Special Relativity Study Questions PHYS 252 Dr. Varriano

1) What comprises a nucleus of an atom? What is the approximate size of a nucleus? What determines the charge of a nucleus? What is an isotope? CLICK FOR ANSWER A nucleus contains protons and neutrons, both of which are referred to as nucleons. A nucleus is on the order of a few femtometers ( m). The charge of a nucleus is determined by the number of protons. charge Q = +Ze where Z is the atomic number (# of protons) and e is the fundamental charge (1.6x C) Isotopes are nuclei that have the same atomic number but different numbers of neutrons.

2) What force binds the nucleons together? Describe the characteristics of this force. CLICK FOR ANSWER Strong Nuclear Force attractive force between nucleons (n-n, n-p, p-p) short-ranged very strong (must overcome p-p repulsion)

3) Why do larger nuclei have more neutrons than protons? CLICK FOR ANSWER More neutrons are needed so that the strong nuclear force can overcome the proton-proton repulsion. 4) Why are all nuclei with more than 83 protons unstable? CLICK FOR ANSWER Adding more neutrons to a nucleus to help overcome proton-proton repulsion works up to Z = 83. After that, all nuclei are unstable. Some have long half-lives but they are still unstable and will decay eventually.

5) What is meant by the binding energy of a nucleus? How do you find the binding energy for a nucleus? CLICK FOR ANSWER

6) Describe the three methods of nuclear decay ( , ,  ). For each decay, state what happens to the following as the parent decays into the daughter: proton number, neutron number, mass number, nuclear charge. CLICK FOR ANSWER  2 p n bound together (He-4 nucleus)  - electron  + positron (antielectron)  gamma ray photon (sometimes emitted after  or β decay from excited daughter) what happensProton # Z Neutron # N Mass # A Charge Q  Big nucleus emits .  2  4  2e -- Neutron decays into proton, electron, and antineutrino. Electron and antineutrino ejected.  1  1 no change  1e ++ Proton changes into neutron, positron, and neutrino. Positron and neutrino ejected.  1  1 no change  1e  Gamma ray photon emitted from excited nucleus after alpha or beta decay. no change no change no change

7) How is the decay constant related to the half-life? Does a long half-life mean a fast or slow decay life-time? CLICK FOR ANSWER

8) Distinguish the following units associated with ionizing radiation: Roentgen, rad, rem. Which unit of radiation exposure best determines the potential health risk? CLICK FOR ANSWER NameDescriptionmks unitcommon unitconversion Exposureamount of ionizing radiation exposed to [C/kg][Roentgen = R][1 C/kg = 3876 R] Absorbed Doseamount of ionizing radiation absorbed by tissues [Gray = Gy][rad][1 Gy = 100 rads] Equivalent Dose absorbed dose times RBE factor [Sievert = Sv][rem][1 Sv = 100 rems] These are common units for the following quantities. The mks units of the quantities are also included. The equivalent dose and the duration of the dose are the most important quantities in determining health risk. Thus, the number of rems is most important.

9) Which is potentially more dangerous: 100 mrad of gamma rays or 100 mrad of alpha-particles? Why? CLICK FOR ANSWER The alpha particle absorbed dose is more dangerous because alpha particles can do more damage once they are inside the body. (They have a larger RBE factor.)

10) What is a typical natural background radiation dose equivalent for one year in the U.S.? CLICK FOR ANSWER Average annual natural background level ~ 310 mrem (2/3 from thoron and radon, 1/3 from other terrestrial sources, cosmic rays, internal isotopes) Average annual artificial background level ~ 310 mrem (mainly from medical procedures) Annual recommended limits beyond background: 100 mrem general public, 5000 mrem occupational ______________________________________________ from U.S. Nuclear Regulatory Commission

11) Do current nuclear power plants utilize fission or fusion? CLICK FOR ANSWER Fission (splitting of a large nucleus immediately after it absorbs a neutron)

12) What isotope is used as the fuel in a nuclear power plant? What is the role of the moderator in a nuclear reactor? What is the role of the control rods? CLICK FOR ANSWER U-235 The moderator slows down the neutrons released in the fissions. Slower-moving neutrons are much more likely to cause other fissions compared to more energetic neutrons. A control rod is comprised of material that absorbs neutrons. To keep the fission process from running away or from dying out, we want one neutron from an initial fission to cause another fission. (We want a “reproduction constant” of one.) Because there are typically 2 to 3 neutrons released per each fission, some of the neutrons in the reactor core must be absorbed by the control rods.

13) Why is Pu-239 used more than U-235 in nuclear bombs? CLICK FOR ANSWER Pu-239 has the following advantages over U-235. It has a greater probability of fission (higher fission cross-section). It will fission fairly easily with high-speed neutrons (unlike U-235). It can be made by having U-238 absorb a neutron in a reactor. It can then be separated out from the U-238 fairly easily and stockpiled. 14) What is a breeder reactor? CLICK FOR ANSWER This is U-235 reactor that has additional U-238 in it so that Pu-239 can be produced in larger quantities. The Pu-239 is separated out and stored.

15) What are the advantages of fusion over fission as an energy source? CLICK FOR ANSWER relatively inexpensive and plentiful fuel supply no weapons-grade fuel or by-poducts no long-lived radioactive by-products that must be stored