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Nuclear Reactions

Chapter 19

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Facts About the Nucleus• Very small volume compared to volume of atom• Essentially entire mass of atom

– Very dense

• Composed of protons and neutrons that are tightly held together– Nucleons

• Every atom of an element has the same number of protons– Atomic Number

• Isotopes are atoms of the same elements that have different masses– Different numbers of neutrons– Mass Number = number of protons + neutrons

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Facts About the Nucleus• The number of neutrons is calculated by

subtracting the atomic number from the mass number

• The nucleus of an isotope is called a nuclide– less than 10% of the known nuclides are

nonradioactive, most are radionuclides

• Each nuclide is identified by a symbol– Element -Mass Number = X-Z

X Element ZA

number massnumber atomic

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Radioactivity• Radioactive nuclei spontaneously decompose into smaller

nuclei– Radioactive decay– We say that radioactive nuclei are unstable

• Decomposing involves the nuclide emitting a particle and/or energy

• During radioactive decay, atoms of one element are changed into atoms of a different element – In order for one element to change into another, the

number of protons in the nucleus must change– All nuclides with 84 or more protons are radioactive

• We describe nuclear changes with using nuclear equations– atomic numbers and mass numbers are conserved

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alpha decay• an particle contains 2 protons and 2 neutrons

– helium nucleus

• loss of an alpha particle means

– atomic number decreases by 2

– mass number decreases by 4

Rn He Ra 21686

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beta decay• a particle is like an electron

moving much faster found in the nucleus

• when an atom loses a particle itsatomic number increases by 1mass number remains the same

• in beta decay a neutron changes into a proton

Pa e Th 23491

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23490

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gamma emission• Gamma () rays are high energy photons• Gamma emission occurs when the

nucleus rearranges• No loss of particles from the nucleus• No change in the composition of the

nucleus– Same atomic number and mass number

• Generally occurs whenever the nucleus undergoes some other type of decay

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positron emission• positron has a charge of +1 c.u. and negligible mass

– anti-electron• when an atom loses a positron from the nucleus, its

– mass number remains the same– atomic number decreases by 1

• positrons appear to result from a proton changing into a neutron

Ne e Na 2210

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electron capture• occurs when an inner orbital electron is pulled into the

nucleus• no particle emission, but atom changes

– same result as positron emission• proton combines with the electron to make a neutron

– mass number stays the same– atomic number decreases by one

Au e Hg 200

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Artificial Nuclear Transformation• Nuclear transformation involves changing

one element into another by bombarding it with small nuclei, protons or neutrons

• reaction done in a particle accelerator– linear– cyclotron

• made-made transuranium elements

Cf n 4 X U 24698

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23892

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Detecting Radioactivity• To detect something, you need to identify

something it does• radioactive rays cause air to become ionized• Geiger-Müller Counter works by counting

electrons generated when Ar gas atoms are ionized by radioactive rays

• radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical

• a scintillation counter is able to count the number of flashes per minute

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Half-Life• Not all radionuclides in a sample decay at once• The length of time it takes one-half the

radionuclides to decay is called the half-life• Even though the number of radionuclides

changes, the length of time it takes for half of them to decay does not– the half-life of a radionuclide is constant

• Each radionuclide has its own, unique half-life• The radionuclide with the shortest half-life will

have the greater number of decays per minute– For samples of equal numbers of radioactive atoms

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Half-Life• half of the radioactive atoms decay each half-life

Radioactive Decay

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time (half-lives)

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Half-Life• First order rate reaction as we’ve done

before

• k = 0.693/t1/2 or

• ln Xo/X = kt

• Activity measured in Bequerels (Bq), number atoms decaying per second

• Also measured in curies (Ci) 3.700 x1010 atoms per second

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Object Dating• mineral (geological)

– compare the amount of U-238 to Pb-206

– compare amount of K-40 to Ar-40

• archeological (once living materials)– compare the amount of C-14 to C-12

– C-14 radioactive with half-life = 5730 yrs.

