Physics:Gemstone irradiation

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Short description: Process to enhance a gemstone's optical properties


A
B
B
B
C
C
The image above contains clickable links
Pure diamonds, before and after irradiation treatment
A Initial (2×2 mm size)
B Irradiated by different doses of 2 MeV electrons
C Irradiated by different doses and then annealed at 800 °C (1,470 °F)

Gemstone irradiation is a process in which a gemstone is artificially irradiated in order to enhance its optical properties. High levels of ionizing radiation can change the atomic structure of the gemstone's crystal lattice, which in turn alters the optical properties within it.[1] As a result, the gem­stone's color may be significantly altered or the visibility of its inclusions may be lessened.

The process, widely practiced in jewelry industry,[2] is done in either a nuclear reactor for neutron bombardment, a particle accelerator for electron bombard­ment, or a gamma ray facility using the radioactive isotope cobalt-60.[1][3] The irradiation treatment has enabled the creation of gemstone colors that do not exist or are extremely rare in nature.[1] However, the process, particularly when done in a nuclear reactor, can make gemstones radioactive. Health risks related to the residual radioactivity have led to government regulations in many countries.

Radioactivity and regulations

Alpha (α) radiation is stopped by a sheet of paper. Beta (β) radiation is halted by an aluminium plate. Gamma (γ) radiation is eventually absorbed as it penetrates a dense material. Neutron (n) radiation consists of free neutrons that are blocked by light elements, which slow and/or capture them.

The term irradiation is a broad one, which covers bombardment by subatomic particles as well as the use of the full range of electromagnetic radiation, including (in order of increasing frequency and decreasing wavelength) infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.[4] Certain natural gemstone colors, such as blue-to-green colors in diamonds[5] or red colors in zircon,[6] are the results of the exposure to natural radiation in the earth, which is usually alpha or beta particle.[5] The limited penetrating ability of these particles result in partial coloring of the gemstone's surface.[5] Only high-energy radiation such as gamma ray or neutron can produce fully saturated body colors,[5] and the sources of these types of radiation are rare in nature, which necessitates the artificial treatment in jewelry industry. The process, particularly when done in a nuclear reactor for neutron bombardment, can make gemstones radioactive.[7][a] Neutrons penetrate the gemstones easily and may cause visually pleasing uniform coloration, but also penetrate into the atomic nucleus and cause the excited nucleus to decay, thereby inducing radioactivity.[8] So neutron-treated gemstones are set aside afterward for a couple of months to several years to allow any residual radioactivity to decay,[3][9] until they reach a safe level of less than 1 nanocurie per gram (37 Bq/g) to 2.7 nanocuries per gram (100 Bq/g) depending on the country.[b]

The first documented artificially irradiated gemstone was created by English chemist Sir William Crookes in 1905 by burying a colorless diamond in powdered radium bromide.[10][11] After having been kept there for 16 months, the diamond became olive green.[10] This method produces a dangerous degree of long-term residual radioactivity and is no longer in use.[12] Some of these radium-treated diamonds—which are still occasionally put on sale and can be detected by particle detectors such as the Geiger counter,[12] the scintillation counter,[13] or the semiconductor detector[13]—are so high in radiation emission that they may darken photographic film in minutes.[14]

The concerns for possible health risks related to the residual radioactivity of the gemstones led to government regulations in many countries.[1] In the United States, the Nuclear Regulatory Commission (NRC) has set strict limits on the allowable levels of residual radioactivity before an irradiated gemstone can be distributed in the country.[3] All neutron- or electron beam-irradiated gemstones must be tested by an NRC-licensee prior to release for sales; however, when treated in a cobalt-60 gamma ray facility, gemstones do not become radioactive and thus are not under NRC authority.[3] In India, the Board of Radiation and Isotope Technology (BRIT), the industrial unit of the Department of Atomic Energy, conducts the process for private sectors.[15] In Thailand, the Office of Atoms for Peace (OAP) did the same, irradiating 413 kilograms (911 lb) of gemstones from 1993 to 2003,[16] until the Thailand Institute of Nuclear Technology was established in 2006 and housed the Gem Irradiation Center to provide the service.[17][18]

Materials and results

Effects of irradiation on
various gemstone materials
Material Starting color Ending color
Amber Light yellow Orangey red,[19]
orangey yellow[19]
Beryl Colorless Yellow[20]
Blue Green[20]
Colorless
to pale pink
(Maxixe-type) 		Deep blue[1]
Diamond Colorless or
yellow
to brown		 Green to blue[21]
Fluorite Colorless Various[20]
Pearl Light colors Brown,[20]
gray to black[20]
or gray-blue[22]
Quartz Colorless to
yellow or
pale green		 Amethyst,[21][20]
brown,[20] rose,[20]
"smoky" (light gray)[21]
Sapphire Pink with
blue tint		 Tint removed[23]
Topaz Yellow
to orange 		Intensify colors[20]
Colorless
to
pale blue 		Brown,[20]
dark blue,[24]
green,[20]
sky blue[24]
Tourmaline Colorless
to
pale colors 		Brown,[20]
green-red (bicolor),[20]
intense pink,[18]
pink,[18][20] red,[20]
yellowish orange[18]
Pink Intense pink,[18]
orangey pink[18]
Blue Purple[20]
Zircon Colorless Brown to red[20]
London Blue (left), one of the neutron-bombarded varieties of topaz,[24] compared to a natural blue topaz (right); Intensely blue topaz does not exist in nature and is the result of artificial irradiation.[25]

