An- Najah National University Faculty of Graduate studies “ Isotops ” By Isra Murrar

1. An-Najah National University Faculty of Graduate studies “Isotops” By IsraMurrar

2. What is an isotope ? • Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. • The term isotope is formed from the Greek roots isos ("equal") and topos ("place"), meaning "the same place". Thus, different isotopes of a single element occupy the same position on the periodic table.

3. Isotope = line of equal Z; nuclides with the same # of protons (therefore they are the same element), but variable N; e.g. 12C, 13C, 14C are isotopes Isotone = line of equal N; nuclides with the same # of neutrons, but variable Z; e.g. 11B, 12C, 13N are isotones Isobar = line of equal mass; nuclides with the same mass number, but variable N and Z; e.g. 12C, 12B, 12Be are isobars

4. Nuclear Stability What makes a nucleus stable is something called its “Binding energy” Mass Defect: = Δm = M – (Σmp + Σmn + Σme) EB = Δm c2 Those nuclei with the largest binding energy per nucleon are the most stable.

5. Binding energy per nucleon as a function of mass number Note that the nucleons of intermediate mass tend to be the most stable.

6. In nuclear physics, properties of a nucleus depend on evenness or oddness of its atomic number Z, neutron number N and, consequently, of their sum, the mass number A. Most notably, oddness of both Z and N tends to lower the nuclear binding energy, making odd nuclei, generally, less stable. • This remarkable difference of nuclear binding energy between neighbouring nuclei, has important consequences: unstable isotopes decay by beta decay (including positron decay), electron capture or alpha decay.

7. The proton:neutron ratio is another factor affecting nuclear stability

8. The plot indicates that lighter nuclides are most stable when the neutron/proton ratio is 1/1. This is the case with any nucleus that has up to 20 protons. As the atomic number increases beyond 20, a different trend becomes apparent. In this range, it appears that a stable nucleus is able to accommodate more neutrons. Stable isotopes have a higher neutron to proton ratio, rising to 1.5/1 for elements having atomic numbers between 20 and 83.

9. Types of isotops • Stable isotops Stable isotopes are those isotopes that do not undergo radioactive decay; thus, their nuclei are stable and their masses remain the same. However, they may themselves be the product of the decay of radioactive isotopes.

10. Radioactive Radioactive isotopes are nuclides that have unstable nuclei that decay, emitting alpha, beta, and sometimes gamma rays. Such isotopes eventually reach stability in the form of nonradioactive isotopes of other chemical elements. Decay of a radionuclide to a stable daughter is a function of time measured in units of half-lives.

11. Types of isotopes by origin 1) Long-lived radioactive nuclides Some radioactive nuclides that have very long half lives were created during the formation of the solar system (~4.6 billion years ago) and are still present in the earth. These include 40K (t½ = 1.28 billion years), 87Rb (t½ = 48.8 billion years), 238U (t½ = 447 billion years), and 186Os (t½ = 2 x 106 billion years, or 2 million billion years).

12. 2) Cosmogenic Cosmogenic isotopes are a result of cosmic ray activity in the atmosphere. Cosmic rays are atomic particles that are ejected from stars at a rate of speed sufficient to shatter other atoms when they collide. This process of transformation is called spallation. Some of the resulting fragments produced are unstable atoms having a different atomic structure (and atomic number), and so are isotopes of another element. The resulting atoms are considered to have cosmogenic radioactivity. Cosmogenic isotopes are also produced at the surface of the earth by direct cosmic ray irradiation of atoms in solid geologic materials. Examples of cosmogenic nuclides include 14C, 36Cl, 3H, 32Si, and 10Be. Cosmogenic nuclides, since they are produced in the atmosphere or on the surface of the earth and have relatively short half-lives (10 to 30,000 years), are often used for age dating of waters.

13. 3) Anthropogenic Anthropogenic isotopes result from human activities, such as the processing of nuclear fuels, reactor accidents, and nuclear weapons testing. Such testing in the 1950s and 1960s greatly increased the amounts of tritium (3H) and 14C in the atmosphere; tracking these isotopes in the deep ocean, for instance, allows oceanographers to study ocean flow, currents, and rates of sedimentation.

14. 4) Radiogenic Radiogenic isotopes are typically stable daughter isotopes produced from radioactive decay. In the geosciences, radiogenic isotopes help to determine the nature and timing of geological events and processes. Isotopic systems useful in this research are primarily K-Ar, Rb-Sr, Re-Os, Sm-Nd, U-Th-Pb, and the noble gases (4H, 3H-3He, 40Ar).

