General Physics (PHY 2140)

1. General Physics (PHY 2140) Lecture 19 • Modern Physics • Nuclear Physics • Nuclear Reactions • Medical Applications • Radiation Detectors Chapter 29 http://www.physics.wayne.edu/~alan/2140Website/Main.htm

2. Lightning Review • Last lecture: • Nuclear physics • Nuclear properties • Binding energy • Radioactivity • The Decay Process • Natural Radioactivity Nuclear density ~ 2.3 x 1017 Kg/m3 Review Problem: An alpha particle has twice the charge of a beta particle. Why does the former deflect less than the latter when passing between electrically charged plates, assuming that both have the same speed? Mass: The alpha particle is 7344 times as massive as the beta particle. Recall:

3. The activity of a newly discovered radioactive isotope reduces to 96% of its original value in an interval of 2 hours. What is its half-life? (a) 10.2 h (b) 34.0 h (c) 44.0 h (d) 68.6 h QUICK QUIZ 29.2 Radioactive Decay:

4. (b). If the original activity is R0=N0, the activity remaining after an elapsed time t is R = R0e-λt = R0e-(0.693/T1/2)t . Solving for the half-life yieldsT1/2 = [-0.693/ln(R/R0)]t. If R = 0.96R0 at t = 2.0 hr, the half-life is:T1/2 = [-0.693/ln(0.96)](2.0 h) = 34 h. QUICK QUIZ 29.2 ANSWER

5. 29.6 Nuclear Reactions • Structure of nuclei can be changed by bombarding them with energetic particles • The changes are called nuclear reactions • As with nuclear decays, the atomic numbers and mass numbers must balance on both sides of the equation

6. Which of the following are possible reactions? Problem A= -17, Z=0 (a) and (b). Reactions (a) and (b) both conserve total charge and total mass number as required. Reaction (c) violates conservation of mass number with the sum of the mass numbers being 240 before reaction and being only 223 after reaction.

7. Q Values • Energy must also be conserved in nuclear reactions • The energy required to balance a nuclear reaction is called the Q value of the reaction • An exothermic reaction • There is a mass “loss” in the reaction • There is a release of energy • Q is positive • An endothermic reaction • There is a “gain” of mass in the reaction • Energy is needed, in the form of kinetic energy of the incoming particles • Q is negative

8. Problem: nuclear reactions Determine the product of the reaction What is the Q value of the reaction?

9. Determine the product of the reaction What is the Q value of the reaction? In order to balance the reaction, the total amount of nucleons (sum of A-numbers) must be the same on both sides. Same for the Z-number. Given: reaction Find: Q = ? Number of nucleons (A): Number of protons (Z): Thus, it is B, i.e. The Q-value is then Endothermic  Need to put in energy = >2.79 MeV

10. Threshold Energy in Endothermic Reactions • To conserve both momentum and energy, incoming particles must have a minimum amount of kinetic energy, called the threshold energy • m is the mass of the incoming particle • M is the mass of the target particle • If the energy is less than this amount, the endothermic reaction cannot occur

11. If the Q value of an endothermic reaction is -2.79 MeV, the minimum kinetic energy needed in the reactant nuclei if the reaction is to occur must be (a) equal to 2.79 MeV, (b) greater than 2.79 MeV, (c) less than 2.79 MeV, or (d) precisely half of 2.79 MeV. QUICK QUIZ (b). In an endothermic reaction, the threshold energy exceeds the magnitude of the Q value by a factor of (1+ m/M), where m is the mass of the incident particle and M is the mass of the target nucleus.

12. Radiation Damage in Matter • Radiation absorbed by matter can cause damage • The degree and type of damage depend on many factors • Type and energy of the radiation • Properties of the absorbing matter • Radiation damage in biological organisms is primarily due to ionization effects in cells • Ionization disrupts the normal functioning of the cell

13. Types of Damage • Somatic damage is radiation damage to any cells except reproductive ones • Can lead to cancer at high radiation levels • Can seriously alter the functional characteristics of specific organisms • Genetic damage affects only reproductive cells • Can lead to defective offspring

14. Units of Radiation Exposure • Roentgen [R] is defined as • That amount of ionizing radiation that will produce 2.08 x 109 ion pairs in 1 cm3 of air under standard conditions • That amount of radiation that deposits 8.76 x 10-3 J of energy into 1 kg of air • Rad (Radiation Absorbed Dose) • That amount of radiation that deposits 10-2 J of energy into 1 kg of air

