Measurements of Natural Radioactivity in Some Granite Samples Using Alpha Spectrometric Analysis

doi:10.4236/ojapps.2013.3860

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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

Email: [email protected], [email protected]

Received September 12, 2013; revised October 13, 2013; accepted October 21, 2013

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.





 





(1)

where, r: is the net count rate.

Figure 1. An example of gamma spectra.

Open Access OJAppS

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:

YsY



 (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:





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-

granite samples.

Sample 238U 234U 234U/238U

S1 40.3 ± 1.1 38.4 ± 1.1 0.95

S3 58.4 60..7

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),

th lower sae volume.

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.

Open Access OJAppS

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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4. Conclusion

A simple analytic

uranium isotopes invali-

dated by testing several IAEA reference samples. The

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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