Elemental analysis of topaz from northern areas of Pakistanand assessment of induced radioactivity level after neutronirradiation for color induction

M. Wasim • W. A. Zafar • M. Tufail •

M. Arif • M. Daud • A. Ahmad

Received: 26 August 2010 / Published online: 26 October 2010

� Akademiai Kiado, Budapest, Hungary 2010

Abstract To study the impurity elements, which render

color-induced topaz long lived radionuclides, three samples

of topaz, from three different cities of the Northern Paki-

stan (Baltistan, Gilgit and Mardan) were analyzed using k0

instrumental neutron activation analysis (k0-INAA). The

samples were irradiated in Pakistan Research Reactor-1

(PARR-1) and PARR-2 at Pakistan Institute of Nuclear

Science & Technology (PINSTECH), Islamabad. The

method was validated by analyzing IAEA-S7 reference

material. In three samples a total of 22 trace level impurity

elements were quantified. Among impurities, 10 elements

including As, Ce, Ga, Ge, La, Na, Sb, Sc, U and Zn were

common in topaz of all the three places. The storage time

has been calculated for each sample required to bring the

induced radioactivity down to permissible level given by

US National Regulatory Commission.

Keywords Topaz � Gemstone � k0-INAA �Trace elements

Introduction

Topaz is a fluorine aluminum silicate with the chemical

formula Al2SiO4(F,OH)2. It exists in the form of ortho-

rhombic crystals [1]. Its structure is not regularly oriented

octahedrons having aluminum in the middle surrounded by

four oxygen atoms and hydroxide or fluoride ions above

and below. These octahedrons are held together by indi-

vidual silicate tetrahedrons but it is the octahedron chain

that gives topaz its crystalline shape. It is found in nature as

large, clear and flawless crystals. Topaz has a hardness of 8

on the Mohs scale, which is the hardest silicate mineral in

nature [2]. It crystallizes from fluorine-bearing vapors in

the last stages of solidification of igneous rocks. It is a

precious mineral due to its application as a gemstone. Pure

topaz is colorless and transparent but is usually tinted by

impurities [1].

Its chemical analysis including major and trace level

impurities have been reported by a limited number of

studies [3–5]. Olabanji et al. [4] used particle induced

X-ray emission (PIXE) and electron microprobe for the

analysis of topaz from Nigeria while Leal et al. [5] applied

k0-NAA for elemental analysis of topaz from Brazil. The

identification of impurities in topaz could lead to better

understanding of the geological environment and its

properties. Every geographic location gives rise to topaz

with slightly different concentrations of trace elements

from which its identity and possibly the origin of a gem

may be determined [6, 7]. The presence of impurities

enabled its use as a solid-state thermoluminescence

dosimeter (TLD) [8] and its color change with irradiation

has enhanced its economical importance [9]. The produc-

tion of color centers or TL in minerals depends on the

amount of trace elements or defects in the crystal structure

of irradiated samples. Impurities provide trapping centers,

which may be shallow or deep, depending upon the nature

of the impurity in the topaz crystal. There are several

methods to induce colors in topaz [9–11]. One such method

is reactor neutron irradiation, which will produce radioac-

tive nuclides in the irradiated sample mostly by (n,c)

reactions with the impurity atoms. Since, some impurity

M. Wasim (&) � M. Arif � M. Daud

Chemistry Division, PINSTECH, Nilore,

Islamabad 45650, Pakistan

e-mail: mwasim@pinstech.org.pk

W. A. Zafar � M. Tufail � A. Ahmad

Pakistan Institute of Engineering and Applied Sciences,

Nilore, Islamabad 45650, Pakistan

123

J Radioanal Nucl Chem (2011) 287:821–826

DOI 10.1007/s10967-010-0879-8

elements produce long-lived radionuclides, which in turn

will require long storage time to bring the induced radio-

activity within the safe limits. It is, therefore, very

important to identify impurities present in a sample before

irradiating it for color induction to be used for commercial

purposes.

Instrumental neutron activation analysis (INAA) is an

attractive technique to analyze geological materials [12].

INAA provides accurate results with high sensitivity and

accuracy for a large number of elements, making this

technique the most attractive for multi-element profiling

[13]. The sensitivity shown by INAA is one of the best

when compared with other analytical techniques [14]. No

study so far has been carried out to investigate impurities in

topaz of Pakistan. The main purpose of this paper is to

implement k0-INAA methodology for the elemental char-

acterization of topaz of Pakistani origin and the determi-

nation of induced radioactivity level produced by impurity

elements present therein by reactor neutron irradiation.

Experimental

Three samples as single crystals, belonging to three dif-

ferent cities of the Northern Pakistan (Fig. 1), were pur-

chased from the open market with the assurance about their

origin. The samples were designated as below:

Sample-1: Color: colorless, Origin: Baltistan, Pakistan

Sample-2: Color: pale yellow, Origin: Gilgit, Pakistan

Sample-3: Color: colorless, Origin: Mardan, Pakistan

To remove impurity inclusions on the surface of topaz

crystal, initially nitric acid (1.5 M) and then doubly dis-

tilled water was used for washings. NAA does not require

powder samples, since the same samples were used as

powder for TLD studies, all samples were therefore,

ground to fine particles. Around 50 g of each sample was

separated from a relatively larger piece of topaz and was

ground to fine powder using vibrating cup mill made of

tungsten carbide whose hardness varies from 8.5 to 9

Mohs while the hardness of topaz is 8 Mohs. The prob-

ability of contamination from grinding equipment is

negligible. The pulverized samples were dried for 24 h at

100 �C.

The k0-INAA standardization requires the determination

of the neutron flux ratio (f), epithermal neutron flux shape

factor (a) and the full energy peak efficiency. A detailed

description of the k0-INAA methodology can be found in

references [15, 16]. The f and a were determined using

Al—0.1% Au wire (IRMM-530RC, Belgium, Geel) and

ZrO2 (99.99%, Aldrich, Wisconsin) powder. Full energy

peak efficiency calibration of the detector was performed

for different detector-to-source geometries using 241Am,133Ba, 137Cs, 60Co and 152Eu point calibration sources. The

efficiency calibration covered the energy range from 59 to

1,408 keV.

Each sample weighing up to 100 mg was wrapped in

small pieces of clean paper and then packed along with the

Au and ZrO2 monitors in a polyethylene rabbit for reactor

irradiation. Irradiation was performed at Pakistan Research

Reactor-1 (PARR-1) and Pakistan Research Reactor-2

(PARR-2). PARR-1 is a 10 MW research reactor with

thermal neutron flux of about 1013 cm-2 s-1 and PARR-2

is a 30 kW Miniature Neutron Source (MNSR) having

thermal neutron flux of 1012 cm-2 s-1. In the present study,

two irradiation channels, RS1 and RS3, of PARR-1 [16]

and two irradiation channels, A2 and B1, of PARR-2 [15]

were used. After irradiation the rabbit was brought to the

laboratory and samples were shifted to pre-weighed clean

polyethylene capsules for counting.

The gamma-ray spectra were acquired using a p-type

coaxial HPGe detector (Eurisys Measures, France) coupled

through a 570 ORTEC resistive-feedback spectroscopy

amplifier to Trump PCI, 8k ADC/MCA card with Gamma

Vision-32 ver. 6 Software (ORTEC, USA). The detector

has 60% relative efficiency and FWHM of 1.95 keV at

1,332 keV gamma peak of 60Co. After acquiring spectra,

further analysis was performed by our in-house written

program GammaLab [17] using k0-INAA standardization.

All samples were corrected for background radiation,

random and true coincidence and spectral interferences.

The true coincidence correction was insignificant because

all samples were counted at more than 12 cm sample-to-

detector height.Fig. 1 Map of Pakistan showing sampling locations of topaz

822 M. Wasim et al.

123

Results and discussion

The f and a was determined for PARR-1 and PARR-2,

at two different irradiation channels of each reactor. The

f is almost double at PARR-1 [f (RS-1) = 41.33 ± 1.69,

f (RS-3) = 41.76 ± 2.33] than its value at PARR-2

[f (A-2) = 20.27 ± 1.22, f (B-1) = 20.79 ± 1.17], which

indicates more thermal than epithermal neutrons at the

irradiation channels of PARR-1. It makes PARR-1 more

suitable for the nuclides having large cross section for

thermal neutrons. In the analysis of samples, irradiations

were performed for 30 s, 1 h and 5 h. The analysis scheme

is presented in Table 1, which shows irradiation, decay and

counting times for all radionuclides measured in this work

along with their corresponding half-lives and gamma

energies used for quantification. Since topaz is mainly

Al2SiO4(F,OH)2, the induced radioactivity decays away

quickly and thus samples can be counted soon after irra-

diation. In this study, all elements were quantified using

only one suitable indicator radionuclide except for Ce and

Sb. In case of Ce, 141Ce and 143Ce were employed and for

Sb, 122Sb and 124Sb were used. Spectral interference was

not frequent; it was found for 122Sb and 134Cs at 563 keV,

for 46Sc and 182Ta at 1,120 keV and for 42K and 233Pa at

311 keV. The presence of Al in a sample interferes with the

quantification of Na by 27Al(n,a)24Na reaction of fast

neutrons. This interference is removed by subtracting

contribution in peak area of 24Na by an amount given by

Table 1 Analysis conditions for IAEA-S7 and three samples of topaz

Element Irradiation

time (Tirr)

