Effect of Moisture on Prompt Gamma-ray
Yield from Soil Samples
[E301]
Syed Haseeb Ali Ahmad
ID #: g201301070
Supervised By:
Dr. Akhtar Abbas Naqvi
1
Effect of Moisture on Prompt Gamma-ray Yield from Soil Samples
Abstract
Effect of moisture on prompt gamma-ray yield was measured in the soil samples via 14
MeV neutron inelastic scattering and thermal neutron capture using a LaBr3:Ce detector.
Water and silica fumes samples were used for the PGNAA (Prompt Gamma ray Neutron
Activation Analysis) Setup energy calibration to identify the hydrogen and silica peaks.
With addition of water in soil, a considerable increase has been observed in gamma ray
yield from hydrogen (H) and oxygen (O); however the gamma ray yield has decreased
from silica (Si). Since we have not change the Si contents in samples, it means decrease
in gamma yield from silica is due to loss of 14 MeV neutrons caused by water. The water
behaves as moderator for neutrons which leads to decrease in gamma ray yield. Finally, it
leads to conclusion that the moisture content in soil may cause an error in determining the
elemental concentration of soil via 14 MeV neutrons inelastic scattering.
2
Table of Contents
Abstract ............................................................................................................................... 2
1.0 Introduction .................................................................................................................. 4
2.0 Experimental Setup ....................................................................................................... 7
2.1 LaBr3:Ce Gamma-ray Detector ................................................................................... 9
2.1.1 Intrinsic Spectrum of LaBr3:Ce Detector ............................................................... 10
3
Prompt Gamma Ray Yield Measurement ………………..........................................11
3.1 Calibration of the PGNAA Setup ............................................................................. 11
3.2 Prompt Gama-ray Analysis of Soil Samples ............................................................ 14
4.0 Results and Discussion ............................................................................................. 15
5.0 Conclusion ................................................................................................................. 17
6.0 Acknowledgement ..................................................................................................... 17
7.0 References .................................................................................................................. 18
3
1.0 Introduction
The concentration of hydrogen-carbon in soil defines the amount of environmental
contamination due to oil spill in the soil. However the hydrogen contents in soil may be
due to moisture/water. Therefore the study of moisture contents in soil is desired to
estimate actual amount of hydrogen in soil contaminated due to oil. In this experiment,
the PGNAA technique is used to analyze the moisture contents in soil using 14 MeV
neutron beam.
There are various techniques to measure moisture contents of the bulk material. Neutron
techniques are also used to determine moisture contents of bulk samples through its
hydrogen contents. There are two neutron techniques namely transmission technique and
moderation technique, which are used to determine moisture contents of the bulk
samples. The transmission technique is more suitable to analyze large size samples. An
accelerator-based fast neutron transmission setup was used to determine moisture
contents of soil samples [1].
Prompt Gamma Ray Neutron Activation Analysis (PGNAA) technique has a wide range
of industrial applications in several disciplines such as process control tasks in
manufacturing industry, contraband detection and well logging in oil exploration industry
[2]. The Prompt Gamma Neutron Activation Analysis (PGNAA) is an isotopic or
elemental radio analytical method. It is based on the radiactive neutron capture (n, γ), a
fundamental nuclear reaction that takes place for each isotope except 4He. In the radiative
neutron capture reaction a target nucleus absorbs a neutron, thus an excited compound
nucleus is formed which de-excites promptly by emitting prompt γ-rays as shown in
figure 1. The excitation energy equals the binding energy in addition to the kinetic energy
of the neutron. The compound nucleus needs about 10-16 s to decay. The nucleus reaches
4
its ground state, typically in 10-9 - 10-12 s, by emitting 2 to 4 gamma rays in a single spill.
Gamma rays (γ) are called prompt, if their decay times following the capture, are much
shorter than the resolving time of the detection system, which typically is in the range of
10 ns to 10 μs. The prompt gamma-radiation is characteristic and its intensity is
proportional to the number of atoms. Most elements emit several hundreds of different
energy prompt gamma rays [3]. If the ground state of the daughter nucleus is stable, the
reaction stops down.
