near to the area (Fig. 2). Considering
the exposure limit at KEK, a few minutes of work isallowed near to the highest point in a day, and a few daysof this kind of work are allowed in one week. Thissuggests that it is necessary to cool down theradioactivities significantly before maintenance work.
Moreover, many people must participate in such radiation
work in order to avoid a large dose to a specific person,and distribute the exposure among many radiation
workers. To achieve this kind of radiation work, themaintenance procedure must be planned in advance.

25/12/2002

10~20
Septum80CD<1PS-MR

10~20 (mSv/h)
E<1EP1

Fig. 1: Dose rate around the septum magnets of the 12GeV-PS for the fast-extraction point. This was measured
on December 25, 2002, after 7 hours of cooling. The
beam loss during operation was estimated to be about 580
W.
14/01/2003

5~10
Septum<1PS-MR15CD

1~3 (mSv/h)
E<1EP1

Fig. 2: Dose rate measured on January 15, 2003 after 3weeks of cooling the extraction point shown in Fig. 1.
There are two Counter Halls: the East Counter Hall(ECH) and the North Counter Hall (NCH), which utilize abeam from the 12-GeV-Ps. The characteristics of NCH
are discussed in this section. NCH has a primary beamline called the “EP1 beam-line”. This beam-line is utilized
in two ways: (1) transport line to the Target Station,
where a proton beam interacts with an aluminium targetto produce neutrinos towards Kamiokande; (2) productionof secondary particles, like π and K particles, which areused for physics experiments in the K5 and K6
experimental areas of NCH. In the case of (1) above, aprimary beam of 3×1012pps extracted by the fastextraction mode from 12-GeV-PS only pass through theEP1 beam-line. In this case, the beam loss along thebeam-line is small, resulting in a dose rate of 1 ~ 5mSv/hafter operation. In the case of (2) above, a primary beamof 1×1012pps is extracted by the slow-extraction mode.
Two production targets (PT, 6cm×2cmφ) can be installedon the EP1 beam-line at the T5 and T6 positions. After
two weeks of irradiation for physics experiments, the dose
rate of the target becomes on the order of 1Sv/h, evenafter 3 days of cooling. Usually, the beam-line
components are highly activated to an order of 50100mSv/h. Since the space of the beam-line tunnel formaintenance is quite small, it is strictly restricted to enter
the tunnel.
Since, from an exposure point of view, access to theprimary beam-line is prohibited, the EP1 beam-line tunnel
comprises a two-story structure: a primary beam-line
tunnel above and a service tunnel below; these tunnels are
separated by 2m thick concrete. From the service tunnel,water supply and electric power can be operated. If arepair of beam-line components, such as a magnet isnecessary, concrete shielding can be removed from theceiling of the primary beam-line tunnel; bolts and nuts
can be handled far from the top; these components can behandled from the top and transferred to a maintenancearea. Thus, exposure due to maintenance work can beeffectively avoided.
The neutrino beam-line is located downstream of theEP1 beam-line. The primary proton beam is transported
through these two beam-lines, and finally the beam isapplied to an aluminium target for neutrino production.
After 30 days of irradiation, the dose rates just after theirradiation are expected to be 10~100Sv/h. After coolingfor about 100 hours, the dose rate becomes about
100~300mSv/h, since Na-24 (half life 15 hours) inducedin the aluminium target decays out quickly. Since thetarget area is shielded with thick concrete shielding andthe influence of the high radiation is quite small,radiation workers inside the target station do not receive asignificant amount of exposure from the target. If
unnecessary, they do not come close to the area.

4 SUMMARY
Usually, the primary beam-line at high-energy proton
accelerator facilities is covered by thick shielding and,thus, the dose rate at the circumference is designed toremain lower during the operation. From the viewpoint ofradiation-control, the dose rate of high-energy neutrons isimportant during operation, and the exposure is kept lowby the shielding design. Generally, exposure caused bymaintenance work inside the primary beam-line tunnel
after operation is of great concerned at high-energy
proton accelerator facilities, and a large part of theexposure record of radiation workers
is due tomaintenance work in the beam-line tunnel. When a beam
is extracted from the 12-GeV-PS, beam loss occurs
around the extraction septum magnet, resulting in a highradiation area. The dose rate is also high around the target
region in the utilizing facility. It is difficult to do handson maintenance around these high dose areas. In order toreduce the exposure due to maintenance work, it isimportant to consider a complete cooling-off period, andthe maintenance procedure must be planned in advance.

5 ACKNOWLEDGEMENT
The author wishes to thank professor Yamane for
helpful discussions and his papers to write this manuscript.

6 REFERENCES[1] I.Yamane, Radiation Protection Measures for theMaintenance Service of JHF 3-GeV Ring, KEK Internal
98-7 (JHF-98-1) (September 1998)[2] I.Yamane, On the Limit of Acceptable Beam Loss fora High Intensity Proton Ring, KEK Report 2001-21(February 2002).