Nuclear Physics

Question 1

Define Activity

Answer 1

the rate of decay of the radioactive nuclei in a given isotope. It is proportional to the total number of nuclei in the sample and is measured in becquerels.

Question 2

Desceribe the Rutherford scattering experiment.

Answer 2

1) A beam of alpha particles was directed at a thin gold foil
2) occurs in a vacuum so that no collisions between air particles and alpha particles can occur.
3) The experiment w

Question 3

What did the alpha particles do when fired at the gold foil.

Answer 3

Most just went straight through the foil
While some were deflected at an angle.
Some were deflected by more than 90 degrees - sending them back from where they came from.

Question 4

What were conclusions of the Rutherford scattering experiment?

Answer 4

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

Question 5

What is the range of alpha, beta and gamma in air?

Answer 5

Alpha - 2-10cm
Beta - 1m
Gamma - infinite range:follows inverse square law

Question 6

What are alpha, beta and gamma absorbed by?

Answer 6

Alpha - paper
Beta - Aluminium foil (3mm)
gamma - several metres of concrete or several inches of lead

Question 7

Which types of radiation get deflected by magnetic fields?

Answer 7

Alpha and beta

Question 8

What are some applications for alpha beta and gamma?

Answer 8

Alpha - Smoke alarms because they have a short range but highly ionising
Beta - controlling thickness of materials
Gamma - used in medicine

Question 9

what is the inverse square law for gamma radiation?

Answer 9

I = k/x^2

Question 10

Explain the experimental verification for the inverse square law.

Answer 10

As gamma radiation moves through the air it spreads out in all directions equally, as you move further away from the source. As you move further away the radiation per unit area will decrease.

The experiment: making measurements of intensity at different distance from the gamma source, using Geiger counter. If distance is doubled the intensity should fall to a quarter - verifying the inverse square law. It would form a straight line verifying the equation.

Question 11

what are some origins of background radiation?
Also how do I get rid of it when I’m conducting an experiment?

Answer 11

The air - radon gas released from rocks.
The ground and buildings - all rocks contain radioactive isotopes
Cosmic radiation
Living things - carbon-14
Man-made radiation - medicines

Corrected count = total count rate - background count

Question 12

What does it mean when radioactive decay has a random nature

Answer 12

Radioactive decay is random and you cannot predict when a nucleus will decay or which nucleus will decay next

Question 13

Define the decay constant

Answer 13

the probability of a nucleus decaying per unit time.

Question 14

How can decay constant be calculated?

Answer 14

Can be calculated by finding the change in number of nuclei (ΔN) of a sample over time (Δt), over the initial number of nuclei (N).

ΔΝ/Δt = -λN
N = N0e^-λt

Question 15

Define half-life

Answer 15

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

Question 16

Draw a graph of the number of unstable nuclei remaining against time.

Answer 16

Check notes

Question 17

Draw the log graph of number of the activity against time.

Answer 17

Check notes

Question 18

Give some applications of organic substances

Answer 18

Radioactive dating of objects - carob 14 used in radioactive dating. Living plants absorb carbon 14, which decays.
Medical diagnosis - radioactive tracers are used to help diagnose patients. Technetium-99m as a gamma source in medical diagnosis.

Question 19

How can long half-lives be dangerous and how should they be stored?

Answer 19

Very long halve-lives mean they will be highly radioactive for a long time, so need to be sealed underground to prevent harm to the environment and people.

Question 20

What does ‘Z’ stand for in element notation?

Answer 20

The proton number (atomic number)

Question 21

What does ‘A’ stand for in element notation?

Answer 21

The nucleon number or mass number

Question 22

How are nuclei held together?

Answer 22

Held together by the strong nuclear force, however protons feel a force of repulsion due to the electromagnetic force and so if the forces are out of balance, the nuclei will become unstable and will experience radioactive decay.