Figure-1: Neutron Captured and Radioactive decay [2]
Besides neutron capture, elastic scattering occurs, both within the sample and from the
sample into the apparatus. The neutron then interacts with the nucleus without exciting it.
The neutron changes its outgoing direction and loses kinetic energy by transferring
energy to the recoil of the scattering nucleus. This energy loss is the mechanism, by
which neutrons are moderated and thermalized with. If the neutron energy is above the
5
energy of the first excited state of the nucleus, it can scatter inelastically by exciting the
nucleus [4].
In this technique, fast neutrons are used to irradiate a material. Some of the fast neutrons
are moderated by the material in an external moderator. A neutron generator is, usually,
used as a neutron source, in which deuterons are accelerated at a few hundred keV and
irradiated onto a deuterium or tritium target, with a yield of about 10-5 neutrons per
deuteron. The neutron energy from the fusion reaction (1) is about 2.5 MeV and from the
fusion reaction (2) is about 14 MeV [3,5].
H + 2H → 3He + n
2
H + 3H → 4He + n
2
(1)
(2)
These neutrons produce prompt γ-rays by interacting with the material through neutron
inelastic scattering (n, n’γ) or thermal neutron capture (nth, γ) reactions. The elemental
composition of the sample can then be determined from the intensity of prompt γ -rays
produced, either through neutron inelastic scattering (n, n’γ) or thermal neutron capture
(nth, γ) or both [4-6].
Prompt gamma rays emitted by the irradiated samples due to inelastic scattering of
neutrons from C, N and O elements. LaBr3:Ce Gamma-ray detector is used to count
gamma rays with good energy resolution. The figure 2 shows a gamma rays decay
scheme due to inelastic scattering of fast neutron from C, N and O elements.
6
Figure-2: Gamma Ray decay from inelastic scattering of neutrons from C, N and O
The intensity of gamma rays is directly proportional to elemental concentration in the
sample. The detection sensitivity of a prompt gamma ray depends upon the gamma ray
detector performance [4].
2.0 Experimental Setup
The schematic diagram of 14 MeV neutron based PGNAA setup is shown in Figure 3.
The experimental setup consists of a cylindrical plastic container filled with a (ϕ90 mm x
145 mm) sample placed 7.0 cm away from a tritium target at a 0o angle with respect to
the 14 MeV neutron beam. The gamma ray detector is placed at a center-to-center
distance of 19 cm from the sample at an angle of 90o-130o with respect to the 14 MeV
neutron beam. Tungsten blocks are inserted between the neutron target and the gamma
ray detector to shield it from the direct beam of 14 MeV neutrons, further the detector
7
was also shielded from 14 MeV neutron-induced gamma ray background through
massive lead shielding inserted between the detector and the tungsten shield [7,8].
Figure-3: Schematic diagram of 14 MeV neutron-based PGNAA setup used for detection
of H concentration in Soil samples [7]
The lead shield was quiet effective in shielding the detector against scattered neutrons
and background gamma rays. However, the gamma ray peaks appeared in the background
spectrum due to inelastic scattering of 14 MeV neutrons from lead shielding was quite
pronounced in the pulse height spectrum. The collected data was processed using
standard NIM electronics modules connected to a personal PC as a multichannel analyzer
(MCA). The NIM electronics block of the experiment is shown in figure 4. The prompt
gamma-ray spectrum of the LaBe3:Ce was recorded for a certain interval of 2500 s [7,8].
8
Figure-4: The block diagram of control electronics for PGNAA experimental setup
2.1 LaBr3:Ce Gamma-ray Detector
Recently developed radiation hardened Lanthanum-Halide (LaBr3:Ce and LaCl3:Ce)
gamma ray detectors with improved light output, decay time and energy resolution have
widened the scope of applications for the Prompt gamma-ray neutron activation analysis
(PGNAA) technique. Even though lanthanum halide detectors have an intrinsic activity
due to radioactive decay of a naturally occurring unstable La isotope, they have been
successfully employed in high count rates studies because this type of detector can handle
higher count rates than the conventional NaI detectors. But, due to their intrinsic activity,
lanthanum-halide detectors may not be suitable in low-level counting experiments [8,9].
