Mrs: Aya Ahmed Abd alrahium saeedMSC &BSC Nuclear medicine
Inaya medical science college
Nuclear Medicine Physics and Equipment 243 RAD
Radioactivity
Radioactivity
• Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear
instability.
Radioactive decay:
• is the process by which an atomic nucleus of an unstable atom loses energy by emitting
ionizing particles (ionizing radiation).
• Henri Becquerel, 1896 Discovered uranium was radioactive by leaving it on
photographic paper and finding the paper to be exposed• There are numerous types of radioactive decay. The general idea:
• An unstable nucleus releases energy to become more stable
Families of nuclei
Isotopes means specific elements with different forms of that elements each containing different numbers of neutrons.
Isotones are atoms of different elements that have the same number of neutrons but having different number of protons e.g. , , &
Isobars are nuclides that have equal weight (mass number) , &
Isomers are atoms that have identical physical attributes as far as the number of protons, neutrons and electrons, they contain a different a mount of nuclear energy.
Rh10146
Ru10145Tc99
43Mo99
42
Rh99
Tc99Ru99
• Isomers are identified by putting an M after the mass number.• M means the atom is currently in metastable state form and will emit
gamma radiation from the nucleus to achieve more stable energy
configuration.
• The most common used radionuclide in nuclear medicine is an isomer.
• Other isomers that have been used in nuclear medicine are &
Tc99M
In113M Sr87M
• The decay, or loss of energy, results when an atom with one type of nucleus,
called the parent radionuclide.
• Transforms to an atom with a nucleus in a different state, or a different
nucleus, either of which is named the daughter radionuclide.
• Often the parent and daughter are different chemical elements, and in such
cases the decay process results in nuclear transmutation. In an example of this,
a carbon-14 atom (the "parent") emits radiation (a beta particle, antineutrino,
and a gamma ray) and transforms to a nitrogen-14 atom (the "daughter").
• By contrast, there exist two types of radioactive decay processes (
gamma decay and internal conversion decay) that do not result in
transmutation, but only decrease the energy of an excited nucleus. This
results in an atom of the same element as before but with a nucleus in a
lower energy state. An example is the nuclear isomer technetium-99m
decaying, by the emission of a gamma ray, to an atom of technetium-99.
• Nuclides produced as daughters are called radiogenic nuclides,
whether they themselves are stable or not.
• The SI unit of activity is the Becquerel (Bq). One Bq is defined as
one transformation (or decay) per second.
• Radioactivity was first discovered in 1896 by the French scientist
Henri Becquerel, while working on phosphorescent materials
Different between radioactivity and chemical radiation
The radioactivity can not affected by extra nuclear condition which affect rates of
chemicals reactions such as:
Temperature.
Pressure.
Chemical form.
Physical state.
So the radioactivity is in sensitivity to extra nuclear condition allows us to characterize
radioactive nuclei by their:
Period of decay (half life).
Mode of decay.
Energy of decay.
• So radioactivity can be described by the
equation :-
Where: A B + X + Q
A = parent nuclei
B = daughter
X = emitted particle
Q = energy released
• The decay rate, or activity, of a radioactive substance are
characterized by:
• Constant quantities:
• half life — symbol t1/2 — the time taken for the activity of a
given amount of a radioactive substance to decay to half of its
initial value.
• mean lifetime — symbol τ — the average lifetime of a
radioactive particle.
• decay constant — symbol λ — the inverse of the mean lifetime.
Activity measurements:-
• The units in which activities are measured are:
Becquerel (symbol Bq) = number of
disintegrations per second; curie (Ci) = 3.7 × 1010
disintegrations per second. Low activities are also
measured in disintegrations per minute (dpm).
Activity
• The quantity of radioactive material, expressed as the number of radioactive
atoms undergoing nuclear transformation per unit time (t), is called activity
(A)
• Described mathematically, activity is equal to the change (dN) in the total
number of radioactive atoms (N) in a given period of time (dt).
• The minus sign indicates that the number of radioactive atoms
decreases with time. Activity is traditionally expressed in units
of curies (Ci).
• In nuclear medicine, activities from 0.1 to 30 mCi of a variety of
radionuclide's are typically used for imaging studies, and up to
300 mCi of iodine 131 are used for therapy.
• The SI unit is the Becquerel (Bq)
1 mCi = 37 MBq
Decay Constant
• Number of atoms decaying per unit time (dn/dt) is proportional to the number of unstable
atoms (n). That are present at any given time:
• Constant of proportionality is the decay constant ().
-dn/dt = n
The minus sign indicates that the number of radioactive atoms decaying per unit
Time (the decay rate or activity of the sample) decreases with time. The decay constant is
equal to the fraction of the number of radioactive atoms remaining in a sample that decay
per unit time. The relationship between activity and can be seen by considering equation
and substituting a for -dn/dt in equation
A = N
Physical Half-Life
• Useful parameter related to the decay constant is physical half-life
(T1/2 or Tp1/2) ; defined as the time required for the number of
radioactive atoms in a sample to decrease by one half.
• The decay constant and the physical half-life are related as follows:
= ln 2/Tp1/2 = 0.693/Tp1/2
• Physical half-life and decay constant are inversely related and unique
for each radionuclide. Half-lives of radioactive materials range from
billions of years to a fraction of a second. Radionuclide's used in
nuclear medicine typically have half-lives on the order of hours or days.
Fundamental Decay Equation
At = A0e-t
where:
At = activity at time t
A0 = initial activity
e = base of natural logarithm
l = decay constant = ln 2/Tp1/2 = 0.693/Tp1/2
t = time
Decay calculations
Hours Decay factor (DF) precalibration factor
0 1.000 1.000
0.5 0.944 1.059
1 0.891 1.122
2 0.794 1.259
3 0.707 1.414
4 0.630 1.587
5 0.561 1.782
6 0.500 2.000
7 0.445 2.247
8 0.397 2.518
9 0.354 2.824
10 0.315 3.174
11 0.281 3.558
12 0.250 4.000
Decay factor for 99mTc
Examples • A vial contains 10 mCi of 99mTc; how much
radioactivity remains after 2 hours?• DF for 2 hours = 0.794• 10 mCi × 0.794 = 7.94 mCi.• How much radioactivity was present 5 hours before
for 10 mCi.• Precalibration DF for 5 hours = 1.782• 10 mCi ×1.782 = 17.82 mCi.• How much of the 10 mCi remains after 36 hours?• 10 mCi × 0.250 × 3 = 0.156 mCi.
Example using general Eq.
• Calculate the activity of 5 mCi/ml of 201Tl after 48 hr (t1/2 = 73 hr).
At = A0e-0693×t/t
1/2
At = 5mCi/(ml)×e-0.9639×(48/73)
At = 3.17mCi/(ml)
• A vial contains a mixture of 20 mCi of 124I (half life 4 days) and 6 mCi of 131I (half life 8 days). What will be the activity in the vial 8 days from now?
• A source of is delivered to the nuclear medicine department calibrated for 100 mCi at 8:00 AM on Monday. If this radioactivity is injected into a patient at noon on Tuesday, what radioactivity will the patient receive?
• A source of 18F (t1/2 = approximately 2 hr) is noted to contain 3 mCi at noon. What was the radioactivity at 8:00 AM that same day?
Thanks