Chapter 18

Question 1

What are the 4 different kinds of radioactive decay?

Answer 1

Alpha, beta-minus, beta-plus, gamma

Question 2

Describe alpha decay

Answer 2

Consists of a helium nucleus: 2 protons and 2 neutrons. +2 charge, mass 4u. Strongly ionising, slow speed, absorbed by a paper/few cm of air, affected by a magnetic field

Question 3

Describe beta decay

Answer 3

2 types: beta-minus and beta-plus. Minus is an emission of an electron; plus is an emission of a positron. Minus is weakly ionising, fast speed, absorbed by 3mm of aluminium, affected by a magnetic field. Plus is annihilated by an electron pretty quickly, so virtually 0 range

Question 4

Describe gamma decay

Answer 4

Short-wave, high-frequency electromagnetic wave. Very weakly ionising, travels at speed of light, absorbed by many cm of lead/several m of concrete, not affected by a magnetic field.

Question 5

What is the absorbed dose?

Answer 5

The amount of energy of radiation absorbed per kilogram of tissue, measured in grays (Gy)
absorbed dose = energy/mass

Question 6

What is the effective dose?

Answer 6

A measure that allows you to compare the amount of damage to body tissues that have been exposed to different types of radiation (takes into account how ionising the radiation is as well as the type of tissue). Measured in sieverts (Sv)
Effective dose = absorbed dose x radiation quality factor

Question 7

How do alpha particles ionise stuff?

Answer 7

Alpha particles are strongly positive and therefore easily pull electrons off atoms, ionising them. This transfers some of the energy of the alpha particle to the atom. The alpha particle quickly ionises many atoms (10000) and loses all its energy.

Question 8

How do beta-minus particles ionise stuff?

Answer 8

Travels at a high speed, so can still knock electrons off atoms, even though isn’t as charged/big as an alpha particle. Each beta particle ionises about 100 atoms, losing energy at each interaction.

Question 9

How do we measure risk in radioactivity?

Answer 9

2 parts: how likely is it that the radiation will cause a problem and how bad it would be if it happened. Nuclear reactor melt down would be really bad but unlikely; radiation can cause cancer, but used in cancer treatments to destroy tumours, prolonging life: benefits outweigh risk.

Question 10

What forces act on the nucleus?

Answer 10

Strong nuclear force holds it together; electromagnetic force pushes protons apart

Question 11

What makes a nucleus unstable?

Answer 11

Too many/too few neutrons; too many nucleons altogether; too much energy

Question 12

When does alpha-emission occur?

Answer 12

In very heavy atoms: nuclei are too massive to be stable

Question 13

When does beta-minus emission occur?

Answer 13

When there are too many neutrons. A neutron decays into a proton, and electron and an antineutrino.

Question 14

When does gamma-emission occur?

Answer 14

From nuclei with too much energy: nucleus is said to be excited (has excess energy); loses this energy in the form of a gamma ray. Often occurs after an alpha or beta decay. No change to nuclear constituents; nucleus just loses excess energy.

Question 15

What is the mass defect?

Answer 15

The idea that the mass of an alpha particle is less than the mass of 2 individual protons + 2 individual neutrons. This is because of Einstein’s equation, E = mc^2. Energy is released as the nucleons bond together, accounting for the missing mass.

Question 16

What is the nuclear binding energy?

Answer 16

Equivalent to the mass defect, is the energy required to separate the nucleons in the nucleus.

Question 17

What do we see when we plot binding energy per nucleon against number of nucleons in the atom?

Answer 17

Nuclear valley. We see that the most stable nuclei (those that have the strongest binding energy, and therefore the least energy in the nucleus) occur around iron, m=56u.

Question 18

What is nuclear fusion?

Answer 18

Joining of 2 small nuclei to create a bigger one with a bigger binding energy, making a more stable nuclei. Large amounts of energy (binding energy overlap) are released per nucleon.

Question 19

What is nuclear fission?

Answer 19

When large nuclei are split in two: get a 2 smaller nuclei, increasing the size of the binding energy and therefore making the nucleus more stable. Energy is released (binding energy overlap), though not as much per nucleon as in nuclear fusion

Question 20

What limits the size of an element?

Answer 20

Spontaneous fission. The larger the nucleus gets, the more likely it is to split into two spontaneously.

Question 21

How do controlled nuclear reactors work?

Answer 21

Harness the energy released in controlled fission reactions
Fuel rods of enriched uranium (containing more U-235 as opposed to U-238 than normal)
U-235 absorbs a neutron, becoming U-236 which then splits into 2 smaller atoms and some neutrons
These neutrons have to be slowed down before they can be absorbed by more U-235, so have a moderator, usually water.
Good to have a moderator that absorbs more neutrons at higher temps: reduces risk of a meltdown
Operate above ‘critical mass’: min. amount of fuel needed to get a continuous chain reaction; use control rods made out of boron to absorb neutrons and prevent it going out of control
Coolant removes heat, usually the same water that acts as the moderator. Steam produced powers a turbine

Question 22

How are the waste products of nuclear fission disposed of?

Answer 22

They are often radioactive due to having a larger proportion of neutrons in their nuclei than normal. Cooled down by placing in cooling ponds, then stored underground in sealed containers until activity has fallen enough to be safe to handle. Some products then have practical applications as medical tracers.

Question 23

How does nuclear fusion occur?

Answer 23

2 light nuclei can only fuse if they have enough energy to overcome the electrostatic repulsion so the nuclear strong interaction can bind them. Typically, they need about 1MeV of kinetic energy: a hell of a lot!

Question 24

Where does nuclear fusion occur?

Answer 24

Core of stars, at around 10^7K. Occurs there because the temperature is high enough to give the required energy for the particles to overcome the electrostatic repulsion. Lots of energy is released due to the lower binding energy of the product, allowing more fusion reactions to take place.

Question 25

What is plasma?

Answer 25

When atoms no longer exist: electrons are stripped away, and you end up with positively charged nuclei and free electrons

Question 26

How does intensity of gamma radiation vary in absorbing material?

Answer 26

I=I(0)e^(-mu x) where I is intensity, I(0) is original intensity, x is thickness of material.