Radioactivity

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.