Radioactivity Physics 12 Adv

Radioactivity Radioactive decay is the emission of some particle from a nucleus which is accompanied by a change of state or type of the nucleus Radioactive decay is the emission of some particle from a nucleus which is accompanied by a change of state or type of the nucleus There are three main types of radiation: There are three main types of radiation: Alpha Alpha Beta Beta Gamma Gamma

Radioactive Isotopes Previously we considered plot of stable nuclei Previously we considered plot of stable nuclei It is also possible to have a nucleus that is unstable It is also possible to have a nucleus that is unstable An unstable nucleus will decay following a few very specific processes An unstable nucleus will decay following a few very specific processes

Alpha Decay An alpha particle is a helium nucleus (two protons and two neutrons) An alpha particle is a helium nucleus (two protons and two neutrons) A nucleus that emits an alpha particle will lose the two protons and two neutrons A nucleus that emits an alpha particle will lose the two protons and two neutrons Large nuclei will emit alpha particles Large nuclei will emit alpha particles They do not penetrate matter well and a sheet of paper or 5cm of air will stop most They do not penetrate matter well and a sheet of paper or 5cm of air will stop most They can free electrons from atoms, meaning they are a form of ionizing radiation They can free electrons from atoms, meaning they are a form of ionizing radiation

Beta Decay When a nucleus emits a beta particle, it appears to lose an electron or positron from within the nucleus When a nucleus emits a beta particle, it appears to lose an electron or positron from within the nucleus There are two types of beta decay (β - and β + ) There are two types of beta decay (β - and β + ) Beta particles can penetrate matter to a greater extent than alpha particles; they can penetrate about 0.1mm of lead or 10m of air Beta particles can penetrate matter to a greater extent than alpha particles; they can penetrate about 0.1mm of lead or 10m of air They are also a form of ionizing radiation but less damaging than alpha particles They are also a form of ionizing radiation but less damaging than alpha particles

Electroweak Theory The strong force acts only on quarks and the strong force carrier, the gluon and does not act on leptons The strong force acts only on quarks and the strong force carrier, the gluon and does not act on leptons The weak force acts on both leptons and quarks and is important to beta decay The weak force acts on both leptons and quarks and is important to beta decay In 1979, Glashow, Salam and Weinberg were awarded the Nobel Prize for the electroweak theory which united electromagnetic and weak interactions In 1979, Glashow, Salam and Weinberg were awarded the Nobel Prize for the electroweak theory which united electromagnetic and weak interactions

Weak Force Carriers While the strong force is carried by a gluon and electromagnetic forces are carried by the photon, the bosons responsible for the weak force have a very large mass While the strong force is carried by a gluon and electromagnetic forces are carried by the photon, the bosons responsible for the weak force have a very large mass The three bosons are W +, W - and Z 0 all have large mass (~81GeV for W and ~92GeV for Z) The three bosons are W +, W - and Z 0 all have large mass (~81GeV for W and ~92GeV for Z) This also requires the existence of a massive spin zero boson called the Higgs particle This also requires the existence of a massive spin zero boson called the Higgs particle

Weak Force Properties Weak interaction can change quark flavours Weak interaction can change quark flavours The weak interaction is not left-right symmetric; that is the physical laws governing weak interaction look different when viewed in a mirror The weak interaction is not left-right symmetric; that is the physical laws governing weak interaction look different when viewed in a mirror The weak interaction is slightly asymmetric to the interchange of particles and anti- particles The weak interaction is slightly asymmetric to the interchange of particles and anti- particles

Beta Decay (β - ) In this type of beta decay, a neutron becomes a proton and a beta minus particle (high energy electron) is emitted In this type of beta decay, a neutron becomes a proton and a beta minus particle (high energy electron) is emitted In addition an antineutrino is emitted (antimatter) along with the beta minus particle In addition an antineutrino is emitted (antimatter) along with the beta minus particle The nucleus’s atomic number increases by one while the atomic mass number remains the same The nucleus’s atomic number increases by one while the atomic mass number remains the same

