Nuclear Physics

Question 1

Forces in the Nucleus:

Answer 1

Within a nucleus, protons exert electrostatic forces of repulsion on each other. These forces can act over a large distance.
The forces of repulsion between the protons can tear the nucleus apart.

However there is another natural force present in the nucleus, called strong nuclear force. Strong nuclear force is a force of attraction that holds the nucleus together. Strong nuclear force is present between every nucleon regardless of their charge. This force is much stronger then electrostatic force of repulsion however it only acts over a short distances.

Question 2

Transmutation

Answer 2

Most atoms that make up the world are present, and are likely to stay like that in the future.

However there are naturally occurring isotopes that are unstable: their nuclei are likely to change in the future.

An unstable nucleus may become stable by changing into a different element by releasing particles as it does so. This is called spontaneous transmutation.

Unstable Nuclei that emit particles and/or radiation are said to be radioactive.

Question 3

Stability of a Nuclei

Answer 3

As the number of a protons in a nuclei increase, more neutrons are needed to keep the nucleus stable.

These neutrons dilute the repulsive forces between the protons by increasing the average distance between them.

Question 4

Alpha Decay:

Answer 4

When an heavy unstable nucleus undergoes radioactive decay it may eject an alpha particle.

An alpha particle consist of 2 protons and 2 neutrons that is positively charged with no orbiting electrons.

Question 5

Beta-Negative Decay:

Answer 5

Beta negative decay is a when a nucleus has a relatively large number of neutrons compared to the number of protons.

A neutron splits into a proton, and electron and a antineutrino particle. The proton stays in the nucleus while the other two are emitted. Since the nucleus has gained a proton it becomes a different element.

Question 6

Beta - Positive Decay:

Answer 6

Beta positive decay is when a nucleus has relatively large number of protons compared to the number of neutrons. A proton in the nucleus decays to a neutron, positron and a neutrino.

The neutron stays in the nucleus while the other two are emitted. Since the nucleus has lost a proton it becomes a different element.

A positron is like a electron in terms of mass but of the opposite charge.

Question 7

Gamma Decay:

Answer 7

Gamma Decay in which some unstable atomic nuclei dissipates excess energy by spontaneously emitted gamma rays.

Gamma rays are photons of extremely short wavelength.

The letter M on the mass symbol shows that it is meta stable and indicates that the nucleus is in its energetic state.

Question 8

Alpha Radiation:

Answer 8

Alpha particles consist of 2 protons and 2 neutrons. They are heavy and slow moving moving at less then 10% of the speed of light.

Because of significant mass and charge it can ionize hundreds of thousands of atoms. Hence it has high ionizing ability.

Each interaction slows it down, and eventually it absorbs two electrons from its environment to become an ordinary helium atom.

Alpha particles can only travel a few centimeters in the air and is easily stopped by paper or the outer layers of the skin. Hence it has low penetrating ability.

Question 9

Beta Decay - Negative:

Answer 9

Beta particles are much lighter then alpha particles and travel much faster at 90% of the speed of light. They also have a smaller charge of +1 instead of +2.

Beta negative particles are fast moving particles, however due to reduced mass and charge compared to alpha they have a moderate ionizing ability.

Because of their negative charge, beta negative particles are prone to be repelled by an atoms electron cloud, as a result they have glancing collisions and loose energy every interaction.

This gives beta negatives particles moderating penetrating ability as it travel through a few centimeters of air but is stopped as little as 1mm of aluminum.

Question 10

Beta-Positive Particles:

Answer 10

Fast Moving Electrons
Positrons are the antimatter counterparts of electrons. This means that when a positron collides with a electron both particles are transformed into pure energy in the form of gamma rays.

Beta positive has very high ionizing ability but very low penetrating.

Question 11

Gamma Rays:

Answer 11

Gamma Rays are electromagnetic radiation with very high frequency.
They have no mass or charge, they travel at the speed of light.
The probability of gamma rays interacting with matter is very low. As a result they have low ionizing ability and very high penetrating ability.

Question 12

Measuring Radiation Exposure:

Answer 12

When ionizing radiation interacts with an organism, it creates ions which can damage tissue or lead to the development of cancer.

Question 13

Half Life

Answer 13

The half life of a radioactive material is the time that it takes for half of the remaining radioactive nuclei in the sample to decay.

Question 14

Fission

Answer 14

Nuclear Fission occurs when an atomic nucleus splits into two or more smaller nuclei.

This is often triggered by the absorption of a neutron.

Nuclides that are capable of undergoing fissions are said to be fissile.

Question 15

Products of Fission

Answer 15

In addition to a new nuclei called daughter nuclei, nuclear fission can also release neutrons.