Question 23

What’s are the 4 reasons why a nuclei will become unstable. (What are the possible decay modes of unstable nuclei?).

Answer 23

1) It has too many neutrons - decays via beta-minus emission
2) It has too many protons - decays via beta-plus emission or electron capture.
3) has too many nucleons - decays via alpha emission
4) too much energy - decays via gamma emission ( nucleus becomes excited and has excess energy).

Also electron capture

Question 24

Draw a rough line of stability for an N - Z graph

Answer 24

Should be from same origin as y=x line but then curves off to the left.

Question 25

Why does the line of stability have this shape for an N-Z graph?

Answer 25

this is because beyond a certain amount of neutrons and protons, the electromagnetic force of repulsion becomes larger than the strong nuclear force keeping the nucleus together, and so more neutrons are needed to increase the distance between the protons in order to decrease the magnitude of the electromagnetic force, therefore keeping the nucleus stable.

Question 26

How can you estimate the nuclear radius of an atom?

Answer 26

calculating the distance of closest approach of a charged particle.

Question 27

How can you determine the exact nuclear radius of an atom?

Answer 27

electron diffraction

Question 28

How does distance of closest approach work?

Answer 28

E.g. alpha particle fired at gold nucleus Initial kinetic energy but as it moves towards positively charged nucleus it will experience electrostatic force of repulsion, slowing down and kinetic energy turns into electric potential energy Occurs to the point where the particle stops and has no kinetic energy and that is the distance of closest approach.

Question 29

What’s the typical value for any nuclear radius?

Answer 29

1x10^-15m

Question 30

What equation is required to calculate the distance of closest approach?

Answer 30

The Coulomb equation. V = 1/4πε0 x Q/r As electric potential is the potential energy per unit charge of a positive charge, if we multiply this by the charge of the fired particle, we get an equation for electric potential energy. E elec = 1/4πε0 x Q1Q2/r

Question 31

How is electron diffraction more accurate than distance of closest approach?

Answer 31

Electrons are leptons They will not interact with nucleons in the nucleus through the nuclear force as an alpha particle would.

Question 32

How does electron diffraction work?

Answer 32

Electrons show wave-particle duality so can be diffracted Accelerated at high speeds so that their De Broglie wavelength is around 10^-15m. λ ≈ hc/E (E = electron energy) Directed at very thin film of material in front of a screen causing then to diffract through the ga[s between nuclei and form a diffraction pattern.

Question 33

Draw a rough graph of intensity against diffraction angle.

Answer 33

check cgp should be similar to single-slit. With Central maximum.

Question 34

What equation can be used to find an estimate for the nuclear radius through electron diffraction?

Answer 34

sinθ = 0.61λ/R Theta - diffraction angle of the first minimum λ - de Borglie wavelength of electrons. R - radius of nucleus electron s were scattered by.

Question 36

Draw a Rough graph of Nuclear radius (R) against Nucleon number (A).

Answer 36

Check cgp or pmt

Question 37

Derive R = R0A^1/3

Answer 37

R = KA^n Where k is a constant Take ln of both sides and use log laws to get large = lnk +nlnA Plot a graph and n should be the gradient of the graph of lnR against lnA n will come out to equal 1/3 Therefore the constant in R = KA^n is now R0 and n is now 1/3

Question 38

How can we show that nuclear density is constant for all nuclei

Answer 38

Substitute the R = R0A^1/3 equation into density = A x m(nucleon) / 4/3πR^3 To get Density = m (nucleon) / 4/3πR0^3 as the variable A is cancelled out and only constant values remain this shows that nuclear density is constant.

Question 39

What is the mass defect?

Answer 39

When measuring the mass of a nucleus and the mass of its constituents the mass of the nucleus is always lower. This is the mass defect / mass difference. The mass that is lost is converted into energy and released when nucleons fuse to form a nucleus.

Question 40

What is binding energy?

Answer 40

Energy required to separate the nucleus into its constituents. Or the energy release when a nucleus is formed from its constituents.

