N. T. Le: Radioprotection 2025, 60( 1), 84 – 90 85 Fig. 1. Top view and side view of neutron calibration room at the Institute for Nuclear Science and Technology.
polyethylene spheres and for the ISO-12789 simulated workplace neutron reference field of the 239 Pu-Be source). The combined standard uncertainties of technical parameters of neutron reference fields were also investigated.
2
Material and method
2.1 Neutron reference field in Viet Nam
The neutron room at the Institute for Nuclear Science and Technology( INST) has inner dimensions of 700 cm 700 cm 700 cm, satisfied the ISO-8529 recommendation( ISO 8529-2, 2000). The layout of the neutron calibration room canbe found in Figure 1. In recent years, a X14-type 241 Am-Be neutron source has been installed at the room’ s center for calibration purposes. The neutron emission rate of the X14-type 241 Am-Be source( supplied by Hopewell Designs, Inc., USA) was 1.299 10 7 s �1 on23 January2015 with the expanded uncertaintyof2.9 %( k = 2, shown in a certificate), it is traceable to the USA National Institute of Standards and Technology. The source anisotropy correction factor at the angle of 90 degrees( perpendicular to the cylindrical neutron source’ s central axis at the source center), F 1( 90) was investigated as 1.030 using the MCNP5 Monte Carlo simulation code, with the standard uncertainty of 0.1 %( k = 1). The half-life of 241 Am-Be source is 432 yr, considered as an economy advantage point for routine calibrations. More detailed information about the source can be found in Ref.( Le et al., 2018).
Based on this neutron source, several neutron reference fields were established at the INST( Hanoi, Viet Nam), following the international criteria: i. e., ISO-8529 series( Le et al., 2018) and ISO-12789 series( Le et al., 2019). These neutron reference fields are routinely used for calibrations of neutron meters at the INST( Le et al., 2019; Nguyen et al., 2020) as well as for delivering standard neutron dose equivalents( Nguyen et al., 2024).
The neutron room at the Military Institute for Chemical and Environmental Engineering( MICEE, Hanoi, Viet Nam) is a cubic one with inner dimensions of 700 cm 700 cm 700 cm.
The room was constructed at an unoccupied area and at basement floor. The values of _ H ð10Þ outside the room were measured before putting the room into operation and yearly investigated to ensure the safety working conditions for radiation workers and public, complied with Vietnamese legal requirements. The MICEE neutron source of 239 Pu-Be was granted by Russia some decades ago. It had been stored as a disused source till being used for establishing the neutron reference field at MICEE by 2020. The neutron source strength of the 239 Pu-Be source was 4.6 10 6 s �1 in 1981 with the standard uncertainty re-assessed within 5.0 %( k = 1). This neutron emission rate is considered as constant during usage process since the source’ s half-life is as long as 2.41 10 4 yr. The cylindrical shape of the 239 Pu-Be source hasaheightof 3.3 cm anda diameter of 2.9 cm, which was installed at the floor’ s center and pumped to the height of 150 cm for irradiation. The source anisotropy correction factor, F 1( 90) was investigated as 1.029 using the MCNP6 Monte Carlo simulation code, with the standard uncertainty of 0.3 %( k = 1)( Nguyen et al., 2024). The layout of the neutron calibration room can be found in Figure 2.
This neutron reference field was established and being used for calibration purposes at the MICEE.
2.2 Bonner sphere spectrometer system
In this work, the Bonner sphere spectrometer( BSS) system was used with different spherical moderators to measure the count rate, M i( unit in count per second, cps; and“ i” stands for the BSS sphere diameter in inches) caused by neutrons from the moderated 241 Am-Be, and 239 Pu-Be sources. The BSS system has a 6 LiI( Eu) thermal neutron sensitive detector and 6 different high density( r = 0.95 g. cm �3) polyethylene Bonner spheres with respective different diameters of 2, 3, 5, 8, 10, and 12 inches( i. e., i = 0, without a moderator; 2; 3; 5; 8; 10; and 12). The cylindrical thermal neutron detector of the 6 LiI( Eu) scintillation crystal has 96 % purity of 6 Li with a height of 0.4 cm and a diameter of 0.4 cm. The BSS system was used to detect neutrons from thermal energy up to 20 MeV.