Low dimensional neutron moderators for enhanced source brightness

Journal of Nuclear Science and Technology, 2002

For a decoupled liquid-hydrogen moderator, optimization studies have been performed on a premoderator and reflector materials (Pb, Be, Fe and Hg) together with several decoupling energies to realize a higher neutronic performance. The result indicated that, among four reflector materials mentioned above, the best neutronic performance could be obtained by adopting a Pb reflector with an optimized premoderator and an appropriate decoupling energy.

Journal of Nuclear Science and Technology, 1991

Although liquid hydrogen is at present a unique cold moderator applicable to high intensity neutron sources, the reported pulsed cold neutron intensity from a liquid hydrogen moderator is significantly lower than that from a solid methane moderator. In this report, a reflected assembly with a thick hydrogen moderator is used to enhance the pulsed cold neutron intensity and the intensity is compared with that from a solid methane moderator. For comparison, 5 and 15 em thick moderators were used for liquid hydrogen moderators. The thicker one was used to obtain the saturated intensity of pulsed cold neutrons. The thickness of a solid methane was 5 em.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2005

The JSNS design criteria for the coupled hydrogen moderator requires a large number of beam lines. We therefore performed neutronic calculations to maximize time-integrated and pulse-peak intensities of low-energy neutrons for 100% para-hydrogen coupled moderators. It was found that a larger number of beam lines increased the total intensity due to the increase of the angular coverage although the average intensity of each beam line decreased a bit due to a finite reflector missing caused by the beam extraction holes in the reflector. In the case of a rectangular moderator, the spatial distribution of the vector flux of low-energy neutrons becomes undesirable at a large beam extraction angle (25 ). As an alternative, we propose a cylindrical moderator to substantially improve the spatial distribution. Timeintegrated and pulse-peak intensities, pulse widths, and pulse decay characteristics were calculated as a function of the moderator diameter. The results indicated that the optimal diameter is about 140 mm. Compared to the rectangular moderator, the cylindrical design gives higher pulse-peak intensities with narrower pulse widths, without penalizing the time-integrated intensities, thus providing a more uniform angle dependence. Some explanations are given for the differences in pulse characteristics between the two moderator shapes.

arXiv (Cornell University), 2019

We report on our efforts to optimize the geometry of neutron moderators and converters for the TRIUMF UltraCold Advanced Neutron (TUCAN) source using MCNP simulations. It will use an existing spallation neutron source driven by a 19.3 kW proton beam delivered by TRIUMF's 520 MeV cyclotron. Spallation neutrons will be moderated in heavy water at room temperature and in liquid deuterium at 20 K, and then superthermally converted to ultracold neutrons in superfluid, isotopically purified 4 He. The helium will be cooled by a 3 He fridge through a 3 He-4 He heat exchanger. The optimization took into account a range of engineering and safety requirements and guided the detailed design of the source. The predicted ultracold-neutron density delivered to a typical experiment is maximized for a production volume of 27 L, achieving a production rate of 1.4 • 10 7 s −1 to 1.6 • 10 7 s −1 with a heat load of 8.1 W. At that heat load, the fridge can cool the superfluid helium to 1.1 K, resulting in a storage lifetime for ultracold neutrons in the source of about 30 s. The most critical performance parameters are the choice of cold moderator and the volume, thickness, and material of the vessel containing the superfluid helium. The source is scheduled to be installed in 2021 and will enable the TUCAN collaboration to measure the electric dipole moment of the neutron with a sensitivity of 10 −27 e cm.

