DISCOVERY OF THE NEUTRINO — COWAN 423 



the annihilation in opposite directions. Thus, quite often one gamma 

 ray would emerge from the top of the water slab, and the other from 

 the bottom. Neutron capture in cadmium produces a burst of many- 

 gamma rays which total about 9 Mev. in energy. These also would 

 emerge from both top and bottom of the slab. By placing large thick 

 tanks of liquid scintillator on either side of the water slab, we could 

 expect to see these events in top-bottom coincidence as well as in time- 

 delayed correlation. A detector of this description was designed but, 

 in a sense, was made twofold. We designed two such slabs and placed 

 them between three thick liquid scintillator detectors, much as the meat 

 is placed between three slices of bread in a club sandwich. This would 

 provide a running check on the equipment, as both detectors must 

 operate in agreement as to what they see. (See pi. 4, fig. 1.) 



Another year's work at Los Alamos went into the construction and 

 testing of the new detector. Dr. John Wlieeler suggested during that 

 time that we make our next measurement at the new Savannah River 

 Plant and arranged for our visit to that laboratory. With the co- 

 operation of the Du Pont scientists there we quickly found an almost 

 ideal spot near one of their reactors. During the year we also de- 

 veloped a new scintillating solution (of triethylbenzene) which was 

 much less hazardous than toluene. (See pi. 4, fig. 2.) 



Wlien completed and sitting in its great lead shield in the physics 

 building at Los Alamos, the detector was about 10 feet high. It 

 occupied a floorspace some 6 feet by 12 feet. The shield around it 

 was made of a steel framework holding walls of lead 6 inches thick. 

 The lead top and bottom were also of this thickness, and hydraulically 

 operated lead doors some 4 inches thick closed the two ends. Three 

 separate scintillation detectors were stacked inside the shield, and 

 between each pair was a flat tank of water and cadmium acetate as a 

 target. 



The detectors were made of rectangular steel tanks which held the 

 liquid scintillator in their center sections. Each was 2 feet thick, about 

 4 feet wide, and some 11 feet long. Each center section of scintillator 

 was 6 feet long. End sections were filled with a clear, nonscintillating 

 liquid to act as shields against radioactivity from the banks of photo- 

 multipliers looking in from each end. There were 55 photomultipliers 

 on each end of each of the 3 detectors. Each photomultiplier was a 

 large 5-inch diameter "eye" which stared fixedly at the sensitive liquid 

 in the tank and reported the faint flashes of light there with electrical 

 pulses. The "compound eye" of the total detector thus had a retinal 

 area greater than 45 square feet. Each of the photomultipliers had 

 been carefully selected and its sensitivity balanced to a standard value. 

 The tanks were painted white inside to conserve every photon possible 

 and reflect it toward the phototubes. (See pis. 2, 6, 7, and 8.) 



