Speaker
Description
We investigated the potential of liquid Hydrogen Deuteride (HD) as a cold neutron moderator to bridge the gap between high-brightness, low-dimensional hydrogen-based sources, and high-intensity, large-volume deuterium-based sources. While cold neutron production traditionally relies on the high moderating efficiency of liquid H2 for brightness or the low absorption of liquid D$_2$ for intensity, HD offers a unique set of midway neutronic properties [1]. Specifically, HD boasts approximately half the absorption cross-section of liquid H$_2$ while maintaining a significantly higher scattering cross-section than D$_2$, potentially enabling the design of moderators that are both compact and intense.
We first validated the Thermal Scattering Library (TSL) for HD at crygenic temperatures based on the work of Huusko, A. [2] using MCNP6.3 to reproduce historical experimental results at the Hokkaido electron linac [3]. The simulations demonstrate an excellent qualitative correspondence with experimental data, accurately capturing the peak energy at 2.4 meV and the cold-to-fast neutron ratios. Furthermore, the model successfully reproduces the pulse shapes and decay times across various energy ranges, providing a solid foundation for a predictive analysis.
In order to better understand the range of applicability of a HD moderator, we conducted a comprehensive computational study across three common large-scale facility configurations: a decoupled pulsed spallation source, a coupled pulsed source with pre-moderation, and a continuous reactor-like source. We tracked the geometric mean between brightness and intensity as Figure of Merit (FOM), as well as other auxiliary metrics, to evaluate the performance across a grid of moderator dimensions.
We found that HD’s performance is highly dependent on the surrounding neutron spectrum:
- in the decoupled source, HD underperforms compared to both natural H2 and pure para-H$_2$ due to its lower proton density (roughly 56% of liquid H$_2$), which limits its efficiency in the fast-neutron environments typical of decoupled systems.
- in the coupled source, HD demonstrates a clear advantage in the large-surface compact regime, particularly for extraction lengths smaller than 15 cm. In these configurations, HD yields a FOM up to 50% higher than para-H2 in the cold range between 4 and 10Å. While the performance is overall quite similar to H$_2$, for neutron wavelengths larger than 10 Å HD provides a 60% gain.
- in the reactor-like source the benefits of HD are most pronounced. In compact geometries, HD provides gain factors of 40-80% over n-H$_2$ and 3 to 4 times the FOM of D$_2$.
The results suggest that in environments with a high thermal-to-fast neutron ratio, HD acts as a neutronically improved n-H$_2$, combining efficient thermal-to-cold conversion with enough transparency to allow for larger, more uniform emission surfaces. We conclude that HD can complement the current neutron source landscape by providing a viable pathway for compact, bright, and intense cold neutron sources in next-generation facilities.
References
[1] Guarini, E. et al, Hydrogen Deuteride for Cold Neutron Production: A Model for the Double Differential Cross Section. Applied Sciences 14, 4718, 2024.
[2] Huusko, Alexander, Calculation of neutron scattering libraries for liquid ortho-deuterium and hydrogen deuteride, Master Thesis, 2022
[3] Sasaki, K. et al, Experimental studies on neutronics of CH$_3$D and HD cold neutron moderators, in: Proceedings of the JAERI-Conf 2001-002 and KEK Proceedings 2000-22, International Collaboration on Advanced Neutron Sources ICANS-XV, Tsukuba, Japan, 2000.