Radioactivity is the spontaneous decay of unstable atomic nuclei through emission of particles or electromagnetic radiation. There are three types of radioactive decay: alpha decay emits helium nuclei, beta decay emits electrons or positrons, and gamma decay emits high energy photons. Not all isotopes of an element are stable, and radioactive decay transforms one isotope into another at a rate characterized by the isotope's half-life and decay constant. The energy and other properties of the emitted particles in radioactive decay depend on the parent and daughter nuclei involved in the transformation.

2. INTRODUCTION
DEFINITION:- An unstable nuclide
spontaneously emits a particle, without the
stimulus of any outside agency, transforming
itself into a different nuclide. Such a nuclide is
said to be radioactive and the process of
transformation is termed as the
RADIOACTIVE DECAY.
The generic name of this process is
RADIOACTIVITY.

3. DISCOVERER
Radioactivity was discovered in 1896 by A.H.
BECQUEREL while studying the fluorescence and
phosphorescence of compounds eradiated with visible
light.
AN INTERESTING PHENOMENON WHICH HE
OBSERVED
After illuminating some pieces of uranium-potassium
sulphate with visible light, he wrapped them in black
paper and separated the package from a photographic
plate by piece of silver. After several hours exposure,
the photographic plate was developed and showed
blackening due to something that must have been
emitted from the compound and was able to penetrate
both black paper and silver.

4. STABILITY OF DIFFERENT ISOTOPES
The nuclear atoms of all isotopes of an element have the
same number of electrons and hence the same chemical
properties.
Some isotopes of an element may be stable while the others
may be unstable.
For example:- hydrogen,the simplest element has three
isotopes,hydrogen,deuterium,and tritium. Of these, the
first two are stable while tritium is unstable.
For a sample of tritium gas in a closed vessel, the
transmutation into 3
He occurs smoothly, and the
concentration of 3
He gradually builds up as tritium
disappears. After about 12 years, half of the sample of
tritium is converted into 3
He .

5. A plot of known nuclides. The dark shading identifies the band of stable
nuclides, the grey shading the radionuclides. Low mass, stable nuclei
have essentially equal number of protons and neutrons but more massive
nuclei have increasing number of neutrons. The figure shows that there are
more stable nuclei for Z>83.
NUCLIDIC CHART

6. The activity of radioactive material is the result of three
different kinds of emanations termed as alpha, B and Y
radiations(rays). The properties of these radiations are-
Alpha- rays are 4
He nuclei, emitted from radioactive nuclei
are completely stopped by a sheet of paper or by a few
centimetres of air. Emission of alpha particle reduces the
mass number of the radionuclide by 4 and its atomic number
by 2.
B- rays are electrons or particles called positrons. After
emission of b particles the mass of the radioactive nucleus is
unchanged but its atomic number is increased or decreased
by one.
Y-rays are energetic photons, which can penetrate through
considerable thickness of lead. Since photons carry no charge
or mass, emission of Y- rays does not change the isotope.

7. LAW OF RADIOACTIVE DECAY
dN / dt = - λN(t)
Or N(t) = N0e-λt
Where λ is the probability per unit time for a
nucleus to decay and N(t) is the number of
radioactive nuclei present at time t.Half- life, T ½
is the time in which one-half of the number of
nuclei decay.
T ½ = In2/λ = 0.693/λ
The decay rate also called the activity of the
sample,
R(t) =λN (t)
SI unit of activity is becquerel, and is equal to
one disintegration per second.

8. SI UNIT OF ACTIVITY
The total decay rate R of a sample of one
or more radionuclides is called the activity of that
sample. The SI unit for activity is the becquerel,
named after the discoverer of radioactivity,
Henry Becquerel.
1 becquerel = 1 Bq = 1 decay per second.
An older unit, the curie, is still in common use:
1 curie = 1 Ci = 3.7 * 1010
Bq
Some other units of activity in common use are :
1mCi(milli curie) = 3.7*107
Bq
1µci ( microcurie) = 3.7*104
Bq.

