Stability band, decay modes and lots of nuclear equations
Doc Brown's Chemistry KS4 science GCSE Physics Revision Notes
In terms of radioactive sources (radioisotopes, radionuclides) what is alpha decay in terms of nuclear equations? and what is beta decay in terms of a nuclear equation? What happens in the nucleus of unstable radioactive atoms? How do we write nuclear equations to represent these nuclear changes? How do we balance nuclear equations? Why is gamma radiation emitted from atomic nuclei? What is a positron? What is positron emission? How do we make artificial (man-made) radioisotopes? Neutron bombardment of a stable isotope to make an unstable, but useful, radioactive-isotopes Balancing nuclear equations for alpha decay, beta minus decay and beta plus decay. What is a cyclotron? What does a cyclotron do and make? These revision notes on how to construct and balance nuclear equations for alpha emission decay, nuclear equations for beta minus (electron) emission decay, nuclear equations for beta plus (positron) emission decay and emission of gamma radiation should help with GCSE/IGCSE physics courses and A/AS level physics courses
(1) The emission of an alpha particle or beta particle leads to a change in the composition of a nucleus in terms of protons or neutrons. The emission of a gamma photon does NOT change the composition of the nucleus, it only lowers the energy associated with the nucleus after the radioactive decay has taken place. Gamma radiation often accompanies radioactive decay by alpha particle or beta particle emission.
(2) The mode of radioactive decay (emission), i.e. alpha (helium nucleus), beta minus (electron) and beta plus (positron), or not at all for a stable nucleus, strongly depends on the neutron/proton ratio, how high the atomic number is and the energy state of the nucleus.
Balancing nuclear equations: The changes due to radioactivity can be represented as nuclear equations and they must balance in mass and charge i.e. what ever the nature of the initial isotopes, for the new isotopes formed and the particles emitted, both the total mass and electric charge must remain the same, that is mass and charge are conserved.
ALPHA DECAY: Alpha particle emission, helium nucleus emitted
BETA- DECAY: beta minus decay, negative electron emitted
GAMMA PHOTON EMISSION
BETA+ DECAY: beta plus decay, positron (positive electron) emitted
NUCLEAR FISSION and NUCLEAR FUSION equations
APPENDIX 1 ISOTOPE STABILITY CURVE GRAPH and MODES of RADIOACTIVE DECAY
These graphs were produced using information from a data book dated 1980. I've used almost every isotope that is stable or radioactive emitting alpha, beta minus (electron) and beta plus (positron) radiation. Many are naturally occurring but I've included artificially produced radioisotopes. I know there are plenty of other isotopes but the data was quite sufficient to show the patterns e.g. the stability bands and the other bands of region of unstable radioactive isotopes with their various decay modes.
(1) Plot of proton number (atomic number) versus neutrons in the isotopes of the elements 1 to 30 (the first point is a neutron with a half-life of 10 mins!).
(2) Plot of proton number (atomic number) versus neutrons in the isotopes of the elements 1 to 70. Its only above atomic number (proton number) of ~57 you begin to see radioactive decay by emission of alpha particle.
(3) Plot of proton number (atomic number) versus neutrons in the isotopes of the elements 1 to 102.
You can see immediately that many isotopes of heavy atoms, particularly for Z >82 (Pb), now decay by alpha particle emission as opposed to just beta plus or beta minus decay. Many radioisotopes of heavy atoms also decay by beta particle emission. Although beta– decay sees the mass number staying the same and the atomic number is raised by 1, ultimately the heavy atoms well above Z = 83, decay via a complex series of changes to more stable isotopes of lead (Z = 82) because with alpha particle emission you lose 4 mass units and the atomic number reduces by 2 units. This 'up and down' of the atomic number (Z) is illustrated below with part of the uranium-238 decay series which occurs naturally in the environment e.g. in rocks containing uranium minerals ...
23892U =α=> 23490Th =β=> 23491Pa =β=> 23492U =α=> 23090Th =α=> 22688Ra =α=> 22286Rn =α=> 21884Po =α=>21482Pb
... and unstable lead-214 then decays by four beta decays and two alpha particle decays to stable lead-206 ...
21482Pb =β=> 21483Bi =β=> 21484Po =α= 21082Pb =β=> 21083Bi =β=>21084Po =α=> 20682Pb ... the half-lives of theses unstable nuclei range from a few minutes to a few million years, so the overall decay process takes many millions of years!
APPENDIX 2 RADIOACTIVE DECAY and QUARKS
APPENDIX 3 Particle accelerators - the CYCLOTRON
A cyclotron is a compact type of particle accelerator machine by which electrically charged particles (usually positive, often protons) are accelerated outwards from the centre along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated along circular paths by a rapidly varying (radio frequency) electric field.
The target stable non-radioactive isotope is placed in the cyclotron bombarded with a beam of accelerated smaller particle e.g. a proton (a hydrogen-1 nucleus), a process sometimes described as 'proton enrichment'. The protons must be accelerated to enormous speeds to have enough energy to be absorbed into another nucleus, thereby raising the atomic number by 1.
After the stable isotopes have reacted with the proton
beam to form radioactive isotopes, these are then taken from the cyclotron,
Cyclotrons are a clean nuclear technology with very little radioactive waste.
Examples of producing positron emitters for PET scanning in medicine (see uses of radioisotopes)
The equations are easy to balance in terms of top left mass numbers and bottom left proton numbers (no complications due to electrons or positrons). The decay equations for (i) to (iii) emitting positrons are given in the beta plus section above.
(i) fluorine-18 18F, is made by bombarding oxygen-18 with protons
(ii) carbon-11 11C, is made by bombarding nitrogen atoms with protons
(iii) nitrogen-13 13N, is made by bombarding oxygen atoms with protons
(iv) oxygen-15 15O, is made by bombarding nitrogen with positive deuterons
A cyclotron can be used positron emitting radioisotopes (beta plus emitters) are made for PET scanning in medicine.
Other uses of particle accelerators
RADIOACTIVITY and NUCLEAR PHYSICS INDEX
RADIOACTIVITY multiple choice QUIZZES and WORKSHEETS
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