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Properties that assist industrial use

Properties that assist medical use

  • In comparison to an X-ray generator, Ir-192 produces the level of energy equal to that of a 460 keV X-ray system
  • This high energy and short wavelength allows Ir-192 to penetrate through thick materials throughout a short exposure time. It therefore is more sensitive to thicker materials (approximately 1 inch) compared to other radioisotopes like selenium-75
  • In addition, Ir-192's high specific activity of 350 curies for every 1 gram means that high amounts of radiation can be utilised from a portable source. That is, a 4x4mm source cylinder can emit up to 200 curies.
  • This means that sources can be placed closer to the film or detector, ensuring high sharpness of radiographs and sensitive detection
  • As well as beta radiation, Ir-192 emits high energy gamma which can effectively destroy cancer cells (utilised in brachytherapy)
  • Ir-192's high specific activity means that a very small source can provide a very high dose rate (HDR), which is essential for brachytherapy.
  • This means that only small sources of Ir-192 need to be used, making it appropriate for the thin and flexible implants used for cancer treatment
  • This versatility allows for a concentrated but localised radiation of tumours, and thus the minimisation of exposure to radiation.

Medicine: Brachytherapy

Industry: Non-destructive testing

  • Ir-192 is used through the radiotherapy of cancer; a significant radioisotope for brachytherapy
  • Brachytherapy is a form of treatment where the radiation source is placed inside or next to the area receiving treatment.
  • Ir-192 specifically is effective for treating cervical, prostate, breast and skin cancer and respective tumours.
  • Treatment is in the form of Ir-192 implants, which are palladium coated Ir-192 wires, that are inserted through a catheter into a body cavity

These cables (known as pigtails) have Ir-192 sources attached at the ends; found in gamma cameras

Ir-192 wires coated with platinum used for implants

Handheld Ir-192 radiography cameras

HDR (high dose rate) after loader machine regulates Ir-192 seeds (pellets)

  • Used in non-destructive testing (NDT) by producing x-ray photographs of metal castings through radiography cameras.
  • This involves the testing and analysis of welds for piping, concrete and pressure vessels (containers holding liquids and gases) in chemical plants and aircraft.
  • Ir-192 is placed in close proximity with the material and produces images via a radiography camera using film, or a detector (placed on the other side of the material)
  • Where there is an imperfection (like a crack), more gamma radiation propagates through the material, producing a high contrast area on film or a signal

Brachytherapy for prostate cancer

Uses

Ir-192 plays a signfiicant role in both the medical and industrial field

Neutron bombardment of Ir-191

Moderation

(cc) image by anemoneprojectors on Flickr

What are radioisotopes?

  • With a source of neutrons, Ir-192 can now be produced
  • However, the neutrons from U-235 fission have high energy and thus must be slowed by water 'moderation' which reduces their speed; slow or thermal neutrons can be much easily absorbed by Ir-191 and iridium

Iridium-192

Nuclear Fission of Uranium

Producing Ir-192

  • Radioisotopes are quite simply isotopes that emit radiation as a result of an unstable neutron to proton ratio
  • Isotopes are forms of elements that have the same number of protons, but a different amount of neutrons
  • As a result, the nuclei of radioisotopes are unstable, resulting in an emission of radiation to become more stable (in the form of gamma radiation, or particles; alpha and beta)

Nuclear Chain Reaction

  • The neutrons used through the neutron activation of Ir-191 or natural iridium originate from the process of nuclear fission of uranium-235.
  • Nuclear fission is fuelled by the bombardment of U-235 nuclei with neutrons, causing them to segregate into smaller atoms (most commonly barium-141 and krypton-92).
  • Two or three neutrons are also produced
  • Subsequently, these neutrons collide with other U-235 atoms, releasing more energy and more neutrons; this initiates a chain reaction.
  • Nuclear reactors control the reaction, meaning that not all the neutrons are absorbed and thus a constant source of neutrons are produced
  • When a neutron collides with the Ir-191/Ir nucleus, it enters an excited state
  • The compound nucleus will then de-excite, becoming more stable through the emission of gamma rays
  • The result is a radioactive nucleus; Ir-192, which then undergoes decay (de-excites) at a much slower rate than before (emitting both beta and gamma). That is, it takes Ir-192 73.88 days for the amount of the source undergoing decay to decrease by a half (half life)

Nature of Iridium-192

How a radiograph is created

  • Ir-192 is a synthetic radioisotope that can be formed from either pure iridium or Ir-191 (a naturally occuring isotope that is preferred over natural iridium as it results in a much greater specific activity)
  • Specific activity refers to radioactivity per unit mass
  • Ir-192 emits both gamma and beta radiation

Overview of Production

  • Ir-192 is produced via the process of neutron activation/bombardment in a nuclear reactor
  • This involves the bombardment of a iridium or Ir-191 core with neutrons at high speeds, which in turn forms an unstable nuclei, emits beta and gamma radiation; forming Ir-192
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