Nuclear physics

Question 1

Describe the history of the atom from 500 BC to 1984

Answer 1

1. Democritus proposed that matter was made up of clumps called atomos in 500 BC
2. Dalton believed that matter was made up of atoms which couldn’t be broken down and each element was made up of a different type in 1984

Question 2

Describe the recent history of the atom

Answer 2

1. JJ Thompson discovered electrons so Dalton was wrong as atoms could be broken up. He proposed a positively charged sphere with electrons scattered throughout
2. Rutherford discovered the nucleus so discovered that the charge and density were not distributed equally

Question 3

Describe the method of Rutherford’s experiment

Answer 3

• Fire beam of alpha particles at a thin gold sheet from an alpha source
• Put a circular detector behind to detect the angle they are deflected

Question 4

What were Rutherford’s predictions for his experiment

Answer 4

All alpha particles would be deflected by a very small amount due to positively charged sphere proposed by JJ Thompson (small amount because charge is very spread out)

Question 5

What actually happened during Rutherford’s experiment

Answer 5

• Most of the alpha particles were not deflected at all
• However, some were deflected by large amounts with a few even by more than 90 degrees i.e. sent back where they came

Question 6

What were the conclusions drawn from the results of Rutherford’s experiment

Answer 6

1. Most of the atom is empty space as most weren’t deflected
2. The nucleus must be very small as only a few alpha particles were deflected
3. The nucleus must have a very large positive charge as some alpha particles were deflected by more than 90 degrees

Question 7

What is the closest approach in Rutherford’s experiment

Answer 7

• Closest approach is the closest distance that the alpha particle gets to the gold nucleus before being deflected back
• This distance is an estimate for the nuclear radius
• At this point the alpha particle is stationary as it needs to stop for a split second before turning back (where the distance would start to increase again)

Question 8

How can you calculate closest approach

Answer 8

It has stopped because the KE of the alpha particle is now = to the Electrical energy between the alpha particle and the nucleus

So put KE = EE and solve for r

Question 9

How can you find the nuclear radius more accurately using electron diffraction

Answer 9

1. Find the de broglie wavelgnth of the elctrons using λ = hc / E where E is the energy of electrons
2. Fire high E electrons (so low λ) at a thin film and electrons will diffract through gaps between nuclei
3. First minimum appears where sinθ = 1.22λ / 2R where R is the nuclear radius

Question 10

What does an electron diffraction interference pattern look like

Answer 10

Maxima are concentric rings with decreasing intensity away from central maxima

Question 11

Why does radioactive decay occur

Answer 11

Nuclei become unstable due to too many protons, neutrons or just energy in general so release energy (alpha beta or gamma) until they become stable

Question 12

Describe the differences in ionising power of alpha, beta and gamma

Answer 12

Alpha is strongly ionising
Beta is weakly ionising
Gamma is very weakly ionising

Question 13

Describe the differences in speed of alpha, beta and gamma particles

Answer 13

Alpha is slow
Beta is fast
Gamma moves at speed of light

Question 14

Which of the 3 types of radiation is not affected by a magnetic field

Answer 14

Gamma

Question 15

What are the relative charges and masses of each type of radiation

Answer 15

• Alpha has a charge of +2 and mass of 4 (2ps 2ns)
• Beta (-) has charge of -1 and mass of 0 (1 e)
• Gamma has charge 0 and mass 0 (EM wave)

Question 16

What are the penetrating powers of each type of radiation

Answer 16

Alpha can travel 2 - 10 cm in air and absorbed by paper

Beta can travel 1 m in air and absorbed by 3mm of aluminium foil

Gamma has infinite range in air and absorbed by several metres of concrete or several inches of lead

Question 17

Describe a use of alpha particles

Answer 17

Used in smoke alarms as they are able to ionise air molecules causing current to flow but when smoke slows down ions, current stops and alarm sounds

Question 18

Describe a use of beta particles

Answer 18

Control thickness of aluminium material in manufacturing process by flattening if too little beta is being detected on the other side and pulling apart if too much

Question 19

Describe 2 uses of gamma radiation

Answer 19

PET scanners where a patient consumes a radioactive substance with a short half life and the scanner detects the gamma rays emitted to produce a detailed image

Killing cancerous cells (but could have side affects by damaging healthy cells)

Question 20

What is background radiation

Answer 20

A Geiger counter would detect some radiation even if there is no source present known as background radiation

Question 21

What are the sources of background radiation

Answer 21

Air, rocks (e.g. in ground and buildings), cosmic rays, all living things

Question 22

What is the inverse square law during the emission of gamma radiation

Answer 22

Intensity is inversely proportional to square of distance from source
so I = k / x^2
This is why we should stay far away from source for safety

Question 23

What is the relationship between nuclear radius and nucleon number

Answer 23

R = R0 A^1/3 where R is nuclear radius, A is nucleon number and R0 is a constant

Question 24

Describe a graph of R against A and R against A^1/3

Answer 24

R against A is curved with decreasing gradient as R ∝ A^1/3 so as A increases R doesn’t increase by as much

R against A^1/3 is a straight line passing through the origin

Question 25

Prove that nuclear density is constant for all atoms

Answer 25

1. V = 4/3 π R^3 and R = R0 A^1/3 so V = 4/3 π (R0)^3 A 2. Mass of nucleus = Au so density = 3u / 4πR0^3 This is a constant which is about = 3.4 x10^17 so very large as most of mass is in nucleus

Question 26

Why is radioactive decay random

Answer 26

Don't know which nuclei will decay when

Question 27

What is the relationship between activity and number of nuclei

Answer 27

A = λN where λ is the decay constant e.g. if you double the sample size, 2x more nuclei will decay per second

Question 28

Define decay constant

Answer 28

Probability of a given nucleus decaying per second

Question 29

How can you calculate decay constant from a graph of N against t

Answer 29

Activity is the gradient as it is number of nuclei decaying per second A =λN so use tangent to graph and use value of N at that point

Question 30

Define half life

Answer 30

Average time taken for number of nuclei or activity to half which is constant at all values of N or A

Question 31

How can you find the half life from a graph of LnN against t

Answer 31

Line is a straight line with negative gradient Positive of this gradient = decay constant half life = ln2 / λ

Question 32

What are the pros and cons of half lifes

Answer 32

pros: - carbon dating - PET scanners of right half life cons: - Long half life of nuclear waste can damage environment

Question 33

How can you calculate the number of nuclei left or activity at a point in time

Answer 33

N = N0 e^-λt A = A0 e^-λt

Question 34

How can different types of unstable nuclei on a graph of number of neutrons against number of protons

Answer 34

- Line of stability is the points on the graph where nuclei are stable, increases with an increasing gradient - Left of line of stability means neutron rich so B- decay occurs - Right of line of stability means proton rich so B+ decay occurs - At the top, too many of both so alpha decay occurs

Question 35

What do energy level diagrams show

Answer 35

Energy on the y axs and no x axis - e.g. the paths that gamma photons take during de-excitation is shown on these but they also show alpha and beta

Question 36

Give an example of an energy level diagram

Answer 36

- Start at Co-60 with a horiztonal line from x=0 - Emit beta particle causing diagonal line down across x axis as energy deay occuring - New horizontal line at new energy - When gamma photon is emitted, vertical line down to ground state, where there is another horizontal line