Thematic Seminar on Methods and Techniques of Neutron Experiments

06-18-2024 11:00 Conference hall (bldg. 119, 3rd floor)

In the last decade, low-dimensional neutron moderators made from nearly pure para-hydrogen (pH2) have emerged. Due to the significant difference in the scattering cross-section of pH2 between thermal and cold neutrons, moderators in the form of tubes or disks provide a substantial increase in brightness compared to traditional cold sources. However, the small size of the moderators can sometimes make it difficult to fully illuminate the sample, leading to an inhomogeneous beam profile on the sample.

To determine the optimal size of the moderator, we have developed a new approach that takes into account instrument parameters such as sample size and angular resolution. This approach is based on phase space considerations, and extensive Monte Carlo simulations have been conducted to validate it. This method is particularly useful in designing new neutron instruments and neutron sources.

We also present a new analytical approach for calculating the brightness of low-dimensional moderators. Our results show that, since the brightness increase is essentially a near-surface effect, rectangular moderators with a high aspect ratio provide even higher brightness of cold neutrons than a moderator optimized for ESS. The obtained results are in excellent agreement with MCNP calculations.

To address the trade-off between increasing brightness and neutron beam intensity in low-dimensional moderators, we propose chessboard and staircase moderator assemblies with a well-developed overall surface. This configuration ensures broad, intense neutron beams while maintaining the high brightness of a narrow moderator. By applying this geometry, a brightness increases of 2.5 to 3.5 times compared to a conventional moderator of the same size can be achieved.