Abstract
A significant contribution to the enhancement of the neutron brilliance achievable with Compact Accelerator-driven Neutron Sources (CANS) can be made by an optimized cold moderator design. When using liquid para-H2 as the moderating medium, the concept of low-dimensional cold moderators can be employed to increase the neutron brightness (as currently foreseen at the European Spallation Source ESS). Para-H2 shows a drop in the scattering cross section by about one order of magnitude around 15 meV, resulting in a large deviation between the mean free paths of thermal and cold neutrons. Taking advantage of this effect, the cold moderator geometry can be optimized to allow the intake of thermal neutrons through a relatively large envelope surface and then extracting them in an efficient way towards the neutron guides. One drawback of this solution is the lack of thermalization of the cold neutrons. In the context of the HBS (High Brilliance Neutron Source) project, efforts are made to overcome this problem by increasing the scattering cross section of the H2 in a defined way. The idea is to admix small amounts of ortho-H2, which maintains its large scattering cross section in the region below 15 meV. Like this, the neutron spectrum can be shifted towards lower energies and adjusted for the needs of the respective instruments. In a cooperation between TU Dresden and FZ Jülich, an experimental setup has been created to prove the feasibility of this concept. The main component of the experimental setup is a LHe-cooled flow cryostat that enables the separate condensation of a para-H2 and a normal-H2 flow and a subsequent mixing of the two in precise proportions. The resulting LH2 mixture at 17 - 20 K is fed into a small cold moderator vessel (approx. 200 ml). In this work, the current status of the setup is presented. The construction and commissioning of the mixing cryostat have been completed and first test runs show that different ortho-para-H2 mixtures can be produced. In the near future, the system will be ready for measurements at a neutron source.
Highlights
The progressive shutdown of existing research reactors in Europe causes an increasing shortage of neutron availability for scattering experiments
The Jülich Centre for Neutron Science (JCNS) is currently developing a future Compact Accelerator-driven Neutron Sources (CANS), the High Brilliance neutron Source (HBS), in which the whole chain from neutron production to detection is to be optimized according to the experimental needs
Liquid para-H2 has been used in cold sources since many years, but recently there has been a new trend in para-H2 moderator design: Thanks to the drop in the scattering cross section of para-H2 at a neutron energy around 15 meV, it is possible to extract neutrons from the cryogenic moderator in a very efficient way due to low self-absorption, if the moderator is implemented as a so-called low-dimensional cold moderator [2]
Summary
The progressive shutdown of existing research reactors in Europe causes an increasing shortage of neutron availability for scattering experiments. Liquid para-H2 has been used in cold sources since many years, but recently there has been a new trend in para-H2 moderator design: Thanks to the drop in the scattering cross section of para-H2 at a neutron energy around 15 meV (see Figure 1), it is possible to extract neutrons from the cryogenic moderator in a very efficient way due to low self-absorption, if the moderator is implemented as a so-called low-dimensional cold moderator [2]. Monte Carlo simulations of moderators using ortho-para mixtures are especially hard to manage and prone to errors [5, 6] To overcome this problem and enable a reliable moderator design, there are endeavors by JCNS to measure the neutron spectra resulting from different ortho-para mixtures (and pure para-H2) experimentally. The cryogenic system required to provide liquid hydrogen with a variable ortho-para ratio is presented
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