Journal of Nuclear and Radiochemical Sciences, Vol. 9, No. 2, pp. 45-47, 2008

Variation of Environmental Neutron Flux with the Depth of Water and Soil
K. Komuraa, N. K. Ahmedb, A. H. El-Kamelc, and A. M. M. Yousef baLow Level Radioactivity Laboratory, Kanazawa University, Wake, Nomi, Ishikawa 923-1224, Japan
Physics Department, Faculty of Science, South Valley University, Kena, EgyptcPhysics Department, Faculty of Science, Assiut University, Assiut, EgyptbReceived: August 1, 2008; In Final Form: December 20, 2008
As a part of interest in the study of the neutron flux in biological environments, variations of slow neutrons withdepth of water and soil were measured through the radioactivity induced in gold by 197Au(n,γ)198Au reaction. The
measurements made for 0–100 cm in fresh water and 20–400 cm in sea water showed that the thermal neutron
flux had a peak at around 10 cm depth and then gradually decreased with depth in water. The depth profile in seawater was almost the same as freshwater. In the case of soil made for 0–60 cm, thermal neutron flux showed apeak at 5 cm and decreased rapidly to 60 cm accompanying a shoulder-like shape during 10–30 cm range.

1. Introduction
Background neutrons are present throughout the earth’s
atmosphere from the interaction of primary and secondary cosmic rays with nitrogen and oxygen nuclei in the air. From a
few hundred meters above the ground to the top of the atmosphere an equilibrium condition exists in which the intensity of
the neutron background remains proportional to the local neutron production. Throughout this region the shape of the neutron energy spectrum remains essentially unchanged. However,near the air/soil or air/water boundary a discontinuity in bothneutron production and scattering properties leads to a nonequilibrium situation.1–3 In particular, neutrons produced inseawater or in air, which diffuse in seawater, are so rapidly
thermalised that near the air/seawater boundary, the thermal
flux is considerably higher than would be expected in free air.1
As cosmic rays interact with any material to produce neutrons,
the background flux measured over seawater or over soil ismade up of neutrons produced in both air and seawater or airand soil. Any massive object serves as a medium for the production of neutrons by secondary cosmic rays and therefore
represents an additional source of background neutrons. For
this reason, background measurements taken at the soil or atsea level may be higher near a large mass of material such asiron or aluminum.1
As a matter of fact, because of the considerable variety ofsoil compositions, particularly its water content, the thermal
neutron energy distribution and the thermal neutron flux in soil
should be variable.1,2 In soil, near the air/soil boundary, theproduction rates of nuclides derived from low-energy neutrons
increase to the maximum and subsequently decrease exponentially with increase in depth.3 In this paper depth profiles ofenvironmental neutrons in water and soil were measured by agold activation method coupled with an extremely low background γ-ray spectrometry.
2. Experimental method
The 197Au(n,γ)198Au reaction was used in the present work to
estimate the thermal neutron flux at different depths of waterand soil. The 411.8 keV γ-ray from 198Au (2.695 d) was measured using extremely low background Ge-detectors installed*Corresponding author. komura@yu.incl.ne.jp. Fax ; 050-13828549

in Ogoya Underground Laboratory (OUL) 4 to calculate thenumber of 198Au atoms produced per unit weight of gold andthen converted to the neutron flux using Monte Carlo CodeProgram.5
Two experiments have been made to know the dependenceof the thermal neutron flux in the depth of water. The first one
was in fresh water at Wake pond, Nomi city. The second one
was in seawater at Marine Research Laboratory of Kanazawa
University at Ogi, Uchi-ura-Machi in Noto Peninsula of Japanseaside.
In the freshwater experiment targets were distributed fromthe surface of water to 100 cm depth. Gold grain targets of 50
g set were exposed to the neutron flux. On the other hand, inthe sea water experiment, the 20 g targets were affirmed at 20,
100, 200, and 400 cm depth. The targets were exposed during
May and June, 1999.
To estimate the variance of thermal neutron flux with thedepth of soil, experimental data were taken at different soil
depths. One sample at 50 cm over the soil and seven samples
inside two holes were used. The first hole was 30 cm depththat contained three samples at 10, 20, and 30 cm depth. Other
four samples were placed at 5, 15, 45, and 60 cm depth in thesecond hole. All samples were 20 g gold grain targets packed
in 3 × 2.5 cm2 polyethylene bags. The targets were exposed tothe environmental neutrons during December, 1999. At thestart of the experiment, water content in soil was 22%.
However this value increased to about 27.5% by rain fall and34% by snow fall.
After more than three weeks of exposure for every group,the targets were collected carefully and every target waswrapped with a cadmium sheet and transferred to the extremely
low background Ge-detectors to measure for more than 4000min.
3. Results and discussion
3.1. Variation of thermal neutrons with the depth of
fresh and seawater. Measured thermal neutron values infresh water and seawater are summarized in Tables 1 and 2 andare plotted in Figure 1 together. As seen from Figure 1, thedepth profile of slow neutrons in seawater is almost the sameas that in fresh water.
Thermal neutron flux at about 10 cm depth increases 1.6
times higher than that in free air. Then it decreases rapidly
with increasing of water depth until 400 cm.

© 2008 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/31/2008

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KomuraJ. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

TABLE 1: Thermal neutron flux in fresh water
198
Au(atoms/g)

flux(10 -3 n cm-2 s-1)

Relativeflux

Free air
01020
3750
60
75
85
100

45.2 ± 6.5
44.9 ± 10.7
71.4 ± 9.6
37.4 ± 7.1
16.6 ± 3.9
15.5 ± 5.1
11.5 ± 3.610.4 ± 2.5
13.3 ± 6.0
9.2 ± 3.1

8.35 ± 1.17
8.29 ± 1.98
13.20 ± 1.77
6.91 ± 1.30
3.07 ± 0.73
2.86 ± 0.95
2.12 ± 0.86
1.91 ± 0.47
2.46 ± 1.11
1.70 ± 0.57

1.00
0.99
1.58
0.83
0.37
0.34
0.25
0.23
0.29
0.20

�����������

�������������������������� ���� ���

Water depth(cm)��� � � �������

TABLE 2: Thermal neutron flux in seawaterAu(atoms/g)

flux(10 -3 n cm-2 s-1)

Relativeflux

20
100
20040028.5 ± 9.0
8.0 ± 5.5
7.6 ± 3.7
6.7 ± 3.3

5.26 ± 1.66
1.48 ± 1.02
1.40 ± 0.69
1.23 ± 0.61

1.00
0.20
0.27
0.23

The present results at the seawater is higher than that calculated ones.1,2 However, they showed that the flux reaches its
maximum value at 8 cm in the water and decreases slowly with
the increase of the water depth until about 40 cm, which is inwell agreement with the present results.
Below 30 cm, the thermal neutron intensity decreases exponentially with depth. Edge 6 found a rapid decrease of slow
neutron intensity with the depth in the first 20 cm layer and aslower decrease at greater depth.
These results show that the thermal neutron flux at 100 cm
in seawater decreased to 28% of the value at 20 cm and to 23%
at 400 cm depth. No fundamental difference of depth profile
was observed between fresh water and seawater. This may be
due to the contribution of dissolved material in seawater(~3.7%) affects only small for absorption of neutron. However,
the rapid decreasing of flux within the first 100 cm is of interest for in future study.
3.2. Variation of thermal neutrons with the depth ofsoil. Table 3 and Figure 2 show the neutron measurement insoil. The thermal neutron flux at 50 cm above soil surface was(6.7 ± 1.1) × 10 -3 n cm-2s-1, and this value increased to a maximum value (8.6 ± 1.2) × 10 -3 n cm-2s-1 at 5 cm depth. This
increase is explained by the thermalization of fast neutrons
mainly by the soil water (22%) in humid surface soil. The
thermal neutron flux begins to decrease at 10 cm depth showing a shoulder-like decease until about 30 cm. This shoulderTABLE 3: Thermal neutron flux in soil
Soil depth(cm)free air
510152030
4560

198
Au(atoms/g)
45.2 ± 7.2
57.7 ± 8.3
44.4 ± 7.1
45.2 ± 9.3
43.8 ± 12.1
35.8 ± 5.6
18.0 ± 6.5
12.4 ± 3.6

flux(10 -3 n cm-2 s-1)
6.72 ± 1.10
8.58 ± 1.23
6.60 ± 1.05
6.72 ± 1.38
6.51 ± 1.80
5.32 ± 0.83
2.68 ± 0.97
1.84 ± 0.54

Relativeflux
1.00
1.28
0.98
1.00
0.97
0.79
0.40
0.37

��

�������������
Figure 1. Depth profile of thermal neutron flux in the freshwater andseawater.

like decease may be explained by thermalization of neurons
mainly caused by soil components. The stability of neutron
flux between 10 and 30 cm depth was not in agreement withthe calculated curves.1,2 This discrepancy may be caused bysnow fall during exposure which increased the water content ofsoil from 22 to 34%.
Wet earth tends to slow down fast neutrons from the air
rapidly. A part of these slow neutrons then diffuse back into
the air and subsequently absorbed by the 14N(n,p)14C reaction.7
Thus the energy spectrum of cosmic neutron changes in soil,resulting in more thermal neutrons.
Both the absorption of thermal neutrons by the components
of soil and the moderation by the large amount of water content
are competed each other. After 30 cm depth, the thermal flux
intensity decreased rapidly again to reach about 27% of its surface value at 60 cm depth.
The present results showed that the thermal neutron flux atthe surface of the soil is (6.7 ± 1.1) × 10 -3 n cm-2s-1 which is in��������������������������� ���� ���198Water depth(cm)���������������

Figure 2. Depth profile of thermal neutron flux in soil.

���Variation of environmental neutron flux with the depth of water and soil
-3-2 -1 2

good agreement with the calculated values (6.6 × 10 n cm s )and (6.4 × 10 -3 n cm-2s-1).3 However, the decrease pattern withthe depth of soil is different from the calculated one1 which
predicts the equilibrium value at about 23–30 cm depth. As
mentioned before, the disagreement of the present result withthe calculated data is probably due to the effect of snow cover.
4. Conclusion
Depth profiles of environmental neutron at air/water and air/soil boundary were measured by 197Au(n,γ)198Au reaction using
gold grain and/or gold sheet. The depth profiles for freshwater(0–100 cm) and seawater (20–400 cm) showed a similar variation with a small peak at around 10 cm. In the soil experimenta small peak was observed at 5 cm and a shoulder-like regionin 10–30 cm and then decreased rapidly.
Acknowledgement. We wish to express our thanks to allmembers of LLRL of Kanazawa Univ. for their grateful help,also to Dr. T. Imanaka, Associate Prof. at Kyoto University for

J. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

47

his kind help to calculate by Monte Carlo code programs.
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