The neutron and gamma photon doses corre
sponding to the neck and eye level from an
“1AmO2source used in thyroid studies have
been theoretically estimated. The radiation haz
ard to the patient is found to be not significant.
Americium-241 sources with activities of a few
curies are used for x-ray fluorescent scanning of the
thyroid gland (1 ) . The 59.6-keV gamma photons
emitted by 241Am are used to excite the characteristic
x-ray emission of iodine atoms in the thyroid. A
typical source for these studies is a pellet compressed
from 241AmO2and aluminum powder; the powdered
aluminum acts as a binder. In such a matrix alpha
particles from 241Am can cause a neutron flux
through (a,n) reactions with the aluminum and oxy
gen atoms of the source complex. This neutron flux
(2) is of some concern from the viewpoint of radia
tion protection. Robinson et al (2) measured the
neutron flux with a neutron “remcounter.― The rem
counter is too large (about 9 in. in diameter) for
making measurements at the short distances from the
source corresponding to the neck and eye regions. As a
check, we thought it worthwhile to calculate the neu
tron dose theoretically from the known (a,n) cross
sections of aluminum and oxygen.
METHOD OF CALCULATION
The neutron yields and spectra resulting from the
interaction of alpha particles with aluminum and
oxygen were calculated separately and then added
together to get the overall neutron yield and spec
trum. Among the natural oxygen isotopes, 18@is the
chiefcontributorto neutronyield from the241AmO2
source. Oxygen-16, although overwhelmingly abun
dant (99.759% ), has a threshold of 15.2 MeV for
the (a,n) reaction. Since the abundance and the
(a,n) cross section of ‘@Oare only about 15% and
10% , respectively, of those of the@ isotope, neu
tron contributions from 170 can also be disregarded.
The calculations were done for the type of source
described by Robinson et al (2) . The activity of
241Am was 5 Ci, and the source was shaped as a
cylinder whose active area was 1.75 in. in diameter
and 0.75 in. thick. Since no information was given
on the ratio of Am to Al atoms, it was taken to be
1:20 since the Am:Be ratio in Am—Be neutron
sources is known to vary between 13 and 30. From
these data the number of Am, 0, and Al atoms per
cubic centimeter of source were calculated.
The neutron yield per alpha particle from the
source is given by
‘Emax
Y=NJ @an(E)dE
E, @dE1
L@-i (1)
where N is the number of target nuclei (aluminum
or oxygen) per cubic centimeter; ffa.i,(E) is the neu
tron-production cross section of the target material,
in barns per atom; E is the energy of the alpha par
tide; dE/dx is the stopping power of the source
material (the AmO@—Almixture) for an alpha par
tide of energy E; E@ is the threshold alpha energy
for neutron production; and Emax @5the maximum
energy of the emitted alpha particles.
The neutron spectrum (3) is given by
FEmax
@ = NJ P(En,E)ffa.n(E)dE
E@ IdEl ‘ (2)
where p(E,@,E) is the probability that a neutron of
energy E,, per unit energy interval is produced when
an alpha particle of energy E interacts with the tar
get medium. These probabilities were calculated as
suming that the neutron is emitted isotropically in the
Received Aug. 19, 1975; revision accepted Nov. 20, 1975.
For reprints contact: 0. Venkataraman, Div. of Radio
logical Protection, Bhabha Atomic Research Center, Trom
bay, Bombay 400 085, India.
408 JOURNAL OF NUCLEAR MEDICINE
RADIATION HAZARDS FROM 241Am SOURCES
USED IN THYROID STUDIES
G. Venkataramanand S. Jayaraman
Bhabha Atomic Research Center, Trombay, Bombay, India
