Neutronium is a hypothetical substance composed purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 for the conjectured "element of atomic number zero" that he placed at the head of the periodic table. However, the meaning of the term has changed over time, from the last half of the 20th century onward it has been used to refer to dense substances resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars. Science fiction and popular literature use the term "neutronium" to refer to a dense phase of matter composed of neutrons. Neutronium is used in popular physics literature to refer to the material present in the cores of neutron stars; this term is rarely used in scientific literature, for three reasons: there are multiple definitions for the term "neutronium". When neutron star core material is presumed to consist of free neutrons, it is referred to as neutron-degenerate matter in scientific literature; the term "neutronium" was coined in 1926 by Andreas von Antropoff for a conjectured form of matter made up of neutrons with no protons or electrons, which he placed as the chemical element of atomic number zero at the head of his new version of the periodic table.

It was subsequently placed in the middle of several spiral representations of the periodic system for classifying the chemical elements, such as those of Charles Janet, E. I. Emerson, John D. Clark. Although the term is not used in the scientific literature either for a condensed form of matter, or as an element, there have been reports that, besides the free neutron, there may exist two bound forms of neutrons without protons. If neutronium were considered to be an element these neutron clusters could be considered to be the isotopes of that element. However, these reports have not been further substantiated. Mononeutron: An isolated neutron undergoes beta decay with a mean lifetime of 15 minutes, becoming a proton, an electron and an antineutrino. Dineutron: The dineutron, containing two neutrons, was unambiguously observed in 2012 in the decay of beryllium-16, it is not a bound particle, but had been proposed as an short-lived state produced by nuclear reactions involving tritium. It has been suggested to have a transitory existence in nuclear reactions produced by helions that result in the formation of a proton and a nucleus having the same atomic number as the target nucleus but a mass number two units greater.

The dineutron hypothesis had been used in nuclear reactions with exotic nuclei for a long time. Several applications of the dineutron in nuclear reactions can be found in review papers, its existence has been proven to be relevant for nuclear structure of exotic nuclei. A system made up of only two neutrons is not bound, though the attraction between them is nearly enough to make them so; this has the abundance of the chemical elements. Trineutron: A trineutron state consisting of three bound neutrons has not been detected, is not expected to exist for a short time. Tetraneutron: A tetraneutron is a hypothetical particle consisting of four bound neutrons. Reports of its existence have not been replicated. Pentaneutron: Calculations indicate that the hypothetical pentaneutron state, consisting of a cluster of five neutrons, would not be bound. Although not called "neutronium", the National Nuclear Data Center's Nuclear Wallet Cards lists as its first "isotope" an "element" with the symbol n and atomic number Z = 0 and mass number A = 1.

This isotope is described as decaying to element H with a half life of 10.24±0.2 min. Neutron matter is equivalent to a chemical element with atomic number 0, to say that it is equivalent to a species of atoms having no protons in their atomic nuclei, it is radioactive. Neutron matter decays into hydrogen. Neutron matter has no electronic structure on account of its total lack of electrons; as an equivalent element, however, it could be classified as a noble gas. Bulk neutron matter has never been viewed, it is assumed that neutron matter would appear as a chemically inert gas, if enough could be collected together to be viewed as a bulk gas or liquid, because of the general appearance of the elements in the noble gas column of the periodic table. While this lifetime is long enough to permit the study of neutronium's chemical properties, there are serious practical problems. Having no charge or electrons, neutronium would not interact with ordinary low-energy photons and would feel no electrostatic forces, so it would diffuse into the walls of most containers made of ordinary matter.

Certain materials are able to resist diffusion or absorption of ultracold neutrons

Yume Wo Katare

Yume Wo Katare is a ramen shop located at 1923 Massachusetts Avenue in the Porter Square neighborhood of Cambridge, Massachusetts. It specializes in Jiro-style ramen; the restaurant is known for its service concept: diners are encouraged to share their dreams and aspirations to their fellow diners after finishing their meal. Yume Wo Katare was opened by Tsuyoshi Nishioka in November 2012, he had worked in ramen restaurants in Kyoto to support his career as a comedian, owned and run five Ramen Jiro branches in Kyoto and Kobe which he sold to their managers before relocating to the United States in 2011. Nishioka considered setting up shop in Hawaii and New York City, but was disappointed by the disproportionately large number of Japanese Americans in both locations, he wanted to set up a restaurant in a location with a large student population. He flew into Boston the day after another guest at his New York hotel suggested he open his restaurant there, decided to set up shop in Porter Square; the restaurant closed over the summer of 2013.

For three non-consecutive days in 2014, Jorgen "Walker" Peterson, a former chemist who worked as the restaurant's manager, launched a vegan version of the ramen recipe as a pop-up restaurant which operated in the Yume Wo Katare space during lunch hours, at a time when the restaurant is closed. Three years Nishioka opened Yume Ga Arukara, a sister restaurant in a nearby part of Cambridge which serves udon, was named by Bon Appétit as one of the top ten new restaurants in the United States for 2018. Nishioka returned to Japan in 2018, where he has since opened up outposts of Yume Wo Katare in Beppu, where he lives, as well as in Okinawa, training around 50 apprentices how to create Jiro-style ramen with the goal of opening dream-centric ramen shops in all 47 Japanese prefectures. Yume Wo Katare has been described as not just being a restaurant, but a "dream factory" and a food-centered aspirational community. Nishioka was affected by the suicide of a partner in the comedy troupe he once performed with in Japan, he has since made it his life's mission to help people achieve their dreams.

To that end, the restaurant offers regular "dream workshops" in addition to its culinary offerings targeting Japanese students in Cambridge to help them figure out their own life dreams and goals. The restaurant only offers one dish: a bowl of ramen either in regular or large sizes which vary based on the amount of chāshū, served alongside cabbage and bean sprouts, as well as options for extra fat and garlic; the ramen base is made from a tonkotsu broth that cooks for 14 hours, giving it a rich flavor, combined with a shōyu broth and hand-cut noodles. The dish itself has been described by The Boston Globe as being "porky, hefty, intense", with prospective diners waiting in line for it every day, sometimes for over an hour. Although Yume Wo Katare is arranged like a classroom, with space for 18 diners at one time, it is known for its interactive and novel yet regimented dining experience; when diners are finished, they are encouraged to share their dreams with all other diners in the room, with everyone cheering and showing support.

Afterward, they are graded by restaurant staff either with a "perfect", "good job", "almost" or "next time" grade based on how much of the dish is left in their bowl, with the goal of pushing people beyond what they may think they're capable of, as well as discouraging wasteful consumption. Official website