Decay

time (Td)

Measurement

time (Tm)

Radionuclide Half-life Energy (keV)

Al 30 s 2 min 20 s 28Al 2.24 min 1778.8

As 1 h 1–5 days 1 h 76As 1.09 days 559.1, 657.05

Br 1 h 1–3 days 1 h 82Br 35.28 h 554.3, 619.1, 776.5, 698.37, 827.8,

1044.0, 1317.5, 1474.9

Ce 5 h 30 days 16 h 141Ce 32.51 days 145.4

1 h 1–3 days 1 h 143Ce 33.04 h 293.3, 664.6

Co 5 h 3–30 days 16 h 60Co 1925.3 days 1173.2, 1332.5

Cr 5 h 3–30 days 16 h 51Cr 27.70 days 320.08

Cs 5 h 3–30 days 16 h 134Cs 2.07 years 563.2, 569.3, 604.7, 795.9, 802.9

Fe 5 h 3–30 days 16 h 59Fe 44.49 days 142.6, 192.3, 1099.2, 1291.6

Ga 1 h 1 days 1 h 72Ga 14.1 h 630, 834, 894.3, 1050.7

Ge 1 h 1 days 1 h 77Ge 11.3 h 211, 264.4

Hf 5 h 3–30 days 16 h 181Hf 42.39 days 133.0, 482.2

K 1 h 1 day 30 min 42K 12.36 h 1524.6

La 1 h 1–3 days 1 h 140La 1.68 days 328.8, 487.0, 815.8, 1596.2

Mn 1 h 2 h 30 min 56Mn 2.58 h 846.8, 1810.7

Na 1 h 2 h–1 days 30 min 24Na 14.96 h 1368.6

Nd 1, 5 h 3–10 days 16 h 147Nd 10.98 days 91.1, 531

Rb 5 h 3–30 days 16 h 86Rb 18.64 days 1077

Ru 5 h 3–30 days 16 h 103Ru 39.26 days 497.1

Sb 1 h 1–3 days 1 h 122Sb 2.73 days 564.2, 692.6

5 h 3–30 days 16 h 124Sb 60.2 days 602.7, 1691

Sc 5 h 3–30 days 16 h 46Sc 83.79 days 889.3, 1120.5

Sm 1, 5 h 3–10 days 1–16 h 153Sm 46.28 h 103.2

Ta 5 h 3–30 days 16 h 182Ta 114.43 days 100.1, 222.1, 1121.3, 1189.1, 1221.4, 1231

Tb 5 h 3–30 days 16 h 160Tb 72.3 days 298.6, 879.4, 966.2, 1177.9, 1271.9

Th 1–5 h 3–30 days 16 h 233Pa 26.98 days 300.1, 311.9, 340.5

U 1 h 1–3 days 1 h 239Np 2.36 days 106.1, 209.7, 228, 277.6, 285.5, 315.9, 334.3

W 1 h 1–3 days 1 h 187W 23.72 h 134.3, 479.6, 551.5, 618.3, 772.9, 985.7

Yb 1 h 1–3 days 1 h 175Yb 4.18 days 282.5, 396.3

Zn 5 h 3–30 days 16 h 65Zn 244.06 days 1115.5

Zr 5 h 3–30 days 16 h 95Zr 64.03 days 724.2, 756.7

Elemental analysis of topaz from northern areas of Pakistan 823

123

A ¼ cNav

kMwepchrF/F 1� e�kti

� �e�ktd 1� e�ktm

� �

where Nav is the Avogadro’s number, c is the fractional

concentration of the target element, M is atomic weight of

the target element, h is the fractional abundance of target

isotope, c is emission probability of the gamma-ray, rF is

cross-section of 27Al(n,a)24Na reaction and /F is fast

neutron flux, k is the decay constant of the radioisotope,

ti is irradiation time, td is decay time and tm is measurement

time. The concentration values based on flux characteristics

from two research reactors were averaged out without

adding any weighting factor to the values based on any

specific reactor.

The k0-INAA methodology was validated by analyzing

the IAEA-S7 (soil) [18] reference material. The results of

26 elements determined in IAEA-S7 are presented in

Table 2 with uncertainties at 1 sigma level. The results of

IAEA-S7 reveal that the determined concentration of only

one element, Co, is slightly out of the reference concen-

tration, while the remaining elements were quantified

accurately within the statistical error. The maximum rela-

tive deviation of -14% was found for Co, which is mainly

due to its small peak area left after background correction.

The data in Table 2 reveals that 10 elements were quan-

tified with B5%, 8 elements with B10% and 6 elements

with [10% relative deviation. Moreover, we have reported

W in IAEA-S7 as information value.

In concentration calculations, uncertainties were esti-

mated using error propagation rules, it included uncertainty

in weight, detector efficiency, k0 factor, Q0 factor, peak

area, f, a and half-life. Significant uncertainties were found

in detector efficiency, peak area, k0 and Q0 factors. The

concentrations of different elements determined in three

Table 2 Concentration

(lg g-1) of the elements

identified in IAEA-S7 and three

samples of topaz

a Information value for IAEA-

S7b No reference value available,

reported in this study

Element Concentration in lg g-1 (or otherwise mentioned)

IAEA-S7 Topaz

Determined Reference Baltistan Gilgit Mardan

Al (%)a 4.66 ± 0.46 4.70 ± 0.18 29.41 ± 2.84 30.37 ± 2.93 29.80 ± 2.88

As 14.3 ± 1.0 13.4 ± 0.4 1.7 ± 0.1 0.69 ± 0.04 0.55 ± 0.03

Bra 7.7 ± 0.5 7.0 ± 1.8 – – –

Ce 59.5 ± 4.5 61.0 ± 3.3 201 ± 15 977 ± 63 5.4 ± 0.4

Co 7.6 ± 0.6 8.9 ± 0.4 – – 2.9 ± 0.2

Cr 66.4 ± 4.9 60.0 ± 3.8 3.3 ± 0.3 6.3 ± 0.6 –

Cs 4.8 ± 0.4 5.4 ± 0.4 4.1 ± 0.2 0.7 ± 0.1 –

Fea 24658 ± 1794 25700 ± 281 – 115 ± 12 –

Gaa 9.3 ± 0.7 10 ± 1 10.0 ± 0.6 4.6 ± 0.3 4.4 ± 0.3

Ge – – 99.1 ± 7.6 138 ± 10 126 ± 10

Hf 5.6 ± 0.4 5.1 ± 0.2 – 6.8 ± 0.4 –

Ka 11155 ± 839 12100 ± 357 495 ± 31 14.7 ± 1.3 –

La 25.1 ± 1.8 28.0 ± 0.5 19.8 ± 1.2 5.2 ± 0.3 4.9 ± 0.3

Mn 605 ± 35 631 ± 12 2.01 ± 0.24 1.15 ± 0.09 –

Naa 2224 ± 162 2400 ± 51 143 ± 6 113 ± 6 98 ± 6

Nd 31.9 ± 2.7 30.0 ± 3.1 – – –

Rb 51.7 ± 3.3 51.0 ± 2.3 15.7 ± 1.2 – –

Ru – – – 1.09 ± 0.09 –

Sb 1.8 ± 0.1 1.7 ± 0.1 4.2 ± 0.3 0.19 ± 0.01 0.19 ± 0.01

Sc 7.9 ± 0.6 8.3 ± 0.5 7.6 ± 0.5 0.014 ± 0.001 0.44 ± 0.03

Sm 4.9 ± 0.3 5.1 ± 0.2 – – –

Ta 0.8 ± 0.1 0.8 ± 0.1 4.5 ± 0.2 – –

Tb 0.7 ± 0.04 0.6 ± 0.1 – – –

Th 8.3 ± 0.5 8.2 ± 0.6 – – –

U 2.7 ± 0.2 2.6 ± 0.3 3.5 ± 0.2 16.0 ± 0.8 2.1 ± 0.1

Wb 1.57 ± 0.11 – 1.3 ± 0.1 – 0.50 ± 0.04

Yb 2.1 ± 0.2 2.4 ± 0.2 – – –

Zn 98.4 ± 8.5 104.0 ± 3.1 64.2 ± 4.3 37.6 ± 2.6 48.8 ± 3.3

Zr – – – 340 ± 30 –

824 M. Wasim et al.

123

topaz samples are presented in Table 2 with uncertainties at

1 sigma level. A total of 18 elements were determined in

sample-1, 19 in sample-2 and 13 in sample-3. The average

relative standard deviation was about 7%. All samples

contained Al, As, Ce, Ga, Ge, La, Na, Sb, Sc, U and Zn as

common elements although in different concentrations. The

data in Table 2 presents sample-2 with the largest number

of elements as compared to the other two samples. Sample-

1 can be identified with the presence of Rb and Ta along

with higher concentrations of K and La. The elements

identified only in sample-2 include Fe, Hf, Ru and Zr. The

concentration of Ce in sample-2 is significantly higher than

for the other samples. Sample-3 can be differentiated not

only on the basis of least number of impurity elements but

also by the presence of Co. Topaz is rarely formed in the

magma and most occurrences are postmagmatic-pneuma-

tolytic. The inclusion of impurity elements takes place

during the crystallization and it further depends upon the

characteristics of the magma, nature of the rock surrounding

the magma and the cooling process.

The presence of certain elements, which form long-lived