Beta Decay (β + ) In this type of beta decay, a proton becomes a neutron and a beta plus particle (high energy positron (antielectron)) is emitted In this type of beta decay, a proton becomes a neutron and a beta plus particle (high energy positron (antielectron)) is emitted In addition a neutrino is emitted along with the beta plus particle In addition a neutrino is emitted along with the beta plus particle The nucleus’s atomic number decreases by one while the atomic mass number remains the same The nucleus’s atomic number decreases by one while the atomic mass number remains the same

Gamma Decay When a nucleus goes through alpha or beta decay, the daughter nucleus is often left in an excited state When a nucleus goes through alpha or beta decay, the daughter nucleus is often left in an excited state In order to reduce the energy of the nucleus, it will go through gamma decay (high energy photon) to return to the ground state In order to reduce the energy of the nucleus, it will go through gamma decay (high energy photon) to return to the ground state Gamma radiation can pass through 10cm of lead or 2km of air Gamma radiation can pass through 10cm of lead or 2km of air It is the most damaging of all due to the energy of the gamma particle It is the most damaging of all due to the energy of the gamma particle

Decay Series When a large nucleus decays by alpha and beta radiation, the daughter nucleus will be more stable than the original nucleus When a large nucleus decays by alpha and beta radiation, the daughter nucleus will be more stable than the original nucleus However, the daughter nucleus may still be unstable and will itself go through alpha or beta radiation However, the daughter nucleus may still be unstable and will itself go through alpha or beta radiation This leads to a decay series This leads to a decay series

Rate of Radioactive Decay It is impossible to predict when a specific nucleus will decay It is impossible to predict when a specific nucleus will decay You can describe the probability of decay You can describe the probability of decay The concept of half life is used with radioactive decay The concept of half life is used with radioactive decay Using the half life equation, it is possible to determine how much of a sample would remain after a given period of time Using the half life equation, it is possible to determine how much of a sample would remain after a given period of time

Half Life N  sample remaining N  sample remaining N 0  original sample N 0  original sample Δt  elapsed time Δt  elapsed time T  half life T  half life

Challenge Problem A neutrino has to pass within a distance D of a quark to have a chance to interact with it. A neutrino has to pass within a distance D of a quark to have a chance to interact with it. What is the area of the circular target centered on the quark through which the neutrino has to pass in order to interact with the quark? What is the area of the circular target centered on the quark through which the neutrino has to pass in order to interact with the quark? If the quarks are located in the nuclei of heavy water molecules, how many quarks are there per molecule with which the neutrino can interact (the neutrino can only interact with d quarks in neutrons)? If the quarks are located in the nuclei of heavy water molecules, how many quarks are there per molecule with which the neutrino can interact (the neutrino can only interact with d quarks in neutrons)? if a heavy water molecule has a radius of 140pm, what is the chance of a neutrino interacting with a d quark in a heavy water molecule? if a heavy water molecule has a radius of 140pm, what is the chance of a neutrino interacting with a d quark in a heavy water molecule? SNO (Sudbury Neutrino Observatory) is a 6m radius vessel filled with heavy water; if water has a density of 1000kg/m^3, what is the chance of a neutrino interacting with a d quark inside the vessel? SNO (Sudbury Neutrino Observatory) is a 6m radius vessel filled with heavy water; if water has a density of 1000kg/m^3, what is the chance of a neutrino interacting with a d quark inside the vessel?

Challenge Problem You are in charge of managing nuclear waste from a CANDU reactor. You know that following the removal of fuel from the reactor, you have radioactive atoms that have a half-life of 20 years. You are in charge of managing nuclear waste from a CANDU reactor. You know that following the removal of fuel from the reactor, you have radioactive atoms that have a half-life of 20 years. How long would you have to wait to ensure that you have no atoms remaining? How long would you have to wait to ensure that you have no atoms remaining? Normally, a radioactive sample is considered safe after ten half-lives; how much of the sample remains after this time? Normally, a radioactive sample is considered safe after ten half-lives; how much of the sample remains after this time?

Practice Problems Page 915 Page