8. Nuclear Physics

Question 1

Describe the Rutherford scattering experiment.

Answer 1

● A beam of alpha particles was directed at a thin gold foil.
● Occurs in a vacuum so that no collisions between air particles and alpha particles can occur.
● The experiment was done in order to determine structure of an atom

Question 2

In the Rutherford scattering experiment it was observed that most of the alpha particles passed straight through. What can we infer from that?

Answer 2

That most of the atom is made from empty space.

Question 3

What evidence was there that suggested that the nucleus had a positive charge?

Answer 3

Because the the nucleus repels the alpha particles and caused it to deflect from its original path, some of them even bounced back.

Question 4

Name 3 types of radiation?

Answer 4

● Alpha
● Beta (plus and minus)
● Gamma

Question 5

Order Alpha, Gamma and Beta radiation starting with the most ionising?

Answer 5

● Alpha
● Beta
● Gamma

Question 6

Order Alpha, Gamma and Beta radiation

starting with the most penetrating?

Answer 6

● Gamma
● Beta
● Alpha

Question 7

A sheet of paper can block which type of radiation?

Answer 7

Alpha radiation.

Question 8

This equation represents which process?
🇦 . . . . 0 . . . . .🇦
🇿 X + -1 e ⟶ 🇿-1 γ + Ve

Answer 8

Electron capture.

Question 9

When a nucleus decays through gamma radiation, how does the atomic number and mass number change?

Answer 9

They remain the same as the number of protons and neutrons remain the same.

Question 10

Why is ionising radiation seen as dangerous?

Answer 10

Because it can kill or mutate cells, which could lead to mutations and lead to things such as cancer.

Question 11

Which radiation is more harmful inside a human body, alpha or gamma?

Answer 11

Alpha radiation - because it has a high ionising power so it would damage more cells. It is also very poorly penetrating, therefore it is not be able to leave the body, whereas gamma radiation is highly penetrating.

Question 12

Give an example of a real life use of Beta decay and explain why Beta is chosen for this.

Answer 12

Beta radiation can be used to measure the thickness of paper or aluminium foil. Alpha isn’t used as it is less penetrative and wouldn’t reach the detector on the other side of the sheet. Gamma radiation is too penetrative and would pass through everything.

Question 13

Which type of radiation follows the inverse square law?

Answer 13

Gamma radiation.

Question 14

What does the inverse square law state?

Answer 14

The intensity is inversely proportional to the square of the distance from the source.

Question 15

What is intensity measured in?

Answer 15

Measured in watts per square meter (W/m^2).

Question 16

Describe an experiment which be used to show the inverse square law and gamma rays.

Answer 16

● Firstly measure background radiation (using Geiger Muller tube), without the gamma source in the room
● Then put the gamma source at a set distance (1m) from the GM tube and measure the count rate per minute. Record 3 measurements for each distance and take an average.
● Do this for many distances going up in 10cm intervals
● Take away the background radiation from each reading
● Square each of the distances
● Plot a graph of the count rate per minute against 1 over distance squared (1/d²)
● If it is a straight line through the origin then it confirms they are directly proportional

Question 17

What is background radiation?

Answer 17

Radiation that is constantly in the surroundings from sources such as rocks and food.

Question 18

What is the decay constant (𝜆)?

Answer 18

The probability of a nucleus decaying per second.

Question 19

What are the units for the decay constant (𝜆)?

Answer 19

s^(-1)

Question 20

What is half life?

Answer 20

The time it takes for half of the unstable nuclei in a substance to decay.

Question 21

What equation can you use to work out the half life of on object?

Answer 21

T1/2= ln(2)/𝜆

Question 22

Complete the equation. 𝜆N =?

Answer 22

Activity.

Question 23

What is activity measured in?

Answer 23

Bq (decays per second).

Question 24

True or false. Radioactive isotope decay exponentially.

Answer 24

True.

i.e . N = N0 e^(-𝜆t)

Question 25

Why is Technetium 99m useful in medicine?

Answer 25

● Because it releases gamma radiation
● It has a short half life therefore it doesn’t stay highly radioactive for long
● Half life of 6 hours: long enough for it to be detected
● It can also be made near to the hospital
● Easy to detect outside the patient
● ‘Clears away’ after a few days

Question 26

What does the graph of N against Z show?

Answer 26

It shows the relationship between proton number and neutron number. The graph shows a stability curve which starts as N=Z until N value of 20. After that the graph curves upward and becomes steeper.

Question 27

Where on the curve does B- decay occur and why?

