Introduction to the World of Nuclear Physics. Lidiya Strautman

7. It was only in this century that aluminium was produced in quantity. to represent; to make shorter; to make clear; to take care; in large amounts; to remove; to keep up with.

The Nucleus

      The atomic nucleus consists of nucleons-protons and neutrons. Protons and neutrons are made of quarks and held together by the strong force generated by gluon exchange between quarks. In nuclei with many nucleons, the effective strong forces may be described by the exchange of mesons (particles composed of quark-antiquark pairs). A proton consists of two up quarks and one down quark along with short-lived constituents of the strong force field. A neutron is similar except that it has two down quarks and one up quark. Although scientists are convinced that nucleons are composed of quarks, a single quark has never been isolated experimentally. Energy brought into a nucleus to try to separate quarks increases the force between them. At high enough energy, the addition of energy creates new particles rather than frees the quarks.

The Discovery of Radioactivity

      In 1896 Henri Becquerel was using naturally fluorescent minerals to study the properties of x-rays, which had been discovered in 1895 by Wilhelm Roentgen. He exposed potassium uranyl sulfate to sunlight and then placed it on photographic plates wrapped in black paper, believing that the uranium absorbed the sun’s energy and then emitted it as x-rays. This hypothesis was disproved on the 26-27th of February, when his experiment “failed” because it was overcast in Paris. For some reason, Becquerel decided to develop his photographic plates anyway. To his surprise, the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Becquerel discovered radioactivity.

      Becquerel used an apparatus similar to that displayed below to show that the radiation he discovered could not be x-rays. X-rays are neutral and cannot be bent in a magnetic field. The new radiation was bent by the magnetic field so that the radiation must be charged and different from x-rays. When different radioactive substances were put in the magnetic field, they deflected in different directions or not at all, showing that there were three classes of radioactivity: negative, positive, and electrically neutral l.

      Ernest Rutherford, who did many experiments studying the properties of radioactive decay, named these alpha, beta, and gamma particles, and classified them by their ability to penetrate matter. Rutherford used an apparatus similar to that depicted in the figure. When the air from the chamber was removed, the alpha source made a spot on the photographic plate. When air was added, the spot disappeared. Thus, only a few centimeters of air were enough to stop the alpha radiation.

      Because alpha particles carry more electric charge, are more massive, and move slowly compared to beta and gamma particles, they interact much more easily with matter. Beta particles are much less massive and move faster, but are still electrically charged. A sheet of aluminum one millimeter thick or several meters of air will stop these electrons and positrons. Because gamma rays carry no electric charge, they can penetrate large distances through materials before interacting – several centimeters of lead or a meter of concrete is needed to stop most gamma rays.

      UNIT 4

      Vocabulary list

      proton accelerator ускоритель протонов

      transform трансформировать; подвергать трансформации, преобразованию, б) преобразовывать один вид энергии в другой (напр., механическую энергию в электрическую)

      transformation 1) видоизменение 2) отображение 3) перегруппировка 4) превращение 5) преобразование

      emit 1) излучать, испускать, выделять (свет, тепло, запах и т. п.); выбрасывать, извергать (пепел, дым, лаву) the rays of heat that are emitted by the warm earth – теплые волны, испускаемые нагретой землей The factory has been emitting black smoke from its chimneys, which is against the law. Из труб завода до сих пор вырывается черный дым, это противозаконно.

      half-life 1) период полупревращения 2) период полураспада 3) полупериод распада

      half-life-decay полураспад, half-life period период полураспада

      momentum 1) количество движения; механический момент, инерция (движущегося тела); кинетическая энергия 2) толчок, импульс; движущая сила to gain, gather momentum – приобретать движущую силу

      binding 1. 1) связывание (действие) 2) соединение (любой предмет, с помощью которого что-л. связывается, соединяется) Syn: bond, band, bandage, fastening binding energy энергия связи, binding kinetics кинетика связывания

      bond activation активация связи

Beta Decay

      Beta particles are electrons or positrons (electrons with positive electric charge, or antielectrons). Beta decay occurs when, in a nucleus with too many protons or too many neutrons, one of the protons or neutrons is transformed into the other. In beta minus decay, a neutron decays into a proton, an electron, and an antineutrino. In beta plus decay, a proton decays into a neutron, a positron, and a neutrino. Both reactions occur because in different regions of the Chart of the Nuclides, one or the other will move the product closer to the region of stability. These particular reactions take place because conservation laws are obeyed. Electric charge conservation requires that if an electrically neutral neutron becomes a positively charged proton, an electrically negative particle (in this case, an ele ctron) must also be produced. Similarly, conservation of lepton number requires that if a neutron (lepton number = 0) decays into a proton (lepton number = 0) and an electron (lepton number = 1), a particle with a lepton number of -1 (in this case an antineutrino) must also be produced. The leptons emitted in beta decay did not exist in the nucleus before the decay – they are created at the instant of the decay. To the best of our knowledge, an isolated proton, a hydrogen nucleus with or without an electron, does not decay. However within a nucleus, the beta decay process can change a proton to a neutron. An isolated neutron is unstable and will decay with a half-life of 10.5 minutes. A neutron in a nucleus will decay if a more stable nucleus results; the half-life of the decay depends on the isotope. If it leads to a more stable nucleus, a proton in a nucleus may capture an electron from the atom (electron capture), and change into a neutron and a neutrino. Proton decay, neutron decay, and electron capture are three ways in which protons can be changed into neutrons or vice-versa; in each decay there is a change in the atomic number, so that the parent and daughter atoms are different elements. In all three processes, the number A of nucleons remains the same, while both proton number, Z, and neutron number, N, increase or decrease by 1. In beta decay the change in the binding energy appears as the mass energy and kinetic energy of the beta particle, the energy of the neutrino, and the kinetic energy of the recoiling daughter nucleus. The energy of an emitted beta particle from a particular decay can take on a range of values because the energy can be shared in many ways among