2. Cobalt Cobalt - metal - may be stable (nonradioactive, as found in nature), or unstable (radioactive, man-made). Most common radioactive isotope is cobalt-60. Hard, brittle, gray metal with a bluish tint. Solid under normal conditions and is generally similar to iron and nickel in its properties. In particular, cobalt can be magnetized similar to iron.

3. 60Co is radioactive, as its nucleus is not stable. One of its neutrons converts to a proton, electron, and anti-neutrino. Anti-neutrinos don't do anything, and we can't detect them without special equipment. The electron picks up the disintegration energy and races out of the 60Co atom at hundreds of millions of miles per hour.

4. The electron is followed by two particles of radiation called gamma rays. The nucleus of the cobalt-60 atom has converted to a nucleus with 28 p and 32 n. It is then a stable nucleus of nickel- 60. This type of decay is called beta-minus decay.

5. The beta decay energy is 2.824 MeV, and gamma rays are produced at 1,173,210 and 1,332,470 eV energies with nearly 100% frequency of occurrence. The maximum electron emission energy is 315 keV. The cobalt-60 nucleus has a half-life of 5.271 years.

6. There are many potential or actual uses of 60Co, some of which are controversial: As a tracer for cobalt in chemical reactions; As a radioactive source for food irradiation; As a radioactive source for laboratory use.

7. Gamma irradiator Note the lead shielding

8. N o natural 60Co in the world today. Nucleosynthesis is the process in which the array of elements on the periodic table were made. These elements were made by a variety of fusion processes as well as radioactive decay, etc. One relevant to the current discussion is the s-process.

9. The s-process occurs in old, massive stars whose cores are full of iron, 56Fe. Due to the high temperature (billions of degrees), many of the nuclei in these stars break up, releasing neutrons. As these n’s collide with 56Fe, they can be absorbed, increasing the nuclear mass.  decay maintains a balance of p’s and n’s as the nucleus grows, giving the elements on the periodic table from Co (Z=27) to Bi (Z=83).

10. Cobalt-60: one of the first steps on the path of the s-process; its decay is directly responsible for the creation of much of the nickel in the world, as well as heavier elements derived from nickel. If this neutron absorption did not occur, we would have far fewer of these heavier elements.

11. Copper (Z=29) is used for cheap, high-conductivity wire. Molybdenum (Z=42) is an essential component of an enzyme used by some bacteria to "fix" nitrogen, that is, to produce a natural fertilizer in terrestrial soil.

12. Iodine (Z=53) is an essential nutrient for human beings. Gold (Z=79) has always been considered a precious metal Mercury (Z=80) is the only metal liquid at room temperature that does not explode on contact with water. It is used in low-friction bearings, electrical relays, thermometers, and barometers.

13. So the world would be different without the derivatives of 60Co produced in the cores of massive stars -- even though there is not a trace of natural 60Co on Earth.

14. We produce 60Co in the same way it is produced in stars: by bombarding a lighter nucleus with neutrons. The best nucleus to use is cobalt-59 ( naturally occurring) 59Co + n ---> 60Co +  (Q = 7.492 MeV)

15. The gamma ray  is not of concern to us because it escapes from the cobalt sample. The 60Co is left behind in the cobalt sample and therefore the cobalt slowly becomes radioactive due to buildup of 60Co.

16. Need a radioactive source, 252Cf (californium-252), which emits neutrons as part of its decay. 252Cf: made in a nuclear reactor, and has a t 1/2 of 2.645 y.

17. There are two decay modes. One is the standard decay chain: 252Cf --> 248Cm +  248Cm --> 244Pu + 

18. The resulting isotope, plutonium- 244, is not stable but takes millions of years to decay so there is essentially no emission from it. You will notice that no neutrons are produced in this process, only alphas. Neutrons are produced by other decay modes of 252Cf; for example, 3% of the 252Cf decays are by spontaneous fission.

19. One example of spontaneous fission would be: 252Cf --> 94Sr + 154Nd + 4n While many variants on this reaction occur, the general idea is clear: the californium nucleus splits ("fissions") into two smaller ones, releasing neutrons (n). An average of 3.8 neutrons is emitted per fission.

20. A typical source that is used emits 8.7 million neutrons per second, which is sufficient to produce a detectable amount of 60Co in about 10 g 59Co.

21. The neutrons produced by californium fission are fast: about 3,000 to 30,000 miles/s (5,000-50,000 km/s). C0-59 will only absorb neutrons which are slower - have to slow them to ~ 10 km/sec (or 6 mi/sec) to get the neutrons to be captured into cobalt-59.

22. So neutron source is immersed in a large tank of water. H 2 O acts as a moderator to slow down neutrons. The Co sits in the water next to the Cf and soaks up the slowed neutrons. (Note: the water tank also serves the purpose of protecting the experimenter from the radiation produced by the Cf.)

23. There are two categories of neutrons which will activate cobalt: the thermal neutrons, with kinetic energies of 0.01-0.1 eV, and the neutrons with kinetic energy ~ 130 eV which undergo resonant absorption.

24. Resonant absorption:Think of the neutron as a wave (in quantum mechanics particles are waves) and imagine the neutron reflecting off different parts of the target 59Co nucleus and adding constructively on itself to increase the energy in the wave. It turnsout nearly all of the activation is from the thermal neutrons, where the absorption cross section is ~ 40 barns.

25. With a NaI or Ge detector you can detect 60Co because its gamma ray emissions at 1.17 and 1.33 MeV are distinctive. The 1461 keV peak in the background is from natural 40K; the 1173 and 1332 keV peaks are actually 60Co.