Journal of Nuclear and Radiochemical Sciences, Vol. 9, No. 2, pp. 49-52, 2008

Some Applications of the Gold Activation by Environmental Neutrons
K. Komura,*,a N. K. Ahmed,b A. H. El-Kamel,c and A. M. M. Yousef ba

Low Level Radioactivity Laboratory, Kanazawa University, Wake Tatsunokuchi, 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
The activation of gold by environmental neutrons was used to study the effect of concrete buildings on the neutron
flux and to estimate the thermal neutron flux in and around the building of Kinki University Training Reactor.
The results showed that three ceilings of 34 g cm-2 thickness decrease the fast neutrons to 26% from its originalvalue. However, the same reinforced concrete decreases the slow flux to only 62% from its original value. On theother hand, the thermal neutron flux at 283 m from the reactor center was two times higher than the environmental background. These results can give attention to the effect of concrete walls as a shielding around the reactors,
especially those that have been hold near to the living environments.

1. Introduction
The vertical cut-off of geomagnetic rigidity and the change
of geographical latitudes affect the distribution of the secondary cosmic radiation including neutrons. However, at sea level
and its vicinity, the measured neutron data are more affectedby the meteorological conditions.1,2 The primary cosmic ray
interacts with all materials to produce secondary cosmic neutrons. The background measurements at sea and land weremade up of neutrons produced, attenuated, reflected, andabsorbed by air, water, or ground.2 The meteorological data
includes air pressure, overhead cloud-amounts, wind speed,
precipitation, and water contents. This means that the airground boundary effects on the spatial and energy distributionsof the neutrons depend greatly upon the type of local surroundings.
The results of altitude dependence measurements of theenvironmental neutron flux made by Yousef3 showed some difference between the measured neutron flux inside and outside
the concrete buildings. This difference appeared as a decreaseof the measured flux inside the buildings comparing with thatoutside the buildings at the same altitude and position. The
attention was directed to the effect of concrete. An advantageof the experiment is that the composition of the concrete can betaken as approximately that of the earth with constant moisture
contents.4 Considering the effect of the concrete, this effect wasmore clearly at the high altitudes than the low altitudes for slow
neutron flux.
As one of the most important applications of the gold activation by environmental neutrons is the study of the neutron fluxin the environment around the strong neutron sources such asreactors, accelerators, and so on. Around these sources, theneutrons contribute with great part of radiation doses to theworkers in these facilities and the other surrounding circumstances.

and high cross section (98.5 ± 0.4 barn) for thermal neutrons,
the 198Au was the most convenient neutron detector as a productof 197Au(n, γ)198Au reaction with 100% abundance. The βdecay of 198Au occurs with the emission γ-rays with 411.8 keV(95.56%). The γ-ray measurements were made using an
extremely low-background γ-ray counting system constructedin the tunnel of former Ogoya Copper Mine (270 mwe depth).5
After neutron exposure, the targets were wrapped with cadmium sheets and brought back to laboratory as early as possible
to start γ-ray measurements. The 411.8 keV γ-ray from 198Au
was measured to calculate the number of 198Au atoms produced
in unit weight of gold and then converted to the neutron flux
using the three-dimensional Monte Carlo code MORSE-CG.6
3. Sampling
To study the effect of concrete buildings on the environmental neutron flux, the experiments were mainly conducted in a
3-storied concrete building of Low Level Radioactivity
Laboratory (LLRL), Kanazawa University. As seen in Figure
1, data were taken at each floor, as well as the roof of the building. Three targets of 50 g gold grains inside paraffin of 7.5 cm
thickness were exposed at the first and second floors and theroof of LLRL. In addition, twelve gold targets of 20 g grains
were distributed on the roof and the three floors of the building. Four target sets composed of bare, Cd (0.5 mm thick) covered, and paraffin (7.5 cm thick) covered were exposed at every
point. Three kinds of the targets in each set were placed at����������������
����������������
�������������
��������������
�����������
����

2. Experimental technique
The environmental neutron measurements were carried out
indirectly through the radioactivity induced in pure gold by
neutron activation. Because of its suitable half-life (2.694 days)*Corresponding author. E-mail: komura@yu.incl.ne.jp������������
���������������

�������

���������� �����������
������������� ���� �������������� ����� � �Figure 1. Target set positions at LLRL.

