Transcript File - Ms. Fox-Lent

Nuclear Reactions
Quick Review
• Atoms are made up of electrons, protons,
and neutrons.
• Isotopes are different “versions” of the
same element with different numbers of
neutrons.
protons
nucleons
neutrons
Strong Force
• Holds the nucleus together, neutrons act like
nucleus glue holding it all together.
• Larger nucueus needs more neutrons to be
stable, this limits atom size.
Neutrons are not stable on their own and will
decay into a proton and electron if left on
their own.
 Most isotopes
naturally are stable.
which
occur
A few naturally occurring isotopes
and all of the man-made isotopes are
unstable.
All elements heavier then Bismuth
(atomic number 83)
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 (a),
• a beta particle (b),
• or a gamma ray(g).
Penetration of radiation
Radioactive decay can also
result in:
• the emission of a positron,
• 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
• Atomic mass decreases by 4
• Atomic # decreases by 2
• This type of radiation can be stopped by a
piece of paper, it is not very penetrating,
BUT if ingested it can be very damaging to
body tissues.
Alpha Decay
A
X
Z
A-4
4
Y
He
+
Z-2
2
unstable atom
alpha particle
more stable atom
Alpha Decay
A
A-4
4
226
222
4
X
Z
Ra
88
Y
+
Z-2
Rn
+
86
He
2
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
The decomposition of a neutron. A beta particle is a fast
moving electron which is emitted from the nucleus of an
atom undergoing radioactive decay.
Electron (fast moving
particle)
Proton (stays in nucleus)
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.
The atomic number, Z, increases by 1 and the mass
number, A, stays the same.
More penetrating, can be stopped by a piece of
aluminium foil.
Beta Decay
218
218
Po
84
At
85
b
-1
0
Beta Decay
A
X
Z
218
Po
84
A
b
-1
218
b
-1
Y
+
Z+1
At
+
85
0
0
Beta Decay
234
A
b
-1
234
234
b
-1
Th
90
Th
90
Y
+
Z
Pa
+
91
0
0
Beta Decay
A
210
b
-1
210
210
b
-1
X
Z
Tl
81
Pb
+
82
Pb
+
82
0
0
Gamma Decay
Gamma rays are not charged particles like a and b
particles.
Gamma rays are electromagnetic radiation with high
frequency.
When atoms decay by emitting a or b 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).
Gamma Radiation
• Emitted along with alpha and beta radiation.
• No mass, no charge, no effect on atomic
mass or atomic number
• HIGH penetration, blocked by lead
• Act like x-rays, but come from a different
source.
Examples of Gamma Radiation
230Th

226Ra
90
234Th
90
4He
+
88

234Pa
91
2
+
0e
-1
+g
+g
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.
What do we mean by “unstable”?
• Nuclear Stability depends on neutron to
proton ratios
• Atomic # <20  wants a ratio of 1 (P=N)
• Atomic # > 20  wants more neutrons then
protons, with a stable ration being around
1.5
• ALL nuclei over 83 are unstable (too big)
How do they become “stable”?
• Too many neutrons  turn N to P(beta
decay)
• Too few neutrons  turn P to N (capture an
electron and emit a positron)
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.
After One ½ Life
• ½ unstable parent
material remains
• And ½ stable daughter
material is present
After Two 1/2 Lives
• ¼ parent material
• And ¾ daughter
material
After Three ½ lives
• 1/8 parent material
• 7/8 daughter material
After Four ½ Lives
• 1/16 parent material
• 15/16 daughter
material
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
Using the Triangle
Examples: