IB DP Physics: HL复习笔记7.1.6 Alpha, Beta & Gamma Particles

• Some elements have nuclei that are unstable
  • This tends to be when the number of nucleons does not balance
• In order to become more stable, they emit particles and/or electromagnetic radiation
  • These nuclei are said to be radioactive
• There are three different types of radioactive emission:
• Alpha (α) particles are high energy particles made up of 2 protons and 2 neutrons (the same as a helium nucleus)
• They are usually emitted from nuclei that are too large

Beta-Minus Decay

• Beta (β⁻) particles are high energy electrons emitted from the nucleus
  • β⁻ particles are emitted by nuclei that have too many neutrons

• During beta decay, a neutron changes into a proton and an electron
  • The electron is emitted and the proton remains in the nucleus

• A completely new element is formed because the proton number changes

Beta-minus decay often happens in unstable nuclei that have too many neutrons. The nucleon number stays the same, but the proton number increases by one

• An electron has a mass number of 0
  • This is because electrons have a negligible mass, compared to neutrons and protons

• Therefore, the nucleon number of the decaying nuclei remains the same
• Electrons have a proton number of -1
  • This means that the new nuclei will increase its proton number by 1 in order to maintain the overall proton number before and after the decay

• The following equation shows carbon-14 undergoing beta decay
  • It forms nitrogen-14 and a beta minus particle

• Beta minus particles are written as an electron in this equation

The carbon nucleus emits a beta particle, and a neutron changes into a proton. This means its proton number increases by +1 and it changes into a new element, nitrogen

Beta-Plus Decay

• Beta (β⁺) particles are high energy positrons (anti-matter of electrons) also emitted from the nucleus
  • β⁺ particles are emitted by nuclei that have too many protons
• During beta plus (β⁺) decay a proton turns into a neutron emitting a positron (anti-electron)
  • The positron is emitted and the neutron remains in the nucleus
• A completely new element is formed because the proton number changes

Beta-plus decay often happens in unstable nuclei that have too many protons. The nucleon number stays the same, but the proton number decreases by one

• A positron has a mass number of 0
  • This is because the positrons have a negligible mass, just like the electron, compared to neutrons and protons

• Therefore, the nucleon number of the decaying nuclei remains the same
• Positrons have an proton number of +1
  • This means that the new nuclei will decrease its proton number by 1 in order to maintain the overall atomic number before and after the decay

• Gamma (γ) rays are high energy electromagnetic waves
• They are emitted by nuclei that need to lose some energy

• If these particles hit other atoms, they can knock out electrons, ionising the atom
• This can cause chemical changes in materials and can damage or kill living cells

When radiation passes close to atoms, it can knock out electrons, ionising the atom

• The properties of the different types of radiation are summarised in the table below
• Note that charge is often described as 'relative charge'

• u is the atomic mass unit (see “Atomic Mass Unit (u)”)
• e is the charge of the electron: 1.60 × 10^-19 C
• c is the speed of light: 3 × 10⁸ m s^-1

ANSWER:     D

Neutrino Emission

• An electron neutrino is a type of subatomic particle with no charge and negligible mass which is also emitted from the nucleus
• The anti-neutrino is the antiparticle of a neutrino
  • Electron anti-neutrinos are produced during β– decay
  • Electron neutrinos are produced during β+ decay