Thermal Fluence Rate Research Articles

Thermal neutron reference radiation facility with high thermalization and large uniformity area

A new thermal neutron facility based on 12 241Am–Be neutron sources has been established at the National Institute of Metrology, China. It has two independent irradiation fields, the inner field and outer field, constructed with high-purity graphite and heavy water as moderators, respectively. Three innovative designs, including the reflection cavity, reflection layer, and homogenizing lens, were introduced to improve the performance of the facility. The reflection layer can increase the thermal neutron fluence rate by about 3–4 times, the reflection cavity also enhances thermal fluence by 3–4 times by taking advantage of multiple scattering of neutrons, and the homogenizing lens mounted on the neutron exit surface of the moderator improve the distribution of thermal neutron. The characteristics of the two fields were investigated by Monte Carlo simulations and experimental measurements. The results show that the new facility has high thermalization and large uniformity area with these innovative designs. For the inner field, the thermal neutron fluence rate at the reference position is (21 433.3 ± 407.2) cm−2 s−1, the cadmium ratio is 20.6, the non-uniformity is 0.1% over the 40 cm × 40 cm region. For the outer field, the thermal fluence rate at the reference position is (2046.0 ± 49.1) cm−2 s−1, the cadmium ratio is 1433, and the non-uniformity is 0.7% in the 70 cm × 70 cm region. The facility is expected to be suitable for the large-size detector, and its excellent performance is attractive for various thermal neutron experiments.

Monte Carlo design and experimental characterization of a moderator device to produce a thermal neutron source from a 241Am/9Be source

A moderator device to produce a uniform thermal neutron field has been designed by Monte Carlo methods using the MCNP6.1 code. It uses a 241Am/9Be neutron source of 111 GBq activity and high-density polyethylene (HDPE) moderator. The main tasks developed were to evaluate the geometry of the moderator and to select the neutron source position, in order to optimize the thermal neutrons flux in the irradiation area, and to assess the dose rate. In the system, named FANT (Fuente Ampliada de Neutrones Térmicos), the neutron moderation and backscattering processes are effective to obtain quite uniform thermal fluence rates above 1000 cm−2 s−1 in a cylindrical irradiation chamber of 32 cm diameter and 34 cm length. The device has been built in the neutron measurements hall of the Energy Engineering Department of Universidad Politécnica de Madrid (UPM), performing several measurements to characterize the neutron field and validate the calculations.FANT can be employed hereafter in different applications requiring a neutron field with a substantial thermal component, like testing and calibration of neutron detectors and neutron dosimeters, or the use NAA (Neutron Activation Analysis) methods for detection of trace substances or materials.

Design by Monte Carlo method of a thermal neutron device using a 241Am/9Be source and high-density polyethylene moderator

A thermal neutron system intended to be used in neutron activation analysis has been designed by Monte Carlo methods. The device is based on a241Am/9Be neutron source of 111 GBq, placed inside a cylindrical cavity open inside a parallelepiped of moderator material. Three different moderator materials, water, graphite and high-density polyethylene (HDPE), were simulated to check what is the most suitable for the detection system, concluding that HDPE reach the better performance. The device achieves an increased thermal neutron flux by taking advantage of neutron moderation in the polyethylene and the neutron scattering in the irradiation chamber walls. The thermal fluence rates obtained were 904 cm−2 s−1, i.e. 8.144 cm−2 s−1 GBq−1, with a fraction of thermal neutrons at the best point of 83% of pristine fast neutrons emitted by the source. The device has been designed by Monte Carlo techniques using the MCNP6 code, and the main tasks developed were to select the moderator material and to maximize the thermal neutrons flux in the irradiation chamber.

Analysis by Monte Carlo of thermal neutron flux from a 241Am/9Be source for a system of trace analysis in materials

