Ch25: Radioactivity

Question 1

What does ionising mean?

Answer 1

Something ionising can ionise atoms, removing some of their electrons and leaving a positive ion

Question 2

What is alpha radiation made up of? What is its charge?

Answer 2

A helium nucleus, 2 protons, 2 neutrons. Charge of +2e

Question 3

What is beta radiation? What is its charge?

Answer 3

Fast moving electrons or positrons. Beta minus has charge -e. Beta plus has charge +e

Question 4

What is gamma radiation?

Answer 4

HIgh energy photons with wavelengths less than 10^-13m. No charge.

Question 5

What is needed to completely absorb alpha particles?

Answer 5

Thin sheet of paper

Question 6

What is needed to completely stop most beta particles?

Answer 6

1-3mm of aluminium

Question 7

What is needed to completely absorb gamma ?

Answer 7

few centimeters of lead

Question 8

What is the typical speed of alpha radiation?

Answer 8

10^6

Question 9

What is the typical speed of beta radiation?

Answer 9

10^8

Question 10

What is the typical speed of gamma radiation?

Answer 10

Speed of light.

Question 11

Briefly describe alpha decay.

Answer 11

A parent nucleus decays into a daughter nucleus, losing 2 protons and 2 neutrons as an alpha particle.

Question 12

What is released during alpha decay?

Answer 12

Energy, an alpha particle(helium nucleus), and daughter nuclei.

Question 13

What causes beta decay?

Answer 13

the Weak nuclear force

Question 14

Briefly describe beta minus decay.

Answer 14

In an unstable radioactive isotope, a neutron decays into a proton, and an electron is emitted as well as an electron anti-neutrino.

Question 15

Briefly describe beta-plus decay.

Answer 15

In an unstable radioactive isotope, a proton decays into a neutron, emitting a positron and an electron neutrino.

Question 16

When does gamma decay occur? What does it change?

Answer 16

Occurs when surplus energy is left after beta or alpha emission. Only releases energy, does not change nucleon make up.

Question 17

Why is radioactive decay considered spontaneous?

Answer 17

• It cannot be predicted when a nucleus will decay.

- Each nucleus in a sample has the same chance of decaying per unit time.

Question 18

Why is radioactive decay considered spontaneous?

Answer 18

Decay of the nucleus is not affected by:

• Presence of other nuclei in the sample
• External factors e.g pressure

Question 19

What is the half life of a sample of an isotope?

Answer 19

The average time taken for half the number of active nuclei to decay.

Question 20

What is the activity of a radioactive source?

Answer 20

The rate at which nuclei decay or disintergrate.

Question 21

What is activity measured in?

Answer 21

Decays per second, Becquerels Bq

Question 22

What is decay constant?

Answer 22

probability of decay of an individual nucleus per unit time.

Question 23

How are half life( t1/2) and decay constant (λ) related?

Answer 23

λ x t1/2 = ln2

Question 24

What are some limitations of carbon dating?

Answer 24

• Assumes ratio of carbon-14 to carbon-12 stays the same over time; this could have been affected by various events.
• Not accurate for very old or small things where activity is close to that of the background rate.