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

50

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

least 80 cm apart from each other to avoid interaction. The
black circles in Figure 1 show the position of the targets in thefirst experiment, while the open circles represent the second
experiment. As a point of notification, the second floor-target
has only one concrete ceiling in the first measurements asshown in Figure 1. The thickness of every reinforced-concrete
layer was estimated to be about 34 g cm-2.
On the other hand, at Kinki University Training Reactor(UTR-Kinki), 25 gold sheet targets were divided to 5 groups toestimate the thermal neutron flux inside and around the reactor(Figure 2). The first group (A1–A4) consisted of 4 targets,
which were exposed inside the reactor body for one-hour operation. Two of these targets were placed in the core of the reactor and the other two were placed at 20 cm above the core. One
target at each place was covered with a 0.5 mm Cd sheet toestimate the Cd ratio in the high intensity flux. The targets
were cooled for 17 days before γ-ray measurement because of
their high radioactivity level. The targets of the second group(3, 6, 8, and 11) were distributed inside the reactor room at four
different distances from the reactor center. This group was
exposed for one hour only during the reactor operation to estimate the thermal neutron flux around the reactor body during
the operation. After 24 h cooling, the γ-ray of targets was measured by Ge detectors. The targets of the third group were
exposed at the same points of the second group for one week toevaluate the average neutron flux over one week operation.
This group consisted of four bare targets (4, 7, 9, and 12) inaddition to two targets with Cd cover (5 and 10). The otherfive gold targets were distributed inside the reactor building torepresent the fourth group (13–17). This group was placed toevaluate the intensity of the thermal neutron flux with which
the workers are exposed for one week. The last group consisted of 6 targets (18–23), which were distributed outside thereactor building until 283 m distance to estimate the thermal
neutron flux in the reactor environment and to evaluate therunaway neutrons from the reactor to the surrounding environ-

�����
���
��

��

��

��

N=

C
1
1
1
,
ε 1–exp(– λt R) exp(– λt D) 1–exp(– λtC)

A = λ N,(1)

where A is the reaction rate of 197Au(n,γ)198Au, λ is the decay
constant of 198Au, C is the peak count, ε is the peak efficiency,t R is the irradiation (exposure) time, t D is the decay (cooling)time before counting, and tC is the counting (measuring) time.
4. Results and discussion
4.1. Influence of concrete buildings on the environmental neutron flux. Tables 1 and 2 and Figure 3 depict theresults of the concrete effect experiments at LLRL. The effectof reinforced concrete is clearly observed from these results.
The neutron fluxes were gradually decreased from the roof to
the first floor exponentially. The fast neutron flux at the firstfloor showed 36% and 26% from its values at the roof in thefirst and second experiments, respectively. As a comprehensive estimation, the second experiment is the more interestingfor discussion. Although in the fast neutrons measurement, thereduction ratio of 3rd floor / roof and 1st floor / 2nd floor wasconstant, about 13%, this ratio increased to 47% for 2nd floor/
3rd floor. The cause of this high reduction is due to the thermalization of the fast neutron by the interaction or scatteringwith the components of the library, such as aluminum shelves,
books, and so on, where the targets were placed. Cadmium
measurements have a similar behavior like the fast neutrons
TABLE 1: Fast neutron flux measured with paraffin
absorber

���
����

��

ment. Two of these targets were placed together at the far
point. This group was exposed for one week, except for one of
the two targets, which was left for one-month exposure to compare with the other target.
All targets were wrapped in Cd sheets during the coolingtime. The short exposure and cooling times were correctedduring the calculations of the production of gold atoms (N)using the known equation 1.

�����������������������

Figure 2. Target set positions at UTR-Kinki.

198

Exposureelevation

Au(atoms/g)

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

Relativeto roofRoof
2nd floor
1st floor

75.7 ± 10.5
57.5 ± 10.6
27.2 ± 7.0

1.95 ± 0.27
1.48 ± 0.27
0.70 ± 0.18

1.00
0.76
0.36

TABLE 2: Environmental neutron fluxes measured with bare state, with paraffin, and Cd covered at LLRL building
Exposure Condition
Slowneutron(Bare)
Fast
neutron(Paraffin covered)Cadmium
measurement(Cd covered)

Exposure elevation
Roof
3rd floor
2nd floor
1st floor
Roof
3rd floor
2nd floor
1st floor
Roof
3rd floor
2nd floor
1st floor

Au (atoms/g)

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

51.6
41.5
34.9
32.4
102
88.4
39.9
26.2
15.1
10.6
5.74
3.46

7.70
6.20
5.20
4.80
1.97
1.70
0.77
0.51
3.20
2.30
1.40
0.75

198

± 8.4
± 9.6
± 10.1± 7.4± 16.2± 14.6
± 8.9
± 7.7
± 4.6
± 4.1
± 3.4
± 3.1

±
±
±
±
±
±
±
±
±
±
±
±

1.24
1.43
1.50
1.10
0.31
0.28
0.17
0.15
1.00
0.88
0.73
0.67

Reduction ratio
1.00
0.81
0.68
0.62
1.00
0.86
0.39
0.26
1.00
0.72
0.44
0.23

Cd ratio

3.4
3.9
6.1
9.4

±
±
±
±

1.2
1.7
4.0
8.6

Some Applications of the Gold Activation by Environmental Neutrons7�����������������

������������������ ���� ��� ���������������������

��������������������������������

����

����

���������

��������� ���������

Figure 3. Slow neutron, fast neutron and Cd covered flux at LLRL.

because its value depends mainly on the epithermal flux. On
the other hand, the slow neutron flux showed a different behavior. The reduction ratio of 3rd floor/roof was about 19% and
13% for 2nd floor/3rd floor, respectively. This ratio decreased to
6% for 2nd floor/1st floor. The reduction at the third floor camefrom the concrete effect, whereas at the second and first floors,
the absorbed slow neutron flux by concrete is compensated by
the thermalization of the high energy neutron flux by the concrete and the other components such as furniture of the building.
The Cd ratio increased gradually from 3.4 at the roof to 9.4
at the first floor, which means that the thermal neutrons are themost predominant than the epithermal neutrons at lower floors.
This effect is a result of the thermalization of fast neutrons by
the concrete. From these results, it is evident that the concrete
is an effective shield for the high-energy neutrons. Three ceilings of 34 g cm-2 thickness decreased the flux to 26% from its
original value. However, the same reinforced concretedecreased the slow flux to only 62% from its original value.
These results can give attention to the effect of concrete walls
as a shielding around the reactors, especially those that havebeen hold near to the living environments.
4.2. Estimation of the thermal neutron flux around the
Kinki University Reactor. The thermal neutron flux intensity
was estimated inside and around the research reactor of Kinki
University. The neutron source for starting the reactor is PuBe (37 GBq). The estimated thermal flux at the core of thereactor is 1.2 × 107 n cm-2 s-1 and 1.2 × 105 n cm-2 s-1 at 20 cm
over the core. On the other hand, the fast neutron flux is 1.3 ×
106 n cm-2 s-1 at the core. According to Prof. Morishima, Cd
ratio is estimated to be 5–7 at the core of the reactor. Table 3
shows the results of one-hour exposure targets. The thermal

51

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

neutron flux at the core of the reactor is 1.24 × 10 n cm s-1
and 5.52 × 105 n cm-2 s-1 at 20 cm above the core. The Cd ratioat the two points is 13.4 and 13.6, respectively. These values
are about 2 times higher than the official value. The large values of the present data may be caused by the thick (0.5 mm)
Cd sheet used for covering the gold target or the target position
was higher than that of the bare gold experiment. The epithermal neutrons are reflected and moderated from the fast neutron
by the components of the reactor or the moderator. The resultsaround the reactor body showed no clear relation between theneutron flux and the distance from the reactor. This is due tothe target position in respect to the neutron source. The targets
3–5 were placed at 3.1 m over the reactor. The others were set
beside the wall of the reactor body (6, 7) or steel door (8–10)
or the concrete outer wall (11, 12). So, the adjacent material
may have some effect on the neutron flux. This effect is moreclearly at 5.7 m where the targets number 8–10 were stock tosteel door. The thermal neutron production over steel bodies isabout 11 times higher than the environmental background.7
This is due to the marked rises of the flux at that point. The
average thermal flux inside the reactor room was about 77.6 ±3.5 n cm-2 s-1 over the reactor and on an average 15.2 ± 2.3 n
cm-2 s-1 around the reactor room.
Table 4 shows that the thermal neutron flux measured for
one week at the same points has the same behavior except thatits intensity was distributed over one week exposure. Thismeans that the measured results for one hour are correct. The
average of the thermal neutron flux over one week decreased to(3.35 ± 0.18) n cm-2 s-1 and (0.68 ± 0.16) n cm-2 s-1 over andaround the reactor, respectively. These results show that the
exposure for one week decreased the average flux intensity to
4.3% and 4.5% over and around the reactor, respectively, comparing with its value during the operation. The Cd ratios at 3.1
m and at 5.7 m were 8.2 and 8.4, respectively. Comparing theseratios with those obtained inside the reactor body, it is shownthat the Cd-shield is more effective at high neutron flux.
The thermal neutron flux distribution inside the reactor
room is plotted together with the flux inside the building in
Figure 4 to show the variation of the intensity with the distancefrom the reactor center. This figure shows obviously that theintensity of the thermal neutron flux decreases exponentiallywith the distance from the reactor. These results indicate thatthe thermal neutron flux inside the control room, where theworkers spend most of the time during the operation of thereactor, was about (0.41 ± 0.04) n cm-2 s-1. This value was measured by targets number 13. The average of the corresponding
values in the hall was about (0.08 ± 0.04) n cm-2 s-1. This value
decreased to (0.034 ± 0.008) n cm-2 s-1at the entrance of thebuilding.
Figure 4 shows also that the thermal neutron flux decreases
gradually with the distance outside the reactor building, but theflux at 283 m is still higher than the environmental background

TABLE 3: Neutron flux measured for one-hour operation of the UTR-Kinki reactor
Target No.

Condition

Distance fromthe reactor center
(m)

A1
A2
A3
A4

Bare
Bare
Cd covered
Cd covered

0
0.2
0
0.2

6
3
11
8

Bare
Bare
Bare
Bare

1.9
3.1
5.2
5.7

198
Au
(atoms/g)

Thermal neutro flux
(n cm-2 s-1)

Cd ratio

Inside the reactor
1.10 × 1011
4.90 × 109
8.20 × 109
3.60 × 108
Inside the reactor room
1.18×105 ± 1.5×104
6.89×105 ± 3.1×104
1.30×105 ± 1.2×104
1.58×105 ± 1.6×104

1.24 × 107
5.52 × 105
1.78 × 106
7.81 × 104
13.3 ± 1.6
77.6 ± 3.5
14.6 ± 1.3
17.8 ± 1.8

13.4
13.6

52

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

TABLE 4: Neutron fluxes inside and around the reactor room
Target No.

Condition

Distance fromthe reactor center
(m)

7
4
12
9
5
10

Bare
Bare
Bare
Bare
Cd covered
Cd covered

1.9
3.1
5.2
5.7
3.1
5.7

13
14
15
16
17

Bare
Bare
Bare
Bare
Bare

6
9.7
12.6
15.7
17.6

19
20
18
21
23
22

Bare
Bare
Bare
Bare
Bare
Bare

12
40
70
140
283
283

198

Au
(atoms/g)
Inside the reactor room
4.85E+03 ± 5.6E+02
2.97E+04 ± 1.6E+03
5.58E+03 ± 3.5E+02
7.63E+03 ± 5.9E+02
3.64E+03 ± 2.0E+02
9.07E+02 ± 1.5E+02
Inside the reactor building
3.66E+03 ± 3.5E+02
8.96E+02 ± 1.2E+02
9.05E+02 ± 1.4E+02
3.60E+02 ± 6.8E+01
3.04E+02 ± 7.3E+01
Outside the reactor building
1.65E+02 ± 3.7E+01
1.89E+02 ± 2.5E+01
2.17E+02 ± 3.8E+01
8.69E+01 ± 2.3E+01
1.42E+02 ± 1.5E+01
1.37E+02 ± 2.3E+01�������������������

������������������������ ���

�����

�����

�������������������

���

8.2 ± 0.6
8.4 ± 1.5

4.12E-01 ± 4.0E-02
1.01E-01 ± 1.4E-02
1.02E-01 ± 1.5E-02
4.10E-02 ± 7.7E-03
3.40E-02 ± 8.2E-03
1.86E-02 ± 4.1E-03
2.13E-02 ± 2.9E-03
2.45E-02 ± 4.3E-03
9.79E-03 ± 2.6E-03
1.60E-02 ± 1.6E-03
1.54E-02 ± 2.6E-03

Acknowledgement. The authors wish to express deep thanks
to Dr. T. Imanaka, Associate Prof. at Kyoto University for hiskind help to calculate the neutron flux using Monte Carlo code
programs, also to all members of UTR-Kinki especially Prof.
Morishima, UTR-Kinki, and to Mr. H. Kofuji, LLRL of
Kanazawa University for their grateful help during the UTRexperiments.

�������

�����

5.47E-01 ± 6.3E-02
3.35E+00 ± 1.8E-01
6.29E-01 ± 4.0E-02
8.60E-01 ± 6.6E-02
7.90E-01 ± 4.4E-02
1.97E-01 ± 3.2E-02

Cd ratiostability of the environmental neutron flux over a long period
on average. From this figure it is evident that the thermal neutron flux rapidly decreases with the increase of the distancefrom the reactor center. This effect is due to the interaction orabsorption of the thermal neutrons with or by the air, concretebuildings and the other surrounding materials. Also the figure
shows obviously that the rise of the environmental neutron fluxin the environment around the reactor is caused by the diffusion of the reactor neutrons or sky shine effect.

� �� � �� ��� �Thermal neutron flux
(n cm-2 s-1)

���

����

���������������������������������
Figure 4. Slow neutron flux inside and outside the building of UTRKinki.

level3 (7.2 ± 0.6) × 10 -3 n cm-2 s-1. The marked increase of theflux at 40 m and 70 m is due to that the targets which wereplaced inside aluminum boxes, where the production of theneutron flux is about 3 times higher than the environmental
background.7 The drop of the flux at 140 m distance is due tothe effect of much concrete buildings around the workshop station, where the target was placed. The thermal neutron flux at
283 m was (16.0 ± 1.6) × 10 -3 n cm-2 s-1, which is about two
times higher than environmental neutron. Target 22 was
exposed at the same point for one month and showed nearly thesame value, (15.4 ± 2.6) × 10 -3 n cm-2 s-1. This result shows theReferences(1) C. Chung, and C.Y. Chen, J. Geophys. Res. 102(D25), 298(1997).
(2) C. Chung, C.Y. Chen, and C. H. Kung, Appl. Radiat. Isot.
49, 415 (1998).
(3) A. M. M. Yousef, Doctoral thesis. South Valley University,
Egypt (2000).
(4) M. Yamashita, L. D. Stephens, and H. W. Patterson, J.
Geophys. Res. 71, 3817 (1966).
(5) K. Komura. Proc. 9 th Intern. Symp. on Environmental
Radiation, eds. T. Tsujimoto and Y. Ogawa, Tsuruga,
Fukui, Japan, p.56 (1998).
(6) M. B. Emmett, MORSE Monte Carlo Radiation Transport
Code System, ORNL4972 (1975).
(7) K. O'Brien, H. A. Sandmeier, G. E. Hansen, and J. E.
Campbell, J. Geophys. Res. 83 , 114 (1978).