Neutron techniques to characterize materials have a wide range of applications, one of the major developments being the identification of terrorist threats with chemical, biological, radiological, nuclear and explosives (CBRNE) materials. In this work, a thermal neutron irradiation system, using a241Am/9Be source of 111 GBq inside polyethylene cylindrical moderators, has been designed, built and tested. The geometry of moderator and the neutron source position were fixed trying to maximize the thermal neutrons flux emitted from the system. Therefore, the system is in fact a thermalized neutron source taking advantage of the backscattered neutrons, achieving thermal fluence rates of up to 5.3x102 cm−2 s−1, with dominantly thermal spectra. Samples can be placed there for several hours and thereafter be measured to identify their component elements by NAA (Neutron Activation Analysis).Through Monte Carlo techniques employing the MCNP6 code (Pelowitz et al., 2014), four different configurations with polyethylene cylinders were simulated to choose the most adequate geometry. The theoretical model was then replicated in the neutronics hall of the Neutron Measurements Laboratory of the Energy Engineering Department of Universidad Politécnica de Madrid (LMN-UPM), carrying out experimental measurements using a BF3 neutron detector. A high agreement between MCNP6 results and the experimental values measured was observed.Consequently, the system developed could be employed in future laboratory experiments, both for the identification of trace substances by NAA and for the calibration of neutron detection equipment.

The thermal neutron facility HOTNES: theoretical design

HOTNES (HOmogeneous Thermal NEutron Source) is a thermal neutron irradiation facility with extended and very uniform irradiation area. A 241Am-B radionuclide neutron source with nominal strenght 3.5×106 s−1 is located on bottom of a large cylindrical cavity (30cm diameter, 70cm in height) delimited by polyethylene walls. The upper part of this volume (30cm diameter, 40cm in height) is used to irradiate samples. A polyethylene cylinder, acting as shadowing object, prevents fast neutrons to directly reach the irradiation volume. Indeed neutrons can only reach the irradiation volume after multiple scattering with the cavity walls. The facility was designed trough extensive calculations with MCNPX. Irradiation planes are disks with 30cm diameter, centred on the cavity axis, and parallel to the cavity bottom. The value of thermal fluence in a given irradiation plane is as uniform as 1–2%. The value of thermal fluence rate simply depends on the height from the cavity bottom. Values of thermal fluence rate in the range 700–1000cm−2s−1 are available, depending on the irradiation plane chosen. The fraction of thermal neutrons is in the order of 90%, also depending on the irradiation plane. The angular distribution of thermal neutrons is roughly isotropic. Taking advantage of the HOTNES design, even large devices can be uniformly irradiated.This work presents HOTNES's design and describes the neutron field in the irradiation volume in terms of spatial, energy and direction distributions.

Modeling of ground albedo neutrons to investigate seasonal cosmic ray‐induced neutron variations measured at high‐altitude stations

AbstractThis paper investigates seasonal cosmic ray‐induced neutron variations measured over a long‐term period (from 2011 to 2016) in both the high‐altitude stations located in medium geomagnetic latitude and Antarctica (Pic‐du‐Midi and Concordia, respectively). To reinforce analysis, modeling based on ground albedo neutrons simulations of extensive air showers and the solar modulation potential was performed. Because the local environment is well known and stable over time in Antarctica, data were used to validate the modeling approach. A modeled scene representative to the Pic‐du‐Midi was simulated with GEANT4 for various hydrogen properties (composition, density, and wet level) and snow thickness. The orders of magnitudes of calculated thermal fluence rates are consistent with measurements obtained during summers and winters. These variations are dominant in the thermal domain (i.e., En < 0.5 eV) and lesser degree in epithermal and evaporation domains (i.e., 0.5 eV < En < 0.1 MeV and 0.1 MeV < En < 20 MeV, respectively). Cascade neutron (En > 20 MeV) is weakly impacted. The role of hydrogen content on ground albedo neutron generation was investigated with GEANT4 simulations. These investigations focused to mountain environment; nevertheless, they demonstrate the complexity of the local influences on neutron fluence rates.

Experimental characterization of HOTNES: A new thermal neutron facility with large homogeneity area

A new thermal neutron irradiation facility, called HOTNES (HOmogeneous Thermal NEutron Source), was established in the framework of a collaboration between INFN-LNF and ENEA-Frascati. HOTNES is a polyethylene assembly, with about 70cmx70cm square section and 100cm height, including a large, cylindrical cavity with diameter 30cm and height 70cm. The facility is supplied by a 241Am-B source located at the bottom of this cavity. The facility was designed in such a way that the iso-thermal-fluence surfaces, characterizing the irradiation volume, coincide with planes parallel to the cavity bottom. The thermal fluence rate across a given isofluence plane is as uniform as 1% on a disk with 30cm diameter. Thermal fluence rate values from about 700cm−2s−1 to 1000cm−2s−1 can be achieved. The facility design, previously optimized by Monte Carlo simulation, was experimentally verified. The following techniques were used: gold activation foils to assess the thermal fluence rate, semiconductor-based active detector for mapping the irradiation volume, and Bonner Sphere Spectrometer to determine the complete neutron spectrum. HOTNES is expected to be attractive for the scientific community involved in neutron metrology, neutron dosimetry and neutron detector testing.

