{'fraction_non_alphanumeric': 0.023574144486692015, 'fraction_numerical': 0.0076045627376425855, 'mean_word_length': 5.482758620689655, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
40 Years of Neutron Scattering: A Perspective

James Chadwick discovered the neutron in 1932, identifying the product of alpha-particle irradiation of beryllium as a neutral particle having about the same mass as the proton. Enrico Fermi led the team that demonstrated the first self-sustaining fission reaction in 1942, after which reactors developed rapidly, optimized for the use of thermal neutron beams for studying materials properties. Eventually, research reactor designs met engineering heat transfer limits that constrain their performance (excellent, though it is). Now, new reactors are optimized more for special purposes than for highest flux. In recognition of reactor flux limitations, accelerator-driven pulsed spallation neutron sources arose, capitalizing on averaging out the heat removal between intense source pulses and on the relatively lower heat dissipation per useful neutron produced in spallation. Pulsed sources and the required time-of-flight methods improved over the years, and now, in terms of the peak available neutron flux, exceed the most powerful reactors. Recently, charged-particle reactions driven by low-energy accelerators have come onto the scene, which are inexpensive to build and operate, and provide needed testing and training venues that support the large sources.