Mass and Energy

The Atomic Mass unit

Kilograms are useful for measuring large masses, but like many of the SI units, on the atomic scale they are far too big.  It’s a bit like a model maker using kilometres rather than millimetres.

The atomic mass unit is far more convenient to use with nuclear masses.  It uses carbon 12 as a reference and is defined as:

Exactly 1/12^th the mass of a carbon 12 atom

The relative atomic mass of an atom is useful.  It is defined as:

___Mass of the atom        ´ 12

 Mass of carbon 12 atom

The relative atomic mass of atoms is usually very close to a whole number, which is consistent with the idea of a nucleus made up of whole numbers of nucleons.

1 atomic mass unit (u) = 1.661 ´ 10^-27 kg.

The table shows particle masses in atomic mass units.  Note that the numbers are expressed to a large number of significant figures as the changes are quite subtle.

We need to be careful to distinguish between the atomic mass and the nuclear mass.

·        The atomic mass is the mass of an atom complete with its electrons;

·        The nuclear mass is the mass of the nucleus alone.  To get the nuclear mass we need to take away the mass of the electrons.

Mass Defect

If we add together the mass of an electron and the mass of a single proton, we get the mass of a hydrogen atom. Let us do the same for a helium atom.

However, if we look in a data book, we see that the atomic mass is 4.002603 u.  There is a difference of 0.030377 u.

All atoms are lighter than the sum of the masses of the protons, electrons, and neutrons.  This is the mass defect, which is the difference between the total mass of the nucleons and the measured mass of the nucleus itself.

To extract a proton or a neutron from the nucleus, we have to pull pretty hard.  Then we find that it will regain its missing mass.  We can use the idea of binding energy to explain this.  The binding energy is defined as the energy released when a nucleus is assembled from its constituent nucleons.  It is equal to the energy needed to tear the nucleus apart into its nucleons.

So with our helium atom, the missing 0.030377 u is released when the nucleons come together.  That energy has to be put back to split the nucleus up again.

This brings us onto the strange idea that mass and energy at the nuclear level are interchangeable and can be related with Einstein’s simple equation:

E = mc²

[E – energy (J); m – mass (kg); c – speed of light (m/s)]

It is important to convert the mass from atomic mass units to kilograms.  The answer we get is in joules.  We can convert this to eV by dividing by 1.6 ´ 10^-19 J/eV

 

Question 2

What is the binding energy of the helium atom whose mass defect is 0.030377 u?

ANSWER

Joules are not convenient units to use at the nuclear level, so we convert to electron volts (eV) by dividing by 1.6 ´ 10^-19 J/eV.

Binding Energy Per Nucleon

If we know the binding energy in a nucleus, and the number of nucleons, we can work out the binding energy per nucleon, which is the average energy needed to remove each nucleon.  The higher the binding energy per nucleon, the more stable is the nucleus.  For helium (⁴He) the binding energy per nucleon is:

                        Binding energy per nucleon = 28.38 MeV ¸ 4 = 7.1 MeV

We can plot a graph of binding energy per nucleon against nucleon number.

From this graph we can see the following:

·        The vast majority of nuclides have a binding energy of 8 MeV per nucleon.

·        Helium has a particularly high value of binding energy per nucleon, much higher than the light isotopes of hydrogen.

·        There is a trend for nuclides of nucleon numbers in multiples of 4 to be particularly stable (i.e. have a high binding energy).

·        Fe is the most stable nuclide.

·        The largest nuclides tend to be less stable, with slightly lower binding energies per nucleon.

Iron has the highest binding energy per nucleon so is the most stable nucleus.  If we look at large nuclei (greater than iron), we find that the further to the right (greater nucleon number) the less stable the nuclei.  This is because the binding energy per nucleon is getting less.  The explanation for this observation lies in that the strong nuclear force that binds the nucleus together has a very limited range, and there is a limit to the number of nucleons that can be crammed into a particular space. 

Radioactive Decay and Binding Energy

Radioactive decay happens when an unstable nucleus emits radiation.  It becomes more stable.  The daughter nuclei always have a higher binding energy per nucleon than the parent nucleus.

Let us look at alpha decay:  

Mass of the thorium nucleus = 227.97929 u

Mass of the radium nucleus = 223.97189 u

Mass of the alpha particle (helium nucleus) = 4.00151 u

Mass on the left hand side = 227.97929 u

Mass on the right hand side = 223.97189 u + 4.00151 u = 227. 97340 u

The right hand side has a mass defect = 227.97929 u  - 227. 97340  u = 0.00589 u

The mass defect can be written in kilograms and the energy can be expressed in joules, but nuclear physicists use a useful little dodge.  The energy equivalence of 1 u = 931.5 MeV.  So the energy given out by this decay is:

E = 931.5 ´ 0.00589 = 5.49 MeV.