ETHERNES: A new design of radionuclide source-based thermal neutron facility with large homogeneity area

A new thermal neutron irradiation facility based on an 241Am–Be source embedded in a polyethylene moderator has been designed, and is called ETHERNES (Extended THERmal NEutron Source). The facility shows a large irradiation cavity (45cm×45cm square section, 63cm in height), which is separated from the source by means of a polyethylene sphere acting as shadowing object. Taking advantage of multiple scattering of neutrons with the walls of this cavity, the moderation process is especially effective and allows obtaining useful thermal fluence rates from 550 to 800cm−2s−1 with a source having nominal emission rate 5.7×106s−1. Irradiation planes parallel to the cavity bottom have been identified. The fluence rate across a given plane is as uniform as 3% (or better) in a disk with 30cm (or higher) diameter. In practice, the value of thermal fluence rate simply depends on the height from the cavity bottom. The thermal neutron spectral fraction ranges from 77% up to 89%, depending on the irradiation plane. The angular distribution of thermal neutrons is roughly isotropic, with a slight prevalence of directions from bottom to top of the cavity. The mentioned characteristics are expected to be attractive for the scientific community involved in neutron metrology, neutron dosimetry and neutron detector testing.

SU‐E‐T‐195: Commissioning the Neutron Production of a Varian TrueBeam Linac

Purpose:The purpose of this work is the characterization of a new Varian TrueBeam™ facility in terms of neutron production, in order to estimate neutron equivalent dose in organs during radiotherapy treatments.Methods:The existing methodology [1] was used with the reference SRAMnd detector, calibrated in terms of thermal neutron fluence at the reference field operated by PTB (Physikalisch‐Technische‐Bundesanstalt) at the GeNF (Geesthacht‐Neutron‐Facility) with the GKSS reactor FRG‐1 [2]. Thermal neutron fluence for the 5 available possibilities was evaluated: 15 MV and 10&6 MV with and without Flattening Filter (FF and FFF, respectively). Irradiation conditions are as described in [3]. In addition, three different collimator‐MLC configurations were studied for 15 MV: (a) collimator of 10×10 cm2 and MLC fully retracted (reference), (b) field sizes of 20×20 cm2 and 10×10 cm2 for collimator and MLC respectively, and (c) collimator and MLC aperture of 10×10 cm2.Results:Thermal fluence rate at the “reference point” [3], as a consequence of the neutron production, obtained for (a) conformation in 15 MV is (1.45±0.11) x10⁴ n•cm2/MU. Configurations (b) and (c) gave fluences of 96.6% and 97.8% of the reference (a). Neutron production decreases up to 8.6% and 5.7% for the 10 MV FF and FFF beams, respectively. Finally, it decreases up to 2.8% and 0.1% for the 6 MV FF and FFF modes, respectively.Conclusion:This work evaluates thermal neutron production of Varian TrueBeam™ system for organ equivalent dose estimation. The small difference in collimator‐MLC configuration shows the universality of the methodology [3]. A decrease in this production is shown when decreasing energy from 15 to 10 MV and an almost negligible production was found for 6 MV. Moreover, a lower neutron contribution is observed for the FFF modes.[1]Phys Med Biol,2012;57:6167–6191.[2]Radiat Meas,2010;45:1513–1517.[3]Med Phys,2015;42:276–281.

A thermal neutron facility for radiobiological studies

Within the Petten BNCT project a thermal neutron facility for radiobiological studies is needed for further test and development of boron compounds. The Argonaut reactor of ECN, the Low Flux Reactor operating at a power of 30 kW, offers opportunities for the development of thermal neutron beams. Optimising calculations have been performed with the Monte Carlo code MCNP, resulting in a design of an extended graphite moderator followed by a lead shield with a bismuth window. This assembly has been built as a modular construction for easy positioning on and removal from an irradiation trolley of the reactor. The facility yields a thermal fluence rate of φ therm = 1.2 · 10 9 cm −2s −1, and a resulting dose rate from the unwanted beam components (γ-rays, fast and epithermal neutrons) lower than 1 Gy/h (physical dose). Experimental validation of the calculations has been performed by neutron activation detectors. Finally, the fluence rates in the vials used for the radiobiological experiments are determined with a set of neutron activation detectors.