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;