– while living, C-14/C-12 fairly constant• CO2 in air ultimate source of all C in body

• atmospheric chemistry keeps producing C-14 at the same rate it decays

– once dies, C-14/C-12 ratio decreases

– limit up to 50,000 years

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Medical Uses of Radioisotopes,Diagnosis

• radiotracers– certain organs absorb most or all of a particular element– can measure the amount absorbed by using tagged

isotopes of the element and a Geiger counter– use radioisotope with short half-life– use radioisotope low ionizing

• beta or gamma

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Mass energy relations• The energy change in a nuclear reaction can be

determined by finding the change in mass and multiplying by the speed of light squared

E = mc2

• To find the change in mass use products – reactants. Use the masses in amu from Table 19.3 (can’t use periodic table because these are average atomic masses)

• Nuclear binding energy– A nucleus weighs less than the protons and neutrons it

is composed of. The difference is called mass defect and the energy change is called binding energy. Use E=mc2 to find

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Other Nuclear Changes

• a few nuclei are so unstable, that if their nucleus is hit just right by a neutron, the large nucleus splits into two smaller nuclei - this is called fission

• small nuclei can be accelerated to such a degree that they overcome their charge repulsion and are smashed together to make a larger nucleus - this is called fusion

• both fission and fusion release enormous amounts of energy

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Fissionable Material• fissionable isotopes include U-235, Pu-239, and

Pu-240• natural uranium is less than 1% U-235

– rest mostly U-238– not enough U-235 to sustain chain reaction

• fission produces about 2.1 x 1013 J/mol of U-235– 26 million times the energy of burning 1 mole CH4

• to produce fissionable uranium the natural uranium must be enriched in U-235

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Fission Chain Reaction• a chain reaction occurs when a reactant is also a

product– in the fission process it is the neutrons– only need a small amount of neutrons to keep the

chain going• many of the neutrons produced in the fission are

either ejected from the uranium before they hit another U-235 or are absorbed by the surrounding U-238

• minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass

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Nuclear Power Plants

• use fission of U-235 or Pu-240 to make heat

• heat picked up by coolant and transferred to the boiler

• in the boiler the heat boils water, changes it to steam, which turns a turbine, which generates electricity

• the fission reaction takes place in the reactor core

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Nuclear Power Plants - Core• the fissionable material is stored in long tubes

arranged in a matrix called fuel rods– subcritical

• between the fuel rods are control rods made of neutron absorbing material– B and/or Cd– neutrons needed to sustain the chain reaction

• the rods are placed in a material used to slow down the ejected neutrons called a moderator– allows chain reaction to occur below critical mass

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Breeder Reactor

• Design common in Europe

• Makes its own fuel by converting U-238 to Pu-239

• Use liquid sodium as a moderator

• Use water filled radiator to transfer heat to boiler

• Plutonium highly toxic and spontaneously combusts in air

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Nuclear Fusion

• Fusion is the process of combining two light nuclei to form a heavier nucleus

• The sun’s energy comes from fusion of hydrogen to produce helium

• Releases more energy per gram than fission• Requires high temperatures and large

amounts of energy to initiate, but should continue if you can get it started

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Factors that Determine Biological Effects of Radiation

The more energy the radiation has the larger its effect can be The better the ionizing radiation penetrates human tissue, the

deeper effect it can have– Gamma >> Beta > Alpha

The more ionizing the radiation, the more effect the radiation has– Alpha > Beta > Gamma

The radioactive half-life of the radionuclide° The biological half-life of the element± The physical state of the radioactive material• The amount of danger to humans of radiation is measured in

the unit rems

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Somatic Damage

• Somatic Damage is damage which has an impact on the organism– Sickness or Death

• May be seen immediately or in the future– Depends on the amount of exposure– Future effects include cancer

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Genetic Damage

• Genetic Damage occurs when the radiation causes damage to reproductive cells or organs resulting in damage to future offspring

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