Open Journal of Applied Sciences, 2013, 3, 514-518
Published Online December 2013 (http://www.scirp.org/journal/ojapps)
http://dx.doi.org/10.4236/ojapps.2013.38060
Open Access OJAppS
Measurements of Natural Radioactivity in Some Granite
Samples Using Alpha Spectrometric Analysis
Hanan Mohamed Diab1, Mohamed Helmy Eweis Monged1, Mahmoud Khattab2
1Egyptian Nuclear and Radiological Regulatory Authority, Cairo, Egypt
2Nuclear Materials Authority, Cairo, Egypt
Received September 12, 2013; revised October 13, 2013; accepted October 21, 2013
Copyright © 2013 Hanan Mohamed Diab et al. This is an open access article distributed under the Creative Commons Attribution
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ABSTRACT
Alpha spectrometry using pulse height analysis has been used for the determination of uranium concentrations in dif-
ferent environmental samples. The concentration of 238U was measured by both destructive and non-destructive tech-
niques with a detection limit of less than 1.8 mBq/kg. However, because of the extremely low 234U concentrations in
environmental samples, it was necessary to use a destructive technique to separate U from the sample matrices as well
as remove interfering elements from the sample solution to determine 238U/234U ratio. In this study, the uranium was
separated from the environmental samples using anion exchangers in (Dowex 1 × 8 Cl− form) and purified via co-pre-
cipitation with Lanthanum fluorides (LaF3) and the alpha source prepared by electrodeposition. The results obtained
were validated using some certified reference samples.
Keywords: Uranium Separation; α-Spectrometry; Alpha Pulse Height Analysis Isotopic Ratio; Granite Samples
1. Introduction
Uranium is a widely distributed lithophile metallic ele-
ment. It may be present as a significant constituent in
some minerals (e.g. uraninite, brannerite and carnotite) or
as an accessory element in others (e.g. zircon, apatite,
allanite and monazite). The natural uranium consists of
three radioisotopes; 238U, 235U and 234U with atomic abun-
dances of approximately 99.275%, 0.72%, and 0.0055%
respectively. All three isotopes comprise the natural ura-
nium and have the same geochemical behavior [1]. The
uranium concentration and U-isotopic ratios are usually
detected and determined in various environmental, sam-
ples by different non-destructive and destructive tech-
niques. The non-destructive techniques are mostly achieved
by γ-spectrometry (e.g. NaI- and HPGe-detectors). They
are carried out on the bulk samples without the need for
complicated and time consuming radiochemical separa-
tions methods [2,3].
Moreover, the destructive techniques are carried out
through several analytical methods (e.g. α-spectrometry,
fluorimetry, kinetic phosphorescence, neutron activation
analysis, etc). Among these techniques, α-spectrometry is
the most common one that measures radioisotopes and
can detect low uranium concentrations (below ng1−1). Its
detection limit is typically 100 to 1000 times lower than
γ-spectrometry [4]. This technique is mostly used for
detection and analysis of U as well as Th radioisotopes,
particularly in the environmental samples, such as natural
waters, which are characterized with low radioactivity
concentration levels [3,5].
The procedure for alpha spectrometry is carried out
through several steps including sample preparation, ra-
diochemical separation, preparation of a thin alpha source,
such as via electro-deposition or co-precipitation, and α-
counting employing high-resolution pulse height analysis
[6-9]. Sample preparation aims to convert the sample into
a thin layered, chemically isolated form that can be placed
into the spectrometer and counted with a minimum alpha
particle energy interferences and self absorption, and alpha
peak broadening due to energy straggling with thick sources.
It is often an extensive process and requires several steps
including: 1) sample digestion (preliminary treatment); 2)
uranium separation and purification 3) alpha source prepa-
ration, and 4) alpha counting over extended periods for
low detection limits. The uranium separation is usually

H. M. DIAB ET AL. 515
carried out using various techniques such as co-precipi-
tation, liquid-liquid extraction, ion exchange and extrac-
tion chromatography. Three main methods are commonly
used for preparation of the sample on a stainless steel
disc (source preparation) namely; 1) direct evaporation
from an organic solvent; 2) electro-deposition and 3)
coprecipitation with NdF. The later method is more pre-
ferred technique but it requires careful preparative steps
to eliminate organic material and to adjust the pH of the
electrolyte. Several methods and flow charts have been
reported in different literatures for sample preparation for
U-analysis using α-spectrometry [6,10,11].