radionuclides in thermal neutron irradiation, may cause

hindrance in the commercialization of irradiated topaz

depending upon their level of concentrations. The induced

radioactivity level in topaz would require storage time to

bring the induced radioactivity down to permissible levels.

A comparison of the elements, which form long-lived

radionuclides, identified in three samples of topaz from

Pakistan with the two samples from Nigeria [4] and 20

samples from Brazil [5] shows that Fe was identified in 23

out of 25 samples, Zn in 20 samples, Co in 16 samples, while

Cs was present in only 7 samples. United States National

Regulatory Commission (USNRC) provides permissible

activity levels of various radionuclides in materials as

Schedule A (Code of Federal Regulations (10 CFR-30.70))

[19], these regulations not necessarily be applicable to other

countries. The storage time for different radionuclides has

been calculated using the permissible levels given in 10

CFR-30.70. It is assumed that all the three topaz samples,

having the composition as shown in Table 2, were irradiated

for 5 h at irradiation channel A-2 of PARR-2. The calcu-

lations reveal that 134Cs formed in topaz from Baltistan

requires 12 years to come down to permissible level of

3.3 Bq g-1. The same is true for topaz from Gilgit, but due

to lower concentration of Cs in this sample, it needs storage

time of 7 years. The third sample of topaz from Mardan does

not contain Cs but it has Co. The Co in the first two samples

(Baltistan and Gilgit) is below the limit of detection, which

is 0.27 lg g-1 for our system. The presence of Co in the

sample from Mardan requires a cooling time of 13 years to

come down to permissible level of activity which is

18.5 Bq g-1. 65Zn, which is the third most long lived

radionuclide formed in all samples, requires 1 year to decay

to permissible level of 37.0 Bq g-1. Similar calculations

were performed in the other way i.e., using the permissible

limits of activity set by USNRC and experimental condi-

tions described above, permissible level of concentrations

for Cs, Co and Zn were calculated, which were 0.7 lg g-1

for Co, 0.07 lg g-1 for Cs and 16 lg g-1 for Zn. Since Cs

and Co together were not identified in all our samples, there

is a possibility to find topaz either without Cs and Co or

having concentrations suitable for shorter decay. The whole

study suggests that elemental characterization by a sensitive

technique is very important step before performing irradi-

ation for color change.

Conclusion

In this study k0-INAA standardization has been applied for

the characterization of topaz. The samples were irradiated

at two different research reactors with different flux char-

acteristics. The method has been validated by the analysis

of IAEA-S7 reference material, which shows that 24 ele-

ments, out of 25, were quantified accurately within the 95%

confidence interval. W has been reported, for IAEA-S7, as

information value. The analysis of topaz revealed 12 to 18

elements as impurities. All samples contained As, Ce, Ga,

Ge, La, Na, Sb, Sc, U and Zn. In these elements only Zn

produces long-lived radionuclide 65Zn, which requires a

few years to decay down to permissible level of specific

activity as defined by USNRC. Two samples, however,

were identified with the presence of Cs and one sample

with Co, which require much longer decays and make these

samples practically less attractive for color induction by

reactor neutron irradiation. This study suggests that the

concentration of Co and Cs must be monitored before

opting for color induction in topaz by reactor neutron

irradiation.

Acknowledgment The authors are grateful for the help of personnel

from Material Laboratory, PIEAS. Yasir Anwar and Sajid Iqbal are

also acknowledged for their help during experimentation.

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