Question 16

E = mc^2

Answer 16

The mass of a particle is directly proportional to its energy, This means that any reaction that absorbs or releases energy must be accompanied by a change in mass. This change is far too small to detect in chemical reactions but its significant in nuclear as the amount of energy releases is significantly higher.

Question 17

Binding Energy

Answer 17

To separate a nucleus into individual nucleons a significant amount of energy would be required.
This amount of energy is called nuclear binding energy of the nucleus.
Adding this energy would increase the mass of the nucleons according to E = mc^2. The difference in mass is the mass defect of the nucleus.

Question 18

Chain Reaction

Answer 18

A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus.

This nucleus in turn produces neutrons and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons).

Question 19

Fuel Rods

Answer 19

When uranium is mined about 99% of it is U-238 which is not fissile. About 0.7 is U-235. Most reactors require fuel that is 3.5% U-235.

The process of refining uranium ore to raise the concentration of U-235 is called enrichment.

Pellets of enriched uranium are then packed into long cylinders called fuel rods.

Question 20

Moderator

Answer 20

When free neutrons are released by a fission reactions, they are moving too fast to cause fission in other atoms. To slow them down, the space between the fuel rods is filled with a material called a moderator. Heavy water is ideal for this as it is much less likely to absorb the neutrons.
Free neutrons slow down as they bounce off the moderators atoms.

Question 21

Control Rods

Answer 21

They are used to slow down a chain reaction when its going too fast.

When the control rods are lowered into the reactor vessel, they begin to absorb some of the neutrons that are being released from the fuel rods. This slows the reaction down and can even stop it entirely. Control rods are made of materials that absorbs neutrons easily, such as boron carbide.

Question 22

Coolant and Shielding:

Answer 22

1. A coolant is circulated through a nuclear reactor to transfer heat from the core to the generators. In some types of reactors the coolant is isolated from the rest of the system because the reactor makes it temporarily radioactive.

2.Reactors are enclosed in a steel case and surrounded by several meters of concrete to prevent radiation from escaping.

Question 23

Radioactive Waste:

Answer 23

Radioactive waste is produced at all stages of nuclear fuel cycle from the mining of uranium ore to the use of nuclear reactors.

1.Low level waste is buried underground.

2.Intermediate level waste is put in steel drums and encased in concrete for storage underground.

3.HIgh level waste is fused into glass blocks and stored undergound.

Question 24

Nuclear Fusion

Answer 24

It occurs when two small nuclei join to form a larger nucleus. This released large amounts of energy. Fusion reactions produce 1000 times more energy per gram than their fission counterparts.

Question 25

Limitations of Fusion

Answer 25

They only occur at very high temperatures and or pressures.

With current technology the amount of energy needed to create a fusion reaction is much greater than the amount that comes out the reaction. This makes it infeasible as energy source. However it produces large amounts of energy with very little waste so its an area of ongoing research.

Question 26

Uses of Radioisotope - Agriculture

Answer 26

1. Small pest can be sprayed with radioactive material which are consumed by the pest. After some time has passed a variety of predators in the area will be captured for analysis.

2.Scientist detect the Prescence of radioisotopes and the idea is that the predator that has consumed the most pest will have the highest concentration of radioactive material.

3.With this knowledge researchers can release more of the identified predator species into the area.

Question 27

Uses of Radioisotopes - Tracers in plants

Answer 27

Phosphorus is an important part of the chemical process involved in growth and reproduction in plants. Phosphorus 32 is a radioisotope of phosphorus however it is chemically similar to Phosphorus 31 (used the same way by plants).

Phosphorus 32 can be injected into the roots of a plant. A Geiger counter is then used to detect movement and concentration of the phosphorus 32 throughout a plant. This helps biologist get an understanding of how the plants grows and reproduces.

Question 28

Uses of Radioisotopes - sterile insect Technique

Answer 28

1.Male insects can be sterilized using gamma radiation and then released into the wild. They mate with females which then lay eggs that are infertile. Overtime this reduces the population of that insect.

Question 29

Uses of Radioisotopes - Manufacturing industry.

Answer 29

1. Can be used to control the thickness of a roller sheet of paper, plastic, or metal

2.With the help of beta radiation is used as it can be stopped by thicker metal but passes through thinner metal. If the material is too thick the reading decreases and the rollers are adjusted to make it thinner. And vice versa.

Question 30

Uses of Radioisotopes - Leaks in underground pipes

Answer 30

Leaks in underground pipelines can be detected by adding a small amount of gamma emitting radioisotope to the fluid. An area above the ground where a high concentration of radiation is detected corresponds to a leak in the pipe. This saves time and effort as only the damaged section needs to be dug up.