Nuclear Decay

Question 1

Background Radiation

Answer 1

• Radioactive gasses are emitted from the ground, can be trapped in buildings and rise to dangerous levels
• Radioactive elements in earths crust release gamma radiation
• cosmic rays from outer space produce showers of particles when they hit the atmosphere and release gamma rays
• 85% of background radiation is from these and naturally occurring isotopes, 15% is artificial

Question 2

Safety

Answer 2

• keep > 30cm away to stop alpha particles
• do not touch the source
• keep sources in lead boxes when not in use
• keep sources pointed away from you at all times
• limit time using sources
• wash hands after use

Question 3

Alpha, Beta, Gamma

Answer 3

• Alpha = a helium nucleus, positive charge, range of 5 cm in air, highly ionising due to mass
• Beta = an electron, negative charge, range of a few metres in air, less ionising
• Gamma = a neutral photon, very long range, weakly ionising

Question 4

Detection

Answer 4

• alpha radiation can be seen with a spark chamber, as it ionises strongly in a small radius
• beta radiation can be detected with a GM tube
• Gamma radiation is hard to detect as it is neutral and weakly ionising

Question 5

Decay processes

Answer 5

• Alpha emission = Rn-204 -> Po-200 + Alpha-4
• Beta emission = Be-10 (IV) -> B-10 (V) + Beta (-I)
• Gama emission = released from an excited nucleus along with alpha and beta

Question 6

Critical Mass

Answer 6

• A specific surface area to volume ratio of radioactive material that causes a rapid release of energy
• nuclear explosives rely on mechanisms that cause critical mass to be suddenly achieved

Question 7

Instability

Answer 7

• either caused by a nucleus too big that the strong nuclear force won’t act over that radius so it decays into a stable nucleus
• imbalance between protons and neutrons in nucleus causes decay

Question 8

Nature of decay

Answer 8

• Spontaneous decay = don’t know which nucleus will decay next
• Random decay = don’t know when the next nucleus will decay

Question 9

Decay equations

Answer 9

• rate of decay = decay constant*Number of radioactive nuclei = - change in nuclei/change in time
• Activity (Bq) = decay constant*number of radioactive nuclei
• half life = ln2/decay constant
• lnA = -decay constant*time + lnA(original)

Question 10

C-14 dating

Answer 10

• living plants have equal ratios of C-12 to C-14 as they gain and lose it through life processes, when they die the C-14 decays away
• using the half life of C-14 you can estimate the age of the plant matter
• unsuitable for anything over 60 000 years old, anything over that requires K-40

Question 11

Nuclear Medicine

Answer 11

• radioactive tracers need long enough half-lives to not be harmful, but also short enough to give patients a minimum dosage, Technetium is a good choice
• cancerous cells are very vulnerable to radiation damage
• meta-stable technetium-99m is most common medical tracer formed from Molybdenum by dissolving it in saline solution and flushing it out gradually