Solving the ⁵⁶Mn puzzle

Marian Jandel, University of Massachusetts, Lowell, MA (Principal Investigator)

Peter Bender, University of Massachusetts, Lowell, MA (Co-Investigator)

Neutron capture reactions play a central role in nuclear science that transcends the boundaries between engineering and academic sciences. In reactors, the capture of neutrons on various materials is crucial for energy generation, control, and activity levels. In astrophysics, radiative capture produces many of the elements beyond iron in high-temperature environments of nucleosynthesis (s-process). In applications, capture gamma-rays are used in prompt gamma ray activation analysis, where discrete transitions, unique to specific isotopes, are used to identify them in samples of interests. This latter technique is used in homeland security interrogation of special nuclear materials in nuclear safety and non-proliferation, in oil-well element assaying and in space exploration missions. For all these needs a reliable knowledge of absolute cross sections is essential, and often taken for granted. This assumption is not always sound, and is the driving motivation behind this proposal.

Manganese is an interesting element for many reasons and provides a good case to illustrate the need for new, better and more precise neutron capture data. Many important structural materials contain manganese. It is present in steel, typically at the 1% level, but some structural steel alloys may contain up to 13%. Aluminum alloys can contain up to 1.5% of manganese. Consequently, manganese is an important marker in nuclear forensics. It is also an important biogenetical element in all known living organisms. As such, manganese is of interest in intra-planetary research, where manganese content can provide the evidence of past life. Currently, active interrogation systems that will use the pulsed neutron generators and high-resolution spectroscopic measurements are under development at NASA. Such systems will be used in the Mars Exploration Program, to search for evidence of past life on Mars. In oil-drilling industry, active neutron interrogation technique is used to explore the soil composition, and manganese is a key element to study. Thus, for many reasons, precisely knowing the thermal neutron capture process and its subsequent decay gamma rays is essential. Close examination of the ⁵⁶Mn capture gamma-ray data in the Evaluated Nuclear Structure Data File (ENSDF) library provides a puzzling picture, where intensities of important capture gamma-ray transitions per neutron capture reaction differ significantly from our preliminary calculations of de-excitation cascades of capture gamma rays ⁵⁶Mn.

To address the ⁵⁶Mn puzzle, we propose new measurements of neutron capture reaction on ⁵⁵Mn using the collimated thermal neutron beam at the University of Massachusetts Lowell’s 1 MW Research Reactor and the Mixed Array of Detectors (MAD), including high-resolution actively-suppressed high-purity germanium detectors. The measurements will be carried out using modern high-density and high-speed digital data acquisition system that enables state-of-the-art coincidence spectroscopy of gamma-rays from excited compound nuclei after neutron capture. The experimental results of neutron capture gamma rays will be compared with large-scale calculations that couple the statistical modeling of capture gamma-ray de-excitation and simulation of gamma-ray radiation interactions with the detectors of MAD. Our work will enable deeper insight into nuclear structure of the ⁵⁶Mn nucleus through validation of theoretical models using the new data and ultimately provide more accurate picture for spectral intensity of gamma-ray transitions not only for discrete state transitions, but also for continuum gamma-ray transitions.