The most commonly irradiated gemstone is topaz, which usually becomes blue after the process.[3] Intensely blue topaz does not exist in nature and is the result of artificial irradiation.[25] According to the American Gem Trade Association, approximately 30 million carats (6,000 kg or 13,000 lb) of topaz are irradiated every year globally, 40 percent of which were done in the United States as of 1988.[26] Dark-blue varieties of topaz, including American Super Blue and London Blue, are the results of neutron bombardment,[24] while lighter sky-blue ones are often those of electron bombardment.[24] Swiss Blue, subtly lighter than the US variety, is the result of a combination of the two methods.[24]

Diamonds are mainly irradiated to become blue-green or green, although other colors are possible.[27] When light-to-medium-yellow diamonds are treated with gamma rays they may become green; with a high-energy electron beam, blue.[21] The difference in results may be caused by local heating of the stones, which occurs when the latter method is used.[21]

Colorless beryls, also called goshenite, become pure yellow when irradiated, which are called golden beryl or heliodor.[1] Quartz crystals turn "smoky" or light gray upon irradiation if they contain an aluminum impurity, or amethyst if small amounts of iron are present in them; either of the results can be obtained from natural radiation as well.[28]

Pearls are irradiated to produce gray blue or gray-to-black colors.[22] Methods of using a cobalt-60 gamma ray facility to darken white Akoya pearls were patented in the early-1960s.[29] But the gamma ray treatment does not alter the color of the pearl's nacre, therefore is not effective if the pearl has a thick or non-transparent nacre.[29] Most black pearls available in markets prior to the late-1970s had been either irradiated or dyed.[29]

Uniformity of coloration

Gemstones that have been subjected to artificial irradiation generally show no visible evidence of the process,[30] although some diamonds irradiated in an electron beam may show color concentrations around the culet or along the keel line.[30]

Color stability

In some cases, the new colors induced by artificial irradiation may fade rapidly when exposed to light or gentle heat,[31] so some laboratories submit them to a "fade test" to determine color stability.[31] Sometimes colorless or pink beryls become deep blue upon irradiation, which are called Maxixe-type beryl. However, the color easily fades when exposed to heat or light, so it has no practical jewelry application.[1]

Notes

a. ^ Generally speaking, an energy of at least 10 MeV is needed to induce radioactivity in a material.[32]

b. ^ (As of 1987), most developed countries regarded 2 nanocuries per gram (74 Bq/g) as safe to release to the public while the U.S. federal release limits for most nuclides were 1 nanocurie per gram (37 Bq/g) or less, and that of the United Kingdom was 2.7 nanocuries per gram (100 Bq/g).[33] (As of 2022), the release limit of the European Union is 2.7 nanocuries per gram (100 Bq/g).[9]

References

Citations

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Hurlbut & Kammerling 1991, p. 170
  2. ↑ Omi & Rela 2007, p. 1
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Nuclear Regulatory Commission 2019
  4. ↑ Nassau 1980, p. 343
  5. ↑ 5.0 5.1 5.2 5.3 King & Shigley 2003, p. 48
  6. ↑ Fielding 1970, pp. 428-429
  7. ↑ Hurlbut & Kammerling 1991, p. 172
  8. ↑ Nassau 1980, p. 346
  9. ↑ 9.0 9.1 Schröck 2022
  10. ↑ 10.0 10.1 Tilden 1917, pp. 145-146
  11. ↑ Hurlbut & Kammerling 1991, p. 158
  12. ↑ 12.0 12.1 Hurlbut & Kammerling 1991, p. 216
  13. ↑ 13.0 13.1 Ashbaugh III 1988, p. 207
  14. ↑ Crowningshield 1981, p. 216
  15. ↑ Parthasarathy 2008
  16. ↑ Office of Atoms for Peace 2006
  17. ↑ Journal of Physics: Conference Series 2019, pp. 1-2
  18. ↑ 18.0 18.1 18.2 18.3 18.4 18.5 Suwanmanee et al. 2021, p. 517
  19. ↑ 19.0 19.1 Li, Wang & Chen 2022, p. 133
  20. ↑ 20.00 20.01 20.02 20.03 20.04 20.05 20.06 20.07 20.08 20.09 20.10 20.11 20.12 20.13 20.14 20.15 20.16 Ashbaugh III 1988, p. 201
  21. ↑ 21.0 21.1 21.2 21.3 21.4 Rossman 1981, p. 70
  22. ↑ 22.0 22.1 Sofianides & Harlow 1991, p. 178
  23. ↑ Soonthorntantikul, Vertriest & Palke 2023, p. 160
  24. ↑ 24.0 24.1 24.2 24.3 24.4 24.5 Jewelers Circular Keystone 1990, p. 39
  25. ↑ 25.0 25.1 Sofianides & Harlow 1991, p. 82
  26. ↑ Ashbaugh III 1988, p. 205
  27. ↑ Skuratowicz & Nash 2005, p. 13
  28. ↑ Rossman 1981, p. 69
  29. ↑ 29.0 29.1 29.2 Department of Geological Sciences 1998
  30. ↑ 30.0 30.1 Hurlbut & Kammerling 1991, p. 127
  31. ↑ 31.0 31.1 Hurlbut & Kammerling 1991, p. 57
  32. ↑ Thomadsen et al.
  33. ↑ Ashbaugh III 1988, p. 212

Works cited



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