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Gamma Radiation and friends. Gülce Maşrabacı. Before the Gamma Radiation, there was... The beginning:. The strong nuclear force.

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Gamma Radiation and friends

Gülce Maşrabacı


Before the Gamma Radiation, there was...

The beginning:

The strong nuclear force

The strong nuclear force (= strong force) is one of the four basic forces in nature (the others being gravity, the electromagnetic force, and the weak nuclear force). As its name shows us, it is the strongest of the four. But, it also has the shortest range, meaning that particles must be extremely close before it performs its effects.


The strong nuclear force

Its main job is to hold together the the subatomic particles of the nucleus = the protons + neutrons = the nucleons.

We have learned, previously, that like charges repel, and unlike charges attract.





The strong nuclear force

If you consider that the nucleus of all atoms except H contain more than one proton in them, and each proton carries a postive charge, why do the nuclei of these atoms stay together? The protons must feel a repulsive force from the other neighboring protons.

This is where the strong nuclear force comes in.


The strong nuclear force

The strong nuclear force is created between nucleons by the exchange of particles called mesons (chargeless hadrons made up of 1 quark and 1 antiquark). This exchange is like constantly hitting a ping-pong ball back and forth between two people. As long as this meson exchange can happen, the strong force is able to hold the participating nucleons together.


The strong nuclear force

The nucleons, though, must be extemely close together in order for this exchange to happen. The distance required is about the diameter of a proton or a neutron. If a proton or neutron can get closer than this distance to another nucleon, the exchange of mesons can occur, and the particles will stick to each other. If they can't get that close, the strong force is too weak to make them stick together, and other competing forces (usually the electromagnetic force) can make the particles move apart.


The strong nuclear force

Beyond barrier: SNF present


The strong nuclear force

In the case of approaching protons, the closer they get, the more they feel the repulsion from the other proton As a result, in order to get two protonsclose enough to begin exchanging mesons, they must be moving extremely fast

(which means the temperature must be really high), and/or they must be under very high pressure so that they are forced to get close enough to allow the exchange of meson to create the strong force.


The strong nuclear force

The nuclear force is independent from charge, which means two protons attract each other the same rate as 2 neutrons or a proton and a neutron. Once the electrostatic barrier is passed, the repulsion force is far too little compared to the strong nuclear force to show its effect anyway.


The strong nuclear force

One thing that helps reduce the repulsion between protons within a nucleus is the presence of any neutrons. Since they have no charge they don't add to the repulsion already present, and they help separate the protons from each other so they don't feel as strong a repulsive force from any other nearby protons. Also, the nucleus is tightly packed so that nucleons can exchange mesons easily. This way, a nucleus is not destroyed.


But where does the energy that

creates this force come from?

When the mass of a nucleus, for example 42He is measured, and when the mass of the nucleons of that nucleus, for this case 2 neutrons and 2 protons, are measured seperately outside of the nucleus, which one do you think was heavier?


But where does the energy that

creates this force come from?

The mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which make it up. When forming a nucleus, the nucleons transform some of their masses into the form of energy. The nuclear binding energy can be measured by Einstein’s favourite formula;

Nuclear binding energy = Dmc2

Where Dm is the difference between the masses of individual nucleons and the nucleus.


Slowly Slowly to nuclear stability

The stability of a nucleus depends mainly on A, the mass number and Z, the atomic number. Up to the mass number 30 or 40, a nucleus has approximately the same nb. of neutrons and protons to be stable. Bigger nuclei must have more neutrons than protons since as Z gets bigger, repulsive forces get bigger.

When nucleus gets big enough, no neutron is enough to keep it stable. After, Z= 82, no nuclei is stable. Such unstable nuclei are radioactive, which means they undergo radiations in order to become stable.


Slowly Slowly to nuclear stability

A nucleus having very much protons compared to neutrons will never be stable, yes, but this does not mean that a nucleus with many neutrons and little protons will be stable. To understand this we may look at this graph, also present in our holy book Zumdahl:


Attempts to become stable:


The changing of one element to another to become more stable through radioactivity is transmutation. It can occur by alpha or beta radiation. (or else some other nuclear reactions such as nuclear bombardment but I will not deal with it now)


So, what is Gamma Radiation?

