Measurement of thermal neutrons reflection coefficients for two-layer

Applied Radiation and Isotopes 135 (2018) 155–159

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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso

Measurement of thermal neutrons reflection coefficients for two-layer reflectors

T

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S. Azimkhania, F. Zolfagharpoura, , F. Ziaieb a b

Department of Physics, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran Radiation Application Research School, Nuclear Science & Technology Research Institute, P.O. Box 11365-3486, Tehran, Iran

H I G H L I G H T S neutrons reflection coefficients are increasing with adding reflector thickness and saturating in certain thickness. • Thermal neutron reflection coefficients of reflectors slightly increase by addition of the second layer. • Thermal maximum value of growth is obtained for lead-polyethylene compound. • The • Suitable agreement have been found between the experimental data and simulation results by using Monte Carlo code shown in our figures.

A R T I C L E I N F O

A B S T R A C T

Keywords: Thermal neutrons albedo Two-layer reflector Polyethylene Excess count Reflection Saturation thickness

In this research, thermal neutrons albedo coefficients and relative number of excess counts have been measured experimentally for different thicknesses of two-layer reflectors by using 241Am-Be neutron source (5.2Ci) and BF3 detector. Our used reflectors consist of two-layer which are combinations of water, graphite, polyethylene, and lead materials. Experimental results reveal that thermal neutron reflection coefficients slightly increased by addition of the second layer. The maximum value of growth for thermal neutrons albedo is obtained for leadpolyethylene compound (0.72 ± 0.01). Also, there is suitable agreement between the experimental values and simulation results by using MCNPX code.

1. Introduction Neutron reflection method commonly used in chemical analysis of bulk samples, neutron dosimetry, detection of land mine and explosive, determination of moisture contents in hydrogenous materials, improvement of neutron beam performance in Boron Neutron Capture Therapy, and enhancement of the multiplication factor in nuclear reactors. Neutron reflectors with different material types are used in the nuclear reactors to reduce the critical size and fuel mass of the reactor core (Dawahra et al., 2015). Reflectors should have a small atomic weight, a high scattering cross section, a high slowing-down power, and a low absorption cross section (Albarhoum, 2011). A reflector is characterized by its coefficient of reflection, or albedo which is defined as the proportion of neutrons leaving the core that are send back towards the core (Reuss, 2008). The neutron reflection depends on the elemental composition of the reflector substance and the geometrical circumstances of the measurement (Csikai and Buczko, 1999). In recent years, there have been attempts to measure the thermal neutron albedo coefficients for different materials and configurations. Thermal neutron

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albedo coefficients have been calculated for monolithic and geometric voided reflectors (Mirza et al., 2006). Also, thermal neutron albedo coefficients have been investigated for multilithic reflectors in copperwood, copper-aluminum, wood-paraffin, and paraffin-iron combinations (Mehboob et al., 2013). Both researches studied increasing of the thermal neutron albedo coefficients for reflectors. However, few studies have been reported the effect of second layers on the reflection properties. In this work, thermal neutron albedo coefficients and relative number of excess counts are measured for different thicknesses of water, graphite, polyethylene, and lead reflectors and determined their saturation thicknesses by using the instruments of 5.2Ci 241Am-Be neutron source (13.47×106 neutron/s), BF3 neutron detector, Cadmium as neutron absorber, and water as neutron moderator. Next, each of used materials have been fixed in its saturation thickness and different thicknesses of other three materials are added as second layers. Then, the effect of second layer are investigated for reflection coefficients of reflectors. Also experimental geometry are designed by using MCNPX code and simulation results are compared with experimental results.

Corresponding author. E-mail addresses: [email protected] (S. Azimkhani), [email protected] (F. Zolfagharpour).

https://doi.org/10.1016/j.apradiso.2018.01.031 Received 16 December 2016; Received in revised form 19 October 2017; Accepted 20 January 2018 0969-8043/ © 2018 Elsevier Ltd. All rights reserved.


S. Azimkhani et al.

Fig. 2. Total, elastic and (N, G) capture cross sections for natural cadmium versus neutron energy (data are taken from ENDF/B-VII.1).

