Nuclear Physics

Question 1

What did John Dalton think particles were like?

Answer 1

In 1804, John Dalton thought that atoms were tiny spheres that can’t be broken up.

He thought that everything was made up of indivisible particles.

Question 2

What did J. J. Thompson think particles were like?

Answer 2

In 1897, J. J. Thompson thought that atoms were positive bodies of matter with negative electrons in them, named the plum pudding model.

Question 3

What did the Rutherford Scattering (1909) experiment imply for the structure of atoms?

Answer 3

Rutherford Scattering suggested that atoms must have a small, positvely-charged nucleus at the centre.

1. Most of the atom must be empty space because most of the alpha particles passed straight through the foil.
2. The nucleus must have a large positive charge, as some positvely-charged alpha particles were repelled and deflected by a large angle.
3. The nucleus must be small as very few alpha particles were deflected back.
4. Most of the mass must be in the nucleus, since the fast alpha particles are deflected by the nucleus.

Question 4

Implications of the Rutherford Scattering experiment:

1. Most of the atom must be _____ _____ because most of the alpha particles passed _____ _____ the foil.
2. The nucleus must have a _____ _____ charge, as some _____-charged alpha particles were repelled and deflected by a _____ angle.
3. The nucleus must be _____ as very few alpha particles were _____ back.
4. Most of the mass must be in the _____ , since the fast alpha particles are deflected by the _____.

Answer 4

Implications of the Rutherford Scattering experiment:

1. Most of the atom must be (empty space) because most of the alpha particles passed (straight through) the foil.
2. The nucleus must have a (large positive) charge, as some (positvely)-charged alpha particles were repelled and deflected by a (large) angle.
3. The nucleus must be (small) as very few alpha particles were (deflected) back.
4. Most of the mass must be in the (nucleus), since the fast alpha particles are deflected by the (nucleus).

Question 5

Describe the plum pudding model.

Answer 5

The plum pudding model described an atom to be a ball of positive charge, with negatively charged electrons evenly distributed throughout it.

Question 6

Describe the Rutherford Scattering experiment.

Answer 6

1. In 1909, Rutherford tried firing a beam of alpha particles at thin gold foil.
2. A circular detector screen surrounding the gold foil and the alpha source was used to detect alpha particles deflected.
3. If the plum pudding model was true, alpha particles would be deflected by a very small amount.
4. Instead, most alpha particles just went straight through the foil and a small number were deflected by a large angle.

Question 7

How can you estimate the closest approach of a scattered particle?

Answer 7

An alpha particle that ‘bounces back’ and is deflected 180° will reverse direction when it’s close to the gold nucleus.

It does this at the point where electric potential energy equals its inital kinetic energy.

So, Ek = Coulomb’s Law/Eelec

Question 8

Give the properties of alpha radiation.

Answer 8

Alpha Radiation:

1. Strongly ionising
2. Slow moving
3. Stopped by a few centimeters of air or paper
4. Positively charged (2p 2n)
5. Deflected in a magnetic field

Question 9

How do smoke detectors work?

Answer 9

Smoke detectors:

Alpha particles ionise the air particles between two metal plates, allowing a current to flow between them.

When smoke enters the detector, the alpha particles can’t ionise the air, causing a reduction in the current.

The reduction in the current triggers the alarm.

Question 10

Give the properties of beta radiation.

Answer 10

Beta Radiation:

1. Mildly ionising
2. Fast moving
3. Stopped by a few millimeters of aluminium
4. Negatively charged
5. Deflected in a magnetic field

Question 11

How do thickness monitors work?

Answer 11

Thickness Monitors:

Beta particles are emitted on one side of the material and, detected on the other.

If the material is too thick, the number of beta particles detected will be too low, and will trigger the machine to reduce the material thickness.

Question 12

Give the properties of gamma radiation.

Answer 12

Gamma Radiation:

1. Weakly ionising
2. Travels at the speed of light
3. Stopped by several centimeters of lead or a few meters of concrete
4. Chargeless
5. Unaffected by magnetic and electric fields

Question 13

What is gamma radiation used for?

Answer 13

Gamma radiation is used to sterilise medical equipment and kill cancerous cells, as well as being used as a medical tracer in diagnosis.

Question 14

How can you be safe whilst dealing with radioactive sources?

