AIR ACTIVATION BY AN ELECTRON SYNCHROTRON
Toshiso KOSAKO * and Takashi NAKAMURA *** Research Center for Nuclear Science and Technolog y,The Universit y of Tokyo** Institute for Nuclear Study, The Universit y of Tokyo
Abstract
The air activatio n of 13 N, 15 0 and ~ 1 Ar in air of the room
housing the 1.3 GeV INS (Institute for Nuclear Study) electron synchrotron (ES) was estimated by an experimen t and a calculatio n.
It is of fundament al interest and importanc e for radiation protection to estimate accuratel y the generatio n of gamma rays and neutrons from a high energy electron accelerat or. Here, we take up INS
electron synchrotr on (see Fig.l.), which was operated at 725 MeV and176 W beam power. Some of the 725 MeV electrons are converted tohigh energy photons for an elementar y particle experimen t by colliding with a platinum internal target and the residual beams (morethan 99%} are decelerat ed in the ring and enter the iron yoke magnet.
The 725 MeV electrons lose their energy accompany ing the electroma gnetic cascade shower in the magnet and produce many photons by theBremsstra hlung. These photons produces 13 N and 15 0 (T 1 12=lO.O m and2.07 m) by (y,n) reaction with air. A part of photons also produces
neutrons by (y,n) reaction of magnet iron and the produced photoneutrons are slowed down in the synchrotr on room. The thermaliz ed neutrons activate ~ 0 Ar (1.286 ojw of air) to ~ 1 Ar (T 1 / 2=1.83 hr) by(n,y) reaction.
The experimen t was done under the condition that the ES opera13 N andtion was long enough for the steady-st ate concentra tions of
15 0 to be reached.
The induced activity of 13 N and 15 0 in the room
was measured with an Nai(Tl) scintilla tor to be 119.7 ~Ci and 47.0
To estimate the~Ci, respectiv ely, immediate ly after the operation .
neutronthermal
the1life,
half
longergeneratio n of ~ Ar, which has
Theflux in the synchrotr on room was measured by Au-activa tion foil.
10~x0.53)(1.06+wasroomon
average neutron flux in the synchrotr
2nth/em /s.
15 0, aFor the analysis of photons and the produced 13 N andevaluatedwasvaluetalexperimen
theanddonewas
model calculatio n
In this calculatio n the emitted photon
within 20 % underesti mation.
Carlo code, EGS, for the electroMontethebydflux were calculatemagnetic cascade showers, and these photon energy spectra were multiplied by the (y,n) photoneut ron reaction cross sections to calculate
the the productio n of 13 N and 15 0. From this analysis, it becomes
clear that the usual method of estimatin g the photons at an electron
accelerat or by using the forward Bremsstra hlung spectrum of a thintarget, is quite inaccurat e when electrons are incident on a bulk
Instructure or impinge on a magnet at a small angle of incidence .
mainlyemitted
arephotons
thethatsuch a case, one must consider
by backscatt ering, and have a considera bly softer spectrum than thethin target Bremsstra hlung.

For the analysis of neutrons, the generatio n of photoneut rons
and the slowing down and the thermaliz ation process caused by high-73-

energy electrons have been analyzed by the combination of an electromagnetic cascade shower calculation and a two-dimensional SN neutrontransport calculation. The calculated thermal neutron flux is 2.11
x 10 4 nth/cm 2 /s and the comparison between experiment and calculation
was fairiy good withinfactor two. This thermal neutron flux isapplied to the estimation of radioactive 41 Ar generation of the electron synchrotron room and the obtained radioactivity of 41 Ar was
25. 5 11Ci.
concrete Sllleld

lr~ ]:

tostorage
ring

7.0 m

12.5 ml:J:Q&Sectlono~~Fig.l. Layout of the electron
synchrotron.
10 m

150

1.460-MeV
K-40 Peck100t •140 sec.6.t • 200 sec0.•.

.

..

\' ,_t=705 sec 6t•200 sec~,00""'..".·

0000

Inner
SpectrLm

"'

Cl>
00

<J5'Channel

Fig.2. Pulse-height distribution of r-rays
from activated air, measured by an Nai(Tl)detector. Symbols L and H indicate energy
acceptance interval for 511 keV peakmeasurements.

Bacl<scattered~Spectrum00"'<>

.•.
.
....
f.

00

oJ

..·-...
...
. --0 0 00 00 0o1t • •00

Electron TraJectoryQi)

Transml tted

CD0Spectrum10"~ '---'---'--L---'---'--

050"(,.. ..

IDO

500

Photon Energy, MeV

Fig.3. Electron beam trajectry on ahorisontal electron orbit plane in theelectron synchrotron ring.

-74-Fig.4. Photon energy spectra of 18 em
thick iron slab injected by a 725 MeV
electron at an incident angle of 7.68
deg, calculated by EGS code.