Radiation & Radioactivity and Half life

1. Radiation & Radioactivity and Half life

2. …a little bit of history… …1808 …1897 …1924 Solid Sphere Model orBilliard Ball Modelproposed by John Dalton Plum Pudding Model orRaisin Bun Modelproposed by J.J. Thomson …1913 …1909 Electron Cloud Model orQuantum Mechanical Modelproposed by Louis de Broglie & Erwin Schrodinger Planetary Model orNuclear Modelproposed by E. Rutherford Bohr Model orOrbit Modelproposed by Neils Bohr

3. The Bohr’s atomic model consists of a central nucleus composed of neutrons and protons, which is surrounded by electrons which “orbit” around the nucleus. By means of Quantum Mechanical Model, proposed by Louis de Broglie and Erwin Schrodinger, the Electron Cloud has beenpostulated. • Protons carry a positive charge, Neutrons are electrically “neutral”, Electrons carry a negative charge. Atoms in nature are electrically neutral so the number of electrons orbiting the nucleus equals the number of protons in the nucleus. • Without neutrons, the nucleus would split apart because the positive protons would repel each other. Elements can have nucleii with different numbers of neutrons in them. For example hydrogen, which normally only has one proton in the nucleus, can have a neutron added to its nucleus to from deuterium, or have two neutrons added to create tritium, which is radioactive. Atoms of the same element which vary in neutron number are called isotopes.

5. Radiation • is energy in transit in the form of high speed particles and electromagnetic waves. • Ionizing radiation • is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from their orbits, causing the atom to become charged or ionized (examples: gamma rays, neutrons) • Non-ionizing radiation • is radiation without enough energy to to separate molecules or remove electrons from atoms. Examples are visible light, radio and television waves, ultra violet (UV), and microwaves with a large spectrum of energies.

6. Energy Scale The energy scale used by most nuclear scientists is electron volts (eV), thousands of electron volts (keV), and millions of electron volts (MeV). An electron volt is the energy acquired when an electron falls through a potential difference of 1 volt. 1 eV=1.602*1012ergs. Masses are also given by their "mass-equivalent" energy (E=mc2). The mass of the proton is 938.27231 MeV. E=mc2 Where: e is energy, m is mass, and c is the speed of light. Einstein's famous equation describes how energy and mass are related. In our animated decays, mass is lost. That mass is converted into energy in the form of electromagnetic waves. Because the speed of light is so great, a little matter can transform into large amount of energy.

7. Common Types of Radiation Alphas An alpha is a particle emitted from the nucleus of an atom, that contains 2 protons and 2 neutrons. It is identical to the nucleus of a Helium atom, without the electrons. Betas A beta is a high speed particle, identical to an electron, that is emitted from the nucleus of an atom Gamma Rays Gamma rays are electromagnetic waves / photons emitted from the nucleus (center) of an atom. X rays X Rays are electromagnetic waves / photons emitted not from the nucleus, but normally emitted by energy changes in electrons. These energy changes are either in electron orbital shells that surround an atom or in the process of slowing down such as in an X-ray machine. Neutrons Neutrons are neutral particles that are normally contained in the nucleus of all atoms and may be removed by various interactions or processes like collision and fission

8. Alpha decay is a radioactive process in which a particle with two neutrons and two protons is ejected from the nucleus of a radioactive atom. The particle is identical to the nucleus of a helium atom. Alpha decay only occurs in very heavy elements such as uranium, thorium and radium. The nuclei of these atoms are very “neutron rich” (i.e. have a lot more neutrons in their nucleus than they do protons) which makes emission of the alpha particle possible. After an atom ejects an alpha particle, a new parent atom is formed which has two less neutrons and two less protons. Thus, when uranium-238 (which has a Z of 92) decays by alpha emission, thorium-234 is created (which has a Z of 90). Because alpha particles contain two protons, they have a positive charge of two. Further, alpha particles are very heavy and very energetic compared to other common types of radiation. Typical alpha particles will travel no more than a few centimeters in air and are stopped by a sheet of paper.

9. Beta decay is a radioactive process in which an electron is emitted from the nucleus of a radioactive atom, along with an unusual particle called an antineutrino (almost massless particle that carries away some of the energy). Like alpha decay, beta decay occurs in isotopes which are “neutron rich” . When a nucleus ejects a beta particle, one of the neutrons in the nucleus is transformed into a proton. Since the number of protons in the nucleus has changed, a new daughter atom is formed which has one less neutron but one more proton than the parent. For example, when rhenium-187 decays (which has a Z of 75) by beta decay, osmium-187 is created (which has a Z of 76). Beta particles have a single negative charge and weigh only a small fraction of a neutron or proton. As a result, beta particles interact less readily with material than alpha particles. Beta particles will travel up toseveral meters in air, and are stopped by thin layers of metal or plastic.