15. More Units • RBE (Relative Biological Effectiveness) • The number of rad of x-radiation or gamma radiation that produces the same biological damage as 1 rad of the radiation being used • Accounts for type of particle which the rad itself does not • Rem (Roentgen Equivalent in Man) • Defined as the product of the dose in RAD and the RBE factor • Dose in REM = dos in RAD X RBE

16. RBE for several types of Radiation Radiation RBE factor

17. Radiation Levels • Natural sources – rocks and soil, cosmic rays • Background radiation • About 0.13 rem/yr • Upper limit suggested by US government • 0.50 rem/yr • Excludes background and medical exposures • Occupational • 5 rem/yr for whole-body radiation • Certain body parts can withstand higher levels • Ingestion or inhalation is most dangerous (Ingested, 1 mCi 90Sr can yield 1000 rem dose! ) LD50 = 400-500 rem whole body

18. Applications of Radiation • Sterilization • Radiation has been used to sterilize medical equipment • Used to destroy bacteria, worms and insects in food • Bone, cartilage, and skin used in graphs is often irradiated before grafting

19. Applications of Radiation, cont • Tracing • Radioactive particles can be used to trace chemicals participating in various reactions • Example, 131I to test thyroid action • CAT scans • Computed Axial Tomography • Produces pictures with greater clarity and detail than traditional x-rays

20. Other Medical Uses • Radionuclide Imaging • Imaging the distribution of radioactively labeled substances in the body • Recent improvement is to use computed tomography techniques • PET scanning (Positron Emission Tomography) • Studying retention, turnover or clearance rates of various substances in the body – labeled vitamins, thyroid uptake and others

21. Therapeutic Applications of Radiation • Cancer Treatment • Various types of methods to get ionizing radiation to cancer cells • External beam – Cobalt-60 or linear accelerators produce gamma rays (photons) in the range of a few MeV to tens of MeV • Brachytherapy – radioactive seeds such as 125I and 103Pd ( photons in the keV range) to 137Cs and 192Ir (< 1MeV) are placed in close proximity to the cancer cells • Proton and ion beams – treatment of Ocular melanoma, radiosurgical procedures, brain metastais, Parkinson’s and others. • Neutron beams

22. Image of Brachytherapy seeds in place

23. More Applications of Radiation • MRI • Magnetic Resonance Imaging • When a nucleus having a magnetic moment is placed in an external magnetic field, its moment processes about the magnetic field with a frequency that is proportional to the field • Weak oscillating field applied perpendicular to DC field • Transitions between energy states can be detected electronically

24. Radiation Detectors • A Geiger counter is the most common form of device used to detect radiation • It uses the ionization of a medium as the detection process • When a gamma ray or particle enters the thin window, the gas is ionized • The released electrons trigger a current pulse • The current is detected and triggers a counter or speaker

25. Detectors, 2 • Semiconductor Diode Detector • A reverse biased p-n junction • As a particle passes through the junction, a brief pulse of current is created and measured • Can be used to measure particle energy • Scintillation counter • Uses a solid or liquid material whose atoms are easily excited by radiation • The excited atoms emit visible radiation as they return to their ground state • With a photomultiplier, the photons can be converted into an electrical signal • Can also be used to measure particle energy

26. Detectors, 3 • Track detectors • Various devices used to view the tracks or paths of charged particles • Photographic emulsion • Simplest track detector • Charged particles ionize the emulsion layer • When the emulsion is developed, the track becomes visible • Cloud chamber • Contains a gas cooled to just below its condensation level • The ions serve as centers for condensation • Particles ionize the gas along their path • Track can be viewed and photographed

27. Detectors, 4 • Track detectors, cont • Bubble Chamber • Contains a liquid near its boiling point • Ions produced by incoming particles leave tracks of bubbles • The tracks can be photographed • Wire Chamber • Contains thousands of closely spaced parallel wires • The wires collect electrons created by the passing ionizing particle • A second grid allows the position of the particle to be determined • Can provide electronic readout to a computer

28. Not all Radiation is bad • An accidental contamination of construction steel with discarded cobalt-60 sources led to the exposure of 10,000 persons to chronic low levels of gamma radiation. The results of a study suggest that long term exposure to radiation at a dose rate of 5 rem/year greatly reduces cancer mortality.

29. Idealized Dose Response Curve • Too much or too little ionizing radiation may not be healthy. 10 rad Ref: Journal of American Physicians and Surgeons, 9(1) Spring 2004