Fig. 1. Photograph of experimental setup.

reflectors and their combinations. The 10B(α, n) reaction produces in result of alpha particles interaction with 10B in BF3 detector. The productions of this reaction are 7Li and 4He which7Li is produced about 94% in the excited state and 6% in the ground state. Q values of the reaction are 2.310 MeV for the excited state and 2.792 MeV for the ground state. Also, because of the wall effect, the discontinuities are observed in 1.47 MeV and 0.84 MeV according to Fig. 3. We obtain the thermal counted neutron values from sum of the counts under the curve of the channel-energy spectrum. In BF3 detector the peak locating in low channels are in association with gamma rays and thermal neutrons have been counted in channels higher than channel number 50. To calculate the thermal neutrons albedo coefficients, the following equation is used

2. Experimental procedures The measuring instrument used in this work is shown in Fig. 1. A cylindrical 241Am-Be neutron source with 5.2Ci activity is placed in water container (150 cm × 100 cm × 100 cm) at distance of 10 cm from one of the container wall. The neutron yield per 106 primary alpha particles of 241Am-Be source is 70 experimentally (Knoll, 2000). Therefore, the activity of used 241Am-Be source is 13.47×106 neutron/ sec. The 241Am-Be neutron source is a fast neutron source and we use a moderator to slow down the fast neutrons to the thermal energy range. So, we used water as neutron moderator. Water container shielded by using lead bricks which have 5 cm thickness. BF3 detector has 2.5 cm in diameter, 20 cm in length and 2320 V operating voltage located at distance of 14 cm from neutron source. The benefit of using the BF3 detector is high efficiency (greater than 90%) for thermal neutrons (with energy up to 0.5 eV) due to the presence of 10B in the detector (Castro et al., 2011). Cadmium plates with dimension 23 cm × 31 cm × 0.4 cm are placed between neutron moderator and detector. In this dimension, counted thermal neutrons without reflectors reach to background amount. Absorption cross section of natural cadmium for thermal neutrons is 2520 barn (Sears, 1992). Cross sections of natural cadmium isotopes have been extracted from Evaluated Nuclear Data File (ENDF, 2011). Then, cross sections of natural cadmium have been calculated according to isotopes abundance of natural cadmium in energy range of thermal neutrons (Vertes et al., 2011). Variation of natural cadmium cross sections with neutron energy are shown in Fig. 2. Cadmium plates as neutron absorber cause to BF3 detector only respond to the thermal neutrons reflecting from the reflector. Multichannel analyzer (MCA) and NTMCA software are used for results analyze. At first, the different thicknesses of water, graphite, polyethylene, and lead reflectors with length 30 cm and width 20 cm located above BF3 detector and the counts of thermal neutrons was recorded for 20 min. Then, the second reflector layers with different thicknesses added to first layers and the thermal neutron counts are recorded by BF3 detector.

α=

Jout Jin

(1)

where Jin and Jout indicate the incoming and scattering neutrons from reflector, respectively (Brockhoff and Shuitis, 2007). The net flow of neutrons in a reactor is described by J, which is called the neutron current density vector (The net number of neutrons passing outward through the surface per cm2/sec). The uncertainty in thermal neutron albedo is given by the following equation (Knoll, 2000)

σα =

(

∂α 2 2 ∂α 2 2 ) σJin + ( ) σJout ∂Jin ∂Jout

(2)

where σJin and σJout are the uncertainty in experimental measurements of Jin and Jout , respectively. Also, the counts of thermal neutrons have been obtained from sum of counts under the curve of the channel-energy spectrum (N1, N2 , …, Nn ), ie (3)

N = N1 + N2+…+ Nn

Therefore, the uncertainty in the counts of thermal neutrons is calculated by

σN =

σN21 + σN2 2 +…+ σN2n

(4)

where

σN1 =

3. Results and discussion

N1 ; σN2 =

N2 ; …; σNn =

Nn

(5)

At first, detector has been located in the reflector place and measured the entered thermal neutrons to the reflector by using BF3 detector (Jin). Then, water, graphite, polyethylene, and lead reflectors

Fig. 3 shows the spectrum of pulse height obtained using BF3 detector for several thicknesses of water, graphite, polyethylene, and lead

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Fig. 3. Recorded pulse height spectrums of the reflected neutrons from water, graphite, polyethylene, and lead reflectors at saturation thickness and some chosen combinations measured by using BF3 detector. The thermal neutron values are obtained from sum of the counts under these curves in channels higher than channel number 50.