10. Gamma 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. Rather than emitting another beta or alpha particle, this energy is lost by emitting a pulse of electromagnetic radiation called a gamma ray. The gamma ray is identical in nature to light or microwaves, but of very high energy. Like all forms of electromagnetic radiation, the gamma ray has no mass and no charge. Gamma rays interact with material by colliding with the electrons in the shells of atoms. They lose their energy slowly in material, being able to travel significant distances before stopping. Depending on their initial energy, gamma rays can travel from 1 to hundreds of meters in air and can easily go right through people. It is important to note that most alpha and beta emitters also emit gamma rays as part of their decay process. However, there is no such thing as a “pure” gamma emitter.

11. Over a century ago in 1895, Roentgen discovered the first example of ionizing radiation, x-rays. Device: a glass envelope under high vacuum, with a wire element at one end forming the cathode, and a heavy copper target at the other end forming the anode. When a high voltage was applied to the electrodes, electrons formed at the cathode would be pulled towards the anode and strike the copper with very high energy. Roentgen discovered that very penetrating radiations were produced from the anode, which he called x-rays. X-ray production whenever electrons of high energy strike a heavy metal target, like tungsten or copper. When electrons hit this material, some of the electrons will approach the nucleus of the metal atoms where they are deflected because of there opposite charges (electrons are negative and the nucleus is positive, so the electrons are attracted to the nucleus). This deflection causes the energy of the electron to decrease, and this decrease in energy then results in forming an x-ray.

12. Making X-rays Where do x-rays come from? An x-ray machine, like that used in a doctor's or a dentist's office, is really very simple. Inside the machine is an x-ray tube. An electron gun inside the tube shoots high energy electrons at a target made of heavy atoms, such as tungsten. X-rays come out because of atomic processes induced by the energetic electrons shot at the target.

13. Properties of Radiation Alpha particles are heavy and doubly charged which cause them to lose their energy very quickly in matter. They can be shielded by a sheet of paper or the surface layer of our skin. Alpha particles are considered hazardousonlyto a persons health if an alpha emitting material is inhaled. Beta particles are much smaller and only have one charge, which cause them to interact more slowly with material. They are effectively shielded by thin layers of metal or plastic and are again considered hazardousonlyif a beta emitter is ingested or inhaled. Gammaemitters are associated with alpha, beta, and positron decay. X-Rays are produced either when electrons change orbits within an atom, or electrons from an external source are deflected around the nucleus of an atom. Both are forms of high energy electromagnetic radiation which interact lightly with matter. X-rays and gamma rays are best shielded by thick layers of lead or other dense material and are hazardous to people when they are external to the body. Neutrons are neutral particles with approximately the same mass as a proton. Because they are neutral they react only weakly with material. They are an external hazard best shielded by thick layers of concrete.

14. Rate of Decay • Beyond knowing the types of particles which are emittedwhen an isotope decays, we also are interested in how frequentlyone of the atoms emits this radiation. • A very important point here is that we cannot predict when aparticular entity will decay. • We do know though, that if we had a large sample of a radioactive substance, some number will decay after a given amount of time. • Some radioactive substances have a very high “rate of decay”,while others have a very low decay rate. • To differentiate different radioactive substances, we look toquantify this idea of “decay rate”

15. #atomsremaining % of atomsremaining Time Half-Life • The “half-life” (h) is the time it takes for half the atoms of a radioactive substance to decay. • For example, suppose we had 20,000 atoms of a radioactive substance. If the half-life is 1 hour, how many atoms of that substance would be left after: 1 hour (one lifetime) ? 10,000 (50%) 2 hours (two lifetimes) ? 5,000 (25%) 3 hours (three lifetimes) ? 2,500 (12.5%)

16. Lifetime (t) The “lifetime” of a particle is an alternate definition ofthe rate of decay, one which we prefer. It is just another way of expressing how fast the substancedecays.. It is simply: 1.44 x h, and one often associates the letter “t” to it.The lifetime of a “free” neutron is 14.7 minutes {t (neutron)=14.7 min.} Let’s use this a bit to become comfortable with it…

17. Lifetime (I) • The lifetime of a free neutron is 14.7 minutes. • If I had 1000 free neutrons in a box, after 14.7 minutes some number of them will have decayed. • The number remaining after some time is given by the radioactive decay law N0 = starting number of particlest = particle’s lifetime This is the “exponential”. It’s value is 2.718, and is a very usefulnumber. Can you find it on yourcalculator?

18. Lifetime (II) Note by slight rearrangement of this formula: Fraction of particles which did not decay: N / N0 = e-t/t After 4-5 lifetimes, almost all of the unstable particles have decayed away!

