C26 Nuclear Physics

Question 1

what is mass

Answer 1

Mass is a property that all energy forms exhibit

Question 2

Einsteins mass-energy equation:

Answer 2

change E = change m x c^2

-Rest mass, mass an object has when an object is stationary.
-Mass increases with any form of energy it holds when not at rest mass (e.g thermal, kinetic and friction).

Question 3

Mass is a form of energy. How does the interaction of an electron - positron pair illustrates this idea well?

Answer 3

Particles completely destroy each other and the entire mass of the particles is transformed into 2 gamma photons.

Question 4

Energy has mass. The change in mass of an object (or system) is related to …

A moving ball has KE, implying that it has greater mass than…

Similarly a decrease in energy in system means that …

Answer 4

the change in its energy.

its rest mass.

the mass of a system must also decrease (e.g hot cup of tea).

Question 5

The total amount of … and … in a system is conserved

Answer 5

mass
energy

Question 6

Since energy is released during decay, there must be an accompanying decrease in mass.

what does this mean in terms of the mass of the parent and daughter nuclei?

Answer 6

Means that the total mass of the daughter nucleus and alpha particle is less than the mass of the parent nucleus.

Beta decay also accompanied by a decreases in mass.

Question 7

Pair Production

Answer 7

The replacement of a single photon with a particle and a corresponding antiparticle of the same total energy.

Question 8

Collision of particles (rest mass and energy).

Answer 8

The total rest mass of particles after a collision is greater than that before. The increase of m multiplied by c^2 must be equal to the minimum KE of the colliding protons.

Question 9

(Binding Energy)
Why is the total mass of separated nucleons in comparison to that mass of the deuterium nucleus, is greater?

Answer 9

As work is done to separate the bond between the nucleons.

Question 10

The mass defect of a nucleus, defined as…

Answer 10

difference between the mass of the completely separated nucleus and the mass of the nucleus.

Question 11

Binding Energy

Answer 11

The minimum energy required to completely separate a nucleus into its constituent protons and neutrons.

Question 12

What is a more useful quantity to use other than just general Binding Energy, and why?

Answer 12

Binding energy per nucleon is a more useful quantity as it compares the binding energy to the no. of nucleons in an atom.

Question 13

The greater the binding energy per nucleon, the more what?

Answer 13

Tightly bound are the neurons within the nucleus, the nucleus is more stable if it has a greater BE per nucleon.

Question 14

… has the highest BE per nucleon, making it what?

Answer 14

Iron

the most stable isotope in nature.

Question 15

Out of all the elements what 3 come across as anomalies, in terms of binding energy, and why?

Answer 15

Helium, oxygen and carbon all anomalies. Have a higher BE per nucleon then expected, making them fairly stable isotopes in comparison to others.

Question 16

How does nuclear fusion occur in stars, and what forces are in play?

Answer 16

-In stars, small nuclei, fused together, produce larger nuclei and energy, must first overcome electrostatic repulsion, requires high temps and pressures (occur due gravitational pressure generated by outer layers of star pressing inwards on star’s core under gravity).

In main sequence stars, stellar fusion occurs via the proton-proton chain in which lone protons in the star’s plasma fuse to form an unstable 2/2 He nuclei. This particle then undergoes beta decay forming deuterium nuclei (½ H , an isotope of hydrogen). The next reaction in the chain fuses deuterium with a further proton to form 2/3 He and so the process continues forming nuclei up to 2/4 He.

In larger stars, fusion occurs predominantly via the CNO cycle in which carbon, nitrogen and oxygen are synthesised. The heaviest elements are synthesised by fusion that occurs as a more massive star undergoes a violent supernova at the end of its life in a process known as supernova nucleosynthesis.

Question 17

why do you need to bring nuclei very close together for nuclear fusion to take place, and how close?

Answer 17

-Bring nuclei close together, 10^-15m, so short-range strong nuclear force attract them together.

-All nuclei +ve charge, so repel, at low temp, nuclei cannot get close enough to trigger a fusion reaction.

Question 18

Artificial Fusion Reactors

Answer 18

Currently being developed (alternative power sources, has no radioactive by-products). However, extremely high temps and pressures needed maintain fusion, can only operated, short periods time.

Question 19

when does fission occur?

Answer 19

Nuclear fission occurs when, nucleus becomes unstable on absorbing another particle (such as a neutron).

Question 20

what occurs during a fission reaction?

Answer 20

Fission, breaking apart of large nuclei into small nuclei, causing reduction total binding energy, a mass defect and energy to be released.

Question 21

How does uranium-235 undergo fission?

Answer 21

-Typically, uranium-235 is used, a fissile material, as it easily undergoes fission and is relatively abundant.

-A low speed, thermal neutron (A neutron in fission reactor with mean kinetic energy similar to thermal energy of particles in the reactor core, also known as a slow neutron), fired at U-235 nuclei which absorbs the extra nucleon, becomes the unstable U-236 isotope.

-Isotope then either decays via fission (85% chance), splitting into two smaller daughter nuclei and more fast neutrons or decays via gamma emission.

-Released neutrons interact with other uranium nuclei, the neutrons cause further reactions.

Question 22

Why is the mass of daughter nuclei, always less than mass before?

Answer 22

Excess mass transformed into energy. Total energy released, combination KE of neutrons and daughter nuclei and the energy of the photons.

Question 23

Why is the binding energy is higher after fission?

Answer 23

As it looses mass, and as a result proves that there has been, release of energy. After decay, daughter nuclei become more stable, hence the binding energy per nucleon, greater than parent nuclei.

Question 24

Environmental Impacts, of Fission Reactors:

Answer 24

-Products fission reactions, typically radioactive, known toxic waste.

-E.g Plutonium-239, half-life ~24,000 years, remains hazardous for millenia. Cannot
simply disposed off, must be contained way that is future proof.

-Waste often buried deep underground, so cannot be accessed. Burial locations must safe from attack, and designed be protected against earthquakes.

Question 25

How is the rate of fission reactors controlled, and why?

Answer 25

Rate reaction, controlled, control rods (e.g. boron, cadmium), absorb thermal neutrons, prevent, further fission reactions. Used, reduce rate reaction, if deemed too high e.g reactor core overheating.

Question 26

How does fission reactors work?

Answer 26

-Fissile materials sit inside a reactor core, (surrounded by a thermal coolant, absorbs the thermal energy from fission and transforms).

-Thermal energy into KE , through turbines, turns the generator, transferring into electrical energy i.e AC.

-Fuel rods within the core contain the fissile material and surrounded by the moderator and control rods.

Question 27

The absorption of neutrons and consequent fission, more likely occur with…

Answer 27

slower neutrons, meaning neutrons released by fission process are too high energy to continue the reaction. Slowed by moderator (e.g. water, graphite), increasing chance, chain of U-235 fission reactions.