added as the second layer on top of the polyethylene has no effect on the thermal neutrons albedo coefficients. The thermal neutrons albedo coefficient of water reflector is better than graphite and lead (0.83 ± 0.01). Therefore, the total thermal neutrons albedo coefficient is constant when graphite or lead as second layer are added to water reflector, but thermal neutrons albedo coefficients increases %3.87 in water-polyethylene combination (0.86 ± 0.01). The subsequent value of thermal neutron albedo is related to graphite (0.75 ± 0.02) and lead (0.62 ± 0.02) reflectors. The maximum value of albedo coefficient is 0.84 ± 0.01 in graphite-polyethylene combination and 0.78 ± 0.01 in graphite-water combination, but albedo coefficient value is constant in graphite-lead combination. When lead is used as first layer in our twolayer reflectors, the maximum increment is observed for total thermal neutrons albedo coefficients. Thermal neutrons albedo coefficient is 0.72 ± 0.01 for lead-polyethylene, 0.70 ± 0.01 for lead-water and I−I 0.66 ± 0.01 for lead-graphite combinations. Also, the formula ( I 0 ) 0 was used to calculate the relative number of excess counts, where I and I0 are the measured counts once by considering reflector and once again without considering the reflector (Papp and Csikai, 2012). The measured and simulated results of relative number of excess counts are a function of reflector thickness for water, graphite, polyethylene, and lead reflectors and their combinations plotted in Fig. 5. As seen in Fig. 5, when reflection coefficient of second layer is higher than firs layer, total reflection coefficient is slightly increasing.

with different thicknesses are located above of the detector and the thermal neutrons are reflected by reflectors and reached to BF3 detector (Jout). Also, the thermal neutrons albedo coefficients have been measured for different thicknesses of two-layer reflectors in water-polyethylene, water-graphite, water-lead, graphite-water, graphite-polyethylene, graphite-lead, polyethylene-water, polyethylene-graphite, polyethylene-lead, lead-water, lead-graphite, and lead-polyethylene combinations. The results of thermal neutrons albedo coefficients for one-layer and two-layer reflectors are shown in Fig. 4 as a function of reflectors thicknesses. Experimental geometry are simulated by using MCNPX code and are calculated thermal neutrons albedo coefficients for one-layer and two-layer reflectors with different thicknesses. The results of simulation are shown in Fig. 4 with solid line curve. It could be observed from Fig. 4, thermal neutrons albedo coefficients are increasing with adding reflector thickness and saturating in certain thickness. The saturation thickness is 8 cm for polyethylene reflector, 7 cm for water reflector, 16 cm for graphite reflector, and 10 cm for lead reflector. At invariable geometrical circumstances, thermal neutrons reflection depends on hydrogenous components of reflector material (Akaho et al., 2001). The hydrogen bound scattering cross section for thermal neutrons is 82.02 barn (Sears, 1992). Polyethylene has higher hydrogen components than other used reflectors. From Fig. 4, it could be seen that the maximum value of thermal neutrons albedo coefficient is for polyethylene (0.95 ± 0.01). So, when water, graphite or lead are

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Fig. 4. The thermal neutrons albedo coefficients as a function of reflector thickness for two-layer reflectors. Because thermal neutrons albedo of polyethylene is higher than water, graphite and lead, therefore, the curve of polyethylene is different and albedo coefficient does not increase by adding second layer to polyethylene.

4. Conclusions

of excess count for lead-polyethylene reflector has been obtained 1.96 ± 0.04 which shows maximum increment in comparison with three others combinations. Using of two-layer reflectors in addition to increasing thermal neutrons reflection coefficients could be effect to neutron shielding. Suitable agreement have been found between the experimental data and simulation results by using Monte Carlo code shown in our figures.

The thermal neutrons albedo coefficients and relative number of excess counts are been investigated for various thicknesses of water, graphite, polyethylene, and lead reflectors and their combinations using 241Am-Be neutron source and BF3 detector. The values of the reflection coefficients for our used reflectors have been found that could be increased by addition a second layer. The measured relative number

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Fig. 5. Relative number of excess counts of two-layer reflectors versus reflector thickness.

References

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