Chapter 20: Nuclear Physics

Question 1

Define mass defect

Answer 1

Mass defect is defined as the difference between the total mass of the nucleons taken separately and the mass of the nucleus

Question 2

What is binding energy?

Answer 2

Binding energy of a nucleus is the energy needed to separate completely the constituent nucleons in the nucleus TO INFINITY.
OR: the energy released when a nucleus is formed by putting all its constituent nucleons together

Question 3

Define Binding energy PER nucleon

Answer 3

Binding energy per nucleon is defined as the average energy per nucleon needed to split a nucleus into separate nucleons

Question 4

What is nuclear fission and fusion?

Answer 4

Nuclear fission is the splitting of a heavy nucleus into 2 or more lighter nuclei. Nuclear fusion is the combining of two lighter nuclei to form a heavier nucleus.

Question 5

What is a spontaneous process?

Answer 5

The half life is the same no matter what external factors such as temperature and pressure are applied.

Question 6

What is a random process?

Answer 6

A random process means the nucleus has constant probability of decay per unit time.
it cannot be predicted which nucleus will decay or when any particular nucleus will decay.

Question 7

Define decay constant

Answer 7

The decay constant is the ratio of the number of decays per unit time to the number of remaining radioactive nuclei. It represents the probability that a radioactive nucleus will decay in a unit time

Question 8

Define half-life

Answer 8

The half-life of a radioactive nuclide is the average time taken for the initial number of radioactive nuclei in that nuclide to decay to one half.
It is also defined as the average time taken for initial number of radioactive nuclei in that nuclide to decay to one half.

Question 9

Define activity (of a radioactive source)

Answer 9

The activity of a radioactive source is the rate of decay of the radioactive nuclei. It can also be defined as the number of disintegrations of the nuclei per unit time

Question 10

Observations of alpha-scattering experiments

Answer 10

1. Most a-particles passed through gold foil with little or no deviation from its original path: The atom is mostly empty space as the nucleus is very small compared to the atom itself
2. A small number of a-particles were deviated through an angle of more than 10 degrees: The nucleus of the atom is positively charged. a-particles are also positively charged, passing through nucleus will experience a repulsive force causing them to deviate
3. An extremely small no. of a-particles (1 in 10000) were deflected through an angle >90 degrees: Nucleus of gold atom have relatively large mass to be able to cause a-particles . colliding almost head-on, to deviate through angles >90 degrees

Question 11

Why cannot use B particles for scattering experiment?

Answer 11

1. Beta particles are emitted with a range of kinetic energies
2. Beta particles have very small mass and are easily deviated by orbital electrons

Question 12

Advantages & disadv
of having a short half life for B-particles

Answer 12

Advantages:
1. The source reduces to a low activity level within a short time for safe disposal
2. High Activity reduces inaccuracies/errors due to background radiation

Disadvantages:
1. The source will need to be replaced frequently
2. May cause detector output to drop rapidly without variation in foil thickness

Question 13

Prediction of existence of antineutrino & neutrino

Answer 13

1.Unlike a-decay, B-particles are emitted w varying speeds, KE graph of B-decay shows continuous spectrum up to a maximum, unlike predictable energy of a-particle, by COE, energies of B-particles should be absolute & predictable

2.nuclear recoil of daughter nucleus not in direction opposite the momentum of electron

Particle emitted along an electron from a nuclei is antineutrino
Emission from a positron (same mass as electron but +ve charge)is called a neutrino

Question 14

Source emitting high-energy B particles placed in aluminium container, although B particles are absorbed, X-ray radiation is emitted from container walls, explain.

Answer 14

B-particles are high speed electrons with high KE, when they collide with aluminium particles, they are decelerated suddenly, producing high-energy photos which are X-ray photons

Question 15

Define radioactive decay

Answer 15

Radioactive decay is a spontaneous and random process in which an unstable nucleus disintegrates into a different nuclei to acquire a more stable state which emits particles and radiation in the process

Question 16

Explain why, when using an experimental exponential decay curve to determine the half-life of a radioactive isotope, it is better to use the part of the graph where the curve is steep.

Answer 16

The steep part of the curve is where activity/no. of nuclei/count rate is significantly higher per unit time, so any background radiation would be insignificant and can be ignored when determining the half-life.

Question 17

Explain how the process of nuclear fusion may result in release of energy

Answer 17

1. When two nuclei of lower nucleon number combine to form a product nucleus of higher nucleon number, it will have larger BE per nucleon
2. BE= BE per nucleon x no. of nucleons
3. Product nucleus will have higher total BE than reactant nuclei
4. Energy released during formation of product nucleus outweighs the energy required for the reaction
5. Hence there is a net release of energy

Question 18

radioactive

Answer 18

The nucleus is unstable and will emit alpha particles or beta particles and/or gamma particles to form a more stable nucleus.

Question 19

Experimental data of alpha particles

Answer 19

Alpha particles are emitted with approximately the same amount if kinetic energy

Question 20

Experimental data for beta particles

Answer 20

Beta particles are emitted with a range of KE or momentum

Question 21

Gamma decay

Answer 21

Occurs when unstable, excited nucleus de-excites to a lower, more stable energy state by emitting a gamma ray photon.

Question 22

Decay constant

Answer 22

The probability of decay of a radioactive nucleus per unit time.
change in N/N