Nuclear Reactions

Alpha, Beta, and Gamma Decay

The Atom

The atom consists of two parts:

1. The nucleus which contains:

2. Orbiting electrons.

protonsneutrons

XA

Z

Mass number

Atomic number

Element symbol

= number of protons + number of neutrons

= number of protons

XA

Z

A = number of protons + number of neutrons

Z = number of protons

A – Z = number of neutrons

Number of neutrons = Mass Number – Atomic Number

U235

92U

238

92

There are many types of uranium:

A

Z

Number of protons

Number of neutrons

A

Z

Number of protons

Number of neutrons

U235

92U

238

92

There are many types of uranium:

Isotopes of any particular element contain the same number of protons, but different numbers of neutrons.

A 235

Z 92

Number of protons 92

Number of neutrons 143

A 238

Z 92

Number of protons 92

Number of neutrons 146

Most of the 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/radionuclides.

Radioactive decay results in the emission of either:

• an alpha particle (),

• a beta particle (),

• or a gamma ray

Radioactive Decay

He42

A. Types of Radiation• Alpha particle ()

– helium nucleus paper2+

Beta particle (-) electron e0

-11-

leadPositron (+)

positron e01

1+

Gamma () high-energy photon 0

concrete

An alpha particle is identical to that of a helium nucleus.

It contains two protons and two neutrons.

Alpha Decay

XA

ZY

A - 4

Z - 2+ He

4

2

Alpha Decay

unstable atom

more stable atom

alpha particle

Alpha Decay

Ra226

88

Rn222

86

He4

2

XA

ZY

A - 4

Z - 2+ He

4

2

Ra226

88Rn

222

86+ He

4

2

Alpha Decay

Rn222

86He

4

2+Po

218

84He

4

2

Rn222

86+Y

A

ZHe

4

2

Alpha Decay

He4

2U

234

92+Th

230

90He

4

2

X A

Z+Th

230

90He

4

2

Alpha Decay

Th 230

90+Y

A

ZHe

4

2

Alpha Decay

He4

2+Ra

226

88He

4

2Th

230

90

X A

Z+Pb

214

82He

4

2

Alpha Decay

He4

2+Pb

214

82He

4

2Po

218

84

Beta Decay

A beta particle is a fast moving electron which is emitted from the nucleus of an atom undergoing radioactive decay.

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.

Beta Decay

Po218

84

0

-1

At218

85

XA

ZY

A

Z + 1+

0

-1

Beta Decay

Po218

84At

218

85+

0

-1

Th234

90Y

A

Z+

0

-1

Beta Decay

Th234

90Pa

234

91+

0

-1

X A

ZPb

210

82+

0

-1

Beta Decay

Tl210

81Pb

210

82+

0

-1

Bi210

83Y

A

Z+

0

-1

Beta Decay

Bi210

83Po

210

84+

0

-1

X A

ZBi

214

83+

0

-1

Beta Decay

Pb214

82Bi

214

83+

0

-1

Gamma Decay

Gamma rays are not charged particles like and particles.

Gamma rays are electromagnetic radiation with high frequency.

When atoms decay by emitting or 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).

Nuclear Decay• Beta Emission

e Xe I 0-1

13154

13153

electronPositron Emission

e Ar K 01

3818

3819

positron

Nuclear Decay• Electron Capture

Pd e Ag 10646

0-1

10647

electronGamma Emission

Usually follows other types of decay. Transmutation

One element becomes another.

Nuclear Decay• Why nuclides decay…

– need stable ratio of neutrons to protons

He Th U 42

23490

23892

e Xe I 0-1

13154

13153

e Ar K 01

3818

3819

Pd e Ag 10646

0-1

10647

DECAY SERIES TRANSPARENCY

Half-Life• is the required for

of a radioisotope’s nuclei to decay into its products.

• For any radioisotope,# of ½ lives %

Remaining

0 100%

1 50%

2 25%

3 12.5%

4 6.25%

5 3.125%

6 1.5625%

C. Johannesson

Half-life• Half-life (t½)

– Time required for half the atoms of a radioactive nuclide to decay.

– Shorter half-life = less stable.

Half-Life• For example, suppose you have 10.0 grams

of strontium – 90, which has a half life of 29 years. How much will be remaining after x number of years?  

• You can use a table:# of ½ lives Time

(Years)Amount Remaining (g)

0 0 10

1 29 5

2 58 2.5

3 87 1.25

4 116 0.625

Half-Life• Or an equation!

C. Johannesson

Half-life Fluorine-21 has a half-life of 5.0 seconds. If you

start with 25 g of fluorine-21, how many grams would remain after 60.0 s?

