Nuclear Fission

Half-lives, reactions and energy

Outline Nuclear Reactions

Conservation of charge Conservation of nucleons

Half-lives Quantum Mechanics Decay rates

Nuclear Fission The process Chain reactions

Nuclear Reactions Radioactive decay involves the

transformation of nuclei from one type to another.

Alpha, Beta and Gamma decay are different ways the nuclei are transformed.

Certain rules are observed during decay.

decay) (Beta 11 eNA

ZNAZ eYX

The conservation rules Energy is conserved: a particle will always

decay into a particle having less mass plus the leftover mass energy as kinetic energy.

Charge is conserved: When a neutral neutron decays into a positive proton, a negative electron is also produced.

Nucleon number is conserved: A nucleon can change from neutron to proton, but it can’t disappear altogether.

Environmental Radiation Radon–a radioactive gas from underground The “daughter” of Uranium decay.

The Radon sequence

e

e

ePoBi

eBiPb

PbHePo

PoHeRn

13021484131

21483

13121483132

21482

132214822

42134

21884

134218842

42136

22286

Exponential Decay A radioactive decay in nature is a

RANDOM process. There is NO way to know exactly when an

unstable atom will decay. On AVERAGE, the members of a group of

identical unstable atoms will decay at a rate proportional to how many there are.

This is called: EXPONENTIAL DECAY.

Nuclear half-lives Particles undergoing exponential decay have a

characteristic time before undergoing decay, called the HALF-LIFE.

The half-life is the time it takes for HALF of a sample of radioactive particles to decay.

We don’t know which half! It’s just the expected average.

Example: The half life of Radon is 3.8 days. If there were originally 100 Radon atoms, after 3.8 days, there would be 50 left.

Examples Sample problem:

The alpha decay of Polonium 218 to Lead 214 has a half-life of 3.1 minutes.

If there are 1000 Polonium atoms at the start, how many will there be after 3.1 minutes, 6.2 minutes, 9.3 minutes….? So, N = (1/2)#half-lives N 0

How many half-lives in 5 minutes? # half lives = 5 / 3.1 = 1.61

How many Po atoms after 5 minutes? N = (1/2)1.61(1000) = 327.

Detection of radiation Radioactive decay releases particles having an

MeV of energy or more. Ionizing radiation has the energy to remove

electrons from hundreds or thousands of atoms. A Geiger tube contains gas, which when ionized

by the passing radiation, creates an electric current. The current is then recorded by the Geiger counter as a single decay event.

Demonstration in class.

Nuclear energy Radioactive decay releases energy in large

amounts. For example, the earth’s core is thought to

be molten largely because of the HEAT from radioactive decay! Therefore, geothermal energy is really nuclear energy!

But is there a form of nuclear energy that we can control – turn on and off at will?

Nuclear Fission

Chain Reaction Each fission reaction releases 150 –200

MeV! Also, about 3 neutrons are released! If the Uranium 235 is abundant enough, a

chain reaction can start.

Ur

Ur

Ur

Ur

3 9 27 Boom!