Init 9/16/2008 by Daniel R. Barnes

a.k.a. “PHC 25 Section Assessment Answers”

Jump to alpha decay

First, let’s clear something up.

Nuclear radiation does not make super-heroes.

Nuclear radiation makes hospital patients and asylum inmates.

http://inmotion.magnumphotos.com/essay/chernobyl

If you want to see more of that, here’s a nice, disturbing link to a photoessay w/spoken narration about the aftermath of the Chernobyl nuclear reactor accident. It’s in two parts. The link takes you to the introduction, but make sure you watch/listen to the main photoessay as well.

Required viewing materials: box of kleenex.

https://www.youtube.com/watch?v=u7QYJKnagas

Here’s a YouTube video that shows a geiger counter being used. You probably don’t need to watch it past the one minute mark.

If an atom’s nucleus has the right balance of protons and neutrons, it can last pretty much forever.

However, if the atom has too many or too few neutrons, the nucleus will be unstable.

At some random, unpredictable moment, the nucleus will “decay”, spitting out a piece of itself.

e = mc2

The three main types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation.

alpha particle

He24 2+

high speed helium nucleus

beta particle

e-10 -

high speed electron

gamma particle

high energy, high frequency

photon

U92238 Th90

234 He24

Uranium-238 undergoes alpha decay to become thorium-234.

The nucleus’ mass number . . . . . . goes down by four.The nucleus’ atomic number . . . . . . goes down by two.

+

e = mc2

e = mc2

C614 N7

14 e-10

Carbon-14 undergoes beta decay to become nitrogen-14.

The nucleus’ mass number . . . . . . stays the same.The nucleus’ atomic number . . . . . . goes up by one.

+

Jump to

fission

Gamma radiation is the most penetrating.

Alpha radiation penetrates 0.05 mm into the human body.Beta radiation penetrates 4 mm into the human body.Gamma radiation shoots right through the human body.In fact, gamma radiation not only shoots right through skin, flesh, and bone, but also through substantial amounts of lead, concrete, or whatever. Gamma rays are very hard to stop. Even air can stop gamma rays, though, if you have enough air.

The 50-mile-thick (or so) layer of air that surrounds our planet shields us nicely from gamma rays that come from outer space.

Alpha particles may be the largest radiated particle that we’re studying here, but they stop the quickest. This seems odd, considering that it’s easier to stop a baseball than it is to stop a freight train.

Alpha particles lose their energy rapidly by ionizing other atoms . . .

Gamma radiation is the most penetrating.

Here’s an ordinary carbon atom minding its own business.

Perhaps it’s part of a keratin protein molecule in your epidermis.

It’s got the usual six electrons oribiting its nucleus.

6 positive protons6 negative electrons neutral atom

Skip ionizing

radiation, please.

Look out, little carbon atom! Here comes an alpha particle!

Aw, gee! That psycho alpha particle just knocked one of your electrons out of orbit!

I’m gonna go mess up

a DNA molecule!

No! Please don’t! You’ll be charged with causing cancer!

I’m CHARGED

already! Look at me! I’m an ION!6 positive protons

5 negative electrons +1 ION!

6 positive protons6 negative electrons neutral atom

Notice something about the alpha particle’s motion . . .

. . . It slows down after it knocks the electron out of orbit.

It loses lots of speed by ionizing the atoms it bumps into, so it doesn’t go very far.

6 positive protons5 negative electrons +1 ION!

6 positive protons6 negative electrons neutral atom

It takes energy to ionize an atom. Knocking electrons out of orbit is hard work.

Alpha, beta, and gamma particles all lose energy when they ionize an atom.

The thing about beta and gamma particles seems to be that they don’t bump into electrons as easily as those huge alphas do, so they get to penetrate deeper into matter before they bump into an electron, lose their energy, and stop. I think.

The neutron-to-proton ratio determines what kind of decay a radioisotope will undergo.

If an isotope has too many neutrons, it will probably undergo beta decay, in which one of its excess neutrons turns into a proton as it spits out a beta particle (an electron).

The opposite of this, electron capture, is when a nucleus with too many protons captures an electron, turning one of its excess protons into a neutron.

Also, if a nucleus is simply too big (atomic number > 83), the nucleus will be radioactive. Such large, unstable nuclei are often alpha emitters, but some are also beta emitters.

WARNING

• The following simulation of radioactive decay focusing on the issue of half-life is somewhat oversimpilfied.

