1. Radioactivity Manos Papadopoulos Nuclear Medicine Department
Castle Hill Hospital
Hull & East Yorkshire Hospitals NHS Trust

2. RADIOACTIVE DECAY
Only certain combinations of nucleons form a stable nucleus
Unstable nuclei
spontaneous nuclear transformation
formation of new elements
emission of radiation
These unstable isotopes are called
radioactive isotopes
The spontaneous nuclear transformation is called
radioactivity or radioactive decay / disintegration

3. RADIOACTIVE DECAY
An unstable “parent” (P) nuclide is transformed into a more stable daughter (D) nuclide through various processes
where d1 + d2 + … signify the emitted particles
The process is usually accompanied by the emission of gamma radiation

4. RADIOACTIVITY

5. ACTIVITY Activity (A) is defined as:
the number of radioactive atoms (N) undergoing nuclear transformations per unit time (t)

6. UNITS OF ACTIVITY Traditionally, expressed in units of curies (Ci)
1 Ci = 3.7 × 1010 disintegrations/second
Typical activities
for imaging: 0.1 to 30 mCi
for therapy: up to 300 mCi
The Système International (SI) unit is the becquerel (Bq)
1 Bq = 1 disintegration/second

7. DECAY CONSTANT Radioactive decay is a random process
The number of atoms decaying per unit time (dN/dt)
is proportional to the number of unstable atoms (N)
where λ is the transformation constant (or decay constant)
being characteristic of each radionuclide

8. HALF-LIFE 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
λ and τ1/2 are related as follows:
where ln2 denotes the natural logarithm of 2
Both λ and τ1/2 are
unique for each radionuclide

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 LAW N0
τ1/2 = 5730y
5730

12. PROBLEM
A nuclear medicine technologist injects a patient with 800 MBq of [99Tcm]-SestaMIBI (τ1/2=6.02 hours). One hour later the patient is imaged. Assuming that none of the activity is excreted, how much activity remains at the time of imaging?

13. SOLUTION
A0 = 800 MBq
λ = 0.693/6.02 hours = hours-1
t = 1 hour

14. 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)
Isomeric transition (ΙΤ)
Electron capture (ε or ec)
Internal conversion (IC)
Spontaneous fission (SF)
Neutron emission (n)

15. ALPHA DECAY Spontaneous emission of an alpha (α) particle
from the nucleus
An α particle is a Helium nucleus
containing two protons and two neutrons

16. ALPHA DECAY Typically occurs  Heavy nuclides (A>150)
Emission of gamma and characteristic X-Rays

17. ALPHA DECAY Alpha particle emitted from the atomic nucleus
Alpha particle and daughter nucleus have equal and opposite momentums

18. ΑLPHA PARTICLES Not used in medical imaging
range in solids and liquids
few micrometres
range in air
few centimetres
Alpha particles cannot penetrate the dead layer of the skin
Health hazard only when enter the body

19. ΒETA DECAY Beta positive (β+) decay: Beta negative (β-) decay:
Proton (p+) → neutron + positron (β+) + neutrino
Beta negative (β-) decay:
Neutron → proton (p+) + electron (β-) + antineutrino

20. β- DECAY converts one neutron into a proton and an electron
no change of A, but different element
occurs with nuclides with an excess number of neutrons

21. β+ DECAY converts one proton into a neutron and a positron
no change of A, but different element
occurs with nuclides with an excess number of protons

22. ΒΕΤΑ PARTICLES Electron (β-) Positron (β+) Anti-particles
As beta particles traverse  lose energy
Positron interacts with an electron
Annihilation radiation
two opposite directed 511 keV photons
threshold for positron decay  2×511 keV = 1.02 MeV
Used in Medical Imaging
Positron emitting radiopharmaceuticals
Positron Emission Tomography (PET)
Anti-particles

23. Positron Emission and Annihilation
ΒΕΤΑ PARTICLES
Positron Emission and Annihilation

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

25. GAMMA DECAY no change of A or Z – same element release of photon
usually occurs in conjunction with other decay

26. GAMMA DECAY
Decay scheme of

27. 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

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

29. Mo-99 DECAY SCHEME

30. 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

31. ELECTRON CAPTURE
Nucleus captures orbital electron (usually a K- or L-shell)
conversion of a proton into a neutron
simultaneous ejection of a neutrino
Emission of
characteristic X-rays
Auger electrons

32. ELECTRON CAPTURE converts one proton into a neutron
no change of A – but different element
occurs with nuclides with an excess number of protons

33. Tl-201 DECAY SCHEME 201Tl decays by electron capture into 201Hg
half-life = 73.1 hours
201Hg-characteristic X-Rays
keV
Emission of characteristic X-Rays used in myocardial perfusion

34. INTERNAL CONVERSION Nucleus in excited state (surplus of energy)
De-excitation through
ejection of a tightly bound electron (K- or L-shell)
alternative mechanism to electron capture
No change of Z – same element

35. SPONTANEOUS FISSION
Heavy nuclei decay by splitting into two daughter nuclei
release of neutrons
release of energy

36. SUMMARY I Half-Life (τ1/2) Decay Constant (λ) Activity
the time required for radioactivity to decay to half its initial value
Decay Constant (λ)
the probability that an atom will decay/transform per unit time
Activity
rate of decay/transformation
At = A0 e-λt

37. SUMMARY II Radioactive Decay Modes Alpha particles Beta particles
depending on the emitted radiation
Alpha particles
Helium nuclei – used in radionuclide therapies
Beta particles
used in imaging (e.g. positrons - PET)
used in therapy (e.g. 131I, 32P)
Gamma ray photons
used in imaging (e.g. 99Tcm, 201Tl)

38. THE END
Any questions ?