Nuclear Reactions and Radioactivity Part II 2

Chemical vs. Nuclear Reactions Chemical Reactions - atoms are rearranged to form new substance(s). Involve changes in electrons Nuclear Reactions - nuclei of atoms change to form a new element. Involve changes in protons and neutrons Nuclear Transmutation

Nuclear Transmutation -Transformations The induced conversion of one nucleus into another (change of one element into another). Usually accomplished using a particle accelerator. 13 27 2 4 15 30 1 Al He P n + ® 98 249 8 18 106 263 1 4 Cf O X n + ® 6

A Schematic Diagram of a Cyclotron Accelerator Used to accelerate particles to produce: radioisotopes used in medical applications synthesis of the transuranium elements studying the fundamental nature of matter. In order to transform an element, the element must be bombarded with a particle with enough velocity/energy to overcome the repulsive forces of the target nucleus. Beginning in the 1930s, particle accelerators were invented to give particles high kinetic energy. The particles were placed in an electric field, usually in combination with a magnetic field. Particles are introduced at one end of the tube and attracted to the other end by a potential difference. When the particle reaches the gap between the two D-shaped electrodes (“dees”), it is repelled by one dee and attracted by the other. The particles move in a spiral path, so the cyclotron can be much smaller than a linear accelerator.

A Schematic Diagram of a Linear Accelerator A series of of separated tubes of increasing length that change their charge from (+) to (-) in unison with the movement of the particle through them. The voltage of each section is alternated, such that a (+) particle is repelled from the section it is leaving and attracted to the section it is entering causing the particle to accelerate in speed.

Sample Problem: Transmutations 94 239 1 Pu n ® + ? He 4 2 ?= X A Z Mass (A) 239 + 4 = ? + 1 243 = 242 + 1 243 = 243 Cm 242 96 ?= Charge (Z) 94 + 2 = ? + 0 96 = 96 + 0 96 = 96

Nuclear Binding Energy Stability of Nucleus Nuclear Binding Energy The amount of energy required to separate the nucleus (1 mole of nuclei) into protons and neutrons. Nucleus + Binding --> Protons + Neutrons Energy The greater the binding energy for a nucleus, the greater the stability

Nuclear Binding Energy Stability of Nucleus: Nuclear Binding Energy According to Einstein's mass energy equation: E=mc2 where m = mass defect This expression indicates that the mass defect (m) which is lost in the formation of stable nucleus is converted into energy. This amount of energy must be released when nucleons are combined to form a stable nucleus. This is the binding energy that holds the nucleons in a nucleus so that despite strong repulsive forces between protons they are forced to unite in the nucleus.

Binding energy per nucleus as a function of mass number Stability of Nucleus: Binding energy per nucleus as a function of mass number

Thermodynamic Stability of Nucleus: The Mass Defect: Careful measurements have shown that the mass of a particular atom is always slightly less than the sum of the masses of the individual neutrons and protons of which the nucleus of the atom consists. The difference between the mass of the atom and the sum of the masses of its parts is called the Mass Defect (m). Mass Defect - The difference between the mass of a nucleus and the sum of the masses of its constituent nucleons. Show overhead

Thermodynamic Stability of Nucleus: The Mass Defect: A much larger mass change accompanies a nuclear process due to the enormous energy required to break the nucleus apart or bind the nucleus together. Energy change =  E = mxc2 2 c E m D = (recall that m = mass defect)

Thermodynamic Stability of Nucleus: The Mass Defect Law of Conservation of Mass and Energy: The total quantity of mass-energy in the universe is constant. When a reaction releases or absorbs energy, a loss or gain in mass must accompany the reaction.

Sample Problem: Mass Defect Thermodynamic Stability of Nucleus Sample Problem: Mass Defect Consider the example of deuteron (1H2) which contains: Number of protons = 1 Number of neutrons = 1 Mass of deuteron (1H2) = 3.3431 x 10-27 kg Mass of proton = 1.6727 x 10-27 kg Mass of neutron = 1.6748 x 10-27 kg Determine the MASS DEFECT.

Problem Solution: Mass Defect Thermodynamic Stability of Nucleus Problem Solution: Mass Defect For deuteron (1H2): Sum of the nuclide masses: = 1.6727 x 10-27 kg + 1.6748 x 10-27 kg = 3.3472 x 10-27 kg Difference of mass: = 3.3472 x 10-27 kg - 3.3431 x 10-27 kg Dm = 0.004 x 10-27 kg This difference in mass is the MASS DEFECT

Fission The splitting a heavy nucleus into two nuclei with smaller mass numbers accompanied by a large release of energy.

Fission Neutrons are also produced during fission which makes it possible to have a self-sustaining fission process, i.e., a chain reaction. If exactly one neutron from each fission event causes another fission event, the process will be self-sustaining. A critical mass of fissionable material is required to obtain the proper number of neutron to be self-sustaining.

Fission - Subcritical Mass (A fission event that does not sustain)

Fission - Supercritical Mass (A fission event that is sustained)

Fission - Supercritical Mass (A fission event that is sustained) Neutrons produced during fission make it possible to have a self-sustaining fission process, i.e., a chain reaction, if exactly one neutron from each fission event causes another fission event, the process will be self-sustaining.

Fusion Fusion is the combining of two light nuclei to form a more stable nucleus accompanied by a large release of energy. Energy He H 3 2 1 + ®

Fusion

Both fission and fusion produce more stable nuclides Stability of Nucleus: Both fission and fusion produce more stable nuclides

Applications of Fission and Fusion Controlled: Nuclear Energy Reactors Uncontrolled: Atomic Bomb FUSION: The hydrogen bomb Development of fusion as a direct energy source

Schematic Diagram (Nuclear Power Generation)

Schematic Diagram (Conventional Power Generation)

Schematic Diagram of the Reactor Core

Fusion as an Energy Source He H 1 4 2 3 + Difficulties: Requires enormous heat energy to give the (+) nuclei enough kinetic energy to force together Gaseous plasma results at these high temperatures making containment very difficult.