Fig. 4.1 . An unstable, heavy nucleus of a radioactive element can disintegrate or decay into a stable, lighter nucleus, with the emission of an alpha particle that carries mass away from the heavy nucleus during its decay. The sub-atomic alpha particle consists of two protons and two neutrons. The nucleus of a helium atom is an alpha particle. Radioactive alpha decay of an individual heavy element like uranium does not occur very often, on average. You would have to wait 704 million years for half of a rock of uranium to change into lead by emitting alpha particles.
Fig. 4.2 . The energy spectrum of cosmic ray particles striking the outer atmosphere of the Earth. The particle flux is plotted as a function of the particle energy in units of electron volts, abbreviated eV, where 1 eV = 1.602 x 10-19 J and 1 GeV = 109 eV, or a billion eV. The most abundant cosmic ray particles are protons with energies of about 1.5 billion eV, and every second about 640 of them enter every square meter of the Earth’s outer atmosphere. They are probably accelerated to high energy during the supernova explosions of massive stars. One cosmic ray proton of 10 billion eV in energy enters each square meter of the Earth’s outer atmosphere every second; the more energetic cosmic ray particles of a hundred thousand million eV are less abundant, with one per square meter every year. Solar flares can emit protons with energies of 10 billion eV or less, and these solar energetic particles can strike the Earth when the solar active region is on the near side of the Sun. Cosmic rays with low flux and very high energy, greater than a million billion eV may be of extragalactic origin.
Fig. 4.3 . An invisible gamma ray photon (top) produces an electron and a positron, short for positive electron, seen by curved tracks in a bubble chamber. Both the electron and the positron are bent into circular tracks by the instrument’s magnetic field, moving in opposite direction because of their opposite electrical charge and spiraling into a smaller circular motion as they lose energy. In this upper pair, some of the photon’s energy is taken up in displacing an atomic electron, which shoots off towards bottom left. In the lower example, all of a gamma ray’s energy goes into the production of the electron-positron pair. As a result, these particles are more energetic than the upper pair, and their tracks do not curve so tightly in the chamber’s magnetic field. (Schematic of a Lawrence Berkeley Laboratory bubble chamber image, reproduced by Frank Close, Michael Marten and Christine Sutton in The Particle Explosion, New York: Oxford University Press 1987.)
Fig. 4.4 . When an alpha particle, or helium nucleus denoted 4He, is sent through a cloud chamber, it usually passes right through it, with a trajectory that marks out a straight line. Very occasionally, the alpha particle will strike the nucleus, 14N1, of a nitrogen atom in the air within the chamber, transforming it into the nucleus, 17O, of an oxygen atom with the emission of a proton, the nucleus of a hydrogen atom and denoted 1H. Such a nuclear transformation was first observed in cloud chamber photographs taken in the early 1920s by Patrick Blackett (1897-1974).
Fig. 4.5 . Two beams of protons have been whirled in opposite directions to nearly the speed of light, each with an energy of 7 trillion electron volts, or 7 TeV = 7 x 1012 eV = 1.12 x 10-6 J, and directed into collision with each other at CERN’s Large Hadron Collider, abbreviated LHC. This image displays the tracks of more than 100 charged particles as they fly away from the point of proton collision. By studying the collision particle debris, including correlations between them, scientists hope to gain an improved knowledge of how sub-atomic particles interact at extremely high energies, including the hot, dense conditions just a small fraction of a second after the big bang. At the point of proton impact, temperatures of more than million million, or 1012, K are generated, exceeding 100,000 times the temperature at the center of the Sun. In particle physics, a hadron is a composite particle made of quarks held together by the strong force; the best-known hadrons are the protons and neutrons, which are components of atomic nuclei. CERN is a French acronym for the Conseil Européen pour la Recherche Nucléaire, or the European Organization for Nuclear Research. The Compact Muon Solenoid, abbreviated CMS, particle detector created this image. (Courtesy of CERN.)
Fig. 4.6 . Photograph of the test of a nuclear bomb, code-named Truckee, delivered by air 16 kilometers south of Christmas Island (now Kiritimati), Pacific Ocean, on 9 June 1962 during Operation Dominic I. The explosion had a force equivalent to 210 kilotons of TNT, about ten times that of the atomic bomb detonated over Nagaski, Japan, on 9 August 1945. Truckee was a prototype test of the W-58 warhead carried on the Polaris A-2 missile and deployed on submarine-launched ballistic missiles. Thermonuclear detonations in the humid Pacific created their own localized weather systems; visible in Truckee’s cloud are multiple cloud condensation structures known as “bells” and “skirts,” as well as the more familiar “cauliflower” structure. The United States Air Force 1352nd Photographic Group, Lookout Mountain Station, took this image from Christmas Island. (Courtesy of Michael Light, also in his book 100 Suns, Alfred A. Knopf, 2003, number 81.)