radioactivity
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Radioactivity
RadiationRadiation: The process of emitting energy in the form of waves or particles.
Where does radiation come from?Radiation is generally produced when particles interact or decay.
A large contribution of the radiationon earth is from the sun (solar) or from radioactive isotopes of the elements (terrestrial).
Radiation is going through you atthis very moment!
http://www.atral.com/U238.html
A. Definitions
• Radioactivity– emission of high-energy radiation from the nucleus
of an atom
• Nuclide– nucleus of an isotope
• Transmutation– process of changing one element into another via
nuclear decay
IsotopesWhat’s an isotope?
Two or more varieties of an element having the same number of protons but different number of neutrons. Certain isotopes are “unstable” and decay to lighter isotopes or elements.
Deuterium and tritium are isotopes of hydrogen. In addition to the 1 proton, they have 1 and 2 additional neutrons in the nucleus respectively*.
Another prime example is Uranium 238, or just 238U.
Radioactivity
By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known:
1) Alpha particles ()
2) Beta particles ()
3) Gamma-rays ()
By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known:
1) Alpha particles ()
2) Beta particles ()
3) Gamma-rays ()
He42
B. Types of Radiation• Alpha ()
– helium nucleus paper2+
Beta-minus (-) electron e0
-11- lead
Gamma () high-energy photon 0 concrete
C. Nuclear Decay• Why nuclides decay…
– to obtain a stable ratio of neutrons to protons
K
K4019
3919
Stable
Unstable(radioactive)
C. Nuclear Decay
• Alpha Emission
He Th U 42
23490
23892
Beta Emission
e Xe I 0-1
13154
13153
TRANSMUTATIONTRANSMUTATIONTRANSMUTATIONTRANSMUTATION
Where do these particles come from ?
These particles generally come from the nuclei of atomic isotopes which are not stable.
The decay chain of Uranium produces all three of these formsof radiation.
Let’s look at them in more detail…
Alpha Particles ()
Radium
R226
88 protons138 neutrons
Radon
Rn222
Note: This is theatomic weight, whichis the number ofprotons plus neutrons
86 protons136 neutrons
+ nnp
p
He)
2 protons2 neutrons
The alpha-particle is a Helium nucleus.
It’s the same as the element Helium, with the electrons stripped off !
Beta Particles ()
CarbonC14
6 protons8 neutrons
NitrogenN14
7 protons7 neutrons
+ e-
electron(beta-particle)
We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus. In symbolic notation, the following process occurred:
n p + e ( + Yes, the same neutrino we saw
previously
Gamma particles ()In much the same way that electrons in atoms can be in an excited state, so can a nucleus.
NeonNe20
10 protons10 neutrons
(in excited state)
10 protons10 neutrons
(lowest energy state)
+
gamma
NeonNe20
A gamma is a high energy light particle.
It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.
A gamma is a high energy light particle.
It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.
Gamma Rays
NeonNe20 +
The gamma from nuclear decayis in the X-ray/ Gamma ray
part of the EM spectrum(very energetic!)
NeonNe20
How do these particles differ ?
ParticleChange in
Mass number
Change in atomic number
Gamma () No change No change
Beta () No changeIncreased by
1
Alpha ()Decreased
by 4Decreased
by 2
Rate of DecayBeyond knowing the types of particles which are emittedwhen an isotope decays, we also are interested in how frequentlyone of the atoms emits this radiation.
A very important point here is that we cannot predict when aparticular entity will decay.
We do know though, that if we had a large sample of a radioactive substance, some number will decay after a given amount of time.
Some radioactive substances have a very high “rate of decay”,while others have a very low decay rate.
To differentiate different radioactive substances, we look toquantify this idea of “decay rate”
Half-Life The “half-life” (h) is the time it takes for half the atoms of a radioactive substance to decay.
For example, suppose we had 20,000 atoms of a radioactive substance. If the half-life is 1 hour, how many atoms of that substance would be left after:
10,000 (50%)
5,000 (25%)
2,500 (12.5%)
1 hour (one lifetime) ?
2 hours (two lifetimes) ?
3 hours (three lifetimes) ?
Time #atoms
remaining% of atomsremaining
D. Half-life• Half-life (t½)
– time it takes for half of the nuclides in a sample to decay
Nuclear Decay
0
2
4
6
8
10
12
14
16
18
20
0 2 4 6 8 10
# of Half-LivesM
ass
of Is
otop
es (g
)
Example Half-lives
polonium-194 0.7 seconds
lead-212 10.6 hours
iodine-131 8.04 days
carbon-14 5,370 years
uranium-238 4.5 billion years
Half-life How much of a 20-g sample of sodium-24 would
remain after decaying for 30 hours? Sodium-24 has a half-life of 15 hours.
GIVEN:
total time = 30 hours
t1/2 = 15 hours
original mass = 20 g
WORK:
number of half-lives = 2
20 g ÷ 2 = 10 g (1 half-life)
10 g ÷ 2 = 5 g (2 half-lives)
5 g of 24Na would remain.
Writing Nuclear EquationsWriting Nuclear Equations
XAZ
Mass Number
Atomic NumberElement Symbol
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
A
Z
1p11H1or
proton0e-1
0-1or
electron4He2
42or
particle
1
1
0
-1
4
2
212Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212Po.
4He242oralpha particle -
212Po 4 He + AX84 2 Z
212 = 4 + A A = 208
84 = 2 + Z Z = 82
212Po 4He + 208Pb84 2 82
23.1
Write Nuclear Equations!
Write the nuclear equation for the beta emitter Co-60.
6060CoCo 00ee ++ 6060NiNi2727 -1 -1 2828
Write an equation to describe the beta decay of a lead-214 nucleus to form a bismuth-214 nucleus.
Write Nuclear Equations!
Write an equation to describe the alpha decay of a radium-226 nucleus to form a radon nucleus.
214Pb 0e + 214Bi82 -1 83
Summary Certain particles are radioactive and undergo decay.
Radiation in nuclear decay consists of , , and particles
The rate of decay is give by the radioactive decay law:
After 5 lifetimes more than 99% of the initial particles have decayed away.
Subatomic particles usually have lifetimes which are fractions of a second…
Certain particles are radioactive and undergo decay.
Radiation in nuclear decay consists of , , and particles
The rate of decay is give by the radioactive decay law:
After 5 lifetimes more than 99% of the initial particles have decayed away.
Subatomic particles usually have lifetimes which are fractions of a second…
A. F ission
• splitting a nucleus into two or more smaller nuclei
• some mass is converted to large amounts of energy
n3 Kr Ba U n 10
9236
14156
23592
10
A. F ission• chain reaction - self-feeding reaction
B. Fusion• combining of two nuclei to form one nucleus of larger mass
• produces even more energy than fission
• occurs naturally in stars
A. Nuclear Power• Fission Reactors
Cooling Tower
A. Nuclear Power
• Fission Reactors
A. Nuclear Power• Fusion Reactors (not yet sustainable)
A. Nuclear Power• Fusion Reactors (not yet sustainable)
Tokamak Fusion Test Reactor
Princeton University
National Spherical Torus Experiment
A. Nuclear Power
• 235U is limited
• danger of meltdown
• toxic waste
• thermal pollution
• Hydrogen is abundant
• no danger of meltdown
• no toxic waste
• not yet sustainable
FIssion
FUSION
vs.
• Choose one of the following to investigate:
– Irradiated Food
– Radioactive Dating
– Nuclear Medicine
– Weapons of mass destruction
– Chernobyl
– Nuclear power future
– Meltdowns/ leaks
• Make a mini-poster to display what you have learned.