Answer 14

Safe use of Radiation:

1. Never directly handle the source
2. Use long-armed tongs to increase your distance from the source
3. Display signs warning others that radioactive sources are in use
4. Keep the time that the sources are being used to a minimum
5. Store in an appropriate lead box when not in use

Question 15

If a beam of high-energy electrons is directed onto a thin film of material in front of a screen, a diffraction pattern is formed.

Give the equation for which the first minimum appears.

(This can be used to find the radius of a nucleus)

Answer 15

For high-energy electrons directed onto a thin film of material in front of a screen, first minimum:

sinθ = 0.61λ/R
not given in exam

Question 16

When you plot a graph of radius of nucleus (R) against the cube root of of the nucleon number (A^1/3), what line do you get?

Answer 16

When you plot a graph of radius of nucleus (R) against the cube root of of the nucleon number (A^1/3), you get a straight line.

R ∝ A^1/3 not given in exam

Or

R = R0A^1/3 given in exam

Question 17

Describe the inverse square law that gamma radiation obeys.

Answer 17

A gamma source will emit gamma radiation in all directions.

This radiation spreads out as you get further away from the source.

This means the amount of radiation per unit area (intensity) decreases the further away you get from the source.

If you took a reading of intensity, I, at a distance, x, from the source you’d find that it decreases by the square of the distance from the source.

Therefore:

I = k/x^2

Question 18

Gamma Radiation Inverse Square Law:

A gamma source will emit gamma radiation in all _____.

This radiation _____ out as you get _____ _____ from the source.

This means the amount of radiation per unit area (intensity) _____ the further away you get from the source.

If you took a reading of intensity, I, at a distance, x, from the source you’d find that it _____ by the _____ of the distance from the source.

Therefore:

_____

Answer 18

Gamma Radiation Inverse Square Law

A gamma source will emit gamma radiation in all (directions).

This radiation (spreads) out as you get (further away) from the source.

This means the amount of radiation per unit area (intensity) (decreases) the further away you get from the source.

If you took a reading of intensity, I, at a distance, x, from the source you’d find that it (decreases) by the (square) of the distance from the source.

Therefore:

I = k/x^2

Question 19

What does the gamma radiation inverse square law tell us?

Answer 19

The gamma radiation inverse square law of intensity with distance squared tells us that intensity is much higher with shorter distances to the source.

This means we should keep a long distance from the source to stay safe.

Question 20

What is the rate of radioactive decay like?

Answer 20

Radioactive decay is completely random.

Every isotope decays at a different rate.

Question 21

What is the decay constant?

Answer 21

The decay constant, λ, is the probability of a given nucleus decaying per second.

The bigger the value of λ, the faster the rate of decay.

Its units is s^-1

Question 22

What is activity measured in?

Answer 22

Activity is measured in becquerels (Bq), where 1 Bq = 1 decay per second.

Question 23

What is half life?

Answer 23

The half-life (T1/2) of an isotope is the average time it takes for the number of unstable nuclei to halve.

Question 24

What does a graph of number of unstable nuclei remaining, N, against time look like?

Answer 24

For a graph of number of stable nuclei remaining, N, against time, the line shows an exponential decay.

Question 25

How can you find the number of unstable nuclei remaining?

Answer 25

The number of unstable nuclei remaining, N, depends on the number originally present, N0. N = N0e^λt *given in exam*

Question 26

How can you find activity for a point in decay?

Answer 26

As a sample decays, its activity goes down. A = A0e^λt Same as N = N0e^λt

Question 27

How is gamma radiation used in medical diagnosis?

Answer 27

Radioactive tracers are used to help diagnose patients. Radioactive sources that emit gamma radiation and have half-lives of around 6 hours (long enough for data to be recorded, short enough to limit radiation) are suitable.

Question 28

Why are long half-lives dangerous?

Answer 28

Isotopes used in nuclear power generation have very long half-lives, so we have to plan ahead about how the waste will be stored (in water tanks or sealed underground). This is to prevent damage to the environment or people for the foreseeable future.

Question 29

What four things must be conserved in nuclear decays?

Answer 29

In every nuclear reaction energy, momentum, charge and nucleon number must be conserved.

Question 30

What is nuclear fission?

Answer 30

Nuclear fission occurs when large nuclei are unstable (usually with at least 83 protons) and can randomly split into two smaller nuclei.

Question 31

How is nuclear fission induced?

Answer 31

Nuclear fission can be induced by making a neutron enter a Uranium nucleus, causing it to become very unstable.

Question 32

Why is energy released during nuclear fission?

Answer 32

Energy is released during nuclear fission because the new, smaller nuclei have a higher binding energy per nucleon.

Question 33

What is spontaneous fission?

Answer 33

Fission is called spontaneous if it just happens by itself. Large nuclei are more likely to spontaneously fission because the larger the nucleus, the more unstable it is.

Question 34

List the components of a nuclear reactor.

Answer 34

A nuclear reactor includes: 1. Fuel rods (rods of uranium) 2. Control rods 3. Moderator (water) 4. Pump 5. Coolant 6. Concrete case

Question 35

Nuclear reactors: What is the critical mass?

Answer 35

The critical mass is the amount of fuel needed for the chain reaction to continue on its own at a steady rate. Any less than the critical mass (sub-critical mass) means that the reaction will just die out.

Question 36

Nuclear reactors: How does the moderator slow down the fission reactions?

Answer 36

The moderator, water, slows down neutrons so that they can be captured by the uranium nuclei and continue the chain reaction. The slowed down neutrons are called thermal neutrons. Collisions with particles of a similar mass to neutrons are more efficient at slowing them down - making water efficient as it contains hydrogen.

Question 37

Nuclear reactors: What do control rods do?

Answer 37

Control rods control the chain reaction by limiting the number of neutrons in the reactor. These absorb neutrons so that the rate of fission is controlled. Control rods are made up of a material that absorbs neutrons, e.g. boron. They can be inserted by varying amounts to control the reaction rate.

Question 38

Nuclear reactors: What does a nuclear reactor do automatically in an emergency?

Answer 38

In an emergency, the reactor will be shut down automatically by releasing the control rods into the reactor. This will stop the reaction as quickly as possible.

Question 39

Nuclear reactors: What is the purpose of coolant?

Answer 39

Coolant is sent around the reactor to remove heat produced in the fission - often this is the same water that is being used as the moderator. The heat from the reactor can then be used to make steam for powering electricity-generating turbines.

Question 40

Nuclear reactors: What is the purpose of the concrete case?

Answer 40

Thick concrete casing acts as shielding - it prevents radiation escaping and reaching the workers.

Question 41

What are some advantages of using nuclear fission to produce electricity?

Answer 41

1. Produces lots of energy | 2. Creates less greenhouse gases than fossil fuels

Question 42

What is the biggest disadvantage to producing electricity with nuclear fission?

Answer 42

There are lots of dangerous waste products.

Question 43

What happens to the waste produced by nuclear fission reactors?

Answer 43

Some waste can be used for tracers in medical diagnosis. Initally when material is removed from the reactor it is very hot, so is placed in cooling ponds until the temp falls to a safe level. It's then stored in sealed containers until its activity has fallen sufficiently. Areas are chosen where there is minimal impact. Residents are consulted.

Question 44

What is nuclear fusion?

Answer 44

Nuclear fusion is when two light nuclei are combined to create a larger nucleus.

Question 45

Why is so much energy needed for nuclear fusion?

Answer 45

All nuclei are positively charged, so there is a repulsive force between two nuclei. Nuclei can only fuse once they have overcome this electrostatic force. About 1MeV of kinetic energy is needed for nuclei to fuse together.

Question 46

What is the mass defect?

Answer 46

The mass of a nucleus is less than the mass of its constituent parts. The difference is called the mass defect.

Question 47

What is binding energy?

Answer 47

Binding energy is the energy needed to separate all of the nucleons in a nucleus. It is equivalent to the mass defect.

Question 48

What is the stability graph for radioactive nuclei?

Answer 48

You can get a stability graph by plotting Z (atomic number) against N (number of neutrons) ______________________________________x_________________________y

Question 49

Describe the line of stability on the stability graph for radioactive nuclei.

Answer 49

Straight line from the origin to (Z = 20, N = 20) Curving upwards to Z = 80; N = 110 –130

Question 50

Where do you find alpha emitters on the stability graph?

Answer 50

Below the line of stability, N > 80 and Z > 60.

Question 51

Where do you find beta-minus emitters on the stability graph?

Answer 51

Any region above and close to the line of stability.

Question 52

Where do you find beta-plus emitters on the stability graph?

Answer 52

Any region below and close to the line of stability.

Question 53

What is the equation for electron capture?

Answer 53

Electron Capture p + e- -> n + ν(e)

Question 54

What is the equation for beta-plus decay?

Answer 54

Beta-Plus Decay p -> n + e+ + ν(e)

Question 55

What is the equation for beta-minus decay?

Answer 55

Beta-Minus Decay ------------------_ n -> p + e- + ν(e)