PGNAA setup employing lanthanum halide detectors are expected to have better
performance than those employing NaI detectors because lanthanum halide detectors
have surpassed conventional NaI detector in terms of light decay time, energy resolution
and high count-rate handling capabilities. This detector also has faster decay time of 60
9
ns and can operate over wide dynamic range of count rate with little variation in the
energy resolution. Moreover, LaBr3 has approximately a factor of two improved energy
resolution as compare to NaI, and with full width half maximum (FWHM) less than 3%
at 662 keV and 30% higher detection efficiency [10].
2.1.1 Intrinsic Spectrum of LaBr3:Ce Detector
The LaBr3:Ce gamma ray detector biased with 588 volts with positive polarity, and its
intrinsic activity was measured, as a reference, using standard NIM electronics modules.
The intrinsic peak was founded in channel number 78. Figure 5 shows the pulse height
spectrum of the detector itself recorded over a period of 2500 seconds. It shows the 1468
(1436+32) keV gamma line of the detector’s intrinsic activity resulting from the sum of
the 1436 keV gamma due to beta decay of
138
La isotope and the 32 keV X-ray
fluorescence peak due to K shell X-ray fluorescence of
capture by
138
137
Ba produced in the electron
La. The intrinsic activity rate was determined from the integrated counts
under the 1468 keV peak [10].
10
250
Gamma Ray Exp. Yield
La (1436 + 32) Intrinsic
200
La (789) Intrinsic
150
100
50
0
40
60
80
Channel Number
100
120
140
Figure-5: Intrinsic activity spectrum of the cylindrical 76mm x 76 mm (diameter X
height) LaBr3:Ce gamma ray detector
3
PROMPT GAMA-RAY YIELD MEASUREMENTS
3.1 Prompt Gama-ray Analysis of Calibration Samples
In this study water and silica fume samples were analyzed to identify prompt gamma ray
peak location of H, Si and O. A pulsed beam comprising 14MeV neutrons was produced
via a T(d,n) reaction using a pulsed deuteron beam that was 200 ns wide and that had a
31kHz frequency. The typical pulsed beam current for the accelerator was 60 μA. The
fast neutron flux from the tritium target was monitored using a cylindrical 76 mm x 76
mm (diameter x height) NE213fast neutron detector that was 1.8 m from the target and
with a130o angle to the beam. The prompt gamma ray spectra from the LaBr3:Ce detector
were recorded for a preset time. The 14.8 MeV neutron flux was measured at 106 n/cm2/s,
and the total gamma ray count rate for the LaBr3:Ce detector with the sample was
approximately 11–12 kHz.
11
To indentify the Si, H, and O elements in soil samples using PGNAA setup, two samples,
namely water and silica fume, were analyzed to calibrate the setup. In water,
concentration of oxygen is very high as compare to hydrogen. Therefore, water can be
used to determine the position of oxygen and hydrogen peaks. The spectrum of water
shows four peaks: Oxygen full peak, single escape SE, double escape DE, and Hydrogen
peaks, respectively from right to left, whereas the silica fume spectrum shows one sharp
peak refers to Si. The intrinsic peaks appear in both spectrums. The pulse height spectrum
of the LaBr3: Ce detector for water and silica fume samples is shown in figure 6.
10
5
9
8
7
6
La (1468) Intrinsic
5
4
Pb SE
__________ Water
.......................Si Fume
Gamma Ray Exp. Yield
3
Si (1780)
2
H (2223)
10
Pb (2610)
4
9
8
7
6
O Double Escape
5
O Single Escape
O (6116)
4
3
2
10
3
100
150
200
250
Channel Number
300
350
400
Figure-6: Full prompt gamma ray spectra of LaBr3:Ce gamma ray detector for Silica
fume and water, taken with 14 MeV PGNAA setup
The spectra exhibits the full energy peaks along with associated escape peaks. For the
1.780 MeV prompt gamma rays from Si, for the 2.223 MeV prompt gamma rays from
hydrogen, the full energy and single escape SE peaks have been detected while for the
6.116 MeV prompt gamma rays of oxygen, the single escape, and double escape DE
12
peaks have been detected along with the full energy peak with equally spaced channel
spacing of order 24.
