R
Ra ad di ia at ti io on n S Sc c ie i en nc ce es s a an nd d A Ap pp pl li ic ca at ti io o ns n s
12 – 16 November 2012/ Hurghada, Egypt
The k
0-NAA Standardization Method Using an Am-Be Neutron Source.
N. F. Soliman, G. Y. Mohamed*, M. Fayez-Hassan, M. A. Ali
Experimental Nuclear Physics Department, Nuclear Research Centre, AEA, Cairo, Egypt
Experimental Nuclear and Reactor Physics Department, AEA, Cairo, Egypt.
*E-mail: [email protected]
ABSTRACT
Instrumental neutron activation analysis is a well established technique for the analysis of trace elements in different samples. Precise elemental concentrations of Al, Mn, Mg and Na in two unknown geological samples were determined by using the k0-standardization method. For such measurements two sets of standard monitors of Gold (Au), Indium (In), Tungsten (W) and Titanium (Ta) were used. One set is bare and the other is cadmium covered. These monitors were used for measuring the irradiation position factors and using the cadmium ratios of the 115In(n, γ)116In and 182Ta(n, γ)183Ta interactions. Neutrons were obtained from CNIF2 facility that uses an Am-Be radio-isotopic neutron source with a modification to have thermal and epi-thermal neutrons.
Measurements were carried out using a gamma-ray spectrometer consisting of a hyper pure germanium detector and necessary associated electronics. The k0- standardization method can be used for quality control tests.
Keywords: Neutron Activation Analysis, Flux Ratio, Irradiation Parameters, Elemental Analysis.
INTRODUCTION
In addition to reactor neutrons, radio-isotopic neutron sources such as
241Am/Be, Pu/Be and 252Cf with suitable activity are also used to study neutron capture reactions (1-4). The main advantages of the radio-isotopic neutron sources are: they are portable, generate stable neutron fluxes (long lifetime), physically small size, rugged construction, easy to use and to shield, low cost, and can be used to study the elements with short half life times. The main goal of the present work is to use the delayed gamma neutron activation analysis (DGNAA) by the k0-standardization method to determine the elemental
concentrations of Al, Mn, Mg and Na in unknown geological samples.
EXPERIMENTAL
Sample preparation is the process of converting the raw form of samples to the suitable form for the used analytical technique. In the present work the geological samples were crushed to tinny granules by mechanical reduction of rock pieces to a smaller particle size in a stepwise sequence, to have a homogeneous powder for each sample. Samples weight of about 80 gm were put in a plastic capsule of 1.5 cm radius and of 5 cm height.
CNIF2 is an irradiation facility(5) that provides neutrons from an Am-Be neutron source. The present activity of the source is about 175 GBq. To determine the neutron flux shape, the factor, and the thermal to epithermal neutron flux ratio, the f factor, the Am-Be neutron source was surrounded by a cylinder of paraffin wax of 2.5 cm thickness in order to moderate fast neutrons emitted from the neutron source to thermal and epi-thermal neutrons as shown in figure (1).
Position of Samples Irradiation (Am-Be) Neutron Source
Paraffin Wax (2.5 cm)
Figure (1): shows the geometrical positions of irradiated samples
The k0- standardization method is achieved by determining the and f factors. The factor corresponds to the deviation of the non-ideal (1/E1+ ) epithermal neutron spectrum from that of the ideal (1/E) behavior, where E is the neutron energy (6). The f factor corresponds to irradiation parameters of thermal and epithermal neutron flux ratio. This was done by using some standard foils of known weights such as Au, W, Ta and In.
As shown in the sketch diagram of figure (1), bare and cadmium covered foils were put on a circle around the paraffin cylinder approximately at the neutron source hot area plane. These foils were irradiated for suitable times and the activated bare and cadmium covered foils were counted with a gamma ray spectrometer.
The spectral characteristics at the irradiation positions are important parameters to determine the
and f factors in the k0-NAA standardization method. These factors can be determined experimentally using different methods such as the cadmium-ratio multi-monitor method and the bare multi- monitor method, the so-called bare triple monitor method.In the present work, a new method was developed to determine the and f factors using the Cadmium-ratio multi-monitor method. Our method depends on irradiation of two sets of certain monitors, with and without cadmium covered, and uses the following equation (5)
α F R 1
Q
f o Cd Cd ,
R
Cd N
Pwt
mSDC N
Pwt
mSDC
Cd (1)where Npis the net peak area under the gamma- ray peak of interest,Sis the saturation factor
1etirr
, is the decay constant, tirr is the irradiation time,D is the decay factor (etd)where td is the decay time,C is a term used for correcting the decay during the counting period and is given by
m t
t e m
1
where tmis the measuring real time, FCd is the cadmium transmission factor for epithermal neutrons (usually FCd =1 but FCd, Au = 0.991)(5). Nuclear data for different combinations of the standard isotopes were used in calculating Qo
αusing the following equation:
1eV
E 1 α 2
0.429 E
0.429 α Q
Q α
Cd α
r o o
(2)
where ECd is the effective Cd cut-off energy (in standard conditions ECd = 0.55 eV) and Er
is the effective resonance energy. The (1eV) term (numerically unity) originates from the definition of the epithermal neutron flux in a 1/E1+ distribution.
The nuclear data for the used standard isotopes are given in Table (1).
The 115In and 181Ta samples were used to calculate the thermal to epithermal neutron flux ratio ( f ) and the neutron flux shape factor for the samples irradiation positions, while the 186W was used to test the measurements accuracy. The 197Au samples were used as a co-irradiated gold monitors.
