lX-1

RADIATION SAFETY PROGRAMS AT NIRS CYCLOTRON FACILITY
Takashi Maruyama and Tadashi Hashizume, Division of Physics,
Yasutaka Kurata, Zenji Murakoshi and Nobuo Okamoto, Division of TechnicalServices, National Institute of Radiological Sciences!.Introduction
The NIRS(National Institute0f Radiological Sciences) cyclotron has beenpricipally used for a clinical trial of fast neutron radiotherapy and for production of short lived radionuclides. For the use of cyclotron in safety,
health physicists should arrange information concerning on the protection ofman and his environment from unwarranted radiation exposure. Health physicists
should be also familiar with the following matters in order of efficiently meet
his reponsibilities and to provide the services required of him; (l)Familiarity
with the immediate program of accelerator operation: (2)Periodic surveys of allradiation produced: (3) Studies of induced radioactivity: (4) Evaluation ofshielding: (5) Proper use, calibration, and interpretation of radiation monitors
including personnel monitors: (6)Radiation safety training: (7) Knowledge ofrules and regulations concerning on personnel and environmental exposure: and(S)Public relations.
Amanual of radiation safety programs in the cyclotron facility were prepared by the working group of radiation safety which consists of health physicicts and staff of radiation safety division. The clinical trial of fast neutron
radiotherapy, production of radionuclides and basic studies of radiologicalphysics and radiobiology have been organized according to the manual of radiation safety programs.
II.Cyclotron Facilities
The cyclotron building is close to the building of hospital. In the cyclotron building, there are four irradiation rooms centering the cyclotron vault:
namely (l)general experimental room where the crinical trial of proton radiotherapy will be started in near future; (2) radionuclide production room; (3)biological irradiation room and (4) fast neutron radiotherapy room. A nuclear
medical investigation room is situated on the second floor. The fast neutron
radiotherapy area is centerized in the basement of cyclotron building jointed
to the hospital through an underpass and the nuclear medical investigation room
is connected with the second floor of hospital building. Therefore, the cyclotron building has three entrances,so that the check of entrants into the building is not quite easy. The entrance and exit at the basement and the second
floor is controlled with a 'Vip Gate System". This system can automatically record when and who passed through the gate by inserting a proper card in the machine. This system may make it possible to check the 'the entrance and exit.
The shielding design of cyclotron building was carried out using an attenuation data of ordinary concrete against D-T neutrons[l]. A maximum thicknessof concrete wall for the cyclotron vault room was 350 em and the ceiling thickness was 200 em. The cyclotron vault room joints with the operation room
through the maze as shown in Fig.lA on the basement. Dose equivalent distribution in the maze measured with a 'rem counter' and an ionization chamber isgiven in the Fig.lA. A maximum wall thickness of irradiation room except for
the general experiment room was 300 em and the thickness of concrete wall adjoining other irradiation room was usually 200 em. The doors of entrances into
the irradiation room consist of 200 em thick steel container filled withwater, so that time is consumed for opening and shutting the door. For thisreason, a maze was constructed in the radiotherapy room and the entrance door
was reduced to a 20 em thick concrete. This can carry on the fast neutron radiotherapy smoothly. The dose equivalent distribution in the maze is shown in Fig.
lB.
In the biological irradiation room, neutrons .<and gamma.c-rays were detected on the line prolonged the beam ~ransport when deuteron beams stopped
on beam shutters placed beflore and behind the switching magnet. These radia183

tions were released from the beam stoppers bombarded with deuteron beams. Aneutron shutter installed inside the concrete wall between the cyclotron vault
Fig lAand the biological irradiation room
·reduced dose equivalent from theleakage radiations as shown in Table
1. Table 1 gives dose equivalents
measured at a point in front of beamtransport for the biological irradiation when the beam stoppers and/or
the neutron shutter were closed.
The neutron shutter made it possible
for users to arrange their experimental materials in the biologicalirradiation room even when the cyclotron is operated for production ofradionuclides and proton therapy.
III.Monitoring System
An area monitor system, which
consists of a cylindrical ionizationchamber(l8cm diam. 27cm long), is
installed in the cyclotron vault roomand each irradiation room. These areavailable to indicate the presence ofradiation in high radiation area. Anda suitable survey meter is used tomonitor unsuspected radiation uponentry to a shut-down cyclotron orhigh radiation area. It shall be theduty of persons working in the cyclotron facilities to take with the survey meter into high radiation area after beam shut-down. Radiation level isdependent on substance and nature of agiven work. As an example, u,; ::a.ys of_dose Tate from induced radioactivity inFig.Z.
the radiation areas are shown in Fig.Z.
The radioactivity induced in air ischecked with a gas monitoring system
placed at the exhaust port of stack.
~
However, cooling air used for radio·:\"nuclide production and a part of air.,in the cyclotron vault room are tempo- &·-~- ---------- Biolo~~~ 13 ~~~v'"
rarily stocked in two special tanks (----.•...___
8 o.1 :each 100 cubic meters x 5 atm.) andEd ; 30 Mev--are released from the stack after about "~·----·---·-- -,._.::!
3 h r.
Radionuclide production ·--.
....
Ed; 1sMev
The environmental monitoring sys- "'terns are used to check leakage radiations from the cyclotron facilities.
Four monitors are situated .around thecyclotron building. The detector consists of BF 1 proportional counter withTime after Irradiation/ min.
10 em paraffin moderator for neutrons
Table 1.
and 1 inch diam. Nai(Tl) scintillation
counter for ganma -rays, respectively. Irrad1at 10n Beam Stopper Neutron
ShutterIII
All the monitoring systems are conditiono.o~ 1 ~~~ ~ 1 ~~~ ~l~~eDeuteroncalibrated with the radioisotope
30 MeV
1400
open
close
open
sources or a substandard chamber
0 •030 "'c l osec l oseopen
r·twice a year.
(1966)
Physics,l2,89Werven;HealthF.J.vanandJ.J.Broerse[1]
References184