Table-1: Prompt Gama ray Energy Calibration
Peak
Channel Number
Corresponding Energy
(keV)
Si
108
1780
H
136.5
2223
O
340
6116
Figure 7 shows the fitted curve of the energy calibration for the LaBr 3:Ce detector. The
calibration equation is shown on the graph where E referred to the prompt gamma energy
while C corresponds to the channel number.
7000
Prompt Gamma Ray Energy (KeV)
6000
5000
E = 18.85 C - 300.1
4000
3000
2000
1000
0
0
100
200
Channel Number
300
Figure-7: Energy calibration curve of the LaBr3: Ce detector
13
400
3.2 Prompt Gama-ray Analysis of Soil Samples
In order to measure the moisture contents in soil, the hydrogen and oxygen gamma ray
yields in three soil samples (dry soil, dry soil + 100 ml H2O, and dry soil + 150 ml H2O)
were measured using PGAA technique. To distinguish the H and Si peaks' locations in
the soil samples, the dry soil sample spectrum was superimposed to the dry soil plus
100ml water sample and dry soil plus 150ml water sample full spectrum as shown in
figure 8.
10
5
9
8
7
La (1468) Intrinsic
6
5
4
H-SE
Gamma Ray Exp. Yield
3
Si (1780)
__________ Dry Soil Sample
Pb-SE
...................... Dry Soil + 100ml H2O Sample
2
__________ Dry Soil + 150ml H2O Sample
H (2223)
10
Pb (2610)
4
9
8
7
6
O Double Escape O Single Escape
5
O (6116)
4
3
2
10
3
100
150
200
250
Channel Number
300
350
400
Figure-8: Full prompt gamma ray spectra of three soil samples taken with 14 MeV
PGNAA setup
Each sample has different concentrations of both hydrogen and oxygen. The height of
peaks is also varying from one sample to another. The height of peak (represents gamma
ray experimental yield) is proportional to concentration of the corresponding element.
[7,11].
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4.0 Results and Discussion
The gamma ray yield spectra for dry soil sample, dry soil plus 100 ml water sample and
dry soil plus 150 ml water sample are shown in figures 8, 9, 10. The gamma ray yield
from hydrogen and silica is shown in figure 9, whereas gamma ray yield from oxygen is
shown in figure 10. The gamma ray yield from hydrogen and oxygen increases as water
was added to samples. However, the gamma ray yield from silica decreases with addition
of water contents to dry soil sample.
14x10
3
__________
Si (1780)
Dry Soil Sample
............................. Dry Soil + 100ml H2O Sample
_____________ Dry Soil + 150ml H2O Sample
12
Gamma Ray Exp. Yield
H-SE
Pb-SE
H (2223)
10
Pb (2610)
8
6
100
110
120
130
Channel Number
140
150
160
Figure-9: Prompt gamma ray spectra of three soil samples taken with 14 MeV PGNAA
setup (Silica and Hydrogen Window)
15
___________ Dry Sample
2400
................................ Dry Sample + 100ml H2O Sample
O-DE
O-SE
Gamma Ray Exp. Yield
______________ Dry Sample + 150ml H2O Sample
2200
O (6116)
2000
1800
1600
290
300
310
Channel Number
320
330
340
Figure-10: Prompt gamma ray spectra of three soil samples taken with 14 MeV PGNAA
setup (Oxygen Window)
With increase in water contents, the gamma yield of hydrogen in pulse height spectra was
increased. After addition of water to soil samples, the pulse height spectra of Silica (Si)
show a larger reduction of gamma yield than that in the dry soil sample. This reduction in
gamma yield is due to the fact that water works as moderator for incident neutrons and
hence causes fast neutrons attenuation. This is the reason why height of silicon peaks
decreases with increase in moisture.
