Download 30.1 Radioactivity The atom is the smallest unit of achemical

Chapter 30
30.1 Radioactivity
The atom is the smallest unit of achemical element, such as gold
or carbon, which has the physical
and chemical properties of the element.
Nucleus
Atoms have a dense center called a nucleus where you can find
protons and neutrons. The nucleus is surrounded by orbiting
electrons.
A atomic nucleus is instable when he is to heavy or when a
balance is missing between the protons and the neutrons. There
are three types of decays: alpha decay, beta decay and gamma
decay.
1- Alpha decay (α)
4
2
•
Two neutrons and two protons (helium nuclei
•
•
•
Charge of +2
Emitted from nucleus of radioactive atoms
Transfer energy in very short distances (few cm in
air)
Shielded by paper or layer of skin
Primary hazard from internal exposure
•
•
He )
•
Alpha emitters can accumulate in tissue (bone, kidney,
liver, lung, spleen) causing local damage
2- Beta Decay (β)
There are two types of the beta decay. The one is the βdecay and the other is the β+ decay.
a- β• neutron decays into a proton
• it has the same charge as electron
• it has the same mass as electron
• it can penetrate with few meters in air. 2 or 3 cm
of wood are enough to protect oneself.
b- Β+
•
•
a proton decays into a neutron
it has the same charge as electron but negative
charge
• it has the same mass as electron
• it can penetrate with few meters in air. 2 or 3 cm
of wood are enough to protect oneself.
3- Gamma decay (γ)
•
•
•
•
High energy electromagnetic radiation
It has no charge
It has no mass
Highly penetrated. You need a big wall of lead to
protect yourself from gamma radiation.
30.2 Half-Life
It is found that, if a given radioactive substance at a certain
time contains N0 nuclei, then at a short time t later a certain
number N have decayed which is given by
N = N 0e
−λ t
Where λ is the decay constant (s-1)
It is sometimes convenient to introduce the half-life,
, T1/2 , of
a substance, defined as the time over which e
exactly
xactly one half of
a substance remains.
N0
−λ t
= N 0e 1/ 2
2
1
−λ t
= e 1/ 2
2
1
−λ t
ln( ) = ln(e 1 / 2 )
2
− ln(2) = −λ t1 / 2
t1/ 2 =
ln(2)
λ
=
0.693
λ
30.9 Radioactive Decays
Standard nuclear notation shows the chemical symbol, the mass
number and the atomic number of the isotope
isotope:
A =
Z
=
number of protons + number of neutrons
number of protons
A – Z =
number of neutrons
Number of neutrons = Mass Number – Atomic
Number
A
Z
X → AZ−−mnY + mny
It is found that nuclei with mass numbers greater than about 100
spontaneously decay into other types of nuclei. Such nuclei are
said to be radioactive, and there are three main types of such
decays.
1- Alpha (α) decay is the spontaneous emission of an alpha
particle (identical to a helium nucleus) from the nucleus
A
Z
X → ZA−−42Y + 24He + Q
Example:
238
92
4
U → 234
90 Th + 2 He
2- Beta (β) decay Beta decay occurs when a neutron changes
into a proton and an electron.
a- β1
0n
1
→
1p
+ 0–1e
(β– emission)
Example:
14
6
C →147 N + β −
b- β+
1
1p →
1
0n
+ 01e
(β+ emission)
Example:
13
7
N →136 C + β +
3- Gamma decay
A nucleus (which is unstable) changes from a higher energy
state to a lower energy state through the emission of
electromagnetic radiation (photons) (called gamma rays).
A
Z
X * → ZA Y + 00 γ