The PULSTAR research reactor and associated instrumentation is a user facility located on the campus of NC State that supports a broad array of interdisciplinary research projects. This 1 MW reactor provides neutrons, for example, for studies of fission fuel failure, neutron imaging of commercial products, materials studies including activation analysis, and studies with positron beams. (Note that licensing for 2 MW operation is also underway after a recent upgrade of the primary cooling loop.) As part of this facility and in collaboration with faculty from Nuclear Engineering, we have constructed an Ultracold Neutron (UCN) source that will support fundamental physics research.
The PULSTAR reactor design is ideally suited to house the UCN source since its under-moderated core has a high leakage of thermal and fast neutrons. These neutrons can be moderated from fast to thermal, thermal to cold, and then cold to ultracold. The source itself is considered next generation because of the 3-step moderation of neutrons and the design is unique compared to other reactor-based UCN sources that rely on a thermal flux moderated in the reactor pool. The NC State source is similar in design to ones used at a spallation source because it streams non-equilibrium neutrons directly from the core to the thermal column port, where the D2O moderates the neutrons to thermal energies. Solid methane at a temperature of T ~ 40 K is located inside D2O tank and is used as the cold neutron moderator. Solid deuterium (SD2) at a temperature of T~5K is utilized in the final moderation step as the UCN converter material.
We are in the final commissioning stages of bringing the NC State source online and estimate that the source will provide a useful UCN density of at least 30 UCN/cc/MW. Such density would be sufficient to support stand-alone physics experiments, development of UCN experiments that ultimately will run at more intensive UCN sources to accelerate the path to physics results, and to provide unique student access for educational purposes. In addition, the flexibility in our source design allows us to clarify questions related to the physics of UCN production in SD2.
Commissioning of all cryogenic components of the UCN source and visual study of solid deuterium crystal were completed outside of the reactor shielding in 2018. Since then, we have proceeded with installation and testing within the thermal column under neutron irradiation from the reactor. The final neutron irradiation tests required for safety approval — finalizing design of the neutron guides and biological radiation shielding — were completed in 2023. The results are published in Journal of Nuclear Engineering(see below).
In the Fall of 2024, we completed the upgrade of the DAQ and programmable logic controller for the He liquefier which is dedicated to the operation of the UCN source. The slow data acquisition system of the source has been updated, a new custom built low-noise power supplies constructed and utilized for the cryostat heaters. At present, we are in the process of developing and testing new control software. Our next step is to commission the system using heaters in the LHe transfer line and mock cryostat cooling loops to simulate realistic cryostat heat loads and facilitate temperature and flow modes favorable for the perfect crystal growth.
We have submitted an experiment request to the reactor safety committee who is scheduled to meet at the beginning of December 2024 to discuss final approval to fully operate the source.
Initial source UCN commissioning tests will utilize a simple geometry to characterize the total UCN flux. We will utilize an existing 3He UCN detector and we plan to fabricate a scintillation UCN detector using the current LANL design. This test will require completion of the UCN exit port design (including a UCN detector) and the associated fabrication. After installation of the source into the reactor biological shielding, we will proceed with UCN production. The plan is to test the UCN production rate first without methane and then with methane at different temperatures to determine the optimum methane temperature for UCN production. The second test will be to study the effect of the deuterium crystal quality and shape on the UCN extraction probability using our knowledge of how to make different shapes and quality of crystals.
One unique feature of our source is the ability to change the solid methane temperature across a wide range. Theoretically, matching the maximum should be when the deuterium phonon spectrum is be around 40~K to 50~K. Scanning the methane temperatures from 30~K to 60~K may shed light on the energy dependence of the UCN production cross section, where the main question is how noticeable is the contribution from cold neutrons with energies above 7 meV. An interesting fact is that a significant gain was predicted for UCN production at the Mainz University source using methane below 22 K, but the experiment showed only a slight increase of 30% as compared to the no pre-moderator measurement. The prediction was performed using a UCN cross section model that ignores >10 meV neutron contributions and solid methane at 20 K. The reason is not yet clear, but it could be an indicator that the 20 K spectrum is too cold for deuterium and that a considerable contribution comes from energies above 7 meV.
UCN sorce publications:
- *External Moderation of Reactor Core Neutrons for Optimized Production of Ultra-Cold Neutrons”,G. Medlin, E. Korobkina, C. G. Teander, B. Wehring, E. Sharapov, A. I. Hawari, P. Huffman, A. R. Young, G. Palmquist, M. Morano, C. Hickman, T. Rao, R. Golub, J. Nucl. Eng. 2024, 5(4), 486-499; https://doi.org/10.3390/jne5040030
- Korobkina, Ekaterina, Igor Berkutov, Robert Golub, Paul Huffman, Clark Hickman, Kent Leung, Graham Medlin et al. “Growing solid deuterium for UCN production.” Journal of Neutron Research Preprint (2022): 1-13.
- “Solid deuterium surface degradation at ultracold neutron sources”. A. Anghel, T. L. Bailey, G. Bison, B. Blau, L. J. Broussard, S. M. Clayton, C. Cude‐Woods, M. Daum, A. Hawari, N.Hild, P. Huffman, T. M. Ito, K. Kirch, E. Korobkina, B. Lauss, K. Leung, E.M. Lutz, M. Makela, G.Medlin, C. L.Morris, R. W.Pattie, D. Ries, A. Saunders, P. Schmidt‐Wellenburg, V. Talanov, A. R. Young, B. Wehring, C. White, M. Wohlmuther, and G.Zsigmond, European Physical Journal A54 148, DOI: 10.1140/epja/i2018‐12594‐2
- “Ultracold neutron source at the PULSTAR reactor: Engineering design and cryogenic testing,” E. Korobkina, G. Medlin, B. Wehring, A.I. Hawari, P.R. Huffman, A.R. Young, B. Beaumont, G. Palmquist, Nucl. Inst. Meth. A767 169 (2014)