This is a beta decay:

Fission

If we look at the graph with binding energy per nucleon, we observe that the large nuclei have a lower binding energy per nucleon.  This means that they are less stable.  This lack of stability is usually shown by radioactive decay, which occurs in a predictable way.  Very rarely a large nucleus will split up spontaneously into fragments.  This splitting of the nucleus is called fission.

The easiest way to explain this is to consider the nucleus as a “wobbly drop”.  Nuclei are not tidy and neatly arranged rows of neutrons and protons; they are microscopic bedlam. 

The strong nuclear force acts between neighbouring nucleons. 

The nucleons are not linked with the same neighbours all the time.  Instead they are constantly swapping about.  However the enough of the nucleons linked to stop the repulsive electromagnetic force tearing the nucleus apart.

Now we imagine the nucleus as a wobbly drop:

Now if the nucleus gets to this shape

The nucleus flies apart in two fragments:

The detail of the mechanism that drives this process is complicated and is based on Heisenberg’s uncertainty principle.  A similar model can be used to explain how alpha decay works.

We can induce fission in large nuclei such as uranium-235.  The most common isotope of uranium, U-238, does not split easily, but the 235 isotope does.  We induce fission by “tickling” the nucleus with a “thermal” neutron.  The neutron has to have the right kinetic energy:

·       Too little kinetic energy means that the neutron will bounce off the nucleus;

·       Too much kinetic energy means that the neutron will go right through the nucleus.

·       Just right means that the neutron will be captured by the strong force, which is attractive between nucleons.  The neutron gives the nucleus enough energy to resonate, and this will make the nucleus neck as shown above

The tickled nucleus flies apart into a number of fragments, leaving on average three neutrons left over.  These too are able to tickle other nuclei and make them split.  Each neutron spawns three more neutrons in each fission, so we get a chain reaction.  

There is a mass defect in the products of the fission so energy is given out.   In an uncontrolled chain reaction, the energy is given out in the form of a violent explosion, which is many times more powerful than the explosive decomposition of TNT.  In an atomic bomb, the mass that is converted to energy is about 20 grams. 

  Nuclear fission has NOTHING whatever to do with radioactive decay.  However the parent nucleus may decay by normal radioactive decay processes, and the daughter nuclei may well be radioactive.  This is a common bear trap.

The daughter fragments may well be highly unstable, and decay by radioactivity.  These form the dangerous fall-out of an atomic bomb detonation, or the waste from a nuclear power station.  Either way, they form some of the nastiest muck known to mankind.

 Compared to the speed of many particles in nuclear and particle physics, this speed is pretty sluggish.

A common bear trap is to say that nuclei are smashed to pieces by neutrons.  The neutrons tickle the nucleus; they do not hammer it.

Some students confuse fission and fusion and use the “fussion”.  It will be marked wrong in the exam, so don’t.

Fusion

Fusion means joining nuclei together, every alchemist’s dream.  It is easier said than done.  The idea is that light nuclei are joined together, increasing the binding energy per nucleon.  This will result in lots of energy being given out.  A possible reaction is:

It is not simply a case of sticking some deuterium and tritium together and shaking it up.  Each nucleus has to have sufficient energy to:

·       Overcome electrostatic repulsion from the protons;

·       Overcome the repulsive strong force which is found outside the region of the strong force.

This means that the gases have to be heated to a very high temperature, 100 million Kelvin.  As all matter at this temperature exists as an ionised gas (plasma), it has to be confined in a very small space by powerful magnetic fields.  Fusion has occurred, but the energy put in to cook the gases enough to make them fuse is far greater than the energy got out by a fusion reaction.

The only use that fusion has been put to is in a thermonuclear device.  The third bomb from the left is a genuine thermonuclear device, now on display (with the nasty bits taken out).  The amount of hydrogen required in the bomb below (430 kilo-tonnes) would fill a small party balloon. 

Some scientists claim to have found fusion at low temperatures.  They had a strange chemical reaction, but it was not fusion.

Fusion, if it could be made to work, has a number of advantages over fission:

·       Greater power per kilogram of fuel used;

·       Raw materials are cheap and readily available

·       No radioactive elements are made by the reaction.

The downside is that materials that make up the reactor will be irradiated with neutrons which will make them radioactive.