Wed Oct 23

Radioactivity

• Can be useful, can be dangerous
• Can be deliberate, can be by-product of other activity
• Can be naturally occurring, can be man made

• Uses of Radiation (itself)
• Uses of Radioactive Materials
  • for the radiation
• then Nuclear Energy
  • radiation is a consequence, not major reason

Gamma Rays (mainly)

As an alternative to X-rays--form images

• used to inspect pipes, welding, etc.
  • small radioactive pellet inside
  • photographic film outside
    • image shows up thin sections
  • much more portable than X-ray unit
  • no attention-- keeps emitting for years
• Can also use to measure thickness
  • metal processing-- use to measure thickness
  • gamma ray source below, sensor above
    • fast, while steel mill operates

Biological Damage: put to use

• gamma rays can kill
  • from bacteria to mammals
• food processing
  • exposure to Gamma rays (or X rays)
    • can kill off bacteria, fungus
    • prevents food spoilage
  • as an alternative to heating/cooking
  • now permitted by US-FDA
    • FDA= Food and Drug Administration
    • e.g., chicken (often bacterial contamination)
  • no obvious degradation of the food material
    • generally much less than cooking
  • no radioactivity remains in the food
• (Could possibly be adapted to treating suspect mail)
  • (without opening)
  • (very fast, automatic process)

Radiation Used Medically:

Radiation Therapy for treatment of cancer and other diseases

• link to Radiation Therapy (Long Island clinic with informative web site
• Exposure to gamma rays (or X-rays)
  • can damage/kill cancerous tissue
• Healthy tissue also damaged
  • fast growing cancers more susceptible
  • can focus effects, reduce unwanted damage
• Typical application
  • most often-- Xrays from a large particle accelerator
    • beam that, say 4-8" wide
    • directed towards tumors
    • rotate body or source
      • spread out risk to neighboring tissue
  • or tiny wires of radioactive elements
    • planted next to tumors
    • left in place for days/weeks
    • most damage to immediate area

Medical diagnosis

• based on chemical properties of the isotope and ability to detect it by the radiation

• Example: thyroid gland (in throat)
  • pictures: two human thyroids (one normal, one with inactive portion)
  • picture: cat, showing thyoid (and brain)
  
  • concentrates much of the iodine we eat
  • low Iodine intake leads to goiter (thyroid diseases)
  • supply small amount of radioactive iodine
    • we can make it from regular iodine
    • expose to neutrons in a nuclear reactor
    • becomes radioactive
      • we can easily detect it by radioactivity
  • body distributes the iodine
    • normally-- most into the thyroid (2-6 hr)
    • abnormal-- poor uptake by the thyroid
  • measure pattern of the radioactivity
    • can form an image of the thyroid
    • can detect abnormal pattern
      • is the size abnormal?
      • is part of gland more active?
      • does gland take up Iodine normally?

risk vs. benefits

• this does expose whole body to radiation
• with sensitive measurement tools,
  • low dosage and low risk
• medical benefits clearly outweigh risks
  • but not a risk free process

Similar Technique used to image the skeleton

• Technetium (odd element)
• chemically safe, acts a bit like Ca
• taken up quickly by bones
  • radioactive isotope of Tc
  • can map where the radiation comes from to within 1/2-1/4 inch
• can form a map of bones
• (X-rays make much clearer bone images)
• may find abnormal uptake
  • arthritis and bone cancer, for example
  • lead to more rapid uptake
  • form strong spots on the bone scan
  • can use to identify and localize disease
• how imaged?
  • array of detectors (each focused to small region)
  • slowly passed over/along your body

Recent Problem (Sept 11 consequences)

• what's a good medical half life?
• if long, takes weeks, months to get enough radioactivity to measure
  • or need large amounts of the isotope
• best signal if half life is, say, 5 minutes
• if need bone uptake (2 hrs),
  • really need half life of several hours
• if half life is 6 hrs, can't keep on the shelf for 1-2 days
• so medical work relies on overnight delivery
  • when airlines don't fly on schedule
  • then medical uses stops

Chemical tracers

• chemists often use radioactive isotopes
• is very easy to trace tiny amounts of material
  • can detect 100-1000 atoms !
  • that's 10^-20 moles, too tiny for any other techniques

Radioactive Dating

• cosmic rays make ¹⁴C isotope
• plants sequester the C from CO₂
• plants show ¹⁴C content (traces)
  • can measure as radioactivity
  • can also measure with mass spectrometer
• loses 1/2 ¹⁴C over 5700 years... figure out age

Radioactive Chemical Analysis

• Neutron Activation Analysis
  • remember Iodine + reactor...produces "hot" I- isotope
  • true for most elements
• sample (say a hair) place in a reactor
  • emerges as radioactive hair
• each element forms unique isotopes
• each isotope has specific gamma rays
  • measure energy, determine the species
  • measure intensity, determine the amounts
    • can, for example, see As or Hg in Napoleon's hair

Special Application

• can detect nitrogen in similar manner
• exposure to specific radioactive material
• produces specific isotope, easily detected
  • used to scan luggage for explosives
  • all explosives are N-rich
  • exposures leave N-residues in the region
• false trigger with N-rich materials
  • heart medicine (nitrogylcerine)
  • fertilizer residues
  • wool or leather (N-in treated proteins)