A gamma ray is simply a high energy photon – a pack of energy-. It is chargeless, pure energy. It has no mass as well.


Still, what is the relationship

between gamma radiation

and radioactive decay?

After alpha or beta decay, a nucleus is often left in an excited state-that is, with some extra energy. It then "calms down" by releasing this energy in the form of a very high-frequency photon, or electromagnetic wave, known as a gamma ray.


Still, what is the relationship

between gamma radiation

and radioactive decay?

After a decay reaction, the nucleus is often in an “excited” state. This means that the decay has resulted in producing a nucleus which still has excess energy to get rid of. So, the emission of gamma rays is a way for a high energy nucleus to reduce its energy and become more stable. (This is due to one of the 2 universal driving forces, the tendency of minimum energy)


The part where words become meaningless

  • As, or if, you have noticed, i have mentioned that the nucleus is left in an excited state, and it returns to the ground state by emitting pure energy, gamma rays. But how, how can a nucleus be in an excited state?
  • It may occur because of a violent collison with another particle- changing the arrangement of nucleons
  • Or more commonly and more related to our business, the nucleus, after a previous radioactive decay may remain in an excited state.

The part where words become meaningless

You may have seen that what I have written seems to suggest that there are energy levels in a nucleus, just like the shells of electrons. Just like an atom, a nucleus itself can be in an excited state, and, when jumping down to a lower state it emits a photon. This can be explained by:


The part where I can not explain

Previously on this slide show:

(***Talking about energy levels in nucleus***)

(...)this can be explained by:


Although not yet clearly explained, it is suggested that the nucleons exist in an interacting, many-body system, and that each nucleon moves in an average field created by all other nucleons. The motion of each nucleon is governed by the average attractive force of all the other nucleons. The resulting orbits form "shells," just as the orbits of electrons in atoms do.


The part where I can not explain

& magic numbers

Yet going on with the explanation...

For nuclei to be stable, there are some “magic numbers”. These are the numbers of neutrons and protons in a nucleus. If a nucleus has that much p or n, it is found to be more stable than the others. This numbers are usually even, for symmetry. (symmetry provides strength in bounds and thus stability.)

These magic numbers are:

For protons: 2, 8, 20, 28, 50, 82.

For neutrons: 2, 8, 20, 28, 50, 82, 126.


So What on Earth are these magic numbers?

Do you remember Noble Gases?

They contained the number of electrons that were completely filling an electron shell. Since the shell was completely filled, they were not active for reacting chemically, thus were called STABLE.

The magic numbers for the nucleus is just like that! Nucleons at that numbers are thought to fill a nuclear shell completely, thus, the nucleus with filled shells are more stable.


Still, higher energy level, lower energy level... What?

When a nucleus of an atom undergoes a nuclear reaction (or a collision), at the end of that reaction, its nucleons can be disorganized. They can be arranged at shells so that they have excess energy. A nucleon can stay at a higher shell, although, say, there is a space at the lower shell. The nucleus rearranges these particles to, as much as possible, completely fill its shells (The lower ones first then the higher ones). By this filling, and jumping down process, the nucleus EMMITS THE EXCESS ENERGY, just like an atom with an excited electron emmiting energy when the electron jumps to a lower level of energy. This energy given off is ultraviolet radiation when an atom goes to a lower energy state, and the energy given off when a NUCLEUS is going to a lower energy state is called, guess what, GAMMA RADIATON!!!!! (phew, hardly made the connection)



If you have questions regarding the previous slides, please ask now since although I felt I should explain the nuclear shell model and tried, I couldn’t and now I will probably be unable to answer any of your questions properly, but to my favor I would like to point out that the nuclear shell model is not yet truly accepted or clearly explained even by scientists. It is a strong theory, though, in my opinion, because with the even numbers it also

suggests SYMMETRY.


