Neutron free decay

The Hot Big Bang theory of the Universe was pioneered by George Gamow, R. A. Alpher and R. C. Herman in the late 1940s and early 50s. They supposed that during the first few minutes of the (then radiation-dominated) Universe, matter was originally present in the form of neutrons and that, after some free decay, protons captured neutrons and successive captures, followed by /3-decays, built up all the elements (Alpher Herman 1950). 

One neglects the free decay of the neutron to the proton because the half-life for that decay (10.6 m) is too long to be relevant.] The neutron-proton ratio, n p, was determined by a Boltzmann factor, that is,... 

Several decades ago the number of elementary particles known was limited, and the system of elementary particles seemed to be comprehensible. Electrons had been known since 1858 as cathode rays, although the name electron was not used until 1881. Protons had been known since 1886 in the form of channel rays and since 1914 as constituents of hydrogen atoms. The discovery of the neutron in 1932 by Chadwick initiated intensive development in the field of nuclear science. In the same year positrons were discovered, which have the same mass as electrons, but positive charge. All these particles are stable with the exception of the neutron, which decays in the free state with a half-life of 10.25 min into a proton and an electron. In the following years a series of very unstable particles were discovered the mesons, the muons, and the hyperons. Research in this field was stimulated by theoretical considerations, mainly by the theory of nuclear forces put forward by Yukawa in 1935. The half-lives of mesons and muons are in the range up to 10 s, the half-lives of hyperons in the order of up to 10 s. They are observed in reactions of high-energy particles. 

Since we now know that a neutron free of the nucleus has a half-life of 17 minutes as it decays to a proton, an electron and an antineutrino of negligible mass, it might be tempting to think of the source of Prout s hypothesis as effectively the total mass of the proton and one electron. The total for the two isolated particles, 1.0078, is still less than that of the neutron. Nonetheless, it might also be tempting to consider the protyle to have the mass of the neutron. 

Baumgartner and Reichold prepared carrier-free Mo(CO)g in high yield by neutron irradiation of powdered mixtures of UjOg and Cr(CO)g. As with their preparation of ° RuCp2, the Cr(CO)g acted only as a catcher for fission-product molybdenum (and for its precursors niobium and zirconium). The yield of 60% found for Mo(CO)6 is higher than the fractional chain yield of Mo in fission, so that the reaction must be partly thermal, starting with molecular fragments which survive j8 decay. 

As an example, consider the decay of free neutrons. A neutron has A — 1, so its decay products must also have A = 1. The stable particle with A = 1 is a proton, and neutron decay results in a proton. The neutron has zero charge, so the sum of the charges of its decay products must also be zero. Because the proton carries a -i-l charge, another particle with a -1 charge is required. This particle must have >1 = 0 to ensure that the mass number is... 

This reasoning is easily applied to the decay of a free neutron. The masses of the three participants are given in Table... 

The two Mossbauer levels of Pt, 99 keV and 130 keV, are populated by either EC of Au(fi/2 = 183 days) or isomeric transition of Pt(fi/2 = 4.1 days). Only a few authors, e.g., [323, 324] reported on the use of Pt, which is produced by thermal neutron activation of " Pt via " Pt(n, y) Pt. The source used in the early measurements by Harris et al. [322, 325] was carrier-free Au diffused into platinum metal. Walcher [326] irradiated natural platinum metal with deuterons to obtain the parent nuclide Au by (d, xn) reactions. After the decay of short-lived isotopes, especially Au(fi/2 = 6.18 days), Au was extracted with ethyl acetate, and the Au/Pt source prepared by induction melting. Buym and Grodzins [323] made use of (a, xn) reactions when bombarding natural iridium with... