Radioactivity of neutron-irradiated catÕs-eye chrysoberyls

S.M. Tang a,*, T.S. Tay b

a Department of Physics, National University of Singapore, Kent Ridge, Singapore 119260, Singaporeb Far East Gemological Laboratory, #B2-02, 19 Tanglin Road, Singapore 247909, Singapore

Abstract

The recent report of marketing of radioactive chrysoberyl catÕs-eyes in South-East Asian markets has led us to use an

indirect method to estimate the threat to health these color-enhanced gemstones may pose if worn close to skin. We

determined the impurity content of several catÕs-eye chrysoberyls from Indian States of Orissa and Kerala using PIXE,

and calculated the radioactivity that would be generated from these impurities and the constitutional elements if a

chrysoberyl was irradiated by neutrons in a nuclear reactor for color enhancement. Of all the radioactive nuclides that

could be created by neutron irradiation, only four (46Sc, 51Cr, 54Mn and 59Fe) would not have cooled down within a

month after irradiation to the internationally accepted level of speci®c residual radioactivity of 2 nCi/g. The radio-

activity of 46Sc, 51Cr and 59Fe would only fall to this safe limit after 15 months and that of 54Mn could remain above

this limit for several years. Ó 1999 Elsevier Science B.V. All rights reserved.

Keywords: Proton-induced X-ray emission; Chrysoberyl catÕs eye; Radioactivity; Neutron irradiation; Gemstone color

enhancement

1. Introduction

The news published in the Fall (1997) issue ofthe journal Gems and Gemology reporting thediscovery of highly radioactive catÕs eye chryso-beryls in Bangkok and Indonesia markets [1] hasaroused worries among consumers and causedconcerns among gem dealers worldwide. One ofthose stones was tested by the Centre for Gem-stone Testing in USA and found to have an ac-tivity level of 52 nCi/g. Such an activity level issigni®cantly higher than the legal release levels set

by the relevant authorities in various countries(between 1 and 2.7 nCi/g). With a Geiger-M�ullercounter, the contact radiation level of the stonewas found to be 11 mR/h and the half-life of itsactivity was measured to be �103 days. Report-edly, the original material came from Orissa, Indiaand it was bombarded with neutrons in a nuclearreactor somewhere in Asia for color enhancement.

Until now, no further work has been reportedin the literature on the types of radioactive nu-clides present in those catÕs-eye chrysoberyls.Chrysoberyl is Al2(BeO4) or beryllium aluminumoxide crystal. The catÕs eye e�ect is due to the rutile(TiO2) needle inclusions lying perpendicular tothe length of the crystal with a cabochon cut. Thethree constitutional elements (O, Be and Al) of the

Nuclear Instruments and Methods in Physics Research B 150 (1999) 491±495

* Corresponding author. Fax: +65-772-6126; e-mail:

phytsm@leonis.nus.edu.sg

0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 1 0 0 0 - 3

catÕs-eye chrysoberyl and the inclusion element(Ti) will yield the following 11 radioactive nuclidesthrough the (n,c), (n,p), (n,a) or (n,2n) reactionswhen irradiated by fast and thermal neutrons: 6He,8Be, 16N, 19O, 24Na, 27Mg, 28Al, 46Sc, 47Sc, 48Sc and51Ti. Most of these radioactive nuclides have veryshort half-lives, ranging from a small fraction of asecond to two days. 46Sc has the longest half-life,which is 83.8 days. In fact, the half-lives of these 11product nuclides are all shorter than the reportedvalue of �103 days observed for one of those ra-dioactive catÕs-eye chrysoberyls. This implies thatsome of the induced radioactive nuclides in theneutron-irradiated gemstone have originated fromits impurities.

The direct way to ®nd out what radioactivenuclides are present in a neutron-irradiated catÕs-eye stone is to get hold of a few newly irradiatedones and analyze them by gamma spectroscopy.However, newly irradiated stones are not easilyavailable and the cooled-down ones may not givethe full information. In this article, we report ourattempt to obtain the information by taking anindirect approach. In this approach, typical im-purities and their concentrations in a catÕs-eyechrysoberyl are ®rst determined by analyzing a fewsamples of catÕs-eye chrysoberyl with PIXE. Theactivities of all the radioactive nuclides that can beproduced from the impurities and the constitu-tional elements by neutron activation are thencalculated using the neutron cross-section dataavailable in the literature. Finally, the activities soobtained are used to estimate the radiation dosethat would be given by an irradiated catÕs-eyechrysoberyl.

