Provides a basic knowledge into the world of radioactivity particularly useful for the students of Radiation Oncology , Radiology & DMRT

1. RADIOACTIVITY
ADDWAY CHAKRABORTY,
1st year Resident,
Department of Radiotherapy,
Medical College Kolkata.

2. HISTORY
• In 1896, Henry Becquerel discovered
radioactivity emitted by uranium compounds.
• The first recorded biological effect of radiation
was due to Becquerel, who while working on a
series of experiments with phosphorescent
materials placed Uranium salts on a
photographic plate wrapped in black paper
and subsequently developed blackening of the
plates.
• From 1896 onwards- Marie and Pierre Curie
pursued the study of Becquerel rays. They
isolated the radioactive elements polonium
and radium.
• In 1901, Pierre Curie repeated the experience
by deliberately producing a radium ‘Burn’ on
his own forearm.

3. STRUCTURE OF A
NUCLEUS
• An atom consists of a small central core called the
nucleus and surrounding cloud of electrons moving in
orbit around the nucleus.
Atomic nucleus- Contain protons and neutrons.
• Stability of the nucleus depends on n/p ratio.
• Stable nuclei in low atomic number range(Z<=20), has
almost same n & p.
• In cases of elements with Z>=20, n/p is >1and <1.6
Instability in nucleus is the source of radioactivity.

4. RADIOACTIVITY
• Radioactivity is defined as the emission of ionizing radiation or
particles caused by the spontaneous disintegration of atomic
nuclei.
Natural Radioactivity Artificial Radioactivity
Spontaneous emission by unstable
nuclei in the form of particles or
electromagnetic radiation or both,
resulting in the formation of stable
isotope. All naturally occurring
elements with atomic number greater
than 83 are radioactive.
The radioactivity of isotopes that
have been artificially produced
through the bombardment of
naturally occurring isotopes by
subatomic particles or by high
levels of x-rays or gamma rays. It is
also called induced radioactivity.

5. RADIOACTIVE DECAY
• Radioactive decay is the process in which an unstable atomic
nucleus loses energy by emitting ionizing particles and
radiation transforming the parent nuclide atom into a
different atom called daughter nuclide .
• Change in number of atoms per unit time is proportional to
number of radioactive atoms at present, so mathematically,
• ΔN/Δt ∝ N or ΔN/Δt= -λN ,
(where λ is the constant of proportionality called the DECAY
CONSTANT).
• We can also derive, N = N0e-λt

6. ACTIVITY
• The number of decaying nuclei per unit time (disintegrations) is
referred to as the activity of a radioactive material.
• If ΔN/ΔT is replaced by A, then the activity can be written as: A= λN.
Thus the equation can be expressed in terms of ACTIVITY: A= A0e-λt
• The SI unit for activity is Becquerel (Bq). 1 Bq= 1 disintegration per
second (dps)
• Conventional unit of radioactivity is Curie (Ci).
• 1 Ci= 3.7× 1010 dps (the Curie was redefined as THE NUMBER OF
DISINTEGRATIONS PER SECOND FROM 1G OF RADIUM.)

7. HALF LIFE
• The term half life ( T1/2 ) of a
radioactive substance is defined as the
time required for either the activity or
the number of radioactive atoms to
decay to half the initial value.
• From the law of radioactive decay,
change in number of atoms per unit
time is proportional to number of
atoms present, so
• N = N0e-λt , by substituting,
• N/N0 = ½
(t=T1/2), e-λT1/2 = ½ or, T1/2 = ln2/λ
• T1/2 = 0.693/λ

8. Average life or lifetime ()
• It is the average lifetime of a radioactive atom before it decays. It is the sum of
lifetimes of all the individual nuclei divided by the total number of nuclei
involved.
• Though theoretically, it will take an infinite amount of time for all atoms to
decay, It can be understood in terms of an imaginary source that decays at a
constant rate equal to initial activity and produces the same total
disintegrations as the given source decaying exponentially.
• 𝞃=1/𝝺. Putting this in the previous equation we get
• = 1.44 x T1/2

9. Ways to reach stability
NUCLEUS UNSTABLE?
Low n/p ratio
High n/p ratio
Large nuclide with
normal n/p ratio
alpha decay Beta+ decay
Beta - decay
Electron
capture
Strong nuclear
forces gets weak
Needs to
decrease
neutrons
Needs to increase n/p

10. Alpha Decay
• Radioactive nuclides with very high atomic
numbers (>82) decay most frequently with the
emission of an ALPHA Particle.
• It is because as the no. of protons in the
nucleus >82, the coulomb forces of repulsion
between the protons become large enough to
overcome the nuclear forces that binds the
nucleons together.
• Thus emits a particle composed of two protons
and two neutrons (Helium nucleus), is called
Alpha particle.
• As a result, the atomic number of nucleus is
reduced by 2, and mass number by 4.

11. Negatron Decay( β – decay)
Negatron emission is preferred in nuclei with excess
neutrons or high n/p ratio to achieve stability.
A neutron in the nucleus breaks into a proton,
electron and antineutrino.
The reaction results in increase in atomic number of
the product nuclei by 1 with no change in mass
number.
Any excess energy in the nucleus after beta decay is
emitted as gamma rays, known as disintegration
energy for the process.

