1. General Physics (PHY 2140) Lecture 19 Modern Physics Nuclear Physics
Nuclear Reactions
Medical Applications
Radiation Detectors
Chapter 29
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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:
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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 half-life? (a) 10.2 h (b) 34.0 h (c) 44.0 h (d) 68.6 h
Radioactive Decay:
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4. QUICK QUIZ 29.2 ANSWER
(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 yields T1/2 = [-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.
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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
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6. Which of the following are possible reactions?
Problem
Which of the following are possible reactions?
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.
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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
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8. Problem: nuclear reactions
Determine the product of the reaction
What is the Q value of the reaction?
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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
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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
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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.
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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
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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
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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
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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
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16. RBE for several types of Radiation
Radiation RBE factor
X-rays and gamma rays
1.0
Beta particles
Alpha particles
10-20
Slow neutrons
4-5
Fast neutrons and protons 10
Heavy ions 20
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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 = rem whole body
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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
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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
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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
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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
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22. Image of Brachytherapy seeds in place
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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
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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
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25. Detectors, 2 Semiconductor Diode Detector Scintillation counter
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
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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
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27. Detectors, 4 Track detectors, cont Bubble Chamber Wire 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
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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.
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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
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