2. Impurities in cats-eye chrysoberyls

2.1. Samples and experimental

Three Indian catÕs-eye chrysoberyls were stud-ied by using the PIXE facility at the NationalUniversity of Singapore [2] to determine their im-purities. Two of the samples are from Orissa Stateand the third one from Kerala State. One of theOrissa stones weighs 1.01 ct or 0.202 g (hereafterreferred to as sample A) and the other weighs 0.95

ct or 0.19 g (sample B). The Kerala stone weighs1.05 ct or 0.21 g (sample C). The samples werecarbon-coated before the PIXE measurements toprevent charging e�ects.

2 MeV protons with a beam current of �3 nAwere employed for the analysis. The time of irra-diation for each sample was approximately 20±30min.. The Si(Li) detector used to collect the X-rayspectra has an active area of 61 mm2 and an energyresolution of 150 keV at 5.9 keV. While collectinga PIXE spectrum, an RBS (Rutherford Backscat-tering Spectrometry) measurement was also car-ried out with a 25 mm2 particle detector placed at160° to obtain information on the total protoncharge. The PIXE and RBS spectra were analyzedusing the PC-based software package NUSDAN[3], which is basically an integration of GUPIX [4]and RUMP [5].

2.2. Results

Besides Ti, observable amounts of Ca, Fe, Cuand Ga were found in all three samples. In sampleA, Cr was also present. Fig. 1 shows the PIXEspectrum obtained from sample A, and Table 1gives the concentrations of all the impuritiespresent in the three samples. The stone fromKerala has signi®cantly higher concentrations ofTi, Fe, Cu and Ga than the two stones fromOrissa.

Fig. 1. PIXE spectrum of sample A (1.01 ct catÕs-eye chryso-

beryl from Orissa, India).

492 S.M. Tang, T.S. Tay / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 491±495

3. Radioactive nuclides produced by neutron irradi-

ation

The activity of a product nuclide immediatelyafter neutron irradiation can be calculated fromthe formula:

A0 � Nr/ 1ÿ ÿeÿkT0

�dis=s;

where N is the number of the parent nuclei, r thecross-section of the reaction in barns, / the neu-tron ¯ux in n/cm2-s, k the decay constant of theproduct nuclei, and T0 the time of irradiation.

To assess the radioactivity of a neutron-irradi-ated catÕs-eye chrysoberyl, we computed the ac-tivities of all radioactive nuclides which would beproduced from the four dominant neutron-in-duced reactions (i.e. (n,c), (n,p), (n,a) and (n,2n))for sample A under the following irradiationconditions:

A 10-day irradiation with a total neutron ¯uxof 5 ´ 1012 n/cm2-s will supply a gemstone with�8000 Mrads of radiation dose, which is normallyrequired for color enhancement. Table 2 shows theresults obtained from our computation. Thoseinduced radio-nuclides with a speci®c activity lessthan 2 nCi/g are not included in the table. Alsoshown in this table are the cross-sections [6], half-lives of the product nuclides, the cool-down time(de®ned as the time for the radioactivity to reduce

to 2 nCi/g) and the types of radiation emitted bythe product nuclides.

As can be seen from Table 2, of all the radio-active nuclides that could be created by neutronirradiation, only six (i.e. 45Ca, 46Sc, 51Cr, 54Mn,55Fe and 59Fe) would not cool down to the level ofspeci®c residual radioactivity of 2 nCi/g within amonth after irradiation. 55Fe has a long cool-downtime of 31 yrs, but it decays by electron captureand emits no radiation. 45Ca emits only a few low-energy gamma rays in every million disintegrations[7]. Therefore, these two nuclides will not pose anyhealth problems. The activities of 46Sc, 51Cr and59Fe would only fall to the safe limit of 2 nCi/gafter 15 months and that of 54Mn would remainabove this limit for as long as 7 yrs.

The activities of 46Sc, 51Cr, 54Mn and 59Fe im-mediately after irradiation are, respectively, 11nCi, 4.62 lCi, 120 nCi and 482 nCi. 51Cr hasthe highest activity but will decay faster since itshalf-life is relatively short (27.7 days). 54Mn has ahalf-life of 312 days, the longest of the four. If ameasurement of the half-life is made by simplytaking the radiation counts at two di�erent times,the outcome could be any value between 27.7 and312 days, depending on how early the measure-ment is conducted after the irradiation.