9. ALPHA DECAY
When a nucleus undergoes alpha decay, it transforms to
different nucleus by emitting an alpha particle (a helium
nucleus, 4
2He ). For Example, when 238
92U undergoes
alpha decay, it transforms to 234
90Th: 238
92U ->
234
90Th + 4
2He . In this process, it is observed that
since 4
2He contains two protons and two neutrons, the
mass number and the atomic number of the daughter
nucleus decrease by four and two respectively. Thus the
transformation of a nucleus A
ZX into a nucleus A-4
Z-2 Y
can be expressed as: A
ZX-> A-4
Z-2 Y + 4
2He . where
A
ZX is the parent nucleus and A-4
Z-2 Y is the daughter
nucleus.

10. The alpha decay of 238
92U can occur
spontaneously (without an external source of
energy) because the total mass of the decay
products 234
90Th and 4
2He is less than the mass of
the original 238
92U. Thus, the total mass energy
of the decay products is less than the mass
energy of the original nuclide. The difference
between the initial mass energy and the total
mass energy of the decay products is called the Q
of the process or the disintegration energy. Thus,
the Q of an ALPHA decay can be expressed as :
Q = (mx - my -mHe ) C2 .

11. A-4
Z-2
Y, and the alpha
particle, 4
2
He. As the parent nucleus
A
Z
X is at rest before it undergoes alpha
decay, the alpha- particles are emitted with
fixed energy, which can be calculated by

12. The potential energy function for the emission of an alpha - particle
by 238
92U . The horizontal black line marked at 4.25 MeV shows the
disintegration energy for the process. Thick grey portion of this line
represents separation R that are classically forbidden to the alpha -
particle. The alpha - particle is represented by a dot both inside and
outside the potential barrier after the particle has tunnelled through.

13. BETA DECAY
A nucleus that
decays spontaneously by emitting an
electron or a positron is said to undergo
beta decay. This is a spontaneous
process, with a definite disintegration
energy and half- life. It is also a
statistical process governed by certain
equations, that are -
32
15P -> 32
16S + e-
+ ν-
( In beta - minus)
22
11Na -> 22
10Ne + e+
+ ν ( In beta plus)

14. The distribution of the kinetic
energies of positron emitted in the
decay of 64
29Cu.

15. SIMILARITY BETWEEN ALPHA
AND BETA DECAY
In both alpha- decay and beta- decay, the
disintegration energy Q is characteristic of the
radionuclide. In the alpha decay of the particle
radionuclide , every emitted alpha particle has the
same sharply defined kinetic energy. However, in
beta decay the disintegration energy, Q, is shared
between the three decay products, the daughter
nucleus, electron or positron and the antineutrino
or neutrino, As a consequence, the kinetic
energy of an electron or a positron in beta
decay process is not unique. It may range from
zero to certain maximum kinetic energy (kmax).
This Kmax of an electron or a positron must equal
the disintegration energy Q.

16. GAMA DECAY
Like an exited atom, an exited nucleus can
make transitions to state of lower energy by
emitting a photon. As the energy of the
nuclear states of the order of million
electrons volts (MeV), the photons emitted
in transitions between nuclear states can
have energy of the order of several MeV.
The wavelenght of photons of such
energy is a fraction of an angstrom. The
short wavelenght electromagnetic waves
emitted by nuclei are called gamma rays.

17. Most radionuclides after an alpha decay or
a beta decay leave the daughter nucleus in
an exited state. The daughter nucleus by
single transition or sometimes by successive
transitions reaches the ground state by
emitting one or more gamma rays. A well-
known example of such a process is that of
60
27Co. By beta emission, the 60
27Co nucleus
transforms into60
28Ni nucleus in its exited
state. The exited 60
28Ni nucleus so formed
than de- excites to its ground state by
successive emissions of 1.17 MeV and 1.33
MeV gamma rays

18. A radioactive nuclide spontaneously
emits a particle, transforming itself in
the process into a different nuclide.
In radioactive decay ,there is absolutely
no way to predict whether any given
nucleus in a radioactive sample will be
among the small number of nuclei that
decay during the next second. Each
nucleus has the same chance of decay.

19. MADE BY:-
SAPNA GARG
AND
NATASHA
GOEL
( XII - D)