5. Nuclear Physics

Question 1

Atom

Answer 1

The smallest unit of an element, made from neutrons, protons and electrons.

Question 2

Proton - (Symbol, charge, mass, location)

Answer 2

Symbol = p
Charge = +1
Mass = 1
Location = In the nucleus

Question 3

Nucleus

Answer 3

The central part of an atom that contains all the protons and neutrons, and so all the mass and positive charge. (pl. nuclei)

Question 4

Neutron - (Symbol, charge, mass, location)

Answer 4

Symbol = n
Charge = 0
Mass = 1
Location = In the nucleus

Question 5

Electron - (Symbol, charge, mass, location)

Answer 5

Symbol = e
Charge = -1
Mass = 0 (1/2000) (tiny mass)
Location = Orbit the nucleus

Question 6

Testing the atomic model

Answer 6

In 1911, Ernest Rutherford proposed the atomic model based on the Rutherfords Gold Foil experiment (aka Alpha Scattering experiment.)

1. Beam of alpha particles was directed through a slit in a flourescent screen towards a piece of thin gold foil (1 atom thick)
2. Experiment carried out in vaccum so particles didn’t hit any air particles –> only collided with foil
3. When the particles hit the circular flourescent screen surrounding the model there was a flash of light

Question 7

Result of alpha scattering experiment

Answer 7

Most particles passed through the gold foil. Some were deflected slightly but still passed out the other side. Very few were completely deflected and hit behind where they were emitted.

Question 8

All 4 atom features

Answer 8

1. The number of protons and electrons are equal.
2. Most of an atom is empty space. The size of the nucleus is incredibly small compared to the size of an atom.
3. The nucleus is positively charged.
4. The nucleus accounts for nearly all the mass.

Question 9

Explanation of Alpha scattering results - (The number of protons and electrons are equal.)

Answer 9

Atoms do not have an overall charge

Question 10

Explanation of Alpha scattering results - Most of an atom is empty space. The size of the nucleus is incredibly small compared to the size of an atom

Answer 10

Most of Rutherford’s α - particles went straight through the gold foil without changing their direction.

Question 11

Explanation of Alpha scattering results - The nucleus is positively charged.

Answer 11

Alpha particles are positive. When some particles come close to the nucleus of the gold foil, they were repelled. The nucleus must therefore also be positive.

Question 12

Explanation of Alpha scattering results - The nucleus accounts for nearly all the mass.

Answer 12

In one experiment the α - particle was deflected back in the opposite direction when a direct hit occurred. This change in momentum could only happen if the tiny nucleus also contains most of the mass.

Question 13

Nuclide

Answer 13

An atom or nucleus characterised by a specific number of protons and neutrons.

Question 14

Nuclide Notation

Answer 14

A notation using symbols for elements along with atomic number and nucleon number to describe the composition of an element’s nucleus.

A
Z X

A - Mass number
Z - Proton number
X - Element

Question 15

Z (proton number) - In relation to charges

Answer 15

Z = relative charge of the nucleus

Question 16

A (mass number) - In relation to mass

Answer 16

A = relative mass of the nucleus

Question 17

Isotope

Answer 17

Atoms of the same element that have different nucleon numbers due to a varied number of neutrons.

Question 18

Nuclear Fusion

Answer 18

Small nuclei fuse to form larger nuclei and release energy.

This is what happens in the sun

Eg.

2 2 4
1 H + 1 H –> 2 He

Total mass is conserved

Question 19

Nuclear fission

Answer 19

Large nuclei split up into smaller nuclei and release energy.

Nuclei with very large nucleon numbers have heavy nuclei and are often unstable. This is why large nuclei often split up.

This can happen independently or can be caused by a single neutron colliding with the nuclei.

Total mass is conserved

Eg. A neutron is fired into the uranium-235 nucleus, causing the nucleon number to increase. A larger mass makes the nucleus unstable. Nuclear fission then occurs and the heavy nucleus splits into krypton and barium with three spare neutrons.

Question 20

Background Radiation

Answer 20

The average level of radiation detectable as part of everyday life, due to a combination of natural and man-made sources.

Question 21

Types of background radiation

Answer 21

Food - (eg. bananas –> dosage of 0.1 micosieverts)

Building materials + rocks - (eg. granite, concrete both contain elements such as uranium, radium and thorium)

Radon gas - (naturally found in atmosphere because of uranium decay)

Cosmic ray - (high radiation waves travelling from outside of our solar system)

Medical procedures - (eg. X-rays, CT scans)

Question 22

Geiger–Müller (GM) tube

Answer 22

Detects alpha, beta and gamma radiation. A ‘click’ can be heard as the radiation is detected and a digital count is recorded.

Question 23

Spark Counter

Answer 23

Detects alpha, beta and gamma radiation with an audible click.

