Modeling of Low-Energy Neutron Scattering in Liquid Hydrogen Using NCrystal

Abstract

The European Spallation Source ESS, under construction in Lund, Sweden, aims to
produce high-brightness and intensity neutrons for a multitude of experiments upon completion. An efficient production of neutrons requires elaborate understanding of the moderation process which slows the neutrons from relativistic speed to a kinetic energy of meV. The moderator used at the ESS will be partially made of water and liquid para-hydrogen, which enables simultaneous production of thermal and cold neutrons. The interaction between neutrons and the moderator is best described by the total neutron scattering cross section which in turn is computationally derived from the scattering kernel. The liquid hydrogen kernel itself includes models of a self- and distinct scattering function which describe the interaction of individual molecules with neutrons as well as interference effects of scattered matter waves from neighboring molecules. Scattering kernels are commonly produced by a software called NJOY, however, an effort at ESS has been made to develop and transition to a new software called NCrystal. Its more flexible design allows for easier implementation of physics models beyond what is currently included in NJOY for more moderator materials. Thus far, NCrystal is primarily aimed at generating scattering kernels for poly-crystalline materials in solid phase, a capability which was now expanded to liquid water and liquid hydrogen. The resulting cross sections were bench-marked against proven software and show good agreement.

Popular Abstract

The challenge of slowing down neutrons the right way
The European Spallation Source ESS, which is currently under construction in Lund, Sweden, is a next-generation neutron research facility. Once completed, it will use neutrons to undertake a multitude of experiments, some of them carrying out fundamental research and others more practical in material science.
The production of neutrons in a spallation source requires an energetic proton beam which is accelerated by a linear particle accelerator to velocities close the speed of light. This beam is impacted into a target made of solid metal. Here, the neutrons are produced by a phenomenon called spallation: the protons are so fast that they break apart the nuclei of the target into smaller nuclei and, crucially, several free and very energetic neutrons. For many applications these neutrons are too fast and thus have to be slowed down to appropriate speeds. But how can you brake particles that move close to speed of light and have no electric charge which you could grab them by? You send them through a moderator. A moderator is a material that acts like a sticky ”syrup” on neutrons and slows them down to the speed of sound before they can escape again and then be used for experiments.
Moderators have been researched and used for decades in efforts to make nuclear energy a safe and reliable power source. Almost every nuclear power plant uses a very familiar moderator: water. However, even after passing through water, the neutrons, now called thermal neutrons, are still too energetic for a lot of material science applications. This is where current knowledge becomes sparser, since different, more rarely used moderators have to be employed, in case of the ESS liquid hydrogen. To design an efficient moderator, the slowing-down process is simulated with scattering software to obtain the scattering kernel, a quantity which describes how energy and momentum is deposited in the moderator when neutrons pass through it. So far, NJOY is the most-used program for this purpose, but as requirements to add more physical models to represent potential future moderating materials grow, limitations in NJOY have become apparent.
As a consequence, a new software called NCrystal, has been developed at the ESS with a more flexible design to make the simulation of different moderators easier. This Master thesis added the functionality of simulating liquid hydrogen, the cold moderator of the ESS. To achieve this, a number of physics models had to be included into the code which describe the interaction between neutrons and the molecules of liquid hydrogen. The final result which is easiest to cross-check against NJOY is the total neutron scattering cross section (see figure), which describes how much a neutron is slowed down by a material depending on the energy of the incident neutron. The results of the thesis show good agreement with existing literature and can thus be used in the future.

• author: Günter, Tim
• supervisor: Douglas DiJulio; Jose Ignacio Marquez Damian; Joakim Cederkäll
• organization: Department of Physics; European Spallation Source ESS AB; Nuclear physics
• course: FYSM30 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: ESS, European Spallation Source, neutron, scattering, moderator, liquid hydrogen, modeling, ncrystal, njoy
• language: English
• id: 9090112
• date added to LUP: 2022-06-17 12:02:49
• date last changed: 2022-06-17 12:02:49