Radioactivity and Nuclear Energy

Question 1

How was the nucleus discovered?

Answer 1

Rutherford’s Scattering
Alpha source directed through thin gold foil
Observe the path of the alpha source

Question 2

Explain the observation of Rutherford’s Scattering

Answer 2

Majority α particles went through without scattering -> atom is mostly empty space
Small amount of α particles deflected by large angle –> centre of the atom is positively charged
Few particles deflected by more than 90° –> centre of atom is very dense

Question 3

Properties of different types of radiation

Answer 3

alpha radiation:
highly ionising so not v penetrating
2-10 cm range in air
deflected by magnetic fields
absorbed by paper

beta radiation:
less ionising but more penetrating
1m range in air
deflected by magnetic fields
absorbed by aluminium foil

gamma radiation
least ionising but most penetrating
infinite range in air
NOT deflected by magnetic fields
absorbed by concrete or lead

Question 4

Uses of alpha and beta radiation

Answer 4

Quality control for thickness of materials
Signal of radiation goes through if thickness is too thin
Signal stops if thickness too thick

Question 5

Uses of gamma radiation

Answer 5

Kill cancerous cells
Sterilise equipment
Detector for radioactive source with short half life

Question 6

Safety precautions for radioactive source

Answer 6

Handle with tongs and at arms length
Store in lead lined container
Limit exposure
Never point source towards other people

Question 7

What are the sources of background radiation?

Answer 7

Radon gas from rocks
Nuclear testing and weapons
Cosmic rays from space
Rocks with naturally occuring radioactive isotopes

Question 8

Describe the nature of radioactive decay?

Answer 8

Random –> can’t predict when the next decay
Spontaneous –> Can’t cause or influence the decay

Question 9

Definition of half life

Answer 9

The average time taken for a radioactive source’s activity to halve

Question 10

Equation for nuclear Activity

Answer 10

A = Initial activity x e^-λt

Question 11

When can the decay constant be used?

Answer 11

Can model the decay of nuclei when there is a large number of nuclei in the sample

Question 12

Uses of half life

Answer 12

C-14 dating –> date organic objects according to half life of C-14

Medical diagnosis –> Short half life isotopes can be used as medical tracers

Question 13

What causes nuclear instability and the type of decay?

Answer 13

Too many neutrons –> Beta minus decay
Too many protons –> Beta plus, electron capture, alpha emission
Too many nucleons –> Alpha emission
Too much energy –> Gamma emission to de-excite the nucleus

Strong nuclear force is not strong enough to overcome the electrostatic repulsion of the protons
More neutrons = stronger nuclear force

Question 14

How to estimate the radius of a nucleus?

Answer 14

Calculate the distance of closest approach which is the distance where there’s no kinetic energy as particle has stopped
Will always be an overestimate
The larger the number of nucleons = larger radius

Question 15

Why is electron diffraction a better estimate of the radius?

Answer 15

Because e- are leptons which don’t react with nucleons unlike alpha particles
electrons are accelerated so de broglie wavelength is similar to diameter of nucleus
Graph of intensity against diffraction angle
sinΦ = 0.61λ/R

Question 16

Definition of Binding energy

Answer 16

Work that must be done to separate a nucleus from its constituent nucleons

Iron has the highest binding energy per nucleon

Question 17

Definition of mass defect and its three conditions

Answer 17

Difference between mass of the atom and mass of its constituent nucleons

Mass defect is small compared to total rest mass of the atom
Energy released is E = mc^2
Mass is in kg, E is in J

Question 18

Describe Induced Nuclear Fission

Answer 18

A thermal neutron is fired at a U-235 nucleus
The U-236 nucleus splits into two daughter nuclei and 2/3 fast moving neutrons
Energy is released because daughter nuclei has higher binding energy per nucleon

Question 19

Definition of critical mass

Answer 19

The minimum mass of fuel required to maintain a steady chain reaction

Question 20

Purpose of Nuclear Reactors’ Features

Answer 20

Moderator: Slows down fast moving neutrons into thermal neutrons through elastic collisions between nuclei/water/ graphite molecules and neutrons

Control Rods: Absorbs neutrons in reactor to control chain reactions using boron or cadmium

Coolant: Absorbs heat released during fission in the core. Heat used to make steam to power electricity-generating turbines. Use water, molten salt or gas

Fuel Rods: U-235 rods that can move up and down, deteriorate overtime and become more radioactive

Question 21

Describe Nuclear Fusion

Answer 21

When two smaller nuclei join to form one larger nucleus
Energy is released due to large binding energy per nucleon of larger nucleus
Can only occur at very high temperature or very high pressures

Question 22

Safety aspects of Nuclear Reactors

Answer 22

Thick concrete shielding around reactor
Fuel rods inserted into reactor remotely to limit worker’s exposure
Emergency: Control rods dropped in completely to stop reaction
Spent fuel rods are most dangerous nuclear waste –> disposed underground with concrete and steel in geologically stable locations