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