Chapter 21 (3150:151 version)
Nuclear Reactions and Their Applications
Everyday chemical properties are governed by an atom’s number and arrangement of electrons.
Energies of nuclear reactions are millions of times greater than those of ordinary chemical
reactions.
Binding Energy
Mass of atom < Sum of masses of particles that make up atom
The “missing” mass (mass defect) was converted to energy, called binding energy
E = mc2
1 gram of mass = 9 × 1010 kJ of energy!
To allow for fair comparison, the mass defect and corresponding binding energy are recorded on
a per nucleon basis.
New (to you) Vocabulary
 nucleons: collective term for protons & neutrons
 nuclide: a single type of nucleus
Radioactive Decay





is a first-order kinetic process (See Chapter 16)
Most known nuclides undergo radioactive decay. (Relatively few are stable.)
Ratio of neutrons to protons determines a nuclide’s stability.
o n/p ratio too large (neutron-rich nuclide): – decay likely
o n/p ratio too small (neutron-poor nuclide): + decay likely
Stable nuclides stop at Z = 83 (bismuth). All elements with Z > 84 are radioactive
(whether naturally occurring or synthetic).
o Z > 83:  decay likely
An unstable nuclide will decay to eventually produce a stable nuclide….could take one
step or many.
23-1
Types of Radioactive Decay
1.

2.

4.
5.
234
90
Th 
 10 e 
234
91
234
90
Pa
– particle = 10 e (an electron)
energy released in decay process creates the – particle (not from an orbital)
net effect: convert neutron to proton
– particles likely to be produced by nuclides with high neutron-to-proton
ratios
example:
K
10 e  38
18 Ar
38
19
+ particle = 10 e (a positive electron)
net effect: convert proton to neutron
+ particles likely to be produced by nuclides with low neutron-to-proton
ratios
+ is “antiparticle” of -
electron capture
example: 80 Hg 
 an inner-orbital electron captured by nucleus
 generally slow
201
gamma () decay



6.
example:
positron (+) decay



Th  2 00
U
42 He 
238
92
 particle = 42 He nucleus (i.e., 42 He 2+)
beta (–) decay




3.
example:
alpha () decay
0
0
example:
60
27
0
1
0
e 
 201
79 Au  0 
Co 
 60
28 Ni 
0
1
e  2 00 
 = high energy photon (electromagnetic radiation)
frequently accompanies other decay types
a way of “draining off” excess energy (product may be in excited state)
 54 Xe  44 Ru  4
spontaneous fission
example: 98 Cf 
 generally slow
 “splitting” of heavy nuclide to lighter ones with similar mass numbers
252
140
108
1
0
n
Applications/Problems
222
Radon
Rn
t½ = 3.8 days
α emitter

222

Radon is colorless, odorless, chemically inert BUT its radioactive decay products
are solids (dust). Any radon that decays in your lungs doesn’t get exhaled.
Rn formed in a series of steps from 238U.
23-2
Smoke Detectors



241
Am
t½ = 458 years
α emitter
α particle ionizes air particles
+ ions move toward – electrode
electrons move toward + electrode
smoke particles also ionized, but recombination of smoke+ ions and e– more
efficient. ˆ current drops → signal to horn
Selected Medical Applications
Considerations for medical applications of radioactive nuclides:
o short half-life
o stable decay products
o small doses
o localization in body
o penetrating ability
131
I
t½ = 8.1 days



18
F
β– emitter
iodine concentrates in the thyroid gland.
131
I used in the treatment of hyperthyroidism; kills some of the thyroid cells.
131
I is one by-product of nuclear power plants.
After the 1986 Chernobyl meltdown, residents downwind of the meltdown were
given KI pills containing non-radioactive iodine so as to saturate the body with
iodine so 131I would not concentrate in the thyroid gland.
t½ = 110 min
β+ emitter

used in Positron Emission Tomography (PET scan)
o choose molecule with high concentration in part of body of interest
o synthesize compound, attach 18F
o administer to patient
o detect reaction of positrons with electrons in surrounding matter
0
0
0
“annihilation”
1 β  1 e  2 0 γ


images metabolic activity very clearly (e.g., Alzheimer’s, epilepsy, CP, stroke)
PET scans still relatively uncommon: 18F short half-life: need to make onsite.
Need a particle accelerator. Cost > $106 (scanner not included).
62
Cu also used in PET scans (blood flow).

o Contrast to Computerized Axial Tomography (CAT scan)
 X-rays passed through body in thin slices.
 Based on different densities of matter/tissue
 No radioactive nuclide involved
 Nothing radioactive administered to patient
23-3
o Contrast to Magnetic Resonance Imaging (MRI)
 Patient inserted into huge magnet
 Based on relaxation of spinning of hydrogen nuclei when magnetic
field turned off
 Excellent for imaging soft tissues (e.g., plaque from MS)
 No radioactive nuclide involved
 Nothing radioactive administered to patient
Fission Power Plants
1. Fuel—a fissionable nuclide
e.g., 235U, 239Pu
 In USA, typically pellets of U3O8 enriched to about 3% 235U. (Naturally
occurring U contains about 0.7% 235U. Uranium-238 doesn’t fission nearly as
effectively.)
 Pellets contained in hundreds of tubes (“fuel rods”).
2. Moderator
 Surrounds fuel rods; slows down neutrons
 In USA, typically water (can also be liquid sodium)
 Does NOT absorb or react with neutrons
3. Coolant
 Carries heat produced to external turbine to produce electricity
 Typically water
4. Control Rods
 Absorb neutrons
 Control number of neutrons colliding with fuel
 Full power: rods up all the way
Shut down: rods down all the way
5. Containment System
USA: ~20% of electricity from nuclear power
France: > 75% of electricity from nuclear power
Nuclear power plants are NOT potential nuclear bombs in and of themselves. The fuel isn’t
nearly pure enough. (~90% fissionable fuel needed for a bomb.)
“Dirty bombs” contain lower levels of radioactive materials than new fuel but enough to cause
some biological harm and lots of psychological harm. Spent nuclear reactor fuel/waste security
is a great concern!
The major problem of fission reactors: the WASTE
Fusion Power Plants?
Advantages: (lack of) waste
unlimited fuel (2H)
Problems:
high temperatures required (~108 K)
containment
23-4
~200 nuclides, ~35 elements