(c) doc b

7a. What happens in alpha & beta radioactive decay?

Stability band, decay modes and lots of nuclear equations

& 7b. Production of radioisotopes - artificial sources

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

RADIOACTIVITY and NUCLEAR PHYSICS INDEX

 

(c) doc b7a. What happens overall in Alpha and Beta Radioactive Decay?

and where does gamma photons fit into the equations?

IMPORTANT NOTES:

(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.

The nucleus for a specific element (specific atomic/proton number) may be unstable due to ....

(i) too many neutrons, too high a neutron/proton ratio, tend to get beta minus decay (beta electron emission)

(ii) too few neutrons, too low a neutron/proton ratio, tend to get beta plus decay (positron emission)

(iii) too high a total of protons and neutrons, tend to get alpha decay with elements of Z > 82 (Pb) (c) doc b

(c) doc b(iv) After a radioactive decay has taken place producing a new nucleus, the new nucleus may have excess energy, and so it loses energy as gamma radiation to attain its more stable state.

When you plot the number of neutrons (N) versus the number of protons (Z) you a slightly curved band of stable isotopes for most elements from Z = 1 to Z = 92. Any isotope lying above or below this stable band will tend to be unstable i.e. radioactive.

Because of all the nuclear changes that are possible it means that it is almost impossible to have a 'pure' radioisotope of any element.

See APPENDIX 1 for data graphs to elaborate on the crude little graph above right, which illustrate (i) to (iii)

(c) doc b 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

  • (c) doc b Alpha Decay e.g. a nuclear equation radioactive decay emitting an alpha particle, usually from heavy nuclei

    • (i) (c) doc b 

    • Uranium-235 is converted into thorium-90 with alpha particle and gamma photon emission

    • In alpha decay a helium nucleus, the alpha particle, of 2 protons and 2 neutrons is emitted at high speed/kinetic energy from the nucleus.

    • The residual atom, often referred to as the politically incorrect 'daughter nuclide', has a mass number of 4 less, and an atomic number of 2 less, compared to the 'parent' or original atom.

      • Sadly, apart from both Nobel Prize winning Marie Curie and her daughter (Irčne Joliot-Curie), in the late 19thC/early 20thC, nuclear physics was dominated by male scientists!

    • Note that the total mass numbers on each side of the equation must be equal to balance and similarly the atomic number totals must also be the same on each side of the nuclear equation to complete the balance.

    • How to balance the mass and charge for alpha decay equations:

      • For alpha emission balancing for (i) total mass = 235 = 231 + 4 and protons = 92 = 90 + 2

      • Note the protons are positive so the 92 = 90 + 2 = conservation of electric charges.

    • Sometimes gamma radiation is also emitted, if so, it doesn't affect the balancing of the nuclear equation because a gamma photon has zero mass and zero charge.

    • All 'heavy' atoms with an proton/atomic number of Z over 82 (Pb) have isotopes that undergo alpha decay.

      • See PLOT (3) in APPENDIX 1, where you will also see that many heavy isotopes also undergo beta decay.

    • Other examples of nuclear equations for alpha decay
      • (ii)  
        • radium-223 is converted to radon-219 with alpha particle and gamma photon emission
      • (iii)
        • americium-243 is converted into neptunium-239 with alpha particle and gamma emission
    • In each case the mass number (nucleon number) drops by 4 and the atomic/proton number decreases by two to give a different element.
    • In all cases of alpha particle emission from an unstable nucleus a new element is formed i.e. the 'transmutation' of one element to another has happened.
    • more on properties of alpha particles & uses of alpha radiation

BETA- DECAY: beta minus decay, negative electron emitted

  • Beta minus decay (electron emission) e.g. a nuclear equation to illustrate radioactive decay by beta particle emission

    • (i) (c) doc b  

    • Carbon-14 is converted into nitrogen-14 with beta particle emission.

    • The neutron/proton ratio changes from 8/6 to 7/7 (1.3 to 1.0).

    • In beta minus decay neutron in the nucleus changes spontaneously into a proton and a high kinetic energy electron forms the emitted beta particle.

    • Note to be balanced, the total mass remains constant and in this case to completely balance the equation, the electric charge must balance i.e. +6 (protons) = +7 (protons) + -1 (electron)

    • Since the proton and neutron have a mass of 1 and the electrons mass is negligible, the mass number stays the same but the atomic (proton) number rises by 1.

    • How to balance the mass and charge for beta minus decay equations:

      • For beta minus emission balancing (i) mass = 14 = 14 + 0 and for protons/beta minus charge = 6 = 7 + (-1).

      • Remember to think of the number of protons as the number of positive charges.

        • So the 6 = 7 - 1 = conservation of electric charge.

    • Beta minus decay tends to happen with isotopes with too many neutrons to be stable (too high an n/p ratio) and lies above the stability curve shown above and now better shown in APPENDIX 1 PLOTS (1) and (2).

    • By changing a neutron to a proton the n/p ratio is reduced to the nucleus of an isotope lying in the stability band.

    • Other examples of nuclear equations for beta decay
      • (ii)    
        • cobalt-60 is converted into nickel-60 with beta particle and gamma photon emission
      • (iii)
        • radium-228 is converted into actinium-228 with beta particle emission
      • (iv)  + 
        • the iodine-131 nucleus changes to a xenon nucleus with the emission of a beta particle and gamma photon.
      • (v)  
        • sodium-24 changes to the isotope magnesium-24 with emission of beta particle and a gamma photon.
    • In each case the mass number (nucleon number) stays the same but the atomic/proton number increases by one to give a different element.
    • In all cases of an beta minus particle emission from an unstable nucleus a new element is formed i.e. the 'transmutation' of one element to another has happened.

GAMMA PHOTON EMISSION

  • (c) doc b Gamma emission: Emission of gamma radiation from a nucleus does NOT involve any change in the atomic (proton) number or mass number i.e. no change in the particle composition of the nucleus.

    • When a 'new' nucleus is formed it tends to have excess energy making it potentially unstable.

    • To become more 'nuclear stable' the nucleus loses some energy as a burst of gamma radiation (a gamma photon) but the proton and neutron numbers do not change.

    • This can be observed with both alpha particle or beta particle emission e.g.

    • (alpha)  (c) doc b 

    • (beta)     
    • more on properties of gamma radiation & uses of gamma radiation

BETA+ DECAY: beta plus decay, positron (positive electron) emitted

  • Positron emission (beta plus decay) e.g. a nuclear equation to illustrate radioactive decay by positron emission

    • e.g. (i)  (c) doc b  

      • and then    e+ + e   ==> 2   

        • annihilation with the nearest available electron, so positrons don't get very far!

        • Note the conservation of electric charge i.e. (+) + (–) = 0 (gamma photons are electrically neutral), but the mass isn't conserved, but energy is, because the two particles are converted into two gamma photons, that is electromagnetic radiation energy (a case of E = mc2 !).

        • A positron is a positive electron and the antiparticle of the more familiar negative electron.

          • A pair of antiparticles have the same mass but opposite electric charges.

        • Positron emitting radioisotopes are effectively gamma photon emitters because the positrons don't get very far due to their almost immediate annihilation.

          • These short-lived isotopes are used in medical physics techniques like PET scanning.

    • magnesium-23 changes to sodium-23 with emission of positron (positive electron)

    • The neutron/proton ratio changes from11/12 to 12/11 (0.92 to 1.1).

    • In positron emission (beta plus decay), a proton changes to a neutron  and  a 'positive electron' called a positron is expelled with very high kinetic energy.

    • A positron has the same mass as an electron but carries a positive charge (it is the 'anti-matter' particle of the electron!).

    • Since the proton and neutron have a mass of 1 and the electrons mass is negligible, the mass number stays the same but the atomic (proton) number falls by 1.

    • How to balance the mass and charge for beta plus decay equations:

      • For beta plus emission balancing (i) mass = 23 = 23 + 0 and for protons/beta plus charge = 12 = 11 + (+1).

      • Remember to think of the number of protons as the number of positive charges.

        • So the 12 = 11 + 1 = conservation of electric charge.

    • Beta plus decay tends to happen with isotopes with too few neutrons to be stable (too low an n/p ratio) and lies below the stability curve shown in the graph above and now better shown in APPENDIX 1 PLOTS (1) and (2).

    • By changing a proton to a neutron the n/p ratio is increased to an isotope lying in the nuclear stability band.

    • Other examples of nuclear equations for beta plus decay (these three isotopes are used in PET scanning)
      • (ii)  189F  ===> 188O  +  0+1e
        • fluorine-18 decays to oxygen-18 plus a positron
          • fluorine-18 is made by bombarding oxygen-18 with protons
          • 188O  +  11H  ===> 189F +  10n
      • (iii)  116C  ===> 115B  +  0+1e
        • carbon-11 decays to boron-11 plus a positron
          • carbon-11 is made by bombarding nitrogen atoms with protons
          • 147N  +  11H  ===>  116C  +  42He
      • (iv)  137N  ===>  136C  +  0+1e
        • nitrogen-13 decays to carbon-13 plus a positron
          • nitrogen-13 is made by bombarding oxygen atoms with protons
          • 168C  +  11H  ===>  137N  +  42He
    • In each case the mass number (nucleon number) stays the same but the atomic/proton number decreases by one to give a different element.
    • In all cases of an beta plus particle emission from an unstable nucleus a new element is formed i.e. the 'transmutation' of one element to another has happened.
    • For quarks and advanced theory of beta plus decay see APPENDIX 2

NUCLEAR FISSION and NUCLEAR FUSION equations

  • NUCLEAR FISSION equations - not radioactive decay, but big nuclei splitting into smaller nuclei e.g.

    • (c) doc b

    • (c) doc b

    • They can be quite complicated and lots of smaller nuclei are formed, I've just picked out equations showing the formation of four possibilities.

    • For more see section 9. Nuclear Fission

  • NUCLEAR FUSION equations - not radioactive decay, but fusing smaller nuclei into bigger nuclei or a big nucleus and a small nucleus, often smaller particles like a neutrons or electrons are produced to e.g.

    • (c) doc b

    • (c) doc b

    • (c) doc b... etc.

    • (c) doc b

    • (c) doc b

    • (c) doc b

    • (c) doc b

    • You find these are balanced in exactly the same way as for radioactive decay, check out the numbers for your self.

    • All the mass numbers should be equal on each side of the equations and note the need for balancing numbers like in chemical equations, except these are nuclear equations!

    • The lower numbers must add up to on both sides of the equation, that is the electric charge must balance (proton positive or minus for electron).

    • For more see section 8. Nuclear Fusion

Advanced Chemistry Page Index and Links

7b. The production of radioisotopes - how to make artificial sources of radioactivity

  • The USES of RADIOISOTOPES are fully described in section 5.

  • Neutron bombardment is a common method used to make artificial or man-made radioisotopes.

    • I remember as a student in 1967 visiting a research reactor at Risley near Manchester, England. A round the nuclear reactor where a circular band of well protected laboratories enabled samples to be inserted into the reactor core. After neutron bombardment in the reactor, the sample could be withdrawn into special fume cupboards and processed in a safe way to extract and purify the desired product.

  • To meet the industrial and medical demand for radioactive-isotopes (as described earlier) many are made by allowing stable isotopes to be hit by neutrons in a small research scale nuclear reactor to make unstable, but useful isotopes.

  • Note again, the balancing of nuclear equations to illustrate the production of radioactive-isotopes e.g.

  • (a) (c) doc b

    • Oxygen-16 atoms are bombarded with neutrons to make the radioisotope carbon-13, used as a chemical tracer carbon in studying the mechanisms in organic chemistry reactions, you can follow what happens to a particular carbon atom i.e. follow what happens to a particular part of a molecule.

  • (b) (c) doc b

    • Sodium-23 atoms are bombarded with neutrons to make the radioisotope sodium-24, which can be used in tracer studies of animal blood circulation, an important diagnostic tool in clinical medicine.

  • (c) (c) doc b

    • Cobalt-59 atoms are bombarded with neutrons to make the radioisotope cobalt-60, used as the gamma source for cancer radiotherapy. The deadly gamma rays from the decay of the unstable cobalt-60 atoms are directed at cancer cells to kill them.

  • (d)

    • Tellurium-130 is irradiated ('bombarded') with neutrons to form the heavier isotope tellurium-131, which then undergoes beta decay to form iodine-131 used in medical tracer studies.

  • Quite a lot of useful radioisotopes are obtained from the spent fuel rods from nuclear reactors.

  • e.g. americium-241 (used in smoke alarms) is a decay product of plutonium-241 and when reprocessing nuclear fuel rods americium-241 along with many other radioisotopes can be separated by a complex chemical processing procedures. The original fuel may be uranium-238.

  • See APPENDIX 3 for the making of positron emitters (beta plus decay) in a CYCLOTRON by bombarding isotopes with protons.

  • The USES of RADIOISOTOPES are fully described in section 5.

 


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

  • Quark changes and radioactive decay

    • Quarks and beta minus decay: emission

    • If an isotope has too many neutrons in the nucleus to be stable (too high a neutron/proton ratio) it undergoes beta– radioactive decay (βdecay) to form a more stable nucleus.

      • The following changes occur in the unstable nucleus to make a more stable nucleus.

      • A neutron changes into a proton and an electron, the latter is expelled from the nucleus as a negative beta particle.

      • neutron ==> proton + electron (beta– particle)

        • Symbolically this can be expressed as ...

