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: [email protected]
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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