15. Types of radioactive decay (return to top) 1) alpha (α) decay results from an excess of mass. In this type of decay, alpha particles (consisting of two protons and two neutrons) are emitted from the nucleus. Both the atomic number and neutron number of the daughter are reduced by two, so the mass number decreases by four. An example is the decay of 238U: 2) ß+ - or "positron decay" results from an excess of protons. In this type of decay, a positively charged beta particle and a neutrino are emitted from the nucleus. The atomic number decreases by one and the neutron number is increased by one. An example is the decay of radioactive 18F to stable 18O: where ß+ is the positron, v is the neutrino, and Q is the total energy given off.

16. 3) ß- - decay results from an excess of neutrons. In this type of decay, a negatively charged beta particle and a neutrino are emitted from the nucleus. The atomic number increases by one and the neutron number is reduced by one. An example is the decay of radioactive 14C to stable 14N: where ß- is the beta particle, v is the antineutrino, and Q is the end point energy (0.156 MeV). 4) electron capture also results from an excess of protons. In this type of decay, an electron is spontaneously incorporated into the nucleus and a neutrino is emitted from the nucleus. The atomic number decreases by one and the neutron number increases by one. Electron capture may be followed by the emission of a gamma ray. An example is the decay of 123I to 123Te:

17. Modern Uses of Radioactive Isotopes Smoke Detectors and Americium-241 What most consumers don't know is that many of these units contain a small amount of americium-241. By utilizing the radioactive properties of this material, smoke from a fire can be detected at a very early stage. This early warning capability has saved many lives.

18. Agricultural Applications - radioactive tracers Radioisotopes can be used to help understand chemical and biological processes in plants. This is true because radioactive forms of the element can be easily detected with a Geiger counter or other such device. Example: A solution of phosphate, containing radioactive phosphorus-32, is injected into the root system of a plant. Since phosphorus-32 behaves identically to that of phosphorus-31, the more common and non-radioactive form of the element, it is used by the plant in the same way. A Geiger counter is then used to detect the movement of the radioactive phosphorus-32 throughout the plant. This information helps scientists understand the detailed mechanism of how plants utilized phosphorus to grow and reproduce.

19. Food Irradiation Food irradiation is a method of treating food in order to make it safer to eat. This process is not very different from other treatments such as freezing and drying. The end result is that the growth of disease-causing microorganismns or those that cause spoilage are slowed or are eliminated altogether. This makes food safer and also keeps it fresh longer.

20. Food irradiated by exposing it to the gamma rays of a radioisotope -- one that is widely used is cobalt-60. The energy from the gamma ray passing through the food is enough to destroy many disease-causing bacteria as well as those that cause food to spoil, but is not strong enough to change the quality, flavor or texture of the food.

21. Archaeological Dating Significant progress has been made in this field of study since the discovery of radioactivity and its properties. One application is carbon-14 dating. Recalling that all biologic organisms contain a given concentration of carbon-14, we can use this information to help solve questions about when the organism died. when an organism dies it has a specific ratio by mass of carbon-14 to carbon-12 incorporated in the cells of it's body. At the moment of death, no new carbon-14 containing molecules are metabolized, therefore the ratio is at maximum. After death, the carbon-14 to carbon-12 ratio begins to decrease because carbon-14 is decaying away at a constant and predictable rate. Remembering that the half-life of carbon-14 is 5700 years, then after 5700 years half as much carbon-14 remains within the organism.

22. Example : If an organism such as a tree contained 1 gram of carbon-14 while it was living, then after 5700 years it would contain half that amount, or 0.5 grams of carbon-14. • This method of dating using carbon-14 is only good for organisms or artifacts that are biological by nature and on the order of tens of thousands or years old.

23. Medical Uses • Bone imaging is an extremely important use of radioactive properties. Supposed a runner is experiencing severe pain in both shins. The doctor decides to check to see if either tibia has a stress fracture. The runner is given an injection containing 99Tcm. This radioisotope is a gamma ray producer with a half-life of 6 hours. • After a several hour wait, the patient undergoes bone imaging. At this point, any area of the body that is undergoing unusually high bone growth will show up as a stronger image on the screen. Therefore if the runner has a stress fracture, it will show up on the bone imaging scan. • This technique is also good for arthritic patients, bone abnormalities and various other diagnostics.