422 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



the doing, need not be so in the telling. "We put our detector very 

 close to the face of the reactor wall, piling all of our shielding around 

 it and all the lead that was available at the Hanf ord plant until the 

 floor sagged, and then we "listened." We restacked our shield and 

 listened again for the double pulses signaling neutrinos when the 

 reactor was operating. (See pi. 5.) 



The lesson of the work was clear: It is easy to shield out the noise 

 men make, but impossible to shut out the cosmos. Neutrons and 

 gamma rays from the reactor, which we had feared most, were stopped 

 in our thick walls of paraffin, borax, and lead, but the cosmic ray 

 mesons penetrated gleefully, generating backgrounds in our equipment 

 as they passed or stopped in it. We had brought large trays of geiger 

 counters to place around and over the detector, so that cosmic rays 

 could be identified as such and rejected from the signal rate. 



We did record neutrinolike signals which, seen in retrospect, were 

 genuine. They appeared and disappeared as the reactor was raised to 

 power and then shut down again. But the cosmic rays with their 

 neutron secondaries generated in our shield were some 10 times more 

 abundant than were the neutrino signals. Under these circumstances, 

 it was quite impossible to test the neutrino signal by changing the 

 number of proton targets in the detector or by altering the cadmium 

 concentration to alter the neutron capture times as we had planned. 



We felt that we had the neutrino by its coattails, but our evidence 

 would not yet stand up in court. We must be more clever than this. 

 We returned to Los Alamos with a gleam in our eyes, for we felt that 

 now we knew how to catch the neutrino. 



PROJECT POLTERGEIST — HI 



It was time to become serious about Project Poltergeist, and so the 

 Laboratory suggested that we set up a formal group for the sole 

 purpose of tracking neutrinos. This we did, taking with us those of 

 the original team who could leave their other work behind, and recruit- 

 ing several new members to the group. 



Looking again at the reaction which signals the capture of an anti- 

 neutrino, we recall that the capture of the particle by a proton changes 

 the proton into a neutron with the emission of a positron. We had 

 used the time correlation of the two pulses produced by positron 

 annihilation and by neutron capture in hydrogen. We would now 

 use the spatial correlation of the various gamma rays as well. This 

 would give us a great advantage over the spurious signals produced 

 by the cosmic rays. 



A new detector was designed in which a large thin tank of water 

 supplied the proton targets, and cadmium acetate dissolved in the 

 water lay in wait to capture the neutrons produced. Positron annihi- 

 lation results in two 0.51 Mev. gamma rays which travel away from 



