Radioactivity

Question 1

What is the atomic number?

Answer 1

The number of protons in the nucleus of an atom.

Question 2

What is the mass number?

Answer 2

The total number of protons and neutrons in the nucleus of an atom.

Question 3

What are isotopes?

Answer 3

Atoms with the same number of protons (same atomic number) but a different number of neutrons (different mass number).

Question 4

What tends to happen to unstable isotopes?

Answer 4

They are radioactive with unstable nuclei, and so emit ionising radiations (alpha particles, beta particles, and gamma rays) randomly and spontaneously.

Question 5

What are the sources of background radiation?

Answer 5

• Artificial: nuclear explosions; nuclear waste; nuclear medicine.
• Natural: outer space - cosmic rays; Earth - air, food, building materials, rocks; living things.

Question 6

What is radioactivity measured in?

Answer 6

becquerels (Bq)

Question 7

What is the structure and charge of alpha particles?

Answer 7

Made up of 2 protons and 2 neutrons with a charge of 2⁺.

Question 8

How penetrating are alpha particles?

Answer 8

Not very penetrating - stopped by mm of paper.

Question 9

How ionising are alpha particles?

Answer 9

Heavily ionising - produce lots of ions and can do lots of damage.

Question 10

What happens during alpha decay?

Answer 10

The atomic number will decreas by two and the mass number will decrease by 4.

Question 11

What is the structure and charge of beta particles?

Answer 11

Made up of an electron with a charge of 1^-.

Question 12

How penetrating are beta particles?

Answer 12

Moderately - stopped by a minimum of mm of aluminium.

Question 13

How ionising are beta particles?

Answer 13

Less ionising than alpha but still moderately ionising.

Question 14

What happens during beta decay?

Answer 14

The atomic number increases by one and the mass number stays the same.

Question 15

What is the structure and of gamma rays?

Answer 15

Electromagnetic waves which do not have a charge.

Question 16

How penetrating are gamma rays?

Answer 16

Very - cannot be stopped but can be reduced by metres of concrete/lead.

Question 17

How ionising are gamma rays?

Answer 17

Not very ionising - tend to pass through things rather than actually collide and interact with atoms.

Question 18

What is the half-life?

Answer 18

• The time it takes for the number of undecayed nuclei to halve.
• The time it takes for the activity of a radiocatice substance to halve (the number of decays per second)

Question 19

What are the dangers of ionising radiations?

Answer 19

• Mutations in living organisms.
• Damage to cells and tissue.

Question 20

How can radioactivity be used in medical tracers?

Answer 20

• A beta/gamma emitting source is injected (or swallowed) into a patient.
• The radiation penetrates the body cells and will show externally - anywhere that radiation cannot get through shows up as somewhere which is not functioning.
• The radioactive source should have a short half-life so that it does not interact with the body cells and cause harm.

Question 21

How can radioactivity be used in industrial tracers?

Answer 21

• To detect a leak in a pipe, a substance which emits gamma rays is injected into a pipe.
• In the areas of the pipe where there is a leak, the gamma rays will have a higher reading.
• Something with a relatively long half-life should be used so that the gamma rays can be detected all the way through the pipe, but they do not cause any damage.

Question 22

How does carbon-dating work?

Answer 22

By comparing the ratio of C¹² to C¹⁴ in an artefact to the ratio in something alive now, you can estimate the age of a substance.

Question 23

What is it called when a neutron collides with the nucleus of uranium-235 and what is produced when this happens?

Answer 23

• (Nuclear) fission.
• Two daughter nuclei are produced as well as a small number of neutrons.

Question 24

How can a neutron colling with uranium-235 lead to a chain reaction?

Answer 24

The nucleus of the uranium splits and releases neutrons, which can be hit/absorbed by different uranium nuclei and produce a chain reaction.

Question 25

What was Rutherford's nuclear model of the atom?

Answer 25

* The atom contained a _small_, _dense_, _positively charged_ nucleus which contained the _vast majority of the mass_ of the atom. * There were _electrons orbiting the nucleus_ (like planets orbiting the sun). * Almost all of the atom is _empty space_ (the atom's radius was much larger than the radius of the nucleus).

Question 26

What were the results of Geiger and Marsden's experiment with gold foil and alpha particles?

Answer 26

* Alpha particles were fired from at a very thing piece of gold leaf. * A detector consisting of fluorescent material, which emitted a flash of light whenever it was hit by an alpha particles, was moved around and showed what path the alpha particles after hitting the gold leaf. * Almost all alpha particles went straight through, but a few deflected at a whole range of angles (some very big) and some were deflected straight back.

Question 27

What form does the energy released when a neutron collides with uranium-235 take?

Answer 27

Kinetic energy of fission products (i.e. daughter nuclei/neutrons).

Question 28

How did Rutherford's observations lead to the current model of the atom?

Answer 28

The 'Plum Pudding model' (a positive sphere contained negative electrons dotted inside) would not explain how the atom would _exert enough force_ on an alpha particle to deflect it back at large angles, because the model says that the mass of the atom is _evenly distributed_ and has a l_ow density_. Rutherofrd came up with his new idea of the atom: * _small_ nucleus because only a few particles were deflected back (therefore mostly empty space) * most of the mass must be _concentrated at the centre_ * centre must be _positive_ so that it can expel the alpha particles

Question 29

What factors affect the deflection of alpha particles by a nucleus?

Answer 29

* The **faster** an alpha particle is travelling, the **less** it will be deflected by the nucleus. * The **more positively charged** a nucleus is (higher atomic number), the **more** an alpha particle will be deflected.

Question 30

What role do the control rods play when the fission process is used to create electricity?

Answer 30

Control rods *(made from boron and cadmium)* can be lowered into the reactor vessel to _absorb_ neutrons and control the _speed_ _of the reaction_ - the more neutrons absorbed, the slower the reaction.

Question 31

What role does the pressurised water play when the fission process is used to create electricity?

Answer 31

* Acts as a _coolant_ and conducts heat from the rods. * Acts as a _moderator_ to _slow down the neutrons_ and therefore increase the likelihood of collisions and nuclear fission occuring.

Question 32

_Useful Energy Transfers in a Nuclear Power Station_ 1. **heat** energy released by the reactor 2. energy in water/steam 3. energy in turbine 4. energy in generator 5. energy in wires

Answer 32

1. **heat** energy released by the reactor 2. _kinetic/thermal/heat_ energy in water/steam 3. _kinetic_ energy in turbine 4. _kinetic/electrical_ energy in generator 5. _electrical_ energy in wires

Question 33

Iodine-131 is a radioactive isotope that emits beta particles. It is used to treat thyroid cancer. The isotope is allowed to enter the tumour. Explain why it is suitable for this treatment.

Answer 33

* Beta particles are (moderately) ionising. * Beta has a short range. * The Iodine-131 has a short half-life. * Iodine is absorbed (easily by the thyroid) and so: * damage to healthy cells is reduced * the radiation does not penetrate out of the body * only tumour cells are killed

Question 34

Describe nuclear fission and how the chain reaction is controlled.

Answer 34

* A nucleus is hit by a neutron (and absorbs it). * The nucleus splits into two daughter nuclei. * Extra neutrons are released. * Kinetic energy is released. * Released neutrons hit further nuclei (and split them). * Moderator slows down the neutrons (and makes it easier for them to be absorbed). * The control rods absorb extra neutrons. * The control rods help to prevent a "runaway" chain reaction.

Question 35

Why are some isotopes described as stable?

Answer 35

They do not emit any ionising radiations (alpha, beta or gamma rays).