        •    (beta – decay)

        • Note that the total initial electrical charge (0) must be conserved, a fundamental law of physics.

          • From a neutral particle the +1 of the proton is balanced by the –1 of the electron.

      • In terms of quark changes, a down–quark changes to an up–quark in the [nucleus]

      • (neutron) ['up–quark + 'down–quark' + 'down–quark'] ==> ['up–quark' + 'up–quark' + 'down–quark'] (proton)

        • e.g. beta- decay: This happens when a molybdenum-98 nucleus decays by beta- particle emission to give an atom of technetium (higher atomic/proton number) and emission of a beta particle (negative electron).

    • Quarks and beta plus decay: emission

    • If an isotope has too many protons in the nucleus to be stable (too high a proton/neutron ratio) it undergoes beta+ radioactive decay (β+ decay) to form a more stable nucleus.

      • The following changes occur in the unstable nucleus to make a more stable nucleus.

      • A proton changes into a neutron and a positron (positive electron), the latter is expelled from the nucleus as a positive beta particle (negative electron).

      • proton ==> neutron + positron (beta+ particle)

        • Symbolically this can be expressed as ...

        •     (beta + decay)

        • Note that the total initial electrical charge (+1) must be conserved, a fundamental law of physics.

          • The original positive +1 proton particle produces +1 positron and a neutral particle.

      • In terms of quark changes, an up–quark changes to a down–quark in the [nucleus]

      • (proton) ['up–quark + 'up–quark' + 'down–quark'] ==> ['up–quark' + 'down–quark' + 'down–quark'] (neutron)

        • e.g. beta+ decay: This happens in the beta+ decay of an oxygen–15 nucleus by positron emission to give nitrogen–15 (lower atomic/proton number) and emission of a positron (positive electron)

        •  

    • For more notes about quarks see the radioactivity -atomic structure page

 


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,
and transformed into positron-emitting radiopharmaceuticals within the facility’s laboratories and are delivered to a nuclear medicine facility where they are used for PET imaging procedures.

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

188O  +  11H  ===> 189F10n

(ii) carbon-11 11C, is made by bombarding nitrogen atoms with protons

147N  +  11H  ===>  116C  +  42He

(iii) nitrogen-13 13N, is made by bombarding oxygen atoms with protons

168C  +  11H  ===>  137N  +  42He

(iv) oxygen-15 15O, is made by bombarding nitrogen with positive deuterons

167O + 21H  ===> 156O  +  32He

 A cyclotron can be used positron emitting radioisotopes (beta plus emitters) are made for PET scanning in medicine.

Other uses of particle accelerators

Particle accelerators are important complex pieces of apparatus that physicists use to investigate the most fundamental structure of nature from atomic nuclei to the various gigantic structures in the universe and how they function e.g. stars, black holes, how the 'Big Bang' began etc. etc.

By getting particles to smash into each other at speeds approaching that of light, all sorts of phenomena can be detected from the most fundamental particles that make up neutrons and protons, to super-heavy elements with life-times a tiny fraction of second.

Its very expensive technology - the Hadron Collider in Europe near Geneva, has cost billions of bounds to build and run. Its the most powerful particle accelerator ever built. It is supported by many countries and enables many scientists to collaborate with each other and share results and theoretical ideas.


RADIOACTIVITY and NUCLEAR PHYSICS INDEX

1. Atomic structure, fundamental particles and radioactivity

2. What is radioactivity? Why does it happen? What radiations are emitted?

3. Detection of radioactivity, measurement, dose units, ionising radiation sources, background radiation

4. The properties and dangers of alpha, beta & gamma radioactive emission

 5. The uses of radioactive Isotopes emitting alpha, beta or gamma radiation

6. Half–life of radioisotopes, how long does material remain radioactive? Uses of decay data & half–life values

7. Nucleus changes in radioactive decay? how to write nuclear equations? Production of Radioisotopes

 8. Nuclear fusion reactions and the formation of 'heavy elements'

 9. Nuclear Fission Reactions, nuclear power energy resources


Advanced Chemistry Page Index and Links

(c) doc b(c) doc bRADIOACTIVITY multiple choice QUIZZES and WORKSHEETS

Easier-Foundation Radioactivity Quiz

or Harder-Higher Radioactivity Quiz

 (c) doc b five word-fills on radioactivity * Q2 * Q3 * Q4 * Q5and ANSWERS!

crossword puzzle on radioactivity and ANSWERS!


ALPHABETICAL SITE INDEX for chemistry     

 Doc Brown's Chemistry 

*

For latest updates see https://twitter.com/docbrownchem

TOP OF PAGE