The Chart of Nuclei N=Z As size of nucleus increases, ratio N/Z increases because more neutrons needed to keep nucleus stable Just above stable region, nuclei have too many neutrons and are unstable.  Decay by emission of an electron (  - )  neutron converted to proton Just below stable region, nuclei have too many protons and are unstable.  Decay by emission of a positron (  + )  proton converted to neutron Very large nuclei simply have too many nucleons and decay by alpha emission (often with a gamma too) NB. A positron is the ‘anti-particle’ for an electron.

Neutron Number Proton Number Using a small section of the chart of Nuclei to plot decay chains Sometimes when an isotopes undergoes radioactive decay, it decays into another unstable isotope, so a ‘decay chain’ occurs.    -- -- ++ This decay chain starts with This emits an alpha to becomeThis emits a beta to become This emits an alpha to become The dotted section shows an imaginary  + decay from.

Energy Levels in the Nucleus The particles in the nucleus are small enough to have quantum properties, like the electrons in the atom. This means that they exist in permitted energy levels, and tend to drop towards the minimum available energy level,giving off electromagnetic radiation in the form of photons as they do so, like the electrons in the atom. Nuclei are often left in a high ‘metastable’ state after emission of an alpha or beta particle. They subsequently drop into a lower, stable state, emitting a photon. The photons which are emitted by nuclei in this way are in the gamma ray part of the EM spectrum. This is how gamma emission occurs. Example : In this case the Al nucleus is still in an excited state with 1.02 MeV of ‘surplus’ energy even after the neutrino has been emitted. The energy levels available in the Al nucleus are arranged such that there are then two possible outcomes, best shown on energy level diagrams 1.02 MeV 0.83 MeV Photon 1.02 MeV 1.02 MeV 0.83 MeV Photon 0.19 MeV Photon 0.83 MeV OR

The existence of energy levels in the nucleus, and the ‘Pauli Exclusion Principle’ which says that only a certain number of particles can occupy each energy level explains why the number of neutrons in the nucleus is limited. The half life of some isotopes in the metastable state is quite long. These isotopes can therefore be isolated to produce pure gamma emitters for medical use. is such an isotope. (The ‘m’ stand sfor ‘metastable’)

N Z Questions Pg a. N Z N = Z Actual Elements  β-β- β+β+ 2. a. X has 82 protons and 126 neutrons

3. a.Electron capture occurs when a proton rich nucleus captures an electron from the inner shell of its atom, thus converting a proton into a neutron. Similarity: Proton converted to Neutron. Difference: Nothing emitted during electron capture. Positron and neutrino emitted during positron emission. b. 4. a MeV γ photon 0.48 MeV γ photon b = 0.21MeV, so energies are 0.21 MeV and 0.27 MeV

The Chart of Nuclei N=Z As size of nucleus increases, ratio N/Z increases because more neutrons needed to keep nucleus stable Just above stable region, nuclei have too many neutrons and are unstable.  Decay by emission of an electron (  - )  neutron converted to proton Just below stable region, nuclei have too many protonss and are unstable.  Decay by emission of an positron (  + )  proton converted to neutron Very large nuclei simply have too many nucleons and decay by alpha emission (often with a gamma too)

Neutron Number Proton Number Using a small section of the chart of Nuclei to plot decay chains Sometimes when an isotopes undergoes radioactive decay, it decays into another unstable isotope, so a ‘decay chain’ occurs. This decay chain starts with This emits an alpha to becomeThis emits a beta to become This emits an alpha to become The dotted section shows an imaginary  + decay from.

Energy Levels in the Nucleus The particles in the nucleus are small enough to have quantum properties, like the electrons in the atom. This means that they exist in permitted energy levels, and tend to drop towards the minimum available energy level,giving off electromagnetic radiation in the form of photons as they do so, like the electrons in the atom. Nuclei are often left in a high ‘metastable’ state after emission of an alpha or beta particle. They subsequently drop into a lower, stable state, emitting a photon. The photons which are emitted by nuclei in this way are in the gamma ray part of the EM spectrum. This is how gamma emission occurs. Example : In this case the Al nucleus is still in an excited state with 1.02 MeV of ‘surplus’ energy even after the neutrino has been emitted. The energy levels available in the Al nucleus are arranged that there are then two possible outcomes, best shown on energy level diagrams 1.02 MeV 0.83 MeV 1.02 MeV 0.83 MeV OR

The existence of energy levels in the nucleus, and the ‘Pauli Exclusion Principle’ which says that only a certain number of particles can occupy each energy level explains why the number of neutrons in the nucleus is limited. The half life of some isotopes in the metastable state is quite long. This isotopes can therefore be isolated to produce pure gamma emitters for medical use. is such an isotope. (The ‘m’ stand sfor ‘metastable’)

Questions Pg a. N Z 2. a. b.

3. a. b. 4. a. b.