81
ev)
et al. U.S. Atomic Energy Commission Report CONF651111 (1966), pp. 205-209.
2 fivans, Robley D. and Evans, Richard O. Studies of SelfAbsorption in Gamma-Ray Sources. Rev. Mod. Phys. 20,
305-326 (1948).
3. Snyder, W.S., Ford, Mary R., and Warner, G. G. Effect of
Size, Shape, Composition, and Density of Body Organs
on the Absorption of Gamma Rays from a Uniform Source
in an Organ. U. 8. Atomic Energy Commission Report
ORNL-4316 (October 1968) p. 274.
“TIME-OF-FLIGHT” GAMMA-RAY CAMERA OF LARGE DIMENSIONS
L. D. Marinelli, G. F. Clemente, I. K, Abu-Shumays,* and O. J. Steingraber
—-
The localization of low levels of radioactivity by means of
time-of-flight techniques is possible if resolutions of the order
ot 5 em are tolerated. A survey is given of the optical, electro ie, and mathematical problems involved.
~
An economical instrument, capable of measuring in
vivo distributions of the order of 0.01 uwCi of y-ray activity, obviously would prove useful in monitoring in-
‘ternal contamination in neutron radioactivation analy-
six /n vivo and peform, over large areas, some of the
tasks performed by the several y-ray cameras available
today, In the case of kK”, with an instrumental efficiency
ol 7 ':, the expected average counting rate would be of
ure 61. A collimator grid whose cubic septa have lumina
from 25 to 100 cm? will cover the array. To the ends of
these rods there are optically coupled fast photomullti-
pliers (P.M.) connected in turn to fast subnanosecond
circuits that measure the time elapsed between the two
signals generated bya single scintillation in the anodes
of the corresponding P.M. pair.
At the present, the measurement of subnanosecond
intervals has become a fine art whose techniques can be
used to advantage. ®) The time ‘‘jitters’”’ Af of these fast
measurements or, equivalently, the spatial resolution
W1;2 of our system, depends on identical factors, except
P.M.
SCINTILLATOR ~
e
»yplacing
enclosure,
s the
ss
WFO NRRL OO
QQ NN
a
~S
rai
~,
OUP
Sp
eee
/
ocated at
_—
PM.
--
SUPPORTS
COLLIMATORS
ae
stitutes 4
THEA TE
= ‘
a:
jon func;
umber of
ch might,
. Investi-,
so
S
__
wt
a
SASSO SS
sh eS
7
SCINTILLATOR ROD
.
|
=
Fic, 61.—Outline of y-ray camera
f source
the order of 0.2 counts/cem2/min; hence, in the presence
afe load, of . comparable background,statistical errors of the
hantomf Orler of 10% would require a counting rate of about 20
has both epin tor 16 hr. Obviously at these levels one cannot
ting Juss Bin good statistical information on activity spread over
ion) and areas much smaller than 100 em?, and this only with the
al disty most. efficient visualization systems available, such as
resulting, the xutofluoroscope. ”
ial cooth
=In order to reduce the number of photomultipliers
ite Usd, we have thought of utilizing prismatic plastic
fireseent rods of 2” x 2” cross section, one or two
MN trrs long and encircling the subject as shown in Fig\pplied Mathematics Division.
that in our case the photons from each scintillation reach
the photocathodes through different path lengths and
that, in effect, the effective decay time of the photocathode burst is lengthened. The dispersion in a long
fluorescent rod can be represented by a roughly
Gaussian distribution described by
Wi = At-S(L) « (Nof(e))~”" f(rare) f(r) F(Z),
where Wy. is the FWHMof the distribution of the time
intervals At obtained with a rod irradiated by a fixed point source finely collimated;
Nj is the number of photons arriving at the
photocathode;