Radioactivity

Question 1

What is ionising radiation

Answer 1

Beta, gamma and alpha radiation are all ionising because they can remove electrons from atoms leaving positive ions

Question 2

How to find the absorption of radiation by different materials

Answer 2

Use a counter to measure the background radiation. Then place the radiation source in a sealed container and place the material a fixed distance away from the radiation source. Place the GM tube in front of the material a fixed distance away from the source. Place counter in front of GM Tube. Minus background radiation from result.

Question 3

What is the absorption of Alpha radiation

Answer 3

Strong Ionisation due to large mass and charge but short range. Stopped by a sheet of paper

Question 4

What is the absorption of Beta radiation

Answer 4

Less ionising due to smaller mass and charge than alpha particles but can penetrate farther. Stopped by a sheet of aluminium

Question 5

What is the absorption of Gamma radiation

Answer 5

Least ionising due to having no charge but has high penetrating power, stopped by lead.

Question 6

What happens during alpha decay

Answer 6

A parent nucleus emits an alpha particle removing 2 protons and 2 neutrons from itself, decaying into the daughter nucleus which is a new element. Energy is released

Question 7

What happens during Beta decay

Answer 7

Beta minus radiation is emitted due too many neutrons causing instability. Weak nuclear force causes a neutron to decay into a proton, emitting an electron and an electron anti neutrino. Proton number increases thus daughter nucleus is a different element. Beta plus radiation is emitted due to excess protons causing instability. Proton decays into a neutron and a positron and electron neutrino is emitted. Daughter nucleus has one less proton than parent, therefore it is a new element.

Question 8

What happens during gamma decay

Answer 8

Gamma photons are emitted if a nucleus has excess energy after alpha or beta emission. Nucleus remains the same.

Question 9

What are the patterns for stability

Answer 9

The stability band shows the stable nuclei. Nuclei to the right of the band have excess protons and will undergo beta plus decay. Nuclei to the left have excess neutrons and undergo beta minus decay. Nuclei with more than 82 protons are likely to undergo alpha decay. Ratio between neutrons and protons increase as protons increase.

Question 10

Why is radioactive decay a random event

Answer 10

Cannot predict when or which nucleus will decay. Each nucleus has the same chance of decaying per unit time

Question 11

Why radioactive decay a spontaneous event

Answer 11

It is not affected by any external factors

Question 12

What is the half life of an isotope

Answer 12

Average time taken for half of active nuclei to decay

Question 13

What is activity

Answer 13

Number of alpha, beta or gamma particles are emitted from the source per unit time. Measured in Bq (Becquerel). Depends on number of un decayed nuclei present in source and half life of isotope. Given by the equation activity = decay constant * Number of nuclei

Question 14

What is the exponential decay equation

Answer 14

Number of nuclei = Initial number of nuclei * e^-decay constant * time

Question 15

Define the decay constant

Answer 15

Probability an individual nucleus will decay per unit time

Question 16

How to measure half life in an experiment

Answer 16

Measure background radiation using counter. Place source fixed distance from counter, place GM tube fixed distance away from counter in front of radiation source. Using a timer, record the count after 30 seconds multiple times. Minus background radiation from counter recording, log both time and counter and plot log counter against log time. Graph gives straight line, gradient equals negative half life.

Question 17

How does radioactive dating work

Answer 17

By measuring the ratio of C-14 to C-12 in the dead tissue and comparing this to the atmospheric composition an estimation for the time since the organism’s death can be calculated. Limitation is that it assumes ratio between c-14 and c-12 has remained constant.