10 Radioactivity and Nuclear Medicine

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10 - Radioactivityand Nuclear Medicine
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Radioactivity

• Involves nuclear changes
• implies unstable nucleus
• Nuclear reaction known as decay
• Rate of decay is not affected by temperature,
  pressure or amount of material
• The energy from nuclear reaction is far greater
  than in normal chemical reactions
• Nuclear reaction of 1.0 g of U-238 ? 8.2 x 107
  kJ
• Burning 1.0 g of methane ? 56 kJ

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Atomic Structure Digression

• Carbon -12 can also be written a C-12
• Z Atomic number protons
• A mass number Protons neutrons
• nucleons

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Isotopes and nuclides

• Isotopes vary only by of neutrons (different
  mass number)
• Each separate isotope is called a nuclide
• Radionuclides are radioactive nuclides
• Find the of protons and neutrons in
• C-13
• C-14
• U-238

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Observations on Nuclide Stability
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The General Findings

• Nuclides will be unstable if
• Z gt 82 (No exceptions)
• Generally, if neutrons and of protons are
  both odd
• There are other rules, but these two are the most
  important for prediction of radioactivity.

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3 Main types of radioactive emissions
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Alpha emission

• a particles
• 2 neutrons and 2 protons
• high ionizing power
• Picks up 2 electrons from what it hits!!
• low penetration power

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Writing nuclear reactions
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Beta emission

• ?-particles
• An electron emitted from nucleus
• a neutron decays into a proton
• medium ionizing power
• medium penetrating power

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Positron emission

• ?-particles
• A positively charged electron emitted from
  nucleus
• a proton decays into a neutron
• medium ionizing power
• medium penetrating power

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Electron Capture

• An electron is captured by a proton in the
  nucleus
• gamma radiation often emitted, but no other
  particles

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Gamma emission

• ? particles are very different from a and ?
• Uncharged and no mass
• A photon of energy
• Generally accompanies a and ? decay
• Taken for granted (not often written in
  equations)
• A Z 0
• U-238 ? Th-234 a ?

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A closer look
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Radioactive particle summary
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Nuclear reaction practice

• Alpha emission from curium-237
• Beta emission from magnesium-28
• positron emission from F-19

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Radioactive Decay Series
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Half-life (t1/2)
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Calculate the age of an object if it shows 20 of
its original C-14 activity.
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Biological half-life

• A medical isotope is also affected by metabolic
  processes
• biological half-life (Tb)
• T effective - combines t1/2 and Tb

Calculate the effective half-life of I-131 if the
half-life is 8.0 days and the biological
half-life is 2.0 days.
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Radiation detectors
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Radiation Therapy

• Useful in cancer treatment
• Irradiate the cancer cells
• Paradoxical, but actual risk of radiation dose
  causing cancer is very small
• Drawback is that healthy cells are also damaged,
• Treatment methods focus on ways to maximize
  cancer exposure, but minimize healthy cell
  exposure
• Irradiation (targeting)
• Brachytherapy - Implantation (distance effect)
• Doses may reach thousands of rems

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Irradiation Treatment

• Expose tumor to gamma rays (gamma knife)
• typically from radioisotopes such as cobalt-60.
• The beam is moved in a circular pattern around
  the tumor
• maximize exposure of the cancer cells
• minimizing exposure of healthy tissues.

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Brachytherapy

• Useful for Cancers of Prostate, Breast, Lung,
  Esophageal, Cervix, Uterine, Anal tumors, Bile
  duct, Sarcomas, Neck tumors, Tongue cancer,
  Nasopharnx
• Permanent seed implantation
• Inject radioactive seeds
• low dose rate over several weeks or months.
• HDR temporary brachytherapy
• placing very tiny plastic catheters
• series of radiation treatments through catheters
• catheters are radioactive material removed

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Diagnostic Use of Radioisotopes

• Tracers
• Bone scans/PET
• Doses in mrem
• MRI (does not use radioactive substances) and is
  not Xray or nuke med.

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Tracers

• Radioisotopes react chemically like the
  non-radioactive elements, so they trace out the
  same pathways in the body
• Ca-47?bone
• I-131?thyroid
• Fe-59?blood
• Co-57? Vit B12
• Tc-99m?brain scan
• Require short half-lives

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Technetium-99 is often used as the radiation
source for bone scans such as this one.
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Positron emission

• Not one of the 3 main particles
• but very important in medicine (PET scans)
• A positron has the same mass as an electron, but
  opposite charge
• A proton converts into a neutron

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PET

• Radioactive tracer is often mixed with glucose
  and then IV or injected into patient
• Glucose taken up by active sites in the body

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Positron Emission Tomography

• What are the benefits vs. risks?
• PET allows study of body function
• detect alterations in biochemical processes that
  suggest disease before apparent with other
  imaging tests, such as CT or MRI.
• Radioactivity is very short-lived, your radiation
  exposure is low.
• The radioactive substance may expose radiation to
  the fetus in patients who are pregnant or the
  infants of women who are breast-feeding.
• Less resolution than CT/MRI

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Positron Emission Tomography

• In cancer, PET can
• distinguish benign from malignant tumors
• stage cancer by showing metastases anywhere in
  your body
• prove whether or not treatment therapies are
  working
• In the brain, PET can
• diagnose Alzheimer's disease for early
  intervention
• locate tumors in the brain and distinguish tumor
  from scar tissue
• locate the focus of seizures for some patients
  with epilepsy
• more accurately assess tumor and other sites in
  the brain for delicate surgery
• In the heart, PET can
• quantify the extent of heart disease
• determine, after a heart attack, if the heart
  muscle would benefit from surgery