U92238 U92

238 U92238 U92

238

U92238 U92

238 U92238 U92

238

U92238 U92

238 U92238 U92

238

U92238 U92

238 U92238 U92

238

Th90234

Th90234

Th90234

Th90234

Th90234

Th90234

Th90234

Th90234 Th90

234

Th90234

Th90234

Th90234

Th90234

Th90234

Th90234Th90

234

4.5 billion years9 billion years13.5 billion years18 billion years? years

0 years

Re

ma

inin

g r

ad

iois

oto

pe

/ %

of o

rig

ina

l am

on

t

Half-lives gone by0 1 2 3 4 5 6 7 8

1/2 = 50% of the original amount

1/4 = 25%

1/8 = 12.5%1/

16 =

6.2

5%1/

32 =

3.1

25%

1/64

= 1

.562

5%1/

128

= 0

.781

25%

1/25

6 =

0.3

9062

5%

Decay Curve for a Radioactive Isotope

Re

ma

inin

g r

ad

iois

oto

pe

/ %

of o

rig

ina

l am

on

t

Half-lives gone by0 1 2 3 4 5 6 7 8

Decay Curve for a Radioactive Isotope

Amount approaches zero but never reaches

zero

U-235U-235

U-235

U-235

U-235

U-235 U-235U-235

U-235U-235U-236Kr-91

Ba-142

U-235U-236Kr-91

Ba-142

U-235U-236Kr-91

Ba-142

U-235U-236Kr-91

Ba-142

e = mc2

The energy released by nuclear reactions is much larger, per gram of explosive material, than the energy relased by chemical explosions.

“The atom bomb dropped on Hiroshima contained 64 kg of uranium, of which 0.7 kg underwent nuclear fission, and of this mass only 0.6 g was transformed into energy.” The 1.5 pounds of uranium that split that day yielded the same explosive energy as 15,000 tons of TNT.

1.5 pounds of uranium = 30,000,000 pounds of TNT.

c = 186,000 mi/s

c2 = 34,500,000,000 mi2/s2

Jump to fission vs

fusion

U-236Ba-142U-235Kr-91

U-235

Without a moderator . . .

. . . fission can not continue

Kr-91

Ba-142

Kr-91

MO

DE

RA

TO

R

U-236Ba-142U-235 U-235U-236

But with a moderator . . .

. . . the chain reaction can continue.

Nice throw! Nice and

slow!

Fusion Boosting

Later models of the atom bomb used the heat and pressure from fission to cause deuterium and tritium to fuse.

The fusion of these forms of “heavy hydrogen” into helium released energy, but not much compared to what the fissioning uranium or plutonium did.

However, the neutrons released by the fusion of heavy hydrogen did help cause more of the fissile material to split, greatly increasing the efficiency and power of the fission chain reaction.Although uranium-238 can not sustain a fission chain reaction, it can be made to split, exothermically, if you shoot neutrons at it. Atom bombs always contained lots of 238U in addition to the main 235U fissile material.

U-236Ba-142U-235Kr-91

Protons don’t like other protons.Positives don’t like positives.

The closer two charged particles get, the stronger the force between them.

Remember how small a nucleus is compared to the atom as a whole?

In a nucleus, protons are VERY close together.

Protons close enough to join and form one nucleus will repel each other with extreme force.

To get protons to get close enough to fuse together, you need to overpower this electrostatic repulsion.

Extreme heat and/or extreme pressure, both of which are found in the centers of stars, can make fusion happen.

If protons are smashed together by exterme heat and/or pressure, they will get close enough for something magical to happen . . .

The “strong force” turns on.

The strong force has a strength of zero until protons get VERY close together, and then it gets VERY strong, very suddenly.

(The “strong force” is sometimes called the “strong nuclear force”.)

The strong force is so powerful, that it overpowers the electrostatic repulsion the protons feel for each other . . .

. . . and it “glues” them together.

U-236Ba-142U-235Kr-91

H

H

H

H

He

Big atom splits in two

Little atoms join together

e = mc2

Nuclear bombs

Profitable electricity

Negative yield electricity . . .

Almost never occurs naturally

Has been happening naturally for billions

of years – a LOT

Atoms change element

Neutrons must not go too fast

High particle speed helps reaction

Atoms do NOT change element. Atoms DO change element.

Electromagnetic forces lower energies

Nuclear forces higher energies

Bonds are broken, bonds are made

Some endo, some exo

Immeasurably small mass changes

Small but measurable mass changes

Matter changes properties

Electrons are the main actors The nucleus is the main actor.

TheEnd

All pages after this are graphical scrapheaps. Just get out of here now.

U-236Ba-142U-235U-236Kr-91

Ba-142U-235Kr-91

U-235

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get a life or something?

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Nuclear Chemistry

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Title Page

Chernobyl kids

Alpha decay animation

Beta decay animation

Half-life animation

Nuclear fission

animation

Fission & fusion animations