Radioactivity Manos Papadopoulos Nuclear Medicine Department Castle Hill Hospital Hull & East...
-
Upload
emma-harrington -
Category
Documents
-
view
218 -
download
0
Transcript of Radioactivity Manos Papadopoulos Nuclear Medicine Department Castle Hill Hospital Hull & East...
Radioactivity
Manos PapadopoulosNuclear Medicine DepartmentCastle Hill HospitalHull & East Yorkshire Hospitals NHS Trust
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
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
...21 ddDP
RADIOACTIVITY
ACTIVITY
Activity (A) is defined as:
the number of radioactive atoms (N) undergoing nuclear
transformations per unit time (t)
dt
dNA
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
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
ANdt
dN N
dt
dN
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
2/12/1
693.02ln
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
t
t eNeNtN
2/1
2ln
00
0
100000
200000
300000
400000
500000
600000
700000
800000
900000
1000000
0 10000 20000 30000 40000 50000
Years
Nu
mb
er o
f 14
C a
tom
s
τ1/2 = 5730y
5730
1/ 2 1/ 2 1/ 20 0 0 0/ 2 / 4 / 8t t tN N N N
N0
RADIOACTIVE DECAY LAW
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?
SOLUTION
A0 = 800 MBq
λ = 0.693/6.02 hours = 0.115 hours-1
t = 1 hour
MBqA
MBqA
eMBqA
eMBqA
eAAhourhours
t
713
891.0800
800
800115.0
1)115.0(
0
1
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)
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
42particle He
ALPHA DECAY
Typically occurs Heavy nuclides (A>150)
Emission of gamma and characteristic X-Rays
DP A
ZAZ
42
ALPHA DECAY
Alpha particle emitted from the atomic nucleus Alpha particle and daughter nucleus have equal and
opposite momentums
241 237 495 93 2Am Np He
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
ΒETA DECAY
Beta positive (β+) decay:Proton (p+) → neutron + positron (β+) + neutrino
Beta negative (β-) decay:Neutron → proton (p+) + electron (β-) + antineutrino
_1
1 vDP AZ
AZ
vDP AZ
AZ
1
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
3 31 2 eH He e
β- DECAY
β+ DECAY
11 116 5 eC B e
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
ΒΕΤΑ PARTICLES Electron (β-) Positron (β+) 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
ΒΕΤΑ PARTICLES
Positron Emission and Annihilation
GAMMA DECAY
Nucleus in excited state (surplus of energy)
Release of excess energy emission of γ-rays
nucleus returns to its ground state
XX AZ
AZ
*
GAMMA DECAY
3 * 32 2He He
no change of A or Z – same element release of photon usually occurs in conjunction with other decay
GAMMA DECAY
Decay scheme of Cs13755
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
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
XX AZ
mAZ
Mo-99 DECAY SCHEME
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
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
energyvYeX AZ
AZ
1
7 74 3
ECeBe e B
ELECTRON CAPTURE
converts one proton into a neutron no change of A – but different element occurs with nuclides with an excess number of protons
Tl-201 DECAY SCHEME
201Tl decays by electron capture
into 201Hg
half-life = 73.1 hours
201Hg-characteristic X-Rays
68.9-80.3 keV
Emission of characteristic X-Rays used in myocardial perfusion
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
SPONTANEOUS FISSION
Heavy nuclei decay by splitting into two daughter
nuclei
release of neutrons
release of energy
SUMMARY I
Half-Life (τ1/2)
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
SUMMARY II
Radioactive Decay Modes 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)
THE END
Any questions
?