1. FRCR II - Radioactivity
Nick Harding
Clinical Scientist
Radiotherapy Department
Castle Hill Hospital
Hull & East Yorkshire Hospitals NHS Trust

2. RADIOACTIVE DECAY Unstable nuclei / isotopes
spontaneous disintegration
transmutation
emission of radiation
These unstable isotopes are called
radioactive isotopes or radionuclides
The spontaneous nuclear transformation is called
radioactivity or radioactive decay / disintegration

3. RADIOACTIVITY

4. RADIOACTIVITY α - particles β - particles γ - rays Helium nuclei
electrons/positrons
γ - rays
electromagnetic

5. ACTIVITY Activity is defined as
the number of radioactive atoms undergoing nuclear transformations per unit time
The Système International unit is the becquerel (Bq)
1 Bq = 1 disintegration/second
Traditionally, expressed in units of curies (Ci)
1 Ci = 3.7 × 1010 Bq = 37 GBq = 37,000 MBq

6. RADIOACTIVE DECAY Radioactive decay is a random process
Number of atoms decaying per unit time
is proportional to the number of unstable atoms

7. RADIONUCLIDE PARAMETERS
The half-life (τ1/2) is defined as:
the time required for the number of radioactive atoms in a sample to decrease by one half
The decay constant (λ) is
the probability that an atom will decay per unit time
Both λ and τ1/2 are
unique for each radionuclide

9. RADIOACTIVE DECAY LAW Number of Nuclei Half-life Parent Nuclei N
1/2
Daughter Nuclei D
Number of Nuclei
1/4
1/8
1/16
1/32
Half-life

10. RADIOACTIVE DECAY LAW
The rate at which a radioactive isotope disintegrates is defined by the following DECAY LAW:
Where
N(t): number of radioactive atoms at time t
N0: initial number of radioactive atoms (at time zero)
τ1/2: half-life
e: base of natural logarithm ( ≈ 2.718)
λ: decay constant

11. RADIOACTIVE DECAY TYPES
Radioactive decays are classified by the types of particles that are emitted during the decay:
Alpha decay (α)
Beta decay (β)
Gamma decay (γ)
Internal conversion (IC)
Isomeric transition (ΙΤ)
Electron capture (ε or ec)
Spontaneous fission (SF)
Neutron emission (n)

12. ALPHA DECAY (α) 42Ηe Ζ = -2 Α = -4 p n n Parent Nucleus
Daughter Nucleus
42Ηe p n
Ζ = -2
Α = -4 n

13. ALPHA DECAY (α) Spontaneous emission of an alpha (α) particle
from the nucleus
An α particle is a Helium nucleus
containing two protons and two neutrons
Typically occurs  Heavy nuclides (A>150)

14. ΑLPHA PARTICLES Not used in medical imaging
range in solids and liquids
few micrometres (10-6m)
range in air
few centimetres (10-2m)
Alpha particles cannot penetrate the dead layer of the skin
Health hazard only when they enter the body eg. Po-210

15. Xofigo® THERAPY Used in medical therapy Ra-223 dichloride (Xofigo®)
Used to treat prostate cancer once it has spreaded to the bones
Radium acts like calcium

16. SMOKE DETECTOR Used in smoke detectors Smoke detector consists of
1 ion chamber
2 electrodes
1 Am-241 source (τ1/2 = 432y)
Am-241 decays via alpha decay
emits alpha particles
ionise air within chamber
electric current

17. SMOKE DETECTOR Used in smoke detectors Smoke detector consists of
1 ion chamber
2 electrodes
1 Am-241 source (τ1/2 = 432y)
Am-241 decays via alpha decay
emits alpha particles
ionise air within chamber
electric current

18. SMOKE DETECTOR Used in smoke detectors Smoke detector consists of
1 ion chamber
2 electrodes
1 Am-241 source (τ1/2 = 432y)
Am-241 decays via alpha decay
emits alpha particles
ionise air within chamber
electric current

19. ΒETA DECAY (β) Beta positive (β+) decay: Beta negative (β-) decay:
Proton (p+) → neutron + positron (β+)
Beta negative (β-) decay:
Neutron (n0) → proton (p+) + electron (β-)
Isobaric transitions – no change in A

20. β+ DECAY β+ Ζ = -1 Α = remains the same Parent Nucleus
Increase N/Z ratio – occurs in neutron poor isotopes
Parent Nucleus
Daughter Nucleus n p β+ n p p
Ζ = -1
Α = remains the same

21. β- DECAY β- Ζ = +1 Α = remains the same Parent Nucleus
Decrease N/Z ratio – occurs in neutron rich isotopes
Parent Nucleus
Daughter Nucleus p β- n p
Ζ = +1
Α = remains the same

22. positron annihilation
β Particles
positron annihilation
photon (511 keV)
β emission
up to “a few mm”

23. TRITIUM 3H

24. TRITIUM ILLUMINATION Tritium decays by beta decay Electrons released
Phosphor atoms excited/ionised
Release of characteristic radiation
Visible light!

25. GAMMA DECAY Nucleus in excited state (surplus of energy)
Release of excess energy  emission of γ-rays
nucleus returns to its ground / stable state

26. ISOMERIC TRANSITION Half-lives from 10-12 sec – 600 years
These excited states are called
metastable or isomeric states
No change in
atomic number
mass number
neutron number

27. ISOMERIC TRANSITION Isomeric transition is a radioactive decay process
excited nucleus decays to lower energy state
gamma radiation emitted
no emission of particles
no capture of particle by the nucleus

28. Mo-99 DECAY SCHEME 99Mo decays by β- decay
into 99Tcm (i.e. 99Tcm metastable state of 99Tc)
half-life = 66 hours
99Tcm decays by isomeric transition
into 99Tc ground state with 6 hr half-life
half-life = 6.01 hours

29. ISOMERIC TRANSITION

30. ELECTRON CAPTURE
An alternative to positron decay for neutron poor radionuclides;
Increase N/Z;
Nucleus captures an orbital electron and converts a proton into a neutron;

31. ELECTRON CAPTURE e- Ζ = -1 Α = remains the same Parent Nucleus`
Parent Nucleus
Daughter Nucleus p n e- p
Ζ = -1
Α = remains the same

32. INTERNAL CONVERSION Nucleus in excited state (surplus of energy)
De-excitation through
ejection of a tightly bound electron (K- or L-shell)
Ejected electron not a beta particle – no change of Z
alternative mechanism to γ-decay

33. SPONTANEOUS FISSION Heavy nuclei decay by splitting into 2 or 3 nuclei
release of neutrons
release of energy

34. Neutron Emission Radioactive decay where neutron ejected from nucleus
No change in Z so same element
Becomes new isotope
Generally short half life

35. Any questions?