IRIDIUM 192: PROPERTIES, APPLICATIONS AND RISKS - CHEMISTRY - 2024

The iridium - 192 is a radioactive isotope of the metal iridium, atomic number 77, belonging to Group 9, period 6, and group d of periodic table. This metal has 42 radioactive isotopes, iridium 192 (¹⁹² Ir) being one of the most prominent.

The ¹⁹² Go has at its core 77 protons and neutrons 115 (totaling an atomic mass of 192 u). It emits a beta particle (β ^-) and gamma radiation (γ) when it decays.

Iridium 192 symbol. Source: me, via Wikimedia Commons

95.13% of the time, ¹⁹² Ir disintegrates through the emission of β ^- to platinum 192 (¹⁹² Pt); and the remaining 4.87% is transformed into osmium 192 (¹⁹² Os) by electron capture.

By emitting a β ^- particle, a radioactive isotope undergoes the transformation of a neutron into a proton, thus increasing its atomic number by one unit. As a consequence of this, the ¹⁹² Ir becomes ¹⁹² Pt; Since, platinum has an atomic number of 78.

The ¹⁹² Ir is the main isotope used in the treatment of cancer by brachytherapy technique. Therefore in this technique the radioactive emitter is placed in the vicinity of the tumor.

Properties of iridium 192

Physical appearance

Silver shiny solid metal. It presents iridescence, which gave rise to its name.

Melting point

2446 ºC

Boiling point

4428 ºC

Density

22.562 g / cm ³. It is, together with osmium, the metal with the highest density.

Heat of fusion

26.1 kJ / mol.

Vapor pressure

1.47 Pa at 2716 K

Electric conductivity

19.7 x 10 m ^-1.Ω ^-1

Enthalpy of vaporization

604 kJ / mol.

Elecronegativity (Pauling scale)

2.2.

Resistance to corrosion

It is resistant to corrosion, including aqua regia.

Characteristics of its radioactive emission

Atomic mass of the radioactive substance

191.962 g / mol.

Nuclear particles

77 protons and 115 neutrons.

Half life time

73,826 days.

Time of life

106.51 days

Radiation type

Particle β ^- and radiation type (γ).

Energy of γ radiation

Average energy of radiation 0.38 MeV, with a maximum energy of 1.06 MeV.

The ¹⁹² Go is produced in a nuclear reactor by bombarding the metal iridium neutrons. This technique avoids the production of unwanted isotopes.

Applications

-Industrial

-It is mainly used in the so-called non-destructive tests (NOD). It is also used as a radio marker in the oil industry, especially in petrochemical plants and pipelines.

-Industrial gamma radiography is used to examine welds, to test and classify their condition in pressurized pipes, pressure vessels, high capacity storage containers and certain structural welds.

-Industrial gamma radiography has also been used in the examination of concrete, also being used to locate reinforcing bars, conduits inside the concrete. Also, this method allows to detect failures in castings.

-The ¹⁹² Ir has been used to examine machined parts and metal plates, and in the determination of structural anomalies due to corrosion or mechanical damage.

For industrial use, the ¹⁹² Ir is placed in sealed containers that emit a beam of gamma radiation that can be directed. These radiation sources are contained within a welded stainless steel frame that contains a number of isotope discs.

The cameras used in these studies are remote controlled. In this case, the gamma radiation source is moved out of the shielded container to an exposure position. This operation is commonly controlled by a Bowden cable.

-In radiotherapy

Brachytherapy

The ¹⁹² Go is one of the main isotope used in brachytherapy. The technique consists of placing the radioactive isotope in the vicinity of the cancerous tumor for its destruction.

When used in brachytherapy, it is usually used in the form of wires, being used in ¹⁹² Ir interstitial implants with low radiation dose (LDR). The radioactive activity of the wire ranges from 0.5 to 10 mCi per cm. Wire is not a sealed source of radiation.

They are also used for high doses of radiation, in the form of tablets placed inside sealed capsules of an alloy of iridium and platinum 3.5 mm in length.

Treatment in patients

The ¹⁹² Go was used to treat 56 patients, between January 1992 and January 1995, by the technique of high activity brachytherapy radioactive temporarily supplied in patients with glioblastoma astrocytoma.

Median survival was 28 months, with the authors of this investigation concluding that brachytherapy can improve control of local tumors and also prolong survival when used in deep malignant brain gliomas.

Of the 40 patients treated with brachytherapy, using ¹⁹² Ir, 70% of the patients showed no evidence of the disease at the end of the follow-up period.

Prostatic carcinoma

A high-dose iridium-192 protocol and a follow-up of up to 130 months is used in patients with prostate carcinoma. Using local anesthesia, five or seven hollow needles carrying the isotope are placed into the prostate, by perineal puncture.

Then, a radiation dose of 9 Gy is initially applied to the prostate and the protocol that includes radiation from outside the body is continued.

Health risks

The International Atomic Energy Agency placed isotope ¹⁹² Ir in Category 2 of radioactive substances. This indicates that it can permanently harm people who handle radioactive material for minutes or hours, and can even cause death within a few days.

External exposure can cause burns, acute radiation sickness, and even death. Accidental ingestion of ¹⁹² Ir seeds or granules can cause burns to the stomach as well as the intestines.

The long-term effects are dependent on the radiation dose, as well as the length of time the radioactive isotope remains in the body.

In the bibliography, there are several cases of contamination of people with iridium-192, among others.

For example, in 1999, in the Peruvian town of Yenardo, a worker extracted a radioactive source; He opened the protection device and placed it in his back pocket.

Soon after, an erythema appeared at the irradiation site, followed by ulceration, bone necrosis, and finally the man died of septic shock.

References

1. Lenntech. (2019). Iridium. Recovered from: lenntech.es
2. Chemistry Reference. (sf). Iridium. Recovered from: chemistry-reference.com
3. Paul R. et al. (1997). Iridium 192 high-dose-rate brachytherapy – a useful alternative therapy for localized prostate cancer? Recovered from: ncbi.nlm.nih.gov
4. Chemistry Learner. (2019). Iridium 192. Recovered from: chemistrylearner.com
5. PubChem. (2019). Iridium IR-192. Recovered from: pubchem.ncbi.nlm.nih.gov
6. Center for preparedness and response. (April 4, 2018). Radioisotope Brief: Iridium-192 (Ir-192). Recovered from: emergency.cdc.gov