Goal: To understand the basics of nuclear physics

Objectives:1) To learn about Atomic number and

weight2) To be able to find the Size of nucleus

3) To understand Fusion and Fission4) To learn about Binding Energy

5) To learn about How all the elements are made

6) To learn about the Hydrogen Bomb and other uses for nuclear power such as

power plants

Atomic number (Z)

• The atomic number is the number of protons an atom has in its nucleus (and electrons if it is not “ionized”).

• Each different element has its own atomic number.

Atomic Weight

• Each atom has some number of neutrons.

• The # of neutrons is N.

• Most atoms lighter than iron have 1 neutron per proton.

• Atoms with a lot more neutrons than protons tend to be unstable.

Size of a nucleus

• Each proton and neutron gets squeezed together.

• The nucleus will almost always have the same density which is the density of matter.

• This is 1014 g/cm3 or 100 trillion times the density of water.

• As for the radius, since the volume of the nucleus depends on the cube of the radius and the number of particles therefore:

• r = r0 * A1/3

• And r0 = 1.2 fm

Atoms

• The density for the total atom includes the electrons, so it is mostly empty space (like finding the density of our solar system).

• Larger atoms have their electrons closer to the atoms, so their densities are larger.

• So, while atoms all have the same density of nucleus, they do not have the same density.

Fusion + Fission

• Fusion is taking two atoms and combining them together.

• This is the power source that powers stars.

• Fission is breaking an atom into more than 1 atom.

• This is what powers nuclear plants.• Often time an “alpha” particle is released –

which is just a helium nucleus.

Binding energy

• It takes some amount of energy to glue the atoms together.

• If you slam them together or break them apart you can either loose energy or gain energy depending on what the new atom needs to be formed.

• Each element has some amount of binding energy.

• If you divide this energy by the # of particles in its nucleus you get a binding energy per nucleon.

Which to do?

• If you go to higher binding energy with larger atoms you gain energy through fusion.

• If you go to higher binding energy with smaller atom you gain energy through fission.

• At what point are you no

longer able to get energy?

How the elements are made:

• There are a few different methods to make different elements.

• Hydrogen was formed in the big bang when energy formed into quarks – and the quarks formed into Hydrogen.

• Nuclear fusion in the early universe created most of the Helium.

• For all of the other elements iron and lighter they were all formed via fusion in the cores of massive stars!

Heavier than Iron

• Once you get to Iron other processes take over.

• The first involves a massive bombardment of neutrons onto a Iron atom.

• This is called the fast process.

• The neutrons then decay and emit an electron to become a proton.

• This is called beta decay.

Reverse

• Sometimes a neutron can capture an electron and become a proton.

• This is called electron capture.

• So, a heavy Nitrogen atom can capture an electron in the nucleus and become a light oxygen atom.

Alpha Decay

• Some atoms are radioactive.• What this means is that in some given time (called a half

life) half of the atoms will release a particle (we have seen the beta decay example already).

• Usually though a mean lifetime is used, after which only 37% of the original atoms stay original.

• Many radioactive materials release a helium nucleus (2 protons + 2 neutrons) in an attempt to become more stable.

• A problem with this is that the nucleus is bigger than it should be in size.

• This is called an “excited state”.• To unexcite itself it will usually emit one or more gamma

rays.

Uses

• We have shown that stars use fusion for power.

• However it is very tough.

• In the core of our sun it is 100 million degrees and 100 times the density of water.

• What usually happens when 2 protons find themselves on a collision course?

NOTHING!

• Even at that temperature their energy is still not enough to collide.

• Eventually their repulsive force brings both to a screeching halt and then they go the other way.

• But, it turns out that there is some small chance that they are really located somewhere else and can fuse – thank you quantum mechanics.

Our uses for fusion

• Well, not much.

• We have tried, but we cannot generate a sustained burst which gives more energy that it takes.

• Heck, even for the sun it takes an average of 10 billion years for each Hydrogen atom to fuse.

Fission

• We use it for nuclear power.• We use it for bombs.• The bombs that were used in WWII would have been

pure fission bombs.• Basically you have some radioactive material that you hit

with a neutron.• That makes it split into 2 atoms + 3 neutrons.• The resulting neutrons then hit other atoms making them

split.• This gives a runaway affect.• But only Uranium 235 reacts this way (neutron in means

3 neutrons out).

Difficulties

• If you had pure U235 this would be pretty easy, but luckily for us U235 comes with a LOT of U238 – which is pretty harmless.

• Also the U238 absorbs neutrons, so if you have a normal breakdown (99.3% U238) then the neutrons quickly all get eaten up by U238 atoms and the chain reaction ends.

• To get it to work you have to “enrich” the U235 to make it a few percent.

• But this is EXPENSIVE!

Nuclear plants

• Now you have what you need to generate power.

• However, the fissioning U235 atoms fire neutrons which move too fast.

• The U235 atoms can absorb them and not fission!

• So, you have to have some substance to slow down the neutrons (such as water).

Reactor at Critical!• Now, how many neutrons do you want? • If you get less than 1 on average, your reactions won’t last long and

will die out. • This is called subcritical.

• If you get on average exactly 1 neutron then you can keep it going at a constant pace.

• This is called critical – and a reactor at critical is actually a GOOD thing – don’t listen to Hollywood.

• Will it blow? Well if you produce more than 1 the reaction rate will INCREASE with time. Other than when it is turned on – this is very bad. If this happens you need to absorb some neutrons by inserting a “control rod”.

• This is called supercritical.

The Worry about N. Korea

• Here is why there is concern about nuclear power being used in countries that cannot be regulated by the UN…

• Sure, you produce energy without greenhouse gasses. • This is good, but:• When the U238 absorbs a neutron (and it will from time

to time – there is a LOT more of it) then it will become U239 – which is not stable.

• The U239 beta decays into plutonium 239.• Pu239 can be used to make nuclear weapons!• This is NOT a good thing clearly.• These can be called “breeder reactors”

H bomb

• The modern version of the atomic bomb uses both fission and fusion.

• The first part of the bomb is a fission bomb (U235 or Pu239).

• This generates a lot of energy – enough to fuse a lighter element such as Hydrogen (which you can provide using water).

• Yes this takes hundreds of millions of degrees!• This fusion reaction is an uncontrolled reaction and

generates even more energy than the original bomb.• This type of bomb destroyed the Bikini atoll.• Stronger bombs than this were banned by the Geneva

convention because any stronger than this and the blast wave would reach outer space and throw some of our atmosphere into space.

Conclusion

• We learned everything there is to know about the basics of nuclear power.

• We can now all apply for Homer Simpson’s job

• (picture from wikipedia)