25, Radioactivity

Question 1

What is alpha radiation?

Answer 1

A helium nucleus, usually from atoms with 82 protons or higher.

Question 2

What is beta radiation?

Answer 2

An electron or positron caused by the weak force when there is too many neutrons or protons.

Question 3

What is gamma radiation?

Answer 3

High energy gamma photons emitted if there is surplus energy following a decay.

Question 4

How can a magnetic or electric field be used to separate the different forms of radiation?

Answer 4

Gamma is unaffected by both of these fields, however alpha and beta are deflected slightly with beta being deflected much more due to its higher mass (despite alpha moving slower and having twice the charge).

Question 5

What experiment could you do to investigate absorption of radiation?

Answer 5

Place a source facing towards a Geiger counter, first take several time samples to find the background radiation then place varying amounts of the absorbent material between the tube and source, again measuring count rate over a set time period.

Question 6

What does it usually take to absorb most alpha radiation?

Answer 6

A few cm of air or a thin sheet of paper.

Question 7

What does it usually take to absorb most beta radiation?

Answer 7

1-3mm of aluminium.

Question 8

What does it usually take to absorb most gamma radiation?

Answer 8

A few cm of lead.

Question 9

What does the term daughter nucleus refer to?

Answer 9

The nucleus left from the decay of the parent nucleus.

Question 10

Why is alpha radiation most ionising?

Answer 10

Because it moves slower and therefore takes longer to react (despite the fact that all radiation has a similar energy level).

Question 11

How can a decay chain be represented graphically?

Answer 11

Using an A-Z plot and arrows showing each decay.

Question 12

What does it mean for decay to be random and spontaneous?

Answer 12

It is random since we cannot predict the next decay and is spontaneous since we cannot affect the chance of a decay.

Question 13

What are the issues with using die to model decay?

Answer 13

Decay happens continuously not over a set time period so does not follow “true” exponential decay.

Question 14

Define activity.

Answer 14

The activity of a source is the rate at which nuclei decay (number of alpha/beta particles per unit time).

Question 15

What is the unit for activity?

Answer 15

becquerel (Bq or s^-1).

Question 16

What does the equation ΔN/Δt = -λN mean?

Answer 16

The rate of change of the No. (N) of undecayed nuclei (i.e. the amount lost per unit time) = decay constant of a isotope * No. nuclei remaining.

Question 17

What does the equation A = λN show?

Answer 17

The activity = decay constant * No. undecayed nuclei.

Question 18

What is the definition of the decay constant?

Answer 18

The probability of decay of an individual nucleus per unit time.

Question 19

How is half life determined for sources with very large half lives?

Answer 19

Take a source, use its mass to find the number of nuclei and use this and its current activity to work out the decay constant then t_(1/2) = ln(2)/λ.

Question 20

How can spreadsheets be used to model decay?

Answer 20

Find the % of decayed nuclei in a time period and repeat the use of A = λN with smaller time periods.

Question 21

How does carbon dating work?

Answer 21

By finding the % of the radioactive carbon-14 relative to the number of carbon-12 and an estimate as to what the original % of carbon-12 should be to find how long it would take for that much carbon-12 to decay.

Question 22

What are the issues with carbon dating?

Answer 22

It assumes the carbon-12 to 14 ratio in living creatures has stayed roughly constant which it may not have due to different levels of C02 in the air.