physics-subatomic-particles

decay . Thus it was necessary to look for some inverse reactions it which neutrino stake part . ,'lthough very few neutrinos out of those which would come near enough t othe other particles would take part in the reaction, it would be possible to detec tthose which did take part . The inverse reactions suggested wer en +v —► e- .4. p ,which is the well-known electron-capture reaction in reverse, an dp+7r—tee+ n .In 1956, twenty-five years after the existence of the neutrino was first postulated ,Cowan, Reines, Harrison, Kruse, and cGuire started their search for it .Their first problem was to find a sufficiently strong source of neutrinos . The ydecided to use the newly-built Savannah River nuclear power station as their source .Here, the uranium: fiseion products produced antineutrinos so that the neutrino flu xwas about 10'1 m32 . They hoped to observe the second of the reactions mentioned above .If this reaction occurred, the positron would produce a trail of ionization in aliquid scintillator (see chapter 8), and when it came to rest, its annihilatio nby a negative electron would produce two garma rays . Furthermore, if some cadmiumcompound was dropped into the scintillator liquid, this would pick up th e slow neutron ,and in doing so, would produce two high-energy gamma rays, which would show up a sflashes in the scintillator . For this reason, the experimenters sandwiched som ecadmium salt solution between two scintillators, so that the three events connecte dwith the positron would all occur practically simultaneously in the cadmium sal tlayer, and after about 10 ps, the flash from the neutron capture would be seen in th ecadmium salt together with the scintillations from the two gamma rays produced by th ecapture . It was necesoary to have a great volume of liquid (, ,-10 tonnes) in th eecintillators, and about five hundred sensitive photomultiplier tubes to detect th etiny flashes of light .Spurious events were sometimes detected by some discrepency in timing and sometime sby the use of a third target and scintillator fixed in anticoincidence with th ephotomultiplier tubes . The experiment ran for around 1400 hours, with about on eevent per hour . When it had finished, various checks were run, such as substitutingheavy water (DL O) for water, in which case no events were detected, and in late 1956 ,the discovery of the neutrino was formally announced .4hile Cowan and Reines were searchirg for neutrinos by the method outlined above ,Davis, Harmer, and Iloffmann, a group of chemists from Brookhaven, were trying t odetect them by other means . It had been shown that when a neutrino interacted wit hthe isotope chlorine-37, the radioactive gas argon-37 was produced, with the emissio nof an electron . In their first experiment, the chemists placed a tank containing 500gallons of carbon tetrachloride (CC14 ) in a heavily-shielded position near the Savanna hRiver plant . Helium was bubbled through the tank to clear it of any argon, and th etank was then left untouched for about thirty days . After this tire, helium war againbubbled through the liquid, and any radioactive argon came out with it . The twomiscible gases were then cooled in a liquid nitrogen cooling apparatus, and wer efractionally separated . The argon was then tested with a Geiger counter, and it wasfound to exhibit traces of K-electron capture radiation .One of the most important reactions thought to be taking place in the sun i sp+p—,rd+e+Y ,and this should produce a neutrino flux of about 1O 15 sh2 on the surface of the earth .For this reason, it was decided that a larger and better neutrino detector should b ebuilt to study these solar neutrinos . Using the same principle as Davis' first detector,a tank containing 500 tonnes of liquid perchloroethylene (CzCl 4 ) was placed in the