physics-subatomic-particles

the proton would be directed into the rest of the equipment . If a given particle wa san antiproton, then it would be stable, and would not decay during its passage throughthe testing e q uipment, and this ser v ed as another test . At the selected momentum, 1 .19GeV, the experimenters knew that pions would have a velocity of about 0 .99 c, kaon sof 0 .93 c, and antiprotons of about 0 .78 c . But only about one in 40 000 particle sproduced was an antiproton, so some method of distinguishing between negative meson sand antiprotons had to be found . The beam was focused to a point, and was recorded whe nit passed through the first of two scintillation counters . 40 feet further on, ther ewas another identical scintillation counter . The readingsfrom both counter ., were thendisplayed on a screen, and the distance apart that the two peaks representing th epassage of a particle were, was proportional to the time taken for that particle t otravel from one counter to the next . It was calculated that a meson would take aroun d40 ns for the journey, and an antiproton around 51 ns . But some method had to b edevised to guard against the possibility that two mesic events might trigger th ecounters at an interval apart typical of an antiproton . The experimenters decided t oset up two velocity-selective Cerenkov counters after the second scintillation counter .A Cerenkov counter works on the principle that when a particle passes through a mediumat a velocity greater than the velocity of light in that medium, it emits photons, th eangle of emission being proportional to its velocity . The first Cerenkov counter,which was made of C B F,, 0, which has a refractive index of 1 .276, was made sensitiveto all particles with a velocity greater than 0 .79 c . The second Cerenkov counter ,made of fused quartz of refractive index 1 .458, was designed to detect particles wit ha velocity of between 0 .75 and 0 .78 c . By this time, any antiproton should have beenslowed down to a velocity of about 0 .765 c by its passage through the two scintillatio ncounters and the first Cerenkov counter . Thus, any antiproton passing through th etesting equipment would trigger the two scintillation counters 51 ns apart, woul dleave no trace in the first Cernkov counter, and would be recorded by the second one .In October 1956, the discovery of the antiproton was officially announced, afte rsixty correct sets of readings had been obtained . A few months later another researc hteam trapped an antiproton in glass, and observed the Cerenkov radiation produced b yits annihilation products. This proved that the particles selected by the team a tBerkeley were true antiprotons . At the same time Chamberlain, Chupp, Segre, Wiegand ,Goldhaber, and some Italian physicists tried to find antiproton disintegration star sin photographic emulsions exposed to beams containing antiprotons . After a short time ,some events were recognised by scanners at Rome, and, when it was realised that th evelocity absorbers used destroyed antiprotons, many more tracks were found .Soon after the discovery of the antiproton, Cork, Lambertson, Piccioni, and Wenze lfound evidence of the antineutron while working at the Lawrence Radiation Laboratory .They employed an improved version of the Chamberlain-Segre antiproton selector t oproduce a beam of antiprotons . This then entered a liquid scintillator in which charg eexchange took place, and any antiprotons became antineutrons . The newly-produced anti -neutrons passed through two more scintillation counters without detection, but anyhigh energy photons, which were also neutral, were revealed by the prescence of alead plate la" thick between the two scintillation counters, which made them materialis einto a virtual pair of an electron and a positron, which was then detected in th esecond scintillation counter . After encounter with the second scintillation counter ,the beam of particles was directed into a large lead-glass Cerenkov counter in whichthe antineutrons decayed . By the time it was announced that the antineutron had bee ndiscovered, 114 flashes of light in the Cerenkov counter from antineutron annihilation