Unit 2 - Radiocarbon Dating

Question 1

What are the naturally occurring isotopes of carbon?

Answer 1

^12C - 98.89%
^13C - 1.11%
^14C - 0.00000000010% (1 in 10^12)

Question 2

How does cosmic radiation work?

Answer 2

1. cosmic rays (from the sun) enter the earth’s atmosphere and collide with an atom, creating an energetic neutron.
2. neutron collides with a nitrogen atom (replacing one of the protons), the nitrogen-14 turns into a carbon-14 atom.
3. plats absorb the CO2 and incorporate Carbon-14 through photosynthesis.
4. animals and humans eat plants, taking in carbon-14
5. following death, wood and bones lose C-14 as it returns to N-14 via beta decay.

Question 3

How to calculate number of nuclei at a certain time?

Answer 3

Nt = N0 x (1/2)^n

(number of nuclei at time t = initial number of nuclei x number of half lives)

Question 4

What is the activity of a radioactive source?

Answer 4

The number of nuclear disintergrations per unit time occurring in a radioactive material

In the international system of units (SI), the becquerel (Bq) is the unit of radioactivity.

One Bq is 1 disintergration per second.
One curie is 37 billion Bq.

Since the Bq represents such a small amount, you are likely to see a prefix used with Bq:
1MBq (27 microcuries), 1GBq (27 millicuries), 37 GBq (1 curie), 1 TBq (27 curies).

Question 5

Three ways to detect radiation

Answer 5

Ionisation (Geiger) counter
Scintillation counter
Accelerator Mass Spectrometry (AMS)

Question 6

Scintillator detector

Answer 6

Effective at measuring gamma radiation.

the scintillator material emits light that is collected in photomultiplier tubes.

Question 7

Accelerator mass spectrometry (AMS)

Answer 7

• ultra sensitive isotope ratio mass spectrometry of small prepared samples
• most advanced way of measuring C-14
  Isotopes typically measured include a handful of long-lived radioisotopes that are naturally lowly abundant due to their instability, yet difficult to measure by their infrequent decay.
  -ions are characterised by combined conventional mass spectrometry and high-energy collisions within the accelerator and in a final detector.

Question 8

AMS vs Radiation counters

Answer 8

AMS:
- counts number of atoms in a sample
- AMS would give 1000 counts in 0.1 seconds.

Scintillation counter:
- measures the infrequent radioactive decay events in the sample ]
- counts would take 5 days.

Question 9

Calculating dates

Answer 9

Relationship between the rate of decay and the number of radioactive nuclei (N)

K is the decay constant and is characteristic of the radionuclide.

Activity = rate of decay = k x N

(Activity proportional to number of nuclei)

Question 10

Exponential decay

Answer 10

Radioactivity follows an exponential decay

The number of nuclei (N) remaining after time t is given by : N = Noe^-kt
Where No is the number of radioactive nuclei present initially, at t = 0.

N and No can be measured in number of nuclei or mass present (as proportional to number of nuclei) or as activity.

(Intereted in ratio so units doesn’t matter)

Question 11

cosmic rays

Give the formula for the nuclear reaction that creates C-14 in the atmosphere

Answer 11

(14:7)N + (1:0)n -> (14:6)C +(1:1)H