4. Radiation dose

One may consider any gemstone with an ac-tivity level above its legal release limit to be unsafe.Alternatively, the safety consideration may be

Time of irradiation: 10 days.Thermal neutron ¯ux: 2.5 ´ 1012 n/cm2-s.Fast neutron ¯ux: 2.5 ´ 1012 n/cm2-s.

Table 1

Concentrations (in ppm) of impurities in three Indian chrysoberyl samples

Impurity Sample A Sample B Sample C

1.01 ct 0.93 ct 1.05 ct

Ca 93 � 20 233 � 28 215 � 46

Ti 783 � 34 1130 � 47 2804 � 09

Cr 182 � 22 ÿa ÿa

Fe 7377 � 181 7955 � 216 18813 � 514

Cu 17 � 8 17 � 16 366 � 42

Ga 166 � 12 244 � 20 1897 � 81

a Below detectable limit of 40 ppm.

S.M. Tang, T.S. Tay / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 491±495 493

based on the exposure dose rate output by thegemstone. The exposure dose rate given by agemstone of activity A lCi at a distance d cm canbe calculated with the formula:

D � A� C=d2 mR=h;

where C is the so-called speci®c gamma-ray con-stant in mR-cm2/h-lCi.

If sample A was irradiated under the conditionsstated in the previous section and was released onemonth after irradiation. It would give an exposuredose rate of 11.6 mR/h at a distance of 0.5 cm atthe time of release. The dose would be mainly dueto the activities of 46Sc, 51Cr, 54Mn and 59Fe as theother radio-nuclides had already decayed to neg-ligible amounts. Table 3 shows the activities of

these four nuclides at the time of release, their Cvalues [8] and the calculated partial dose from eachof them at a distance of 0.5 cm. If one places hishand at 0.5 cm from this hypothetically neutron-irradiated stone at the time of its release, his handwill get an absorbed dose of 11.6 mrem in onehour. This absorbed dose rate is 20 times thepermissible dose rate of 0.57 mrem/h set by ICRP(International Commission on Radiological Pro-tection) for skin, hands and feet. The stone ishence de®nitely hazardous. Two years later, 46Sc,51Cr and 59Fe would have decayed away and theactivity of 54Mn would become �30 nCi. The stonewill only give an exposure dose rate of <0.5 mrem/h at a distance of 0.5 cm and can then be consid-ered not hazardous.

Table 2

Estimated radioactivity of chrysoberyl catÕs eye sample A at the end of 10-day irradiation with a total thermal and fast neutron ¯ux of

5 ´ 1012 n/cm2-s. Also listed are the reaction cross-sections, half lives of product nuclides, the cool-down times (time for the radio-

activity of a product nuclide to reduce to 2 nCi/g), and the types of radiation emitted by the product nuclides

