TIME VARIATIONS OF EXPOSURE RATES FRGr1 RESIDUAL
RADIOACTIVITIES PRODUCED IN SHIELD CONCRETES
K. Kondo, H. Hirayama, S. Ban, tt Taino and H. Ishii
National Laboratory for High Energy Physics
Oho-machi, Tsukuba-gun, Ibaraki-ken, 305, JapanRESULTS AND DISCUSSION

ABSTRACTThe exposure rates due to the residual activities
is calculated by the following equation:

The concrete constituent (magnetites ore, pyritesore, marble, gravel and Portland cement) were irradiated by secondary particles at the slow extracted beamline of the KEK 12 GeV proton synchrotron, and thelevels of photon exposure rates from these constituentswere estimated on the basis of the saturated activities
for individual nuclides produced. It is suggested thatthe use of marble concrete in the inside walls at highenergy accelerator tunnels can reduce markedely theexposure doses to the accelerator operating crew andmaintenance workers, compared with heavy and ordinary
concretes commonly used.

X= l: fi· Ai • (1- e-Ai.Tirr). e-Ai·Twi

wherexexposure rate [ (R/h) g-l perphoton
seconddary particles: 2 30 MeV /s)l at 1em from the sample,
specific gamma-ray constant (R/h · Ci-) at
1 em for the ith nuclide,saturated activity of the ith nuclides[Ci· g-1 per(secondary particles: ~ 30 MeV/s)],decay constant of the ith nuclide
cooling time,r.l

INTRODUCTION
The tunnel shielding of the slow extracted proton
beam line of the 12-GeV proton synchrotron at theisNational Laboratory for High Energy Physics (KEK)
composed of magnetite heavy concrete. After about six
year's operation, the induced radioactivities in theconcrete have been one of te major causes of radiation
exposure to the operating crew and maintenance workers,
and is so great as to sometimes restrict their work.l)
The residual activities produced in an accelerator
are widespread and in practice it is very
enclos~redifficult to control the radiation dose from activatedconcretes, while control of the photon
shieldingexposure rates from locally hot machine components isaccomplished by shielding and/or perio(:licusuallyreplacement. Thus it is very important in designing
accelerator shielding to choose theenergy
highshielding materials which are less susceptible toactivation.
In this paper, Magnetite ore, pyrites ore, gravel
cement and marble were irradiated by secondary particles produced by the primary beam at KEK 12-GeV proton
synchrotron, and the time variations of exposure rates
from these concrete constituents were examined exA discussion on the optimum shielding
tensively.
materials at high energy accelerators is given on thebasis of the above results.

The published values of AiZ) were used in thecalculation. As an example, Fig. 1 shows the time
variation of exposure rates thus calculated for theconstituents irradiated by the secondary
concreteparticles for one week. Except .for the cement, theexposure rates rank in the following order; magnetite
regardless ofore > gravel > marble,
ore > pyrites
irradiation time and decay time. The exposure rates ofthe marble are about one-tenth of those of the graveland one-hundredth of those of the magnetites andpyrites ores at Tw ~ 10 4 hours. Furthermore the ratios
of exposure rates of irradiated concrete constituentsto those of, the marble become greater with an incresein irradiation time and in decay time. Fig. 2 shows
the time variations of exposure rates for various typeof concretes, here the mixing ratios of aggregate to
cement were assumed to be (by weight): 11.5 forpyrites heavy concretes, 5.25 forandmagnetiteordinary concrete and 5.67 for marble concrete. These
ratios are commonly used in making concretes. As Fig.
2 shows, the overall variation of exposure rates are
essentially similar to those for individual aggregates.
10"'.----------,--------~---------.

EXPERIMENTALYear lrrodiation
Secondory Particles

The powdered (40- 80 mesh) concrete consituentsof magnetites ore, pyrites ore, gravel, marble andPortland cement are packed in acrylic case (25 mmdiameter, 2 mm thick), and te stacked samples areirradiated by secondary particles at EP2. The samples
were irradiated for 201.5 hours at the position on theEP2 floor about 5.5 m downstream from the K2 target (Pt
1 em diameter, 4 em long) and at about 30° to thedirection. The 12-GeV proton beam intensity
beamhitting the Pt target was about 5.49 x 10 11 protons/s
during the cycle of machine operation. The flux ofhigh energy hadron (2 30 MeV) was determined to be(1.01 ± 0.07) particles/cm- 2 7 s-' from the 22 Na yield in
the irradiated Al disc (2.0964g), using the value of9.85 mb for the spallation reaction of 27 Al. Highenergy neutrons are considered to be principal particles for ·this reactions. The measurement of radioactivities and calculation of saturated activities
were made in a similar manner as those previously
reported .2)

I

Cement A

2 Morble
3 Gra\11!!1E•~

10-11"~; 0-12~!

IOJ\0to'to'"" ..tO'1

Time After Snutdowo U-ioufl

Fig. 1373

Time variations of exposure rates [(R/h)·g- 1 per(secondary particle/s) from the concrete_constituents
irradiated by the secondary particles (E>30 MeV)
(1-yr irradiation).

t~"'r----------,----------r---------,(]) Magnetite conaele~ Pyrites concrete
G Ordinary concrete® Marble concrete {cementA)<S Marble coocrete lcementBI

E~0~a.

'00

10"""

concrete slob

withh=-:0a.

infinitebreadtha:aJQOcm

~

a:0.5

1 Week Irradiation
Secondary Particles!

IOL-----~----~----~----~~--~~--~~

05

10thickness

15

20of marble concrete

2530lcml

Time After shutdown (Hour)Fig. 2Time variations of exposure rates [(R/h)
g- 1 per (secondary particle/s) from heavy,

ordinary and marble concretes irradiated by
the secondary particles (E > 30 MeV) (1week irradiation).

Fig. 3

These results are suggesting that the use of marble
concrete for the inside walls of accelerator tunnels
can reduce markedely personnel doses of accelerator
maintenance workers. The complex concrete block ofpyrites and marble concretes shown in Fig. 3 wasdevised in order to reduce radiation doses from teactivated concretes as low as possible. Exposure rates
at the position P are shown in Fig. 3 as a function of
the thickness of marble concrete, by assuming that onlyMn is present and distributed uniformly in the pyrites.
concrete,and linear attenuation coefficients formarble and pyrites concretes are 0.185 cm- 1 and 0.274
crrr 1 for 0.83 MeV gamma-ray of 54 Mn. About 15 em
thickness is enough to shield the radiations fromheavily activated pyrites concrete. The concrete block
thus designed was made actually to .examine the compressive strength. Fig. 5 shows the cross-sectional viewof the complex concrete thus made. After standard
curing and aging to 28 days, its strength was morethan 235 kg/cm 2 3~ being about the same as those ofordinary and heavy concretes.

Fig. 4

Relationship between thickness of marble
concrete and exposure rates at theposition P

REFERENCES
1.

2.
3.

The complex concrete block composed of pyritesand marble concretes
left: pyrites concrete, right: marble concrete

374

Reports on Administrative Works at the Division ofRadiation & Safety Controls in 1980, KEK Internal
Rep. No. 81-12.
K. Kondo, H. Hirayama, S. Ban, M. Taino and
H. Ishii, Health phys., 46 (1984) 1221.
M. Kawada, Private communication, 1982, Dowa MineCo., Tokyo, Japan.