Physics 8 - Nuclear Physics

Question 1

Describe the Rutherford scattering experiment.

Answer 1

• A beam of alpha particles was directed at a thin gold foil.
• It occurs in a vacuum so that no collisions between air particles and alpha particles can occur.
• The experiment was done in order to determine the 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 inform 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 nucleus repels the alpha particles and caused it to deflect from its original path, some of them were even bounced back.

Question 4

Name three types of radiation?

Answer 4

• Alpha
• Beta (plus and minus)
• Gamma

Question 5

Order Alpha, Beta and Gamma radiation in order of most to least ionising.

Answer 5

• Alpha
• Beta
• Gamma

Question 6

Order Alpha, Beta and Gamma radiation in order of most to least penetrating.

Answer 6

• Gamma
• Beta
• Alpha

Question 7

A sheet of paper can block which type of radiation?

Answer 7

Alpha.

Question 8

The equation ᴬzX+⁰₋₁e→ᴬz₋₁Y+Vₑ represents which process?

Answer 8

Electron capture.

Question 9

When a nucleus decays through gamma radiation, how does the atomic 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, since it has a high ionising power so it would damage more cells. It is also very poorly penetrating, so it would not be able to leave the body. Compared to the less ionising and more penetrating Gamma radiation, Alpha is more likely to do harm.

Question 12

Give an example of a real life use of Beta decay and explain why Beta decay 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 react the detector on the 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 meter (W/m²).

Question 16

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

Answer 16

• Firstly measure background radiation (using a 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 the distance squared (1/d²).
• If it is a straight line through the origin then it confirms that they are directly proportional.

Question 17

What is background radiation?

Answer 17

Radiation that is constantly in the surrounding 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. It has the units s⁻¹.

Question 19

What is half life?

Answer 19

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

Question 20

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

Answer 20

T₁/₂=ln(2)/λ

Question 21

What is the equations for nuclear radioactivity?

Answer 21

A=λN
Where:
A=Activity (Bq)
λ=Decay constant (s⁻¹)
N=Number of particles

Question 22

What is radioactivity measured in?

Answer 22

Bq, becquerels (decays per second).

Question 23

True or false; “Radioactive isotopes decay exponentially.”? (Give a formula)

Answer 23

True;
N=N₀e^(λt)
N=Number of particles at time t
N₀=Initial number of particles
e=Mathematical constant
λ=Decay constant (s⁻¹)
t=Time (s)

Question 24

Why is Technetium 99m useful in medicine?

Answer 24

• 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 be made near to the hospital.
• It is easy to detect outside the patient.
• It “clears away” after a few days.

Question 25

What does the graph of N against Z show?

Answer 25

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

Question 26

Where on the N against Z curve does Beta minus decay occur and why?

Answer 26

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

Question 27

Where on the N against Z curve does Beta plus decay occur and why?

Answer 27

Below the stability line, because the isotopes found there contain too many protons. Therefore when beta plus decay occurs the protons turn into neutrons.

Question 28

How do heavier nuclei often decay?

Answer 28

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 29

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

Answer 29

We can work out how close a particle will get to a nucleus: we know the kinetic energy 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=(QₐQₙ)/(4πε₀r)
Eₖ=½mv²
r=qQ/2πε₀mv²

Question 30

How is electron diffraction used to determine the diameter of an atom?

Answer 30

• An electron bean 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 31

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

Answer 31

R=r₀A¹/³

Therefore nuclear radius is directly proportional to the cube root of the nucleon number.

Question 32

True or false; “The density of a nucleus is independent of the radius of the nucleus.”?

Answer 32

True, as V=(4/3)πr₀³A
Therefore; density=m/((4/3)πr₀³A)
m=Au
So density=Au/((4/3)πr₀³A)
Cancel out the A's so density=u/((4/3)πr₀³)