Reaction Cross-section Half-life Activity Cool down time Radiation

r (mb) T1=2 A TC

9Be (n,a) 6He 32.8 0.807 s 2.13 mCi 18 s bÿ9Be (n,2n) 8Be 144 ~7 ´ 10ÿ17 s 9.33 mCi ~1.7x10ÿ15 s 2a18O (n,c) 19O 0.16 26.9 s 82.9 nCi 3.4 m bÿ, c16O (n,p) 16N 0.019 7.13 s 4.91 lCi 1.6 m bÿ, c27Al (n,c) 28Al 232 2.25 m 30.1 mCi 59 m bÿ, c27Al (n,p) 27Mg 4 9.45 m 518 lCi 3.2 h bÿ, c27Al (n,a) 24Na 0.725 15.0 h 94.0 lCi 11 d bÿ, c44Ca (n,c) 45Ca 1100 163 d 16.6 nCi 2.4 y bÿ, c48Ca (n,c) 49Ca 1100 8.72 m 32.8 nCi 55 m bÿ, c40Ca (n,a) 42K 13 12.4 h 241 nCi 4.8 d bÿ, c50Ti (n,c) 51Ti 179 5.76 m 1.25 lCi 1.1 h bÿ, c46Ti (n,p) 46Sc 12.5 83.8 d 11.0 nCi 1.1 y bÿ, c47Ti (n,p) 47Sc 20 3.35 d 175 nCi 29 d bÿ, c48Ti (n,p) 48Sc 0.315 43.7 h 30.6 nCi 11 d bÿ, c50Cr (n,c) 51Cr 16000 27.7 d 4.62 lCi 1.0 y c54Cr (n,c) 55Cr 380 3.50 m 249 nCi 32 m bÿ, c52Cr (n,p) 52V 1.09 3.76 m 26.3 nCi 23 m bÿ, c54Fe (n,c) 55Fe 2500 2.73 y 1.15 lCi 31 y e58Fe (n,c) 59Fe 1140 44.5 d 482 nCi 1.2 y bÿ, c54Fe (n,p) 54Mn 82.5 312 d 120 nCi 7.0 y bÿ, c56Fe (n,p) 56Mn 1.07 2.58 h 1.06 lCi 1.2 d bÿ, c54Fe (n,a) 51Cr 0.6 27.7 d 8.81 nCi 123 d c63Cu (n,c) 64Cu 4400 12.7 h 6.76 lCi 7.4 d e, b�, bÿ, c65Cu (n,c) 66Cu 2200 5.1 m 1.46 lCi 1.0 h bÿ, c69Ga (n,c) 70Ga 1700 21.1 m 20.2 lCi 7.1 h bÿ, c71Ga (n,c) 72Ga 4700 14.1 h 36.1 lCi 9.7 d bÿ, c69Ga (n,p) 69mZn 0.496 13.8 h 5.90 nCi 2.2 d bÿ, c

494 S.M. Tang, T.S. Tay / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 491±495

5. Concluding remarks

In all the three Indian catÕs-eye chrysoberylsstudies, the impurities Ca, Ti, Fe, Cu and Ga werefound. Particularly, Fe was present in a very highconcentration of �10 000 ppm. Our computationsrevealed that a 1-ct stone with such a concentra-tion of Fe would give rise to �480 nCi of 59Fe viathe (n,c) reaction and �120 nCi of 54Mn from the(n,p) reaction after receiving �8000 Mrads ofneutron dose in a reactor. 59Fe has a half-life of44.5 days and would decay down to negligibleamounts in 15 months. However, 54Mn has a rel-atively long half-life of 312 days and will not decaydown to the level of 2nCi/g until 7 yrs later. Theproperties of these two radio-nuclides ®t the ra-dioactivity characteristics of the neutron-irradiat-ed catÕs-eye chrysoberyls tested by the Centre forGemstone Testing.

Because of the variability in the type and con-centration of impurity among catÕs eye stones fromdi�erent localities, some catÕs eye stones might bemore radioactive than others after neutron irra-

diation. To measure the radioactivity of a neutron-irradiated gemstone, accurate gamma spectro-scopic analysis is de®nitely required. However,PIXE is a valuable technique for assessing thesuitability of color enhancement by neutron irra-diation for various types of gemstone.

References

[1] Gem News, Gems and Gemology 33 (3) (1997) 221.

[2] F. Watt, I. Orlic, K.K. Loh, C.H. Sow, P. Thong, S.C. Liew,

T. Osipowicz, T.F. Choo, S.M. Tang, Nucl. Instr. and

Meth. B 85 (1994) 708.

[3] K.K. Loh, Master thesis, National University of Singapore

1993.

[4] J.A. Maxwell, J.L. Campbell, W.J. Teesdale, Nucl. Instr.

and Meth. B 43 (1989) 218.

[5] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344.

[6] IAEA Technical Reports Series No. 156, Handbook on

nuclear activation cross-sections (IAEA 1974).

[7] C.M. Lederer, V.S. Shirley, Table of Isotope, 7th ed., Wiley,

New York, 1978.

[8] J. Villforth, G. Shultz, Health Handbook, US Department

of Health, Education and Welfare, 1970.

Table 3

Activities of the four major neutron-induced radio-nuclides in a hypothetically irradiated catÕs-eye chrysoberyl (sample A) one month

after the irradiation, C values of the nuclides, and exposure dose rates output by the nuclides at a distance of 0.5 cm

Nuclide Activity C Exposure dose

1 month after irrad. (mR-cm2/h-lCi) rate at 0.5 cm (mR/h)

46Sc 8.6 nCi 10.9 0.3751Cr 2.19 lCi 0.16 1.4059Fe 302 nCi 6.4 7.7354Mn 112 nCi 4.7 2.11

S.M. Tang, T.S. Tay / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 491±495 495