physics-subatomic-particles

not by any measurable amount . Since neutrinos are massless, they must propagate throug hspace at the velocity of light in vacuo and in straight lines, and they must hav eDoppler effects similar to those of light . Neutrino astrophysics promises to be one o fthe most interesting and fruitful branches of physics in a few y ears'time, when bette rneutrino d e tectors have been built .Let . us now consider the nature of neutrinos themselves . Various problems concerningthe weak interaction, which we shall discuss in chanter 7, led Gsll-Lynn and Feinberg ,in 1961, to the idea that for some reason the neutrino and the antineutrino in muo ndecay can not annihilate each other . They suggested that this was becaus e there existtwo distinct types of neutrinos, those associated with the electron, and thos eassociated with the muon . In 1959 3.Ponteeorvo had calculated that it would be possibl eto produce a medium-energy neutrino beam from a particle accelerator, and in 1960 ,Schwartz, Steinberger, and Lederman worked out that the newly-built AGS at Brookhave nwould be able to do this . Therefore, with the support of the Atomic i]nergy Commission ,Schwartz, Steinberg, Lederman, Danby, Goulianos, Nistry, and Gaillard, s e t up thei rexperiment to test the two-neutrino theory . They were going to produce what they hope dto be muon-neutrinos from pion decay, and then mace these react with protons an datomic nuclei in order to produce muons . If there was only one type of neutrino, ruon sand electrons would be produced by this process in equal numbers .They decided to use half the available beam energy : 15 GeV . They directed proton sof this energy at a beryllium target, where they produced pions and kaons, with energie sof around 3 GeV . Any of the resultant p articles which were inside a 14 0 cone, entere dthe experimental apparatus . The particles, predominantly pions, core first directe ddown a 21 m decay tube in which about 107 ; of the pions decayed, and the resultan tmuons and neutrinos were mostly collimated into the forward direction by centre-of-mas smotion . Then all the particles struck 13 .5 m of steel from the armour-plating of anold battleship . All the pions were captured by about the first 30 ems of this, but th emuons survived slightly longer . 'The only particles which emerged from the other end ofthis target were the neutrinos . They then entered a series of ten spark chambers (se echapter 8) with dimensions 1 m x 14 m x m, each composed of nine 25 mm thic kaluminium plates . The total mass of material in the spark chambers was about 10 tonnes .One of the major problems of the experiment was to eliminate as much backgroun dcosmic radiation as possibl e . The first p recaution taken was to place scintillation :counters at the top and ends of the spark chambers, and connected so that only n ocount in the u pp er counters and a count in the counters nearest the target woul dtrigger the spark chambers . however, about 80 cosmic ray particles still penetratedper second . '1'herfore it was decided that the synchrotron beam should he pulsed i n25)as pulses, at 1 .2 s intervals . between which time the spark chambers could not betriggered by any particle . The pulses themselves consisted of twelve bunches, eac h22 ns long, and separated by a time of 220 ns . During the running of the experiment ,between Sept e mber 196] . and June 1962, Some 1 700 COO puls e s were accepted, althoughthe total running time of the synchrotron was only 5* s .The experimenters had estimme ted that about 10 4 neutrinos would pass through th eirapparatus, of which about 25 would yield useful reactions . The counters triggered th espark chamb ers about ten times an hour, thus producing a total of about 500(' pictures .Out of these, over half were blank, but the reason for this is not understood . Of th eremainder 480 were caused by cosmic ray particles, leaving 51 photographs fo rstudy. By removing 1'.- m of shielding, it was possible to test whether any of th etracks were caused by particles from the primary beam, and by replacing it with lead,