2. Experimental Work
2.1. Non-Destructive Analysis
Six granite samples were collected from eastern desert of
Egypt and were prepared for destructive and nondestruc-
tive technique. For nondestructive analysis, the samples
were mechanically pulverized and passed through 0.8
mm mesh sieve. Samples were collected using the stan-
dard methods to get composite sample that represents
each site. The dried and sieved portion of the samples
were transferred to Marinelli beakers of 100 or 1000 ml
volume and sealed at least for 4 weeks to reach secular
equilibrium between radium and thorium, and their proge-
nies. 226Ra (238U series), 228Ra (232Th series), and K ac-
tivities were measured using gamma-spectrometry based
on hyperpure germanium detectors. The HPGe detector
had a relative efficiency of 40% and full width at half
maximum (FWHM) of 1.95 keV for 60Co gamma energy
line at 1332 keV and operated with Canberra Genie 2000
software for gamma acquisition and analysis. The gam-
ma transmissions used for activity calculations were
351.9 (214Pb), 609.3, 1120.3 and 1764.5 keV (214Bi) for
the 226Ra series, 338.4, 911.1 and 968.9 keV Ac) for the
Th-series and 1460.7 keV for 40K presented in gamma
specrtrum below in Figure 1. The gamma-spectrometers
were calibrated using both 226Ra point source and potas-
sium chloride standard solutions in the same geometry as
the samples [12].
2.2. Destructive Analysis
For destructive analysis, 10 g of ashed soil sample was
spiked with uranium tracer (232U) for chemical yield
monitoring. The dried samples were ashed at 550˚C for
eight hours. The ashed sample was dissolved in 40 ml of
65% HNO3, 15 ml of 37% HCl and 10 ml of 40% HF
acids. Uranium in the dissolved sample solution was ex-
tracted from most of the matrix elements with 25 ml of
0.2 M TOPO/Cyclohexane (Trioctyl-phosphine oxide)
and then back-extracted with 25 ml of 1 M NH4F/0.1 M
HCl solution. The solution is co-precipitated by LaF3 (25
mg/ml of La(NO3)3 with HF 40%). Then, the solution is
centrifuged and the formed precipitate is dissolved in hot
boric acid (saturated solution) and HNO3. The uranium is
re-oxidized to the hexavalent state by adding H2O2. This
followed by evaporation of the solution to dryness and
the obtained residue is dissolved in 9 M HCl, and then
passed through a conditioned anion exchange resin col-
umn (15 cm long; its inner diameter is 8 mm) at a flow
rate of 1 ml/minute. The used resin is 2 g Dowex 1 × 8
Cl− form, 50 - 100 mesh (strongly basic gel type poly-
styrene resin) with appropriate functional groups. To
elute U from the column, 0.5 M of HNO3 is passed
through the column with a flow rate of 1 ml/minute and
the eluted U is evaporated to dryness in a crystallizing
dish using 1 ml of concentrated HCl. The eluted uranium
is transferred into the electrolysis cell from the crystalli-
zation dish with 0.4 ml of 4 M HCl, three times by 1 ml
of (NH4)2C2O4 (4%) and then once 0.6 ml distilled water.
The electrolysis is carried out for 3 hours at 300 mA (0.3
A), and then 1 ml of ammonia solution (NH4OH with
25% conc.) is added. After one minute, the electrolysis
current is cut off. The ammonia increases the OH con-
centration which prevents re-dissolution of the hydroxide
from the cathode surface and then measured by alpha
spectrometry [13]. Schematic radiochemical procedure of
uranium is shown in Figure 2.
2.3. Uncertainty Calculation
The error associated with any particular counting result is
determined by the use of the following equation.
2
NN
tt t


 


r
(1)
where, r: is the net count rate.
Figure 1. An example of gamma spectra.
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H. M. DIAB ET AL.
516
Figure 2. Schematic representation of the radiochemical
procedure of uranium.
In this case we are interested in subtracting one count
from another (gross counts minus background counts)
and determining the resulting % error of the NCPS (Net
Count Per Second) based on the standard deviation σ
value. Counting instruments typically have a confidence
interval of 95%. Thus Equation (2) is written as:
0
0
YsY
n
s
rr
tt
  (2)
where,
-r0Y, rsY : are the net count rate at the gamma line (Y)
for the background and the sample respectively,
-to, ts: are the real counting time of the background and
the sample respectively.
2.4. Validation of the Method
The precision and accuracy of the method were deter-
mined by analyzing reference materials: soil IAEA-326,
IAEA-375 and sediment IAEA-300.The precision achieved
was 6.7% for U isotopes. Typical lower limits of detec-
tion for the alpha measurements were 1.6 mBq/kg for
238U and 1.8 mBq/kg for 234U. Blank samples and reagent
blanks were processed and measured at the beginning of
the analysis to batch to trace any cross contamination
which might occur during the analysis steps. The data
obtained shows good accuracy without any sign of cross
contamination.
2.5. Apparatus
The alpha spectrometry system employed 450 mm2 sili-
con surface barrier detectors, (ORTEC model 576 A) with
450 mm2, USA. The silicon surface barrier detector was
characterized by high resolution performance, low back-
ground, excellent stability and high permissible counting
rates. The detector resolution was about 25 keV for 241Am
and the detector efficiency was approximately 23% with
no significant variation in the range interval 2.5 - 8.8
MeV. It was determined using the following equation:
c
N
A
t
 
where, η is the detector efficiency, N is the counts of the
alpha peak, A is the activity of the radionuclide and tc is
the counting time.
