More About Decay Modes

Nuclear Physics Lesson 9

HomeworkHomework

Revise for the skills test.Revise for the skills test.

HWK days: HWK days:

Tues Wk 1 Period 4Tues Wk 1 Period 4

Mon Wk 2 Period 2Mon Wk 2 Period 2

Thurs Wk 2 Period 4Thurs Wk 2 Period 4

Nuclear Physics Test - some time before Nuclear Physics Test - some time before half term.half term.

Learning ObjectivesLearning Objectives

To interpret the N-Z chart for radioactive To interpret the N-Z chart for radioactive isotopes.isotopes.

To know how To know how αα, , ββ-- and and ββ+ + decay are decay are represented on the N-Z chart.represented on the N-Z chart.

To apply this to a radioactivity series.To apply this to a radioactivity series.

To know how to represent nuclear energy To know how to represent nuclear energy levels.levels.

The N-Z GraphThe N-Z Graph

N-Z PlotN-Z Plot

Notes on the N-Z PlotNotes on the N-Z Plot

For light elements (Z < 20), N is roughly For light elements (Z < 20), N is roughly equal to Z for stable protons, i.e. equal equal to Z for stable protons, i.e. equal numbers of protons and neutrons.numbers of protons and neutrons.

As the elements get heavier, more As the elements get heavier, more neutrons are needed for the strong force neutrons are needed for the strong force to overcome the Coulomb repulsion (50% to overcome the Coulomb repulsion (50% more N than Z for the heaviest elements).more N than Z for the heaviest elements).

so the line of stability bends upwards.so the line of stability bends upwards.

Notes on the N-Z PlotNotes on the N-Z Plot

αα emitters (Z > 60) emitters (Z > 60)

ββ-- emitters (above the stability line) emitters (above the stability line)

ββ++ emitters (below the stability line) emitters (below the stability line)

Decays Represented on Decays Represented on N-Z PlotN-Z Plot

You need to know how to show You need to know how to show αα, , ββ-- and and ββ++..

THIS GRAPH IS THE WRONG WAY THIS GRAPH IS THE WRONG WAY ROUND!ROUND!

ExerciseExercise

Draw the N-Z graph for known isotopes.Draw the N-Z graph for known isotopes.

Indicate the line of stability and describe Indicate the line of stability and describe how and why it changes as Z is increased.how and why it changes as Z is increased.

Indicate where the Indicate where the αα, , ββ-- and and ββ+ + emitters emitters will appear on the graph.will appear on the graph.

Explain why they appear in these regions Explain why they appear in these regions of the graph.of the graph.

Nuclear Energy LevelsNuclear Energy Levels

After a nucleus has emitted an alpha or a After a nucleus has emitted an alpha or a beta particle, the daughter nucleus may beta particle, the daughter nucleus may be in an excited state.be in an excited state.

This will cause it to emit a photon as it This will cause it to emit a photon as it de-excites.de-excites.

It works just like electrons making It works just like electrons making transitions between atomic energy levels transitions between atomic energy levels – – but it takes place in the nucleusbut it takes place in the nucleus

Calculate the photon Calculate the photon wavelength?wavelength?

ExerciseExercise

Describe the two ways Describe the two ways that a 0.83 MeV that a 0.83 MeV photon can be photon can be emitted?emitted?

Explain how the beta Explain how the beta decay of Mg-27 can decay of Mg-27 can result in either 1 or 2 result in either 1 or 2 photons being emitted.photons being emitted.

Calculate the 3 photon Calculate the 3 photon frequencies possible.frequencies possible.

A Bigger PictureA Bigger Picture

SummarySummary

A graph of neutron number against proton A graph of neutron number against proton number shows that there are more number shows that there are more neutrons in larger nuclei.neutrons in larger nuclei.

This is needed to ensure stability of the This is needed to ensure stability of the nuclei. nuclei.

Excited nuclei give off gamma rays after Excited nuclei give off gamma rays after emitting an alpha particle or a beta emitting an alpha particle or a beta particle.particle.

RecapRecap

How are gamma rays emitted by the How are gamma rays emitted by the nucleus?nucleus?