EPJ Web of Conferences, 2020

Low-dimensional moderators were designed for the European Spallation Source (ESS), and the same concepts can be applied to compact sources. At ESS, quasi two-dimensional (2D) high-brightness moderators will serve all the instruments of the initial suite. The design of the moderators is influenced by several factors, such as the layout of the facility, the requirements for beam extraction, and the number of instruments; these constraints led to the choice of the 2D “butterfly” moderator for ESS. In an accelerator-based compact source, such moderators can be designed in an even more efficient way than for high-power sources, taking advantage of the lower heat loads and of the more compact arrangement of the target-moderator system, which gives more freedom in the optimization of the geometrical setup. Some promising design options have been explored.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2005

Neutronic studies of decoupled hydrogen moderators were performed by calculations taking into account para hydrogen content, decoupling energy, moderator dimensions/shapes and reflector material. Low-energy parts of calculated spectral intensities with different para hydrogen contents were analyzed by a modified Maxwell function to characterize neutron spectra. The result shows that a 100% para hydrogen moderator gives the highest pulse peak intensity together with the narrowest pulse width and the shortest decay times. Pulse broadening with a reflector was explained by time distributions of source neutrons entering into the moderator through a decoupler. Material dependence of time distribution was studied. A decoupling energy higher than 1 eV does not bring about a large improvement in pulse widths and decay times, even at a large penalty in the peak intensity. The optimal moderator thickness was also discussed for a rectangular parallelepipe-shaped and a canteen-shaped moderator.

2018

A series of experiments were conducted at the research reactor of Kinki University, UTR-KINKI, Japan using various arrangements of neutron moderators and absorbers for evaluating the energy responses of wide energies neutrons at different layers and positions. Among various arrangements of moderators and absorbers, an arrangement (22mm PE+22.5mm B4C+10mm PE +50mm PE) has been optimized where the responses of neutron energies are found to be significant compared to all other arrangements, especially for the intermediate energy of neutrons. The experimental results are also checked and compared to the experimental result with the same arrangement using the 252 Cf neutron bare source at National Institute for Fusion Science (NIFS), Japan. It is assumed that the result of this study would be contributed for designing a lighter weight multi-layers moderator and absorber based cylindrical or spherical dose monitor for wide energy response of neutron.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2004

We have measured neutronic performance of the moderators at the Manuel Lujan Jr. Neutron Scattering Center, Los Alamos National Laboratory in order to provide the neutron beam characteristics for the neutron scattering experiments at the Lujan Center as well as to study the validity of neutronic calculation codes. The absolute neutron intensities, the energy spectra, and the neutron pulse shapes (neutron emission time distributions) were measured for the H 2 O coupled moderator, the high-intensity H 2 O decoupled moderator, the high-resolution H 2 O decoupled moderator, and the partially coupled liquid-hydrogen moderator. The overall characteristics of the moderators were found to agree with the designed performance. r

1998

A liquid H{sub 2} moderator has been operational at the Manual Lujan Jr. Neutron Scattering Center (Lujan Center) since 1985. Detailed Monte Carlo calculations have been made to estimate the neutronic performance of the liquid H{sub 2} moderator cold source, using the state-of-the-art Los Alamos LAHET Code System and liquid H{sub 2} scattering kernels. The absolute neutron spectrum leaking from the moderator has been measured. In thermal equilibrium at 20 K, the moderator would convert to 100% para-hydrogen on some time frame. The authors show that the performance of the Lujan Center cold neutron source is sensitive to small fractions of ortho-hydrogen, and display these results as a function of the ortho/para H{sub 2} concentration. Integral data from the measured neuron spectrum have been combined with calculated predictions to estimate the ortho/para H{sub 2} concentration in the moderator. From these analyses, they estimate the ortho-hydrogen concentration in the liquid H{sub 2}...

The Synthetic Scattering Function (SSF) allows a simple description of the incoherent interaction of slow neutrons with hydrogenous materials. Its original formulation, essentially aimed at describing molecular gases, has been extended to include the important case of molecular solids. In any case, the main advantages of the Synthetic Model reside in the analytical expressions that it produces for double-differential cross sections and energy-transfer kernels, which in turn permit the fast evaluation of neutron scattering and transport properties. The SSF routines were integrated into the NJOY code, in such a way that the cross sections can be generated with the same format either from its standard library or from our model. We discuss in this work some aspects of the formalism developed to include the case of molecular solids in a more detailed manner, and present applications of the original and extended Synthetic Model to recent experimental results on liquid Hydrogen, Mesitylene...