Question 24

Cloud Chamber

Answer 24

Shows the paths of alpha and beta particles by producing a vapour trail behind each particle.

Question 25

How to calculate the radioactivity of a sample.

Answer 25

Count rate (counts/s) = Measured count rate (counts/s) - Background radiation (counts/s)

Question 26

Alpha particles

Answer 26

A alpha particle is made up of 2 protons and 2 neutrons (like a helium nucleus)

Total mass = 4
Total charge = 2+

Strong ionising radiation –> large kinetic energy + large charge + concentrated mass

Least penetration - Will only travel approx. 5 cm in air + stopped by thin paper or skin

Alpha particle in an electric field is attracted to the negative plate bc of positive charge.

a (a-4) 4
z X –> (z-2) Y + 2 a

Question 27

Beta Particle

Answer 27

A high energy electron

During beta decay a neutron decays into a proton and an electron is released. –> thus mass stays the same but the proton number increases.

Mass = almost none (approx. 1/2000)
Charge = -1

Mildly ionising

Mildly pentrative - Can travel through skin but stopped by a couple cm of aluminium foil.

Beta particle through an electrical field

Attracted to the positive plaete bc of negative charge.

a a + 0 0
z X –> z +1 Y + -1 e

Question 28

a
z X

Answer 28

X - element symbol
a - mass number
z - proton number

Question 29

Gamma rays

Answer 29

Gamma emissions are electromagnetic waves. Gamma emissions are of very high frequency and have very high energy.

Mass = 0
Charge = 0

Most penetrating - can only be stopped by several cm of lead or thick concrete

Weakly ionising - have high energy so still slightly ionising

Through an electrical field

No charge so the emissions aren’t attracted or repelled.

a a 0
z X –> z Y + 0 y

2nd is still the same element/ isotope as X

Question 30

Half-life

Answer 30

Half-life is the time taken for the count rate of a radioactive source to decrease by half.

Half-life is the time taken for the number of radioactive nuclei to decrease by half.

Question 31

Number of remaining nuclei (half-life formula)

Answer 31

Number of remaining nuclei = original amount / 2^n

n = number of half-lifes that have occurred.

(after approx. 5 halflifes 99% of the sample has decayed.)

Question 32

Half life - (axis)

Answer 32

Y-axis = could be mass, nuclei number, count rate…

X-axis = is always Time (units)

Question 33

How to handle / store alpha particles

Answer 33

Alpha particles can be stored in thin package sicne they are weakly penetrating.

Since they are highly ionizing protective clothing made of lead must be used just in case of penetration.

Question 34

How to handle / store beta particles

Answer 34

Beta particles can be stored in lead / similarly dense metal container.

Protective clothing + gas mask worn.

Question 35

How to handle / store gamma particles

Answer 35

Highly penetrative so buried deep underground to stop radiation causing problems.

Weakly ionising but still very penetrative so protective clothing should definetely be used. Usually robots handle gamma to protect humans.

Question 36

Smoke Detectors - Uses of radiation

Answer 36

• Alpha particles emitted from sample –> land on detector. Bc of the 2+ charge a current flows.
• Smoke disrupts particles so breaks the current –> alarm gets set off.

Question 37

Thickness measurement - Uses of radiation

Answer 37

• Beta particles emitted towards metal sheet. Detector placed underneath sheets. Depending on the amount of particles let through, the rollers are adjusted.
• Beta particles used because they are medium penetrative.

Question 38

Fault detection - Uses of radiation

Answer 38

• Pipes wrapped in x-ray films. Radioactive sample inside the pipes.
• When radiation reaches the x-ray film it gets developed in those places.
• Gamma radiation used bc very penetrative + small half-life needed so as not to cause issue later on.

Question 39

Irradiating food - Uses of radiation

Answer 39

• Large ‘blasts’ of UV-light (weakly ionising) directed at food.
• Kills single celled organisms –> extends shelf life + makes food safer for pateints with low immune systems to eat –> hospital food.

Question 40

Cancer treatment - Uses of radiation

Answer 40

• Gamma or x-rays –> directed at tumour –> kills cancerous cells.

Question 41

Organ function testing - Uses of radiation

Answer 41

• Radioactive tracer with small half-life injected into bloodstream. Patient put under radiation detector –> shows any places with cluster of radiation –> blood clots
• Gamma used bc its weakly ionising + penetrates skin.

Question 42

Safety strategies when dealing with radioactivity

Answer 42

• Limit exposure time (less radiation + ionisation then)
• Sheilding (place barrier between radioactive material + human)
• Distance (increase distance from radioactive source –> less intense)

Question 43

Sterilising medical equipment - Uses of radiation

Answer 43

• Gamma rays directed at equipment kills bacteria by breaking down their DNA.
• Gamma used bc its highly penetrative.
• This limits infections during surgery etc.