Question 41

define (1) atomic mass unit

Answer 41

1/12th of the mass of a carbon-12 atom.

Question 42

Define nuclear fission

Answer 42

The splitting of a large nucleus into tow daughter nuclei. Occurs in very large nuclei which are unstable (such as uranium), and occurs completely RANDOMLY.

Question 43

Why is energy released during fission?

Answer 43

because the smaller daughter nuclei have a higher binding energy per nucleon.

Question 44

Define nuclear fusion

Answer 44

Where two smaller nuclei join together to form one larger nucleus. it only occurs in fairly small nuclei.

Question 45

Why is energy released during fusion?

Answer 45

Because the larger nucleus has a much higher binding energy per nucleon.

Question 46

Does fusion or fission release more energy?

Answer 46

Fusion releases far more energy than fission, however fusion can only occur at extremely high temperatures. E.g. stars, this is due to a massive amount of energy is required to overcome the electrostatic force of repulsion between nuclei.

Question 47

Define binding energy per nucleon

Answer 47

The binding energy of a nucleus divided by the number of nucleons in the nucleus.

Question 48

Plot a rough graph of binding energy per nucleon against nucleon number.

Answer 48

Check cgp or pmt

Question 49

What can a graph of binding energy against nucleon number tell us?

Answer 49

Whether an element can undergo fusion or fission. Peak of graph occurs at nucleon number 56 (iron) which has the highest binding energy per nucleon Nuclei smaller than iron can undergo fusion Nuclei larger than iron can undergo fission Look back at the definitions of fusion and fission and why energy is released for each of them if you don’t understand why.

Question 50

How does the understanding of nuclear physics apply to society?

Answer 50

Understanding the nuclear physics behind the production of nuclear power allows society to make informed decision about how electricity should be generated.

Question 51

What are thermal neutrons?

Answer 51

Have a low energy messing they can induce fission whereas neutrons with a higher energy rebound away from uranium - 235 after a collision and so not cause a fission reaction.

Question 52

What is induced fission?

Answer 52

Firing thermal neutron into the uranium nucleus causing it to become extremely unstable. The products o f fission are two daughter nuclei and at least one neutron. The neutrons released during fission go on to cause more fission reactions forming a chain reaction, where each fission goes on to cause at least one more fission.

Question 53

Define critical mass

Answer 53

Minimum mass of a fuel required to maintain a steady chain reaction. Using exactly the critical mass of a fuel will mean that a single fission reaction follows the last, while using less than the critical mass would lead the reaction to eventually stop.

Question 54

What is the function of a moderator?

Answer 54

Slows down the neutrons released in fission reactions to thermal speeds through elastic collisions between the nuclei of the moderator atoms and the fission neutrons.

Question 55

what’s a key characteristic of a moderator atom? What is often used as a moderator in a nuclear reactor?

Answer 55

The closer the moderator atom are in size to a neutron, the larger the proportion of momentum which is tranferred, therefore the lower the number of collisions required to get the neutrons to thermal speeds. Water is often used as it contains hydrogen, Inexpensive Not very reactive Also graphite is sometimes used.

Question 56

What is the function of control rods?

Answer 56

absorbs neutrons in the reactor in order to control chain reactions.

Question 57

Why is the height of the control rods controlled in a nuclear reactor? What are they usually made of?

Answer 57

In order to control the rate at which fission reactors occur to control the amount of energy produced. Made of materials which absorb neutrons without undergoing fission such as boron.

Question 58

What is the function of a coolant in a nuclear reactor?

Answer 58

absorbs the heat released during fission reactions in the core of the reactor.

Question 59

What is usually used as a coolant in nuclear reactors?

Answer 59

Water is both moderator and coolant as it has a higher specific heat capacity meaning it can transfer large amounts of thermal energy. This heat is they used to make steam which powers electricity-generating turbines.