Transcript Document

General Physics (PHY 2140)
Lecture 19
 Modern Physics
Nuclear Physics
Nuclear Reactions
Medical Applications
Radiation Detectors
Chapter 29
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http://www.physics.wayne.edu/~alan/2140Website/Main.htm
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Lightning Review
Last lecture:
1. Nuclear physics
 Nuclear properties
 Binding energy
 Radioactivity
 The Decay Process
 Natural Radioactivity
A
Z
X
r  r0 A
1/ 3
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:
mE
r
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qB 2
2
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
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(b) 34.0 h
(d) 68.6 h
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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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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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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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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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Problem: nuclear reactions
Determine the product of the reaction 3 Li  2 He  ?  n
What is the Q value of the reaction?
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8
7
4
X
Determine the product of the reaction 3 Li  2 He  Y ?  n
What is the Q value of the reaction?
Given:
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.
Number of nucleons (A):
Number of protons (Z):
Thus, it is B, i.e.
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Find:
Q=?
7  4  X  1  X  10
3 2  Y  0 Y  5
Li  24He  105 B  01n
The Q-value is then


Q   m  c 2  m 7 Li  m 4 He  m10 B  mn c 2  2.79MeV
Endothermic  Need to put in energy = >2.79 MeV
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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
KEmin


 m
 1   Q
 M
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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QUICK QUIZ
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.
(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.
KEmin
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 m
 1   Q
 M
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Radiation Damage in Matter
Radiation absorbed by matter can cause damage
The degree and type of damage depend on many factors

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Type and energy of the radiation
Properties of the absorbing matter
Radiation damage in biological organisms is primarily due to
ionization effects in cells
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Ionization disrupts the normal functioning of the cell
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Types of Damage
Somatic damage is radiation damage to any cells except
reproductive ones
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Can lead to cancer at high radiation levels
Can seriously alter the functional characteristics of specific
organisms
Genetic damage affects only reproductive cells
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Can lead to defective offspring
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Units of Radiation Exposure
Roentgen [R] is defined as
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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)

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That amount of radiation that deposits 10-2 J of energy into 1 kg
of air
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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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RBE for several types of Radiation
Radiation
X-rays and gamma rays
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RBE factor
1.0
Beta particles
1.0-1.7
Alpha particles
10-20
Slow neutrons
4-5
Fast neutrons and protons
10
Heavy ions
20
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Radiation Levels
Natural sources – rocks and soil, cosmic rays

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Background radiation
About 0.13 rem/yr
Upper limit suggested by US government
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0.50 rem/yr
Excludes background and medical exposures
Occupational

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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
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Applications of Radiation
Sterilization
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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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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
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Computed Axial Tomography
Produces pictures with greater clarity and detail than traditional
x-rays
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Other Medical Uses
Radionuclide Imaging
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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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Therapeutic Applications of Radiation
Cancer Treatment
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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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Image of Brachytherapy seeds in place
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More Applications of Radiation
MRI
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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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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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Detectors, 2
Semiconductor Diode Detector
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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
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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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Detectors, 3
Track detectors

Various devices used to view the tracks or paths of charged
particles
Photographic emulsion
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Simplest track detector
Charged particles ionize the emulsion layer
When the emulsion is developed, the track becomes visible
Cloud chamber
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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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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
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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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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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