19. Lifetime (III) • Not all particles have the same lifetime. • Uranium-238 has a lifetime of about 6 billion (6x109) years ! • Some subatomic particles have lifetimes that are less than 1x10-12 sec ! • Given a batch of unstable particles, we cannotsay which one will decay. • The process of decay is statistical. That is, we can only talk about either, 1) the lifetime of a radioactive substance*, or 2) the “probability” that a given particle will decay.

20. Lifetime (IV) • Given a batch of 1 species of particles, some will decay within 1 lifetime (1t), some within 2t, some within 3t, and so on… • We CANNOT say “Particle 44 will decay at t =22 min”. You just can’t ! • All we can say is that: • After 1lifetime, there will be (37%) remaining • After 2 lifetimes, there will be (14%) remaining • After 3 lifetimes, there will be (5%) remaining • After 4 lifetimes, there will be (2%) remaining, etc

21. Lifetime (V) • If the particle’s lifetime is very short, the particles decay away very quickly. • When we get to subatomic particles, the lifetimesare typically only a small fraction of a second! • If the lifetime is long (like 238U) it will hang around for a very long time!

22. Lifetime (IV) What if we only have 1 particle before us? What can we sayabout it? Survival Probability =N / N0 = e-t/t Decay Probability = 1.0 – (Survival Probability)

23. Half-life is the time required for the quantity of a radioactive material to be reduced to one-half its original value. All radionuclides have a particular half-life, some of which a very long, while other are extremely short. For example, uranium-238 has such a long half life, 4.5x109 years, that only a small fraction has decayed since the earth was formed. In contrast, carbon-11 has a half-life of only 20 minutes. Since this nuclide has medical applications, it has to be created where it is being used so that enough will be present to conduct medical studies.

24. When given a certain amount of radioactive material, it is customary to refer to the quantity based on its activity rather than its mass. The activity is simply the number of disintegrations or transformations the quantity of material undergoes in a given period of time. The two most common units of activity are the Curie and the Becquerel. The Curie is named after Pierre Curie for his and his wife Marie's discovery of radium. One Curie is equal to 3.7x1010 disintegrations per second. A newer unit of activity if the Becquerel named for Henry Becquerel who is credited with the discovery of radioactivity. One Becquerel is equal to one disintegration per second. It is obvious that the Curie is a very large amount of activity and the Becquerel is a very small amount. To make discussion of common amounts of radioactivity more convenient, we often talk in terms of milli and microCuries or kilo and MegaBecquerels.

25. Common Radiation Units – SI • Gray (Gy) - to measure absorbed dose ... the amount of energy actually absorbed in some material, and is used for any type of radiation and any material (does not't describe the biological effects of the different radiations) • Gy = J / kg (one joule of energy deposited in one kg of a material) • Sievert (Sv) - to derive equivalent dose ... the absorbed dose in human tissue to the effective biological damage of the radiation • Sv = Gy x Q (Q = quality factor unique to the type of incident radiation) • Becquerel (Bq) - to measure a radioactivity … the quantity of a radioactive material that have 1 transformations /1s • Bq = one transformation per second, there are 3.7 x 1010 Bq in one curie. • __________________________________________________________________________________ • Roentgen (R) - to measure exposure but only to describe for gamma and X-rays, and only in air. • R = depositing in dry air enough energy to cause 2.58E-4 coulombs per kg • Rad (radiation absorbed dose) - to measure absorbed dose • Rem (roentgen equivalent man) - to derive equivalent dose related the absorbed dose in human tissue to the effective biological damage of the radiation. • Curie (Ci) - to measure radioactivity. One curie is that quantity of a radioactive material that will have 37,000,000,000 transformations in one second. 3.7 x 1010 Bq

26. Terms Related to Radiation Dose Chronic dose … means a person received a radiation dose over a long period of time. Acute dose … means a person received a radiation dose over a short period of time. Somatic effects … are effects from some agent, like radiation that are seen in the individual who receives the agent. Genetic effects … are effects from some agent, that are seen in the offspring of the individual who received the agent. The agent must be encountered pre-conception. Teratogenic effects … are effects from some agent, that are seen in the offspring of the individual who received the agent. The agent must be encountered during the gestation period. Stochastic effects … are effects that occur on a random basis with its effect being independent of the size of dose. The effect typically has no threshold and is based on probabilities, with the chances of seeing the effect increasing with dose. Cancer is a stochastic effect. Non-stochastic effect … are effects that can be related directly to the dose received. The effect is more severe with a higher dose, i.e., the burn gets worse as dose increases. It typically has a threshold, below which the effect will not occur. A skin burnfrom radiation is a non-stochastic effect.

27. Thank you for your attention