General Physics (PHY 2140)

Transcription

1 General Physics (PHY 2140) Lecture 19 Modern Physics Nuclear Physics Nuclear Reactions Medical Applications Radiation Detectors Chapter

2 Lightning Review Last lecture: 1. Nuclear physics Nuclear properties Binding energy Radioactivity The Decay Process Natural Radioactivity r = r A A X 1/3 Z 0 Nuclear density ~ 2.3 x Kg/m 3 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: r = me 2 qb 2

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

4 QUICK QUIZ 29.2 ANSWER (b). If the original activity is R 0 =λn 0, the activity remaining after an elapsed time t is R = R 0 e -λt = R 0 e -(0.693/T 1/2)t. Solving for the half-life yields T 1/2 = [-0.693/ln(R/R 0 )]t. If R = 0.96R 0 at t = 2.0 hr, the half-life is: T 1/2 = [-0.693/ln(0.96)](2.0 h) = 34 h. 4

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 5

6 Problem Which of the following are possible reactions? ΔA= -17, ΔZ=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. 6

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 7

8 Problem: nuclear reactions Determine the product of the reaction What is the Q value of the reaction? Li + He + n ? 8

9 Determine the product of the reaction What is the Q value of the reaction? Li + He + n 7 4 X 3 2 Y? Given: reaction Find: Q =? 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. Number of nucleons (A): Number of protons (Z): Thus, it is B, i.e. The Q-value is then 7+ 4= X + 1 X = = Y + 0 Y = 5 Li + He B + n ( ) ( ) n 2 2 Q= Δ m c = m + m m m c = 2.79MeV Li He B Endothermic Need to put in energy = >2.79 MeV 9

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 KE min = 1 + m M Q 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 10

11 QUICK QUIZ If the Q value of an endothermic reaction is 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. (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. KE min = 1 + m M Q 11

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 12

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 13

14 Units of Radiation Exposure Roentgen [R] is defined as That amount of ionizing radiation that will produce 2.08 x 10 9 ion pairs in 1 cm 3 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 14

15 More Units RBE (Relative Biological Effectiveness) The number of rad of x-radiation x 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 15

16 RBE for several types of Radiation Radiation RBE factor X-rays and gamma rays 1.0 Beta particles Alpha particles Slow neutrons 4-5 Fast neutrons and protons 10 Heavy ions 20 16

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 90 Sr can yield 1000 rem dose! ) LD 50 = rem whole body 17

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 18

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

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 20

21 Therapeutic Applications of Radiation Cancer Treatment Various types of methods to get ionizing radiation to cancer cells ls 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 125 I and 103 Pd ( photons in the kev range) to 137 Cs and 192 Ir (< 1MeV) are placed in close proximity to the cancer cells Proton and ion beams treatment of Ocular melanoma, radiosurgical procedures, brain metastais,, Parkinson s s and others. Neutron beams 21

22 Image of Brachytherapy seeds in place 22

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 23

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 24

25 Detectors, 2 Semiconductor Diode Detector A reverse biased p-n 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 b radiation The excited atoms emit visible radiation as they return to their r ground state With a photomultiplier,, the photons can be converted into an electrical signal Can also be used to measure particle energy 25

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 26

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 ed Can provide electronic readout to a computer 27

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. 28

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