Transcript No Slide Title

Nuclear Reactions
and
Half-life
 Most isotopes which occur naturally
are stable.
A few naturally occurring isotopes
and all of the man-made isotopes are
unstable.
Unstable isotopes can become stable
by releasing different types of
particles.
This process
is called radioactive decay and the
elements which undergo this process
are called radioisotopes/radio nuclides.
Radioactive Decay
Radioactive decay results in the
emission of either:
• an alpha particle (α) = Helium nucleus
• a beta particle (β-) = electron
• a gamma ray (γ) = high energy
photon
• the emission of a positron (β+)
• or in the capture of an electron.
Alpha Decay
An alpha particle is identical to that of a
helium nucleus.
It contains two protons and two neutrons.
Alpha Decay
A
X
Z
unstable atom
226
Ra
88
A-4
Y
Z-2
+
more stable atom
222
Rn
+
86
4
He
2
alpha particle
4
He
2
Alpha Decay
222
226
Ra
88
Rn
86
4
He
2
Alpha Decay
222
Rn
86
222
Rn
86
A
4
Y
He
+
Z
2
218
Po
+
84
4
He
2
Alpha Decay
A
230
4
234
230
4
X
Z
U
92
Th
He
+
90
2
Th
He
+
90
2
Beta Decay
A beta particle is a fast moving electron which
is emitted from the nucleus of an atom
undergoing radioactive decay.
+
β-
Beta decay occurs when a neutron changes
into a proton and an electron.
Beta Decay
As a result of beta decay, the nucleus
has one less neutron, but one extra
proton.
+
β-
The atomic number, Z, increases by 1 and
the mass number, A, stays the same.
Beta Decay
A
X
Z
218
Po
84
A
Y
+
Z+1
218
At
+
85
0
e
-1
0
e
-1
Beta Decay
218
218
Po
84
At
85
β
-1
0
Beta Decay
234
Th
90
234
Th
90
A
Z
234
Y
+
Pa
+
91
0
e
-1
0
e
-1
Beta Decay
A
210
210
210
X
Z
Tl
81
Pb
+
82
Pb
+
82
0
e
-1
0
e
-1
Gamma Decay
• Gamma rays are not charged particles like
alpha and beta particles.
• Gamma rays are electromagnetic radiation with
high frequency.
• When atoms decay by emitting alpha or beta
particles to form a new atom, the nuclei of the
new atom formed may still have too much
energy to be completely stable. This excess
energy is emitted as gamma rays (gamma ray
photons have energies of ~ 1 x 10-12 J).
Examples of Gamma Radiation
230Th
90
234Th
90


226Ra
88
234Pa
91
+ 4He +
2
+
0β
-1

+
Gamma Decay
•Nuclear reactions can also occur
when the nuclei of atoms react
when bombarded by particles
like neutrons, charged particles,
etc..
Positron Decay
A positron is a particle with the mass
of an electron but it has a positive
charge. A positron may be emitted
as a proton changes to a neutron as
in the case above.
Electron Capture
Electron capture is one process that unstable atoms
can use to become more stable. During electron
capture, an electron in an atom's inner shell is
drawn into the nucleus where it combines with a
proton, forming a neutron and a neutrino. The
neutrino is ejected from the atom's nucleus.
Neutrino
Neutrinos are similar to the more familiar
electron, with one crucial difference:
neutrinos do
not carry
electric
charge.
Half-Life
The half-life is the amount of time it takes
for half of the atoms in a sample to
decay. The half-life for a given isotope is
always the same; it doesn't depend on
how many atoms
you have or on how
long they've been
sitting around.
Calculating Half-life
A = A0 (1/2)n
A0 = Initial amount of radioactive material
A = Final amount of radioactive material
n = number of half-lives
Practice Problem
Nitrogen-13 emits beta radiation and
decays to carbon-13 with a half-life of 10
minutes.
Assume that your
starting off with 2 g
of the radioisotope
nitrogen-13.
How many grams of
the isotope will still
be present after
30 minutes?
Solution
A = A0 (1/2)n
A0 = 2g
n=3
Solve for A
A = 2 g (1/2)3
A = 0.25 g of nitrogen-13