The system was vacuum controlled (anti-recoil) and
controlled with ORTEC software for calculation of the
radionuclide activity. The counting time used for meas-
urements was 4 - 8 days, depending on the sample activ-
ity, to achieve a detection limit of about 0.002 Bq per
sample. The counting time can be reduced by increasing
sample weight, but it was found that it is cost effective
because as the sample weight increase the chemicals and
acids needed for digestion will increase and the time re-
quired for sample digestion will also increase. The meas-
ured 238U, 235U and 234U activity concentrations were
reported in Bq/kg as shown in Figure 3. The chemical
yield for the process involved in alpha sample analysis is
around 70%. The system energy calibration was performed
with a mixed alpha source containing 239Pu (Eα = 5.1
MeV), 241Am (Eα = 5.48 MeV) and 244Cm (Eα = 5.8 MeV)
radionuclides. They have the same chemical composition,
concentration, geometry as well as counting configura-
tion. The detection limit of the α-spectrometry was about
as 0.002 Bq per sample [14,15].
3. Results and Discussion
3.1. Non-Destructive Analysis
The three most common primordial radionuclides inves-
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H. M. DIAB ET AL. 517
Figure 3. An example of U-isotopes measured by Alpha Spectrometer.
tigated in the study area were K, U (Ra) and Th.
3.2. Destructive Analysis
5U, and 238U as well as ac-
ction
lim
Table 1. The activity concentration in (Bq/kg) for the ana-
40 238226232
The tabulated activity for the naturally occurring ra-
dionuclides 238U (226Ra) and 232Thare the average of the
activities of most abundant photo peaks of the decay pro-
ducts of the uranium series (351, 609, 1120, 1764 keV)
and thorium series (238, 583, 911 keV). The specific
activities of 238U, 232Th and 40K for the collected samples
were shown in Table 1. 238U, 232Th and 40K concentra-
tions ranged from 44.9 to 149 Bq/kg, from 4.8 to 79.6
Bq/kg and from 19.7 to 334.1 Bq/kg respectively. The
high concentrations of 238U activity might be due to its
geological formation.
The concentrations of 234U, 23
tivity ratios of 234U/238U, and 235U/238U measured were
shown in Table 2. The specific activities of 238U ranged
from 13.2 to 66.1 Bq/kg with an average of 37.2 Bq/kg,
while the specific activities of 234U ranged from 13.8 to
62.6 Bq/kg with an average of 36.7 Bq/kg. The ratios of
234U/238U ranged from 0.94 to 1.05 Bq/kg with an aver-
age of 0.98 Bq/kg. A very good correlation (r2 = 0.998)
exists between 234U and 238U as shown in Figure 4.
The specific activities of 235U were below the dete
it of the system. The isotope ratio involving the minor
isotope namely 234U, can be obtained with a reasonable
accuracy of about 5%, which is promising especially
lyzed samples measured by HpGe Detector.
Sample 226Ra (238U-series) 228Ra (232Th-series) K-40
S1 91.1 ± 3.9 52.0 ± 1.6 288.0 ± 6.0
S2 77.2 ± 3.1 4.8 ± 0.2 195.7 ± 1.
S3 129.9 ± 5.1 79.6 ± 2.1 334.1 ± 6.9
S4 149.0 ± 5.3 52.0 ± 1.7 288.2 ± 6.1
S5 139.3 ± 5.0 75.6 ± 1.9 360.8 ± 3.9
S6 44.9 ± 3.9 25.3 ± 1.5 152.8 ± 5.3
Table 2. The acentratiog) fo
lyzed samples measured by-ectrometry in reference and
ctivity conn in (Bq/kr the ana-
sp
granite samples.
Sample 238U 234U 234U/238U
S1 40.3 ± 1.1 38.4 ± 1.1 0.95
S3 58.4 60..7
IAE
IAE
6 ± 1.2 ± 21.02
S6 13.2 ± 0.6 13.8 ± 0.4 0.95
A-326 32.0 ± 1.4 31.4 ± 3.4 0.98
A-375 13.2 ± 0.4 13.9 ± 0.4 1.05
IAEA-300 66.1 ± 4.8 62.6 ± 5.1 0.94
wimpl
The validations of the meod were tested using IAEA
26 (soil), IAEA-375 (soil),
an
th lower sae volume.
th
reference materials IAEA-3
d IAEA-300 (sediment) samples. The values obtained
were in good agreements with the reference values re-
ported by IAEA indicating robustness of our procedure.
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H. M. DIAB ET AL.
518
Figure 4. Correlation between U-238 and U-234 measured
by alpha spectrometry.
al technique for the determination of
soil samples was developed and
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A simple analytic
uranium isotopes invali-
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results obtained using current procedure indicates that
these radioactivity concentrations are of natural origin.
The isotopic values of approximately one between 234U
to 238U indicate secular equilibrium between these two
isotopes in the soil samples.
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