Since we have not changed the Si contents in the samples, it means decrease in gamma
ray yield from silica is due to decrease in neutron flux attributed to water moderation
effect. The water behaves as moderator for neutrons which leads to decrease in gamma
ray yield. The hydrogen contents in measurement may be due to either moisture or any
other H-containing contaminants. Hence an expected error in measurement of
contamination (C/H ratio) by oil is observed due to moisture.
16
5.0 Conclusion
Effect of moisture on prompt gamma-ray yield was measured in the soil samples via 14
MeV neutron inelastic scattering using a LaBr3:Ce detector. The PGNAA technique was
used to identify the Si, H, and O elements in soil samples. With addition of water in soil,
a considerable increase has been observed in gamma ray yield from hydrogen (H) and
oxygen (O); however the gamma ray yield has decreased from silica (Si). Since we have
not change the Si contents in samples, it means decrease in gamma yield from silica is
due to decrease in neutron flux attributed to water moderation effect. It concluded that the
measured hydrogen contents may be due to either moisture or any other H-containing
contaminants. Hence an expected error in measurement of contamination (C/H ratio) by
oil is observed due to moisture. Finally, it leads to conclusion that the moisture content in
soil may cause an error in determining the elemental concentration of soil via 14 MeV
neutrons inelastic scattering.
6.0 Acknowledgement
I would like to express my special thanks and appreciation to Dr. Akhtar Abbas Naqvi,
who personally take interest to make my research successful. I am also very thankful to
the all people who supported me during this research, Dr. Fatah Z.Khiari, Mr. Rashid,
and Mr. Khokhar.
17
7.0 References
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technique, Nuclear Instruments and Methods in Physics Research A 497 (2003) 569–576
[2] Gabor L. Molnar, Handbook of Prompt Gamma Activation Analysis with Neutron Beams.
Kluwer Academic Publishers, 1st edition (2004)
[3] A.A. Naqvi, M.S. Abdelmonem, Ghada Al-Misned, Hanan Al-Ghamdi, Performance
improvement of keV Neutrons-based PGNAA setups, Applied Radiation and Isotopes 64 (2006)
1631–1636
[4] Glenn F. Knoll, Radiation Detection and Measurement, John Wiley & Sons, Inc, 3rd
edition(1999)
[5] Naqvi A. A., M. Maslehuddin,M. A. Garwan, M.M. Nagadi, O.S. B. Al-Amoudi,M. Raashid,
and Khateeb-ur-Rehman, "Effect of Silica Fume Addition on the PGNAA Measurement of
Chlorine in Concrete." Applied Radiation and Isotopes 68(2010), 412-417
[6] Krane, K. S, Introductory Nuclear Physics. New York: John Wiley & Sons, Revised edition
(1988)
[7] Naqvi, A.A. , Al-Matouq, F.A., Khiari, F.Z., Isab, A.A., Raashid, M., Khateeb-ur-Rehman .
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[8] Naqvi A. A., Fares A. Al-Matouq, F. Z. Khiari, A. A. Isab. Khateeb-ur-Rehman, M. Raashid.
Prompt gamma tests LaBr3:Ce and BGO detectors for detection of hydrogen, carbon oxygen in
bulk samples. Nuclear Inst. and Methods in Physics Research, A 684 (2012) 82-87
[9] M. Balcerzyk, M. Moszynski, M. Kapusta, Nuclear Instruments and Methods in Physics
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[10] Naqvi A. A., ZameerKalakada,, M.S. Al-Anezi, M. Raashid, Khateeb-ur-Rehman, M.
Maslehuddin and M. A. Garwan , F.Z. Khiari, A. A. Isab and O.S. B. Al-Amoudi. Detection
Efficiency of Low Levels of Boron and Cadmium with a LaBr3:Ce Scintillation Detector. Nuclear
Inst. and Methods in Physics Research, A 665 (2011) 74–79
[11] A. Favalli, H.C. Mehner, V. Ciriello, B. Pedersen, Investigation of the PGNAA using the
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18