WJEC Physics for AS: Student Bk

82 WJEC AS Level Physics: Unit 1 The opposite process can also happen: if it possesses enough energy, a high energy photon can create an electron-positron pair. It also needs to interact with another particle (usually an atomic nucleus) to enable energy and momentum to be simultaneously conserved. In Fig. 1.7.2 a high energy photon enters from the top and interacts with a hydrogen atom at A (it is in a bubble chamber, which consists of a tank of liquid hydrogen), ejecting a high energy electron, creating a low energy electron-positron pair and a second photon which continues to B where it creates a second (higher energy) e – e + pair. A magnetic field at right angles to the page makes the charged particles travel in curves: the opposite charges of electrons and positrons results in the typical ‘ram’s horn’ effect at A. 1.7.4 The evidence for neutrinos Neutrinos are neutral particles of very low mass which only interact via the weak force. This means that they need to come within ~10 –18 m to interact, so interactions hardly ever happen: for example, a typical solar neutrino could expect to penetrate 1–2 light- years thickness of lead! The first evidence for their existence came from studies in the 1930s of the energy spectrum of beta particles. Phosphorus-32, 32 15 P , decays by β – emission. Before neutrinos were known the complete reaction was expected to be: 32 15 P  32 16 S + 0 –1 e The energy release in the decay is 1.5 MeV . If we apply the principle of conservation of momentum, we can calculate that the beta particles should take nearly all the energy (>99.9%), with the much heavier sulphur nucleus taking a tiny fraction. Compare this with the actual energy spectrum in Fig.1.7.3: 1.5 MeV is indeed the maximum beta particle energy but there is a continuous spectrum of energies with the peak being less that 0.5 MeV . This energy spectrum is only possible if a third particle is also produced, which can share the energy with the beta particle. This particle is called the neutrino (strictly, the electron anti-neutrino) and the complete reaction is: 32 15 P  32 16 S + 0 –1 e +  v e The photograph in Fig. 1.7.4 is of the β -decay of a nucleus of He-6 . As with Fig.1.7.2 this is in a bubble chamber. The curved track is the β -particle; the short fat track, the recoil of the resulting Li-7 nucleus. In order to conserve momentum a particle (a neutrino) must be emitted downwards in the photograph. The neutrino doesn’t interact and so leaves no track. 1.7.5 Building heavy particles Electrons, being leptons, are elementary particles; that is they are not composed of other particles. Hadrons , e.g. protons and neutrons (which are together also called nucleons), on the other hand, are composed of quarks, bound together with the strong force (see Section 1.7.6). Evidence for the existence of quarks is indirect. Single quarks are never detected. They are always seen in combination (see Section 1.7.8). There are three different types of hadron: • Baryons , such as protons and neutrons, are composed of three quarks. First generation baryons are composed entirely of a mixture of up (u) and down (d) quarks. • Antibaryons such as antiprotons are composed of three antiquarks. • Mesons , are composed of a quark and an antiquark. Fig. 1.7.2 e – e + pair production Fig. 1.7.3 P-32 b -spectrum Fig. 1.7.4 He-6 decay in a bubble chamber Fig. 1.7.5 Quark structure of nucleons u u u d d d proton neutron relative frequency beta particle energy / MeV 0 0.5 1.0 1.5 A B e _ e + e _ e +