Chemical Damage from Radiation

• gamma rays can ionize, break bonds
• treat simple polyethylene (soft)
  • break some C-H bonds
  • it forms new bonds (C-C)
  • makes tougher plastic
  • no remaining radioactivity

• A few more links if you are interested
• more on thyroid scans
• Treatment of Overactive Thyroid (for cats; similar treatment for people)
  • Radioactive Iodine
  • The treatment of choice involves the use of radioactive Iodine to selectively destroy only the part of the thyroid gland that has a problem. It has the advantage of a very high success rate and does not require anesthesia. It will also destroy the diseased thyroid tissue that has moved towards the chest. The disadvantage is the fact that your cat must be boarded at the treatment facility for up to 10 days after treatment to minimize radiation exposure to others. Approximate cost is $700. This treatment is done at a special center, and is available only on a referral by one of our doctors. The center that performs the scan to make the diagnosis is the same place that will institute this treatment.
  • Before your pet is treated with radioactive iodine we will test its kidney function with Tapazole to determine if kidney function will be adequate after the radioactive iodine destroys the abnormal thyroid tissue.

Radiation as a Bad Thing....

• as we already noted, radiation damages tissues
  • often a risk/benefit situation

• Sources of unwanted radiation
  • simple storage of radioactive materials
  • created in normal operation, nuclear reactors
    • can be catastrophic (e.g., Chernobyl)
    • serious problem with reactor wastes
  • produced, distributed in nuclear weapons
    • we still detect isotopes from A-tests of the 1950's
    • many isotopes have half lives of 25-1000 years
  • associated with mining
    • radioactive materials like uranium
    • present near other minerals (by-product)
  • some naturally occurring
    • local mineral deposits
    • cosmic rays (at high altitude)
• specific example
  • military makes projectiles (large bullets) from Uranium
    • most dense and destructive projectiles
  • has had radioactive materials extracted
    • called delpeted uranium
    • but still has residual activity , explosive risk is greater
    • risk for handlers,risk for those at receiving end
    • risk afterwards (exploded shells, scrap metal)

Radiation Dosage and Risk

units of radioactivity: Curie and rem, millirem

• at very low levels
  • very low risk
  • body has some repair features
• can try to set legal limits for exposure
• can monitor exposure
  • relatively easy for employees, visitors
  • film badges, regular testing
• can compare animal tests
  • level where one detects medical problems
  • set thresholds much lower, best estimate

Medical Consequences

• immediate-- high levels of radiation destroy tissue
  • see clear burn patterns (skin damage, blisters)
  • see inflammation
    • (partly reaction to abnormal chemical fragments)
    • also true for ordinary burn victims
  • extent depends on exposure levels
• some immediate, some emerges slowly
• at lower levels the risks are more subtle
  • typically show up years later
  • as increased risk of specific disease
    • often as cancer
    • cancer due to risk of DNA abnormality
• very difficult medical problem
  • some human information from extreme cases
    • Hiroshima and Nagasaki
    • Chernobyl
    • Nuclear workers in the 1950's
    • Occasional accidents
  • controlled animal tests
    • at higher levels
    • then try to predict effects
      • at much lower levels
      • to humans
  • would be simpler if condition were unique to radioactive injury
    • rarely the case
    • typically many or unknown causes
      • (say of specific cancer)
      • radiation is one factor
      • often one of the lesser factors
      • at worst, slight increase in incidence
      • often, concealed in other effects
    • at low rates, randomness can hide effect
      • cancer clusters= suspicious
      • but will sometimes occur randomly

Nuclear Weapons

• Clearly a social and individual concern
• Lessened as cold war faded
• Still lots of weapons available
• Shifted from fears of global warfare to nuclear terrorism
• Perhaps less worry from China and Russia
  • more from Pakistan, Iran, Iraq

• No evidence anyone tried and failed with a nuclear weapon
  • difficulty is cost of gathering right materials
  • not in creating a crude weapon

Nuclear weapons

• A-bomb or fission bomb
• H bomb for fusion bombs

• How do they work?
• Need critical mass of suitable isotope
• ²³⁵U is only useful natural isotope
  • present at low concentration
  • ²³⁸U absorbs neutrons, stops the fission
• need to separate ²³⁵U from ²³⁸U
  • chemically, identical
  • differ only in mass (slight difference)
• UF₆ (gas)
  • tiny hole
  • slightly more ²³⁵U-F₆ escapes
  • slight concentration in gas
  • repeat several thousand times
  • get enriched uranium
• Now done with centrifuges

• Also, plutonium
  • made from ²³⁹U in reactors
  • chemically easier to separate from U
    • (different element)

A-Bomb:

• if more than 1 neutron kept, start more reaction
• then process builds
• if rapid, then abrupt release of heat
• = bomb
• energy released is massive
• rated at megatons of TNT
• i.e.., millions times more energy per pound
• than conventional chemical explosives
• if most neutrons escape, process is very slow
  • happens with small samples of ²³⁵U
  • subcritical mass
  • (mass is classified data, but is 5-10 kg)
• if assemble critical mass, no control
  • if slowly assembled, heat drives it apart before it explodes
  • typically, divide mass into 3-10 parts
    • use explosives to drive the parts together
    • explosion holds mass together for a few msec
• this is the argument that says a nuclear reactor won't explode