Radiation

• Objectives

– Understand the concepts of ½ life and ½ thickness in radiation

– Differentiate between fusion and fission

– Describe the processes involved in radioactive decays (alpha, beta, and gamma)

Elementary Particle Properties

Particle Symbol Charge Weight Weight Ratio (e-)

proton p+ +1 1.007 amu 1838 heavy

electron e- -1 0.0005 amu 1 light

neutron n0 0 1.008 amu 1840 heavy

1 Atomic Mass Unit (amu) = 1.66 x 10-24 grams

Atoms are electrically neutral with no net charge.

Ions are atoms that have been stripped of one

or more of their electrons and have a net charge.

Isotopes• Identical Chemical Properties, Different Atomic Weight• Difference = presence of number of neutrons in the nucleus

Hydrogen = 1.0079 amu

ISOTOPE Constituents Atomic Mass Occurrence

H 1 proton 1 electron 1.0070 amu 99.985 %

Deuterium 1 proton 1 electron 2.0141 amu 0.014 %

1 neutron

Tritium 1 proton 1 electron 3.0220 amu 0.001 %

2 neutrons

Hydrogen 11H 1 proton, 0 neutrons

Helium-4 42He 2 protons, 2 neutrons

Uranium-235 23592U 92 protons, 143 neutrons

neutron 10n

proton 11p

electron 0-1e

Isotope Designations

Radiation

• Three types of Nuclear Radiation.

• Revealed by their deflection in a magnetic field as neutral, charged negative or charged positive.

NS

Radioactive Particles• Alpha

Ejection of 2 protons and 2 neutrons from an

unstable nucleus. 42He =

• Beta Ejection of an electron from an unstable

nucleus

as part of the decay of a neutron. 0-1e =

• Gamma Atomic nucleus transition, yielding high

energy

photons.

Nuclear Reactions 226

88Ra 22286Rn + 4

2He

Note that the numbers all add-up (conservation of particles).

AZX

A nucleons Total particles in nucleus 226 = 222 + 4

Z protons Number of protons 88 = 86 + 2

Alpha Decay• Alpha = 4

2He

Parent 22688Ra Radium

Daughter 22286Rn Radon

Radiation 42He

22688Ra 222

56Rn + 42He

Most of the energy is with the lighter particle, in this case

the alpha particle.

Beta Decay• Beta = 0

-1e

10n 1

1p + 0-1e

The decay of a neutron into a proton and electron.

Beta Decay• Beta = 0

-1e

10n 1

1p + 0-1e

The decay of a neutron into a proton and electron.

146C 14

7N + 0-1e (Radioactive Carbon)

9038Sr 90

39Y +

Gamma Radiation• Gamma E = h fVery high energy photons are emitted from the nucleus.

Excess radiation emitted from an excited nucleus….

8738Sr* 87

38Sr +

Excess radiation emitted as part of another process….60

27Co 6028Ni + +

Radioactive Decay

If you start out

with a sample of

parent atoms (No),

after some time

there will be fewer

because of

radioactive decay

into the daughter

atoms.

ShieldingWe can detect the radiation from a

radioactive source.

Say we get X counts/minute (cpm).

Geiger Counter

ShieldingWe can shield the source with various

materials to test their usefulness in

protecting against the radiation.

Geiger Counter

Half Thickness

Half Thickness: The thickness of a material needed to

cut the count rate by 1/2.

If a 4 inch thickness of lead, drops a count rate

from 1000 cpm to 500 cpm, the half thickness must

be 4 inches.

Half Thickness

Half Thickness: The thickness of a material needed to

cut the count rate by 1/2.

If 12 inches of wood, drops a count rate

from 2000 cpm to 125 cpm, the half thickness must

be 3 inches.

2000 1000 500 250 125

0 3 6 9 12

Shielding Efficiency

Cotton Fabric Wood Lead

’s are the most penetrating type of radiation.

Nuclear Energy Generation B

indi

ng E

nerg

y/nu

cleo

n

Atomic Mass Number

Uranium

Iron

Hydrogen

Fission

Fusion