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Neutron-to-Proton Ratio• The proton has high mass and high charge.• Therefore the proton-proton repulsion is large.• In the nucleus the protons are very close to each other.• The cohesive forces in the nucleus are called strong
nuclear forces. Neutrons are involved with the strong nuclear force.
• As more protons are added (the nucleus gets heavier) the proton-proton repulsion gets larger.
Patterns of Nuclear Patterns of Nuclear StabilityStability
Neutron-to-Proton Ratio• At Bi (83 protons) the belt of stability ends and all nuclei
are unstable.– Nuclei above the belt of stability undergo -emission. An
electron is lost and the number of neutrons decreases, the number of protons increases.
– Nuclei below the belt of stability undergo +-emission or electron capture. This results in the number of neutrons increasing and the number of protons decreasing.
– Nuclei with atomic numbers greater than 83 usually undergo -emission. The number of protons and neutrons decreases (in steps of 2).
Patterns of Nuclear Patterns of Nuclear StabilityStability
Radioactive SeriesFor 238U, the first decay is to 234Th (-decay). The 234Th undergoes -emission to 234Pa and 234U. 234U undergoes -decay (several times) to 230Th, 226Ra, 222Rn, 218Po, and 214Pb. 214Pb undergoes -emission (twice) via 214Bi to 214Po which undergoes -decay to 210Pb. The 210Pb undergoes -emission to 210Bi and 210Po which decays () to the stable 206Pb.
Patterns of Patterns of Nuclear StabilityNuclear Stability
• 90Sr has a half-life of 28.8 yr. If 10 g of sample is present at t = 0, then 5.0 g is present after 28.8 years, 2.5 g after 57.6 years, etc. 90Sr decays as follows
9038Sr 90
39Y + 0-1e
• Each isotope has a characteristic half-life.• Half-lives are not affected by temperature, pressure or
chemical composition.• Natural radioisotopes tend to have longer half-lives than
synthetic radioisotopes.
Rates of Radioactive Rates of Radioactive DecayDecay
• Einstein showed that mass and energy are proportional:
• If a system loses mass it loses energy (exothermic).• If a system gains mass it gains energy (endothermic).• Since c2 is a large number (8.99 1016 m2/s2) small
changes in mass cause large changes in energy.• Mass and energy changed in nuclear reactions are much
greater than chemical reactions.
Energy Changes in Energy Changes in Nuclear ReactionsNuclear Reactions
• For every 235U fission 2.4 neutrons are produced.• Each neutron produced can cause the fission of another
235U nucleus.• The number of fissions and the energy increase rapidly.• Eventually, a chain reaction forms.• Without controls, an explosion results.• Consider the fission of a nucleus that results in daughter
• At critical mass, the chain reaction accelerates.• Anything over critical mass is called supercritical mass.• Critical mass for 235U is about 1 kg.• We now look at the design of a nuclear bomb.• Two subcritical wedges of 235U are separated by a gun
barrel.• Conventional explosives are used to bring the two
subcritical masses together to form one supercritical mass, which leads to a nuclear explosion.
• A tokamak is a magnetic bottle: strong magnetic fields contained a high temperature plasma so the plasma does not come into contact with the walls. (No known material can survive the temperatures for fusion.)
• To date, about 3,000,000 K has been achieved in a tokamak.
Radiation Doses• The SI unit for radiation is the becquerel (Bq).• 1 Bq is one disintegration per second.• The curie (Ci) is 3.7 1010 disintegrations per second.
(Rate of decay of 1 g of Ra.)• Absorbed radiation is measured in the gray (1 Gy is the
absorption of 1 J of energy per kg of tissue) or the radiation absorbed dose (1 rad is the absorption of 10-2 J of radiation per kg of tissue).
Biological Effects of Biological Effects of RadiationRadiation
• The -particles produced have a high RBE.• Therefore, inhaled Rn is thought to cause lung cancer.• The picture is complicated by realizing that 218Po has a
short half-life (3.11 min) also:218
84Po 21482Pb + 4
2He
Biological Effects of Biological Effects of RadiationRadiation