12. Positron Decay( β+ decay)
• Positron emitting nuclides have a deficit of neutrons, their n/p
ratios are lower than line of stability.
• A proton in the nucleus changes to neutron, a positron and a
neutrino and release of disintegration energy.
• The atomic number of the product nuclei decreases by 1, no
change in the mass number.
• Positron released here combines with an electron
producing annihilation of the particles and to release
2 gamma rays, each of 0.511 MeV. This phenomenon
is known as positron-electron annihilation which is
used in PET CT scan.(18F emits positron that are
annihilated by electrons in the tissue, producing
photons which are detected and intervening anatomy
are reconstructed by computer.

13. ELECTRON CAPTURE
• Another method to gain stability in nuclides deficient
in neutrons or low n/p ratio.
• One of the orbital electrons( mostly from K shell) is
captured by the nucleus, transforming a proton into a
neutron.
• The atomic number of the product nuclide decreases
by one.
• The vacancy in shell is filled by an electron in higher
orbit, resulting in release of characteristic/ fluorescent
x rays or ejection of an outer shell electron called
Auger electrons by absorption of characteristic x rays.

14. Beta+ decay and Electron Capture
• Beta + decay and electron
capture can happen in a
same radioactive element
with n/p ratio below the
line of stability.
• Example is 22Na which
decays 90% by positron
decay and 10% by
electron capture.

15. INTERNAL CONVERSION
• After a nuclear transformation, a nucleus
may be left in an excited state and would
eventually get rid of the excess energy by
production of γ rays.
• In an alternative mechanism, the excess
energy is passed to an orbital electron
which is ejected.
• Kinetic energy of the ejected electron is
equal the energy difference b/w the energy
released by nucleus and binding energy of
the shell.
• Can be crudely described as an internal
photoelectric effect.
• Similar to electron capture, the ejection of
an electron by internal conversion will
create a vacancy in the orbit ultimately
producing characteristic x rays or auger
electrons.

16. ISOMERIC TRANSITION
• The excited state in a daughter nucleus during most
radioactive transformation is released by γ rays or
internal conversion immediately.
• In some cases, the excited state of the daughter
nuclide persists called as metastable state.This state
is an isomer of the final product nucleus.
• Some excited states may have a half-lives ranging up
to more than 600 years. However, some with very
short half lives are utilised in Nuclear Medicine, like
99mTc (T1/2=6 hours)

17. DECAY SERIES
• Sometimes when a nucleus decays, the product is not stable either (radioactive
isotope)and it will decay. The series of disintegration until a stable nuclide is
reached is called a decay series.
• All naturally occurring radioactive elements have been grouped into 3 series
Uranium series
Actinium series
Thorium series
Primary element Half life of primary
element
Terminating Element
Uranium Series 238U 4.5 * 109 years 206Pb
Actinium Series 235U 7.13 * 108 years 207Pb
Thorium Series 232Th 1.39* 1010 years 208Pb

18. • The figure showing Uranium
series starting from U238 and
terminating at Pb82

19. Decay scheme
of Cobalt 60
• The figure shows a decay
scheme of 27Co60.
• It has been made radioactive
by bombarding 27Co59 with
0n1.
• It first emits a β- particle and
then in two successive jumps
emits packets of energies
known as photons.

20. RADIOACTIVE EQUILIBRIUM
Many radioactive nuclides undergo successive transformations in
which the original nuclide called the parent, gives rise to a
radioactive product nuclide called the daughter.
If the half life of the parent is longer than that of the daughter, then
after a certain time, a condition of equilibrium will be achieved, that
is the ratio of the daughter activity to the parent activity will be
constant.
The apparent decay of the daughter nuclide is then governed by the
half life or disintegration rate of the parent.
It is of the following two types:

21. 1.Transient Equilibrium
• If the half life of the parent is not much longer
than the daughter, then the type of equilibrium
established is called TRANSIENT
EQUILIBRIUM.
• Common example is equilibrium between parent
99Mo ( T1/2 = 67 hrs) and daughter 99mTc ( T1/2 = 6
hrs).
• After approximately 23 hours the Tc-99m activity
reaches a maximum, at which time the
production rate and the decay rate are equal and
the parent and daughter are said to be in
transient equilibrium

22. 2.Secular Equilibrium
• If the half life of the parent is much longer
than the daughter, then it is known as
secular equilibrium.
• Common example- Parent 226Ra(T½=1622
years ), Daughter 222Rn (T½=3.8days).
• In secular equilibrium, the activity of the
parent and the daughter are the same if all
of the parent atoms decay directly to the
daughter .
• Once secular equilibrium is reached, the
daughter will have an apparent half- life
equal to that of the parent.

23. TO SUMMARIZE
• The atomic nucleus consists of positive charged protons and
neutral neutrons being held together by strong nuclear forces.
• As number of nucleons increases, the strong nuclear forces
fail to act leading to unstable nucleus.
• Emission of radiation from a nucleus in the form of particles
and gamma rays is called radioactivity.
• Activity is rate of decay per unit time. Si unit is Becquerel = 1
dps.
• Half life is defined as the time required for either he activity
or number of radioactive atoms to decay to half the initial
value.

24. TO SUMMARIZE
• Modes of decay
1. 𝝰 decay- helium nucleus, product of decay of high atomic number
nuclide.
2.β decay- Negatron decay in high n/p ratio, positron decay in low
n/p ratio nucleus.
3.Electron capture- in low n/p nucleus
4.Internal conversion and isomeric transition- In excited nucleus,
ejection of electron or gamma photon.
• Two types of radioactive equilibrium exists when the ratio of
daughter’s activity to parent’s activity remain constant;
Transient and Secular.