Back to Gamma Rays

Gamma usually comes with friends

Previous knowledge:After a nuclear interaction, when the nucleus of the reactant had undergone a beta or an alpha radiation, the nucleus still has excess energy. Instead of having another alpha or beta radiation, the nucleus gives out the excess energy in the form of gamma rays. So gamma rays frequently accompany natural decay reactions and particle reactions.


Why not another beta or alpha but gamma?

Why not giving off the excess energy directly

but in 2 steps, (beta/alpha + gamma) ?

The presence of gamma decay is favoured by the theory that energy is quantized in atomic level; that is: Energy is given off in discrete amounts called quanta. Instead of giving off a high amount of energy at once, it is more probable and easy for an excess energy to be given in steps. So, a nucleus, instead of giving off its whole excess energy at once by beta or alpha, gives some by beta or alpha and the rest by gamma. This way, to rearrange its particles after giving off energy is much easier.If the nucleus had given off all its energy at once, rearranging the nucleons would have been harder, nuclear orbits would be shuffled a lot as to cause hardships reorganizing. Plus, the kinetic energy of a beta particle, an antineutrino or an alpha particle may not be as high as for the nucleus to give off all its excess energy, an additional particle with high kinetic energy may be needed.


How can 512B decay to the ground state of 612C?

Right path- most probable

Left path –least probable

1 megaelectron volt = 1.60217646 × 10-13 Joules



In gamma decaya nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons).


Properties of Gamma Radiation

  • Gamma photons have no mass and no electrical charge-they are pure electromagnetic energy.

Properties of Gamma Radiation

  • Because of their high energy, gamma photons travel at the speed of light

Properties of Gamma Radiation

  • Their wavelength is short and frequency high showing they are really fast and of high energy.
  • The number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter atoms are the same chemical element.

Properties of Gamma Radiation

  • Highly concentrated gamma-rays can kill living cells

High-energy radiation kills cells by damaging their DNA, thus blocking their ability to grow and increase in number.


Properties of Gamma Radiation

  • In cancer treatments, focused gamma rays can be used to eliminate malignant cells, known as radiotherapy

Properties of Gamma Radiation

  • Needs a lead block or a thick concrete block to be stopped. (Lead has a high density and it is not radioactive.)

Properties of Gamma Radiation

  • Has weak ionizing property (no charge no mass)

Radiation that falls within the “ionizing radiation" range has enough energy to remove tightly bound electrons from atoms, thus creating ions.


An example:

An example of a nuclear interaction that results with gamma emission:

A gamma ray is released to lower the energy state of Thorium. As seen, the atomic and mass numbers of Thorium stays the same, only on the right side of the equation it is more stable.


Balancing a nuclear equation:


  • Total Nucleon Number (TOP VALUES) =Total number of protons and neutrons
  • Total electric charge (BOTTOM VALUES)
  • Are kept the same.

Type of radiation emitted & symbol

Nature of the radiation

(higher only)

Nuclear Symbol

(higher only)

Penetrating power, and what will block it (more dense material, more radiation is absorbed BUT smaller mass or charge of particle, more penetrating)

Ionising power - the ability to remove electrons from atoms to form positive ions


a helium nucleus of 2 protons and 2 neutrons, mass = 4, charge = +2

Low penetration, biggest mass and charge, stopped by a few cm of air or thin sheet of paper

Very high ionising power, the biggest mass and charge of the three radiation's, the biggest 'punch'!


high kinetic energy electrons, mass = 1/1850, charge = -1

Moderate penetration, 'middle' values of charge and mass, most stopped by a few mm of metals like aluminium

Moderate ionising power, with asmaller mass and charge than the alpha particle


very high frequency electromagnetic radiation, mass = 0, charge = 0

Very highly penetrating,smallest mass and charge, most stopped by a thick layer of steel or concrete, but even a few cm of dense lead doesn't stop all of it!

The lowest ionising power of the three, gamma radiation carries no electric charge and has virtually no mass, so not much of a 'punch' when colliding with an atom

Differences between

alpha, beta and gamma rays:


What have we learned?

Question 1:

What is the nature of a gamma ray?


Question 2:

What is the mass of a gamma ray? Compared to alpha and beta particles, therefore, is it more or less energetic?


