WJEC Physics for AS: Student Bk

86 WJEC AS Level Physics: Unit 1 4. The delta particles generally usually decay into either a proton or a neutron plus a charged pion ( π + or π – ) in about 10 –24 s . See the end of Section 1.7.6 for the decay of  – . This decay can be written in two ways:  –  n + π – and ddd  udd +  ud Write equations for the decays of the other  particles in the same way. What indications are there that these decays take place by the strong interaction? 5. A second generation baryon has the quark structure uus. It decays into first generation particles (a baryon and a meson) with a decay time of ~ 2.6 × 10 –10 s . (a) What indications are there that this decay proceeds via the weak interaction? (b) There are two modes of decay: into two charged particles or into two uncharged particles. Write these equations at both the composite particle and the quark level. (c) Following one decay, the meson subsequently decays into two photons. Identify, giving a reason, which of the two modes in (b) was followed. 6. One method of detecting neutrinos is to use dry-cleaning fluid! This fluid is chlorine rich: 25% of chlorine atoms are the Cl-37 isotope. Occasionally an electron neutrino (e.g. from the Sun) interacts with a particle in a Cl-37 nucleus, converting the nucleus into Ar-37 . The proton numbers for chlorine and argon are 17 and 18 respectively. (a) Identify the particle in the chlorine nucleus which is changed and the particle it is changed into. Explain your answer. (b) Write the equation for the interaction just including the relevant particles (i.e. ignore the rest of the nuclei). (c) Huge numbers of solar neutrino pass through the Earth every second. Why is this only an ‘occasional’ interaction? 7. Only one-third of solar neutrinos, arriving at the Earth, consist of electron neutrinos. The other two-thirds consist of muon and tauon neutrinos in roughly equal numbers. Suggest why the muon and tauon neutrinos cannot be detected as in Q6. [Masses in MeV / c 2 : m e ~ 0.5 ; m m ~ 105 ; m t ~ 1800 .] 8. The K + particle is a second generation meson with quark composition u¯s . It decays into a π + meson and X which is another first generation particle. (a) Identify X by considering the relevant conservation laws. (b) Write the decay reaction, K +  π + …at the quark level. & S C Stretch & Challenge The figure shows the energy spectra of the solar neutrinos arising from the following reactions: pp : p + p  d + e + + v e pep : p + e – + p  d + v e 7 Be : 7 Be + e –  7 Li + v e 8 B : 8 B  8 Be + e + + v e where d is a deuteron, the heavy hydrogen nucleus, 2 1 H . (a) Account for the fact that the pp and 8B spectra are continuous but the others each consist of single lines. (b) The left-hand edge of each of the blue areas gives the low energy limit of detection of three types of neutrino detectors: gallium, chlorine and Kamiokande. By researching the detection methods, explain the energies of these lower limits. Neutrino Flux at Earth (cm –2 , s –1 , MeV –1 ) Neutrino Energy (MeV) 10 12 10 10 10 8 10 6 10 4 10 2 0.1 0.3 1 3 10 7 Be 7 Be pep 8 B Gallium Chlorine Kamiokande