Gamma Radiation and friends

1. Gamma Radiation and friends Gülce Maşrabacı

2. 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.

3. 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. R R A

4. 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.

5. 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.

6. 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.

7. The strong nuclear force Beyond barrier: SNF present

8. 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.

9. 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.

10. 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.

11. 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?

12. 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.

13. 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.

14. 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:

16. Attempts to become stable: Transmutation 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)

17. Transmutation

18. 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.

19. 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.

20. 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)

22. 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.

23. 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:

24. The part where I can not explain Previously on this slide show: (***Talking about energy levels in nucleus***) (...)this can be explained by: THE NUCLEAR SHELL MODEL 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.

25. 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.

26. 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.

27. 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)

28. Warning! 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.

29. 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.

30. 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.

31. Excited nucleus ground state nucleus +  00

32. 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

33. Gamma Decay of He-3

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

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

37. Properties of Gamma Radiation • Because of their high energy, gamma photons travel at the speed of light

38. 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.

39. 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.

40. Properties of Gamma Radiation • In cancer treatments, focused gamma rays can be used to eliminate malignant cells, known as radiotherapy

41. 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.)

42. 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.

43. 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.

44. Balancing a nuclear equation: Protactinium • Total Nucleon Number (TOP VALUES) =Total number of protons and neutrons • Total electric charge (BOTTOM VALUES) • Are kept the same.

45. 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 Alpha 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'! Beta 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 Gamma 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:

46. Penetration of the 3 radiations

47. Again Penetration

50. What have we learned? Question 1: What is the nature of a gamma ray? Answer Question 2: What is the mass of a gamma ray? Compared to alpha and beta particles, therefore, is it more or less energetic? Answer Question 3: What is needed to stop the penetration of a gamma ray? Answer