Question 3:

What is needed to stop the penetration of a gamma ray?



What have we learned?

Question 4:

Is gamma ray a short or a long range force?


Question 5:

Does the parent nucleus change into a different element nucleus during gamma radiation?


Question 6:

Give the magnetic (electrical) difference between alpha, gamma and beta



What have we learned?

Question 7:

The sum of the values at the top and the bottom are the same in a radioactive decay reaction. What are these? The Atomic Number at the bottom and the Mass Number at the top?


Question 8:

What happens to the parent nucleus when it undergoes gamma decay?



What have we learned?

Question 9:

Tell a use of the gamma rays that we have learned


Question 10:

What is transmutation? Does it occur in an individual gamma decay?


Question 11:

The free nucleons or the nucleus consisting of the nucleons, which has greater mass? How does the force between nucleons in a nucleus arise?



What have we learned?

Question 12:

Is the strong nuclear force a long or a short range force? What about the electrical force? Tell how these two fight in a nucleus and say which overcomes.


Question 13:

Explain the theory of how the attraction force between two nucleons is attained in the nucleus.



Answer Key

Answer 1:

Gamma rays are high energy photons, they are chargeless pure energy



Answer Key

Answer 2:

Massless gamma rays have higher energy than alpha and beta. Alpha and Beta are particles with high kinetic energy, but gamma itself IS energy.



Answer Key

Answer 3:

A lead block or a thick concrete wall



Answer Key

Answer 4:

It is a very long range force, it is effective in great distances and has high penetrating ability



Answer Key

Answer 5:

No, the parent nucleus stays the same, it only gives off excess energy and becomes more stable.



Answer Key

Answer 6:

Alpha: positively charged (+)

Beta: negatively charged (-)

Gamma: Chargeless



Answer Key

Answer 7:

Nooo! Neither mass nor number of protons is necessarily conserved in a nuclear reaction. What is conserved is the nucleon number (top) and the electrical charge (down)



Answer Key

Answer 8:

A parent nucleus that undergoes gamma decay gives off the remaining excess energy after a nuclear reaction and jumps to a lower energy state,becoming more stable,hopefully entering the zone of stability. (nuclear shell model)



Answer Key

Answer 9:

Since gamma rays can kill living cells, we focus them to kill malignant tumour cells! (The enemy of my enemy is my friend)



Answer Key

Answer 10:

Transmutation is the changing of one element to another to become more stable through radioactivity. It can occur by alpha or beta radiation but NOT gamma radiation, since we have learned, hopefully, that gamma radiation is only related to energy and does not change the number of nucleons.



Answer Key

Answer 11:

The free nucleons have greater mass. The difference is the measure of the nuclear binding energy, nucleons forming a nucleus lose some of their mass into energy for binding.



Answer Key


Answer 12:

Strong nuclear force: Short ranged

Electrical force: Long ranged

Although Electrical force is effective in greater distances and the protons in the nucleus would tend to repel each other, the strong nuclear force is greater than electrical force. Once a nucleon passes into the region where SNF is effective, the 2 nucleons stick together, overcoming the repulsive force between (+)ly charged protons. Before passing into the effective SNF region, the closer the 2 nucleons get, the greater the repulsive force, however this fact can be eliminated since the particles move very fast and quaickly enter the SNF region.


Answer Key

Answer 13:

The nucleons are bound to each other by constant particle exchange, these very small particles are called mesons, and as long as meson exchange goes on, the attractive nuclear force goes on. As if you are playing volleyball with a friend and you are bound until one of you quits or the ball falls.


The End.....

gülce maşrabacı




More Resources,

Even More Resources


Unanswered Questions :(

  • How do neutrons help keep the nucleus together? What is the gluing effect of neutrons?
  • If the neutrons help keep nucleus together, why, in beta decay, a neutron is transformed into a proton?


  • Giancoli Physics Text Book
  • Zumdahl Chemistry Text Book
  • Student Presentations: /inspired by Negehan Demirci’s Nuclear Stability presentation.

More Resources


Even More Resources