Answer 27

Above the stability line, because the nuclei found there contains too many neutrons. Therefore when beta minus decay occurs the neutron turns into a proton and it becomes more stable.

Question 28

What type of decay occurs below the stability line and why?

Answer 28

Beta plus decay. As the isotopes found here often have too many protons. Therefore when beta plus decay occurs the proton turns into neutrons.

Question 29

How does the heavier nuclei often decay?

Answer 29

Through alpha decay. This is because alpha decay emits a helium nucleus (2 protons and two neutrons) therefore causing the nuclei to become less heavy and more stable.

Question 30

A alpha particle is fired at a nucleus, with the kinetic energy at the start known. How can you use energy conservation to find closest approach of of particle?

Answer 30

We can work out how close a particle will get to a nucleus: we know the kinetic energy as it heads to the nucleus and the electrostatic potential energy that repels it. At some point these equal and this is the closest approach
the particle can get.

Ek = Ep
Ep = 1/(4π𝜀0) x (QaQn)/r
Ek = 1/2 mv^2
r = (QaQn)/(2π𝜀0mv^2)

Question 31

How is electron diffraction used to determine the diameter of a radius?

Answer 31

● An electron beam is fired at a thin sheet of the desired atom.
● A diffraction pattern is produced on a screen behind.
● Using the angle of a minimum we can use equations to calculate the diameter.

Question 32

What is the relationship between nuclear radius(R) and nucleon number (A)?

Answer 32

R = r0 A^(1/3)
Therefore nuclear radius is directly proportional
to the cube root of the nucleon number.

Question 33

True or false. The density of a nucleus is independent of the radius of the nucleus.

Answer 33

True. 
As V = (4/3) 𝜋 r0^3 A. 
Therefore Density = m/(4/3) 𝜋 r0^3 A).
m = Au
 so density = Au/((4/3) 𝜋 r0^3 A). 
Cancel out the A’s
Density = u/((4/3) 𝜋 r0^3).

Question 34

What equation is used to convert mass to its energy equivalent?

Answer 34

E = mc^2

Question 35

What is the mass defect?

Answer 35

The difference between the total mass of all the nucleons separately compared to the mass of the nucleus.

Question 36

Why is there a mass defect?

Answer 36

Because energy is needed to bring the constituent parts of a nucleus together, therefore the mass equivalent of the energy is lost and the total mass decreases.

Question 37

What is binding energy?

Answer 37

The energy required to separate a nucleus into its constituent parts.

Question 38

What is nuclear fission?

Answer 38

● Where a unstable nucleus splits into 2 smaller nuclei.
● Often occurs with the larger nuclei.
● The binding energy per nucleon increases when fission occurs therefore the overall process releases energy.

Question 39

What is fusion?

Answer 39

When two small nuclei fuse together to create a larger nuclei. The new nucleus has a larger binding energy per nucleon than the old nuclei therefore energy is released in the process.

Question 40

Why is it difficult to make fusion occur on earth?

Answer 40

There is a large repulsion between the two positively charged nuclei, therefore a lot of energy is required to overcome the repulsion and fuse them together.

It is hard to get a material that can withstand the heat and be cost effective.

Question 41

How is fission used in nuclear reactors?

Answer 41

Rods of uranium-235 absorb neutrons and become unstable and then split into two daughter nuclei. It also releases 2/3 more neutrons. These then go on to be reabsorbed by another uranium-235.

Question 42

What is the purpose of a moderator?

Answer 42

To slow down the neutrons so they travel slow enough to be absorbed by the uranium.
They do this through elastic collisions between the moderator and the nucleus.

Question 43

Why is control rods essential for a nuclear power station?

Answer 43

They stop the chain reaction from being out of control.
They absorb neutrons so that only 1 of the neutrons released in each reaction can go on to be absorbed by another uranium.
If not then the nuclear reactor would overheat as too many reactions would happen at once.

Question 44

Is Boron used as a control rod or a moderator?

Answer 44

Control rod.

Question 45

Give an example of a material that can be used as a moderator?

Answer 45

Water.

Question 46

What is the purpose of using water as a coolant?

Answer 46

It allows heat from the nuclear reactor to escape, which stops the reactor from overheating.

Question 47

What is the critical mass?

Answer 47

The minimum mass of fuel needed for a chain reaction to occur.

Question 48

Which waste products from a nuclear reactor cause the highest risk?

Answer 48

Spent fuel rods.

Question 49

How is high-level waste disposed of?

Answer 49

● They are first stored in cooling ponds

● Then they are put in sealed steel containers and put deep underwater.

Question 50

What is low level waste contained in?

Answer 50

They are sealed in containers and put underground until it is safe again.