Table (1): Nuclear data for some standard foils used to determine the "f" and " "
factors.
Target
isotope Q0 Er, eV Product
isotope T1/2 E , keV k0;Au 115In 16.8 1.56 116mIn 54.3 min 1293.5 2.29
181Ta 33.3 10.4 182Ta 114.4 d 1121.3 8.27E- 2
186W 13.7 20.5 187W 23.72 h 685.7 3.71E- 2
197Au 15.7 5.65 198Au 2.695 d 411.8 1
Using equation (2) for different suggested values of the parameter (α) ranging from -0.1≤ α ≤ +0.1, the irradiation parameters and f can be determined from the intersection of the f values versus as shown in the plot of figure (2) .This was done by using two standard foils of known weight such as Indium (In) and Titanium (Ta). The result of Figure (2) indicates that f = 30.8 and = 0.06.
-0.10 -0.05 0.00 0.05 0.10 0.15 0.20
20 25 30 35 40 45
f
In Ta
Figure (2): Deviation of Alpha Values of the Epithermal Neutron Energy Power from the Ideal (1/E) Spectrum.
Accuracy of the Method.
In order to test the applied technique, a pure tungsten foil of known weight is used as a monitor to evaluate the measurements accuracy. Table (2) presents the results of the calculated concentrations using the k0 standardization- method at α=0.06, compared by its certified values. A gamma ray spectrum of the standard Tungsten sample is shown in Figure (3).
Table (2): A comparison between the measured and the certificate value of Tungsten (W) standard sample.
Standard Sample Certificate Concentration Calculated concentration
Tungsten (187W) 100 % 94.91 ± 5.9 %
0 400 800 1200 1600 2000
Energy, keV 0
20 40 60 80 100
Counts 134.2 keV 479.6 keV 685.7 keV 1460 keV (BG)
Figure (3): The Gamma-ray energy spectrum of the Tungsten (187W) standard sample.
Elemental Concentration for Geological Samples.
The Nuclear Material Authority of Egypt provided us with two geological samples (basalt and feldspar) from the eastern desert of Egypt. The samples were irradiated for 39 day at the neutron source irradiation position as shown in figure (1). The induced gamma ray activities of each sample were measured for 500 and 2000 seconds using an HPGe γ-ray spectrometer as shown in figures (4) and (5). The k0 standardization method was used in determining precisely the elemental concentration of the constituents using the comparator method. The activation by epithermal neutrons was taken into consideration in addition to that by thermal neutrons.
0 500 1000 1500 2000 2500 3000 Energy, keV
0 5 10 15 20 25 30 35 40 45
Counts 2754.0 Na-24
1460.0 K-40
1368.6 Na-24
843.8 Mg-27 Irradiation time: 95.52 hr Cooling time: 57.96 sec Measuring time: 500 sec
Feldspar
1778.9 Al-28
Figure (4): The Gamma-ray energy spectrum of neutron activated Feldspar sample. (tm = 500 sec) .
0 500 1000 1500 2000 2500 3000
Energy, keV 0
40 80 120 160 200
Counts
Basalt Irradiation time: 94.65 hr Cooling time: 513 sec Measuring time: 2000 sec
846 Mn - 56 1368.6 Na - 24 1460.0 K - 40 1810 Mn - 56 2754.0 Na - 24
Figure (5): The Gamma-ray energy spectrum of neutron activated Basalt sample.
(tm = 2000 sec)
The delayed gamma ray neutron activation analysis using the Am-Be neutron source has to be a valuable technique for the qualitative and quantitative determination of elements in such samples.
In the ko standardization method of NAA, the concentration
a of an analyte"a" is obtained from the equation (3)
p,aAu p, a o,
Au o, Au
o, Au m P
a m P
a ε
ε α Q f
α Q f a k
1 SDC Wt N
SDC Wt δ N
(3)
Where "Au" refers to the co-irradiated gold monitor. The results are illustrated in Table (3) for elemental concentration in ppm of 24Na, 28Al, 27Mg and 56Mn isotopes.
Table (3): The average elemental concentrations of some Egyptian geological samples (in ppm).
Sample Element Eγ keV Concentration, ppm
24Na 1368.6
2754 18381 ± 4 %
28Al 1778.9 724.34 ± 8.9 % Basalt
56Mn 1810.7 1471.77 ± 10.5 %
24Na 1368.6
2754 21782.07 ± 5 %
27Mg 843.8 124825.09 ± 8.3%
Feldspar
28Al 1778.9 1505.86 ± 7.7 %
The results indicate that, Sodium was identified by its two gamma-ray lines 1368.6 and 2754 keV of 24Na. The gamma ray line 1778.9 keV due to 28Al indicates the presence of Aluminum. Magnesium (27Mg) was identified by gamma ray energy 843.8 keV. The gamma ray lines 848, 1810 keV and 2113 keV of 56Mn indicate the presence of Manganese.
CONCLUSIONS
On view of the designed and calibrated DGNAA assembly using an Am-Be neutron source for analyzing geological samples we conclude that, the designed system can be used for different industrial applications. The delayed gamma ray spectra obtained in the present work were good enough for qualitative and quantitative elemental analysis for such complex samples. The design of shielding materials and neutron flux measurements at different places around the assembly has been done in optimum conditions. The increase of the neutron flux at the target position is recommended to have more promising data.
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