Week 4

Question 1

what is the cause of radioactivity and radioactive decay?

Answer 1

as a nucleus gets bigger and number of protons increase, more neutrons are required to balance out the electrostatic repulsion and at some point, the atom becomes unstable. nuclei then spontaneously rearrange their structure to become stable

Question 2

what is alpha decay?

Answer 2

2 protons and 2 neutrons (strongly bound) are ejected from the nucleus. this is an alpha particle.

mass number -4 atomic number -2

Question 3

what is beta/positron decay?

Answer 3

beta minus decay- when there are too many neutrons in a nucleus, a neutron decays to a proton and a negatively charged electron (beta particle) is ejected as well as an anti-neutrino

atomic number +1

positron decay- when there are too many protons in a nucleus and a proton decays into a neutron and emits a positively charged electron (positron) and a neutrino

atomic number -1

Question 4

what is gamma decay?

Answer 4

when atoms are in meta state/an excited state (after alpha/beta decay), they will instantaneously decay to their ground state, emitting one or more gamma rays (equal to the difference in energy)

no change, excited state goes away

Question 5

what is characteristic x-ray emission?

Answer 5

when an electron is removed from one of the inner shells, an outer electron moves down to fill this position. due to the difference in energy levels between shells, an x-ray photon is emitted equal to the difference in energy. this energy is unique for elements and thus, characteristic

Question 6

how can the half-life be used to characterise radioactive rate of decay?

Answer 6

the half-life is the time taken for half the nuclei in the original sample to decay. half-life is used as radioactive decay is exponential so this gives a measure of the rate of decay

Question 7

what are the similarities and differences between gamma rays and x-rays?

Answer 7

gamma rays occur in the nucleus and involves a release in energy from the nucleus, x-rays originate from electron transitions, gamma rays are usually higher energy but there is a significant overlap. both are photons of electromagnetic radiation

Question 8

what is activity in a sample?

Answer 8

number of disintegrations per second
calculated in becquerel (Bq) = 1 dis/sec
depends on: number of nuclei in sample (N), decay parameter (λ)

Question 9

list the equations for activity (A), number of nucleus (N), and half life (T 1/2)

Answer 9

A = Ao x e^(-λt)
N = No x e^(-λt)
T 1/2 = ln(2)/λ