Chapter 17Radioactivity and Nuclear Changes

17

Why do I have to know this stuff?

• We could all be vaporized.• Or…nuclear energy could be our

salvation.• How old is that fossil?• Can we treat that tumor?• Radon (a bigger problem elsewhere)• Food preservation (refrigerated, is it

safe after 3 days or 3 weeks?)

                                                                                                                               

                                  

PET images courtesy of the Alzheimer's Disease Education and Referral Center/National Institute on Aging; Postmortem images

courtesy of Edward C. Klatt, Florida State University College of Medicine

Positron emission tomographyAge: 20 -- 80 Normal -- 80 with Alzheimer’s

Postmortem Coronal Sections

NormalAlzheimer’s

Is grandma losing her mind?

There are important differences between chemical and nuclear reactions; let’s begin

with this chemical reaction.

CCl4 + H   CCl3H + Cl  

Chemical Reaction—let’s do it by Lewis Dots

Note: no atoms lost, just rearranged.

Now here’s a nuclear reaction.

CCl4 NCl4+ + 1 e- 

Carbon turned to Nitrogen. Can it do that?

Nuclear Reactions Chemical Reactions

New elements often formed 

Elements stay the same

Particles in the nucleus are involved

Only electrons are involved

Huge energy available

Normal amounts of energy

Not affected by temperature, pressure, etc.

Affected by surroundings

The number of nucleons remains the same

The number of atoms remains the same

Sum of atomic numbers (positive charge) remains the same.

Number of electrons remains the same

Becquerel’s Experiment

*Mr. B was studying phosphorescent light. *Was it the same as ordinary light? *Would it expose photographic film in the same way? *Today, we know it will…but Mr. B’s source was rocks. *He placed certain “ordinary” non-phosphorescent

rocks….and they exposed the film, too! This was true even if the film was shielded from normal light.

Antoine Henri BecquerelComptes Rendus 122, 420 (1896)

Nobel: 1903 with P. & M. Curie

Image: Wikipedia

Serendipity means lucky.

The greatest results often come just when we aren't looking for them. In fact, looking for some specificresult can blind you to more important things.

Most scientists struggle with this problem, especially today because science has become a means to an end.

Nuclear Reactions Convert Small Amounts of Mass to Energy

E = mc2

Chocolate bar: 35 g = 0.035 kg

Nuclear Energy: (0.035 kg)(3 x 108 m/s)2 = 3 x 1015 Joule

Chemical Energy: 200 food cal = 200,000 cal = 800,000 Joule

Nuclear energy 4 billion times greater! But it is not so easy to convert chocolate to energyespecially if you want to do it quickly.

Why atomic weapons are so powerful

It is possible to convert a fairly small portion of some isotopes of some elements (uranium, plutonium, hydrogen) into energy.

Even though the portion of matter converted is small, the “bang” is pretty large because….

the conversion process is very quick. Energy changesthat happen quickly imply high power.

A populace that is burning millions of candy bars can equal the energy of a nuclear bomb, but not the power.

Also, the A-bomb is small, concentrated energy, whereasall the people to burn those candy bars would be spread out.

Atomic weapons & results—friend or foe?

“Runt 1”, a very largenuclear weapon

Castle Romeo Test Shot

It is possible for such weapons to produce craters 250 feet deep, more than one mile wide.

Is it possible for them never to be used again?

From the Gallery of U.S. Nuclear Tests http://www.fas.org/nuke/hew/Usa/Tests/index.html

Meet the Nucleons.(kind of a repeat)

Name Penetrates Symbol What is it? Alpha, several cm air 2

4He helium nucleus, +2 charge

Beta, paper 10e electron

Gamma, 1/4” lead 00 very energetic

photon neutron armor 0

1n neutron(**)

positron like electron 10p or e01 one form of

antimatter

The symbols are designed to help us track nuclear reactions.

ch emass amuConvenientSymbolarg

/

All symbols have this meaning. Upper left: mass in multiples of hydrogen mass Lower left: charge Main symbol: an atomic symbol or n for neutron, etc.

There are only 100 or so elements, but many have multiple isotopes,

some are radioactive.

“stable”and “unstable” may be inverted on this figure. Someone want to look that up?

Figure 5.3 (stable & unstable isotopes)

All those isotopes are why the mass numbers for an atom are not simple

integers.

average mass of carbon = 12.01

the 12 isotope is very common, and only small portions of the fatter isotopes are found. 

the 13 isotope is useful for identifying compounds in nuclear magnetic resonance.

the 14 isotope is useful for radioactive dating of ancient objects. 

Rules for nuclear reactions

The total mass does not change*The total charge does not change

*Except for the tiny amount converted to pure energy, as discussed already.

Work some of the examples in the notes—write them on ELMO

Types of nuclear processes are referred to by the ray or particle

they produce.

Nuclei can eject protons or helium nuclei & get lighter (alpha decay). They can swallow a neutron & spit out the electron to get heavier (beta decay).

Whenever the # of protons changes, the atom type has changed.

Figure 5.6 Uranium Decay Path

Where does a 900-pound gorilla sit?

When does radioactive decay occur?

Decay events—e.g. electrons detected for beta decay—most probable at first

As radioactive molecules become depleted, detected events decrease.

Half-life

Half-life Problems

Practical nuclear reactions

Fission example (atomic bombs, power plants) 235U + 1n 134Xe + 100Sr + 2 1n + energy

Fusion example (sunlight, future perfect energy source?)

http://www.lbl.gov/abc/Basic.html

ν2γ2e2HeH4 00

00

01

42

11

4 hydrogens 1 alpha + 2 positrons + 2 gammas + 2 neutrinos + ENERGY

LSU Peptide-based Alzheimer Inhibitors

AMY-1 x = 1, y = 6AMY-2 x = 6, y = 1AMY-3 x = 1, y = 1

Mediators Developed by Professor Robert Hammer & Professor Mark McLaughlin

MCP 1

MCP 2

K L V F F

K L V F F

Stop here?