GIVEN:

t½ = 5.0 s

mi = 25 g

mf = ?

total time = 60.0 s

n = 60.0s ÷ 5.0s =12

WORK:

mf = mi (½)n

mf = (25 g)(0.5)12

mf = 0.0061 g

Half-Life

• Example 1: If gallium – 68 has a half-life of 68.3 minutes, how much of a 160.0 mg sample is left after 1 half life? ________

2 half lives? __________ 3 half lives? __________

Half-Life

• Example 2: Cobalt – 60, with a half-life of 5 years, is used in cancer radiation treatments. If a hospital purchases a supply of 30.0 g, how much would be left after 15 years? ______________

Half-Life

• Example 3: Iron-59 is used in medicine to diagnose blood circulation disorders. The half-life of iron-59 is 44.5 days. How much of a 2.000 mg sample will remain after 133.5 days? ______________

Half-Life

• Example 4: The half-life of polonium-218 is 3.0 minutes. If you start with 20.0 g, how long will it take before only 1.25 g remains? ______________

Half-Life

• Example 5: A sample initially contains 150.0 mg of radon-222. After 11.4 days, the sample contains 18.75 mg of radon-222. Calculate the half-life.

Nuclear Fission and Nuclear Fission and FusionFusion

Nuclear power• Power can be obtained two ways.

• Fission Splitting atoms• Get energy if the nucleus is big.

• The smaller ones are more stable.

• What we do in nuclear reactors.

• Fusion Joining atoms• Get energy if the nuclei are small.

• The larger one is more stable.

• This is how the sun works.

NUCLEAR FISSION

A reaction in which an atomic nucleus of a radioactive element splits by bombardment from an external source, with simultaneous release of large amounts of energy, used for electric power generation

Nuclear FissionFission is the splitting of atoms

These are usually very large, so that they are not as stable

Fission chain has three general steps:

1. Initiation. Reaction of a single atom starts the chain

(e.g., 235U + neutron)

2. Propagation. 236U fission releases neutrons that

initiate other fissions

3. Termination.

Nuclear Fission

• A very heavy nucleus splits into more stable nuclei of intermediate mass.

• The mass of the products is less than the mass of the reactants.

• Missing mass is converted to energy

Fission of 238U

Neutron induced in U235

Fission is Exothermic

The sum of the masses of the resulting nuclei is less than the original mass (about 0.1% less)

The “missing mass” is converted to energy according to E=mc2

Neutrons may:1 - Cause another fission by colliding with a U235 nucleus

2 - Be absorbed in other material

3 - Lost in the system

If sufficient neutrons are present, we may achieve a chain reaction

Each split (fission) is accompanied by a large quantity of E-N-E-R-G-Y

• Creates two smaller nuclides and free neutrons• The free neutrons potentially collide with nearby U235 nuclei• May cause the nuclide to split as well

Uranium IsotopesUranium Isotopes

Naturally occurring Uranium contains Naturally occurring Uranium contains two major isotopestwo major isotopes

Uranium-238 (99.3%)Uranium-238 (99.3%)Uranium-235 (0.7%)Uranium-235 (0.7%)As it turns out the only isotope of As it turns out the only isotope of

Uranium that undergoes fission is Uranium that undergoes fission is Uranium-235Uranium-235

235235U FissionU Fission 235235

9292U + U + 1100n n 236236

9292UU**

and 10and 10-14-14 seconds later... seconds later... 236236

9292UU** 92923636Kr + Kr + 141141

5656Ba + Ba + 3 3 1100nn + + ENERGYENERGY

50 possible sets of fission products (sum of 50 possible sets of fission products (sum of atomic numbers = 92)atomic numbers = 92)

3 neutrons released for ONE 3 neutrons released for ONE 2352359292UU

each neutron can split another each neutron can split another 2352359292UU

CHAIN REACTIONCHAIN REACTION POSSIBLE POSSIBLE If amount of If amount of 235235

9292U is sufficient (U is sufficient (CRITICAL MASSCRITICAL MASS) ) then the number of neutrons generated is high then the number of neutrons generated is high enough to result in a enough to result in a nuclear explosionnuclear explosion ) )

Where does all this Where does all this energy come from?energy come from?

E = mcE = mc22

E = Energy (joules)E = Energy (joules)m = mass (kg)m = mass (kg)

c = speed of light c = speed of light = =

3 x 103 x 1088 m/s m/s

Nuclear Fission & POWER

• Currently about 103

nuclear power plants in

the U.S. and about 435

worldwide.

• 17% of the world’s

energy comes from

nuclear.

Fusion

• Light-mass nuclei combine to form a heavier, more stable nucleus.

• More energetic than fission reactions

• Source of energy for the H-bomb

• Origin of the elements

Nuclear Fusion

FUSIONFUSION

411H 4

2He + 2 ? + + energy

Stars energy is produced through fusion

reactionsFusion occurs until Fe is produced because less energy is released than

required to fuse Fe nuclei = _____ ____ ____

Star burns out

The most destructive force on the planet

H-bombs 1000s of times more powerful than A-bombs

Cold Fusion:Efforts are

being made to start and sustain a fusion reaction at lower temperatures, in other words with a lower amount of input energy