Nuclear Physics Tutorial 6 - Nuclear Radius

 

 

The key idea to the use of particles and radiation to investigate the structure of matter lies in the de Broglie (pronounced ‘de Broy’) relationship, which states that particles have wave properties.  It is the logical extension of the particulate nature of electromagnetic wave phenomena (see Unit 1).

When we investigate large objects, light is a good way to do this.  We have eyes to see with, and we can make direct observations with a magnifying lens or a microscope.  On the atomic and nuclear level, microscopic objects like fleas and bacteria are very large objects indeed.  However the light is limited by its wavelength to resolving objects about 1 mm across.  Much less than that, then diffraction becomes important.  Waves will not travel through a gap less than a wavelength.

 

Electrons can be shown to have wave properties by the simple use of an electron diffraction tube.  A slice of carbon is placed in a beam of electrons so that the electrons diffract.

 

 

This has led to the development of the electron microscope, which allows magnifications much bigger than was ever possible with the light microscope.  A good light microscope can magnify up to 1000 times.  The electron microscope can magnify up to about 1 million times, and can reveal the existence of individual atoms.  The electron beams are focused by magnets just like the lenses on a microscope.

 

The existence of the nucleus was determined about 100 years ago by Ernest Rutherford (1871 – 1937) in his famous alpha scattering experiment.

 

 

The idea that inspired the experiment was that the alpha particles were considered to be nuclear sized bullets that would smash the atoms in the gold foil like water melons.  Instead they found that many alpha particles were deflected, while a few came back in the direction they came from.  It led to two important conclusions:

 

Further calculations led to an estimate of nuclear size of about 1 × 10-15 m or 1 femtometre (fm).

 

Question 1

Use Coulomb’s Law (Module 4) to work out the force of repulsion between two protons 1 × 10-15 m apart.  Comment on your result. 

Answer
Question 2

Use the value you worked out in Question 1 to comment on the magnitude of the strong force.  Explain your answer.   

Answer

   

X-ray Diffraction

X ray diffraction has been a useful tool to discover the structure of solid materials.  It was perfected by the father and son team of William and Lawrence Bragg.  X rays are of course electromagnetic waves, which are scattered by diffraction by the crystal lattices of materials.  A sample of the material is placed in the beam of X-rays, and the resulting scattering pattern is picked up on a photographic plate.  The X rays are diffracted in a cone:

 

 

By use of a simple equation we can determine the separation of layers of atoms.  The equation (which you won't be asked about in the AQA exam) is:

 

nl = 2d sin q

 

There is a simple student experiment in which microwaves are used in conjunction with a lattice of 4 cm polystyrene spheres.  It gives students experience in the use of this equation to determine the layer separation.

 

More complex analysis is needed for determination of the structure of the crystal lattices, and this is covered in the discipline of X-ray crystallography.  Some very complicated structures indeed have been worked out using such techniques.

 

 

To get the resolutions required to look at the nucleus, we need de Broglie wavelengths of 10-15 m, and this is not possible with electrons.  If we use more massive particles, we can obtain much shorter de Broglie wavelengths, hence more resolution.  That’s the theory.  In practice, the nuclei bombarded with high energy particles have tended to break up.  It has been described as finding out how a watch works by smashing it with another watch, and guessing how the pieces fit together.

 

Particle physics experiments have been able to interpret the sub nuclear particles that make up the nucleons within the nucleus.  They cannot be seen directly of course, but inference has been drawn from the patterns observed in collisions between particles.  Physicists use powerful computers to explain these phenomena.

 

 

Nuclear radius

Rutherford estimated the radius of a nucleus as 3.0 × 10-14 m from the data obtained in alpha scattering experiments.  He used Coulomb’s law in his calculations:

 

 

The point P is the closest that the alpha particle gets to the nucleus before being repelled.  Its kinetic energy is 0 because it is stationary.  All its energy is potential.  So we can use our knowledge of electrostatic potential energy to calculate the distance.

 

            Ep =  potential at P × charge of the alpha particle =    1__ . Ze × 2e

                                                                              4pe0       rC

 

Rearranging gives us:

 

            rc  2Ze2         = 9 × 109 × 2 × 79 × (1.6 × 1019) = 2.96 × 10-14 m

                  4pe0 Ep                             1.23 × 10-12

 

 

This calculation is based on alpha particles of 7.68 MeV energy which converts to 1.23 × 10-12 J.

 

There are one or two points to bear in mind from this calculation:

  

Question 3

Describe the principal features of the nuclear model of the atom suggested by Rutherford.

Answer

 

A more accurate estimate of the nuclear radius has been determined by the use of a technique called electron scattering.  The electrons interact with the nucleus entirely by the electromagnetic interaction whereas the alpha particles interact by the strong nuclear interaction, which is not well understood.

 

The scattering of electrons is treated like the diffraction of waves around a spherical object.  The in depth analysis of the results is quite complicated, but a reasonable estimate can be obtained with a relatively simple equation (which you don't need for the exam):

 

sin q = 0.61l

           R

  The term l is the de Broglie wavelength of the high-energy electrons, q is the angle of diffraction, and R is the nuclear radius.  This gives a result of the nuclear radius being 2.65 × 10-15 m.

 

We need to note the following:

 

The radius depends on the nucleon number through a simple relationship:

 

      R = r0A1/3

 

 

[The term A1/3 means the cube root of A, the nucleon number.  The term r0 is a constant with the value 1.4 × 10-15 m.  R is the nuclear radius.]

 

Example

 

What is the nuclear radius of gold with a nucleon number of 197?
 R = r0A1/3 = 1.4 × 10-15 m × 3Ö(197) = 1.4 × 10-15 m × 5.82 = 8.15 × 10-15 m

We can plot the radius against the cube root of the nucleon number, which gives us a straight line plot as shown on the graph.

 

 

So we can see a nice linear progression.  We need to note one or two points.

Question 4 

What is the nuclear radius of an iron atom, of which the nucleus contains 56 nucleons?  Would it be any different to a cobalt-56 nucleus?

Answer

 

 

Nuclear Density

We are used to the density (mass per unit volume) of solid materials being in the order of 1000 to 10000 kg m-3.  Since most of the atom is empty space, and the vast majority of the matter in an atom is contained in the tiny space of a nucleus, it makes sense that the density of nuclear material is very high indeed.

 

Question 5 

What is the nuclear density of an iron atom, of which the nucleus contains 56 nucleons? 

Mass of a nucleon = 1.67 × 10-27  kg

Answer

 

This calculation gives you a really big number, which works out at about 100 million tonnes per cubic centimetre.  Drop that on your foot, and you really will have tears in your eyes.

 

Put another way, the mass of the Earth would fit in a sphere of radius 100 metres; it would fit on top of your school.

 

When large stars die, they can do so in a cataclysmic explosion called a supernova.  Strange things happen: the remains of the star collapse in on themselves under gravity, and the material is squashed so much that electrons are absorbed by protons to make neutrons.  Neutron stars are the result, which have an intense gravity field.  If the star is really big, then the neutrons are squashed into a really tiny space where the laws of physics no longer apply, and even light cannot escape.  We have a black hole.

Summary

The structure of matter can be investigated with electron beams, X ray photons, or particles.

 

These methods use the wave properties of particles.

 

Nucleus is very tiny compared to the rest of the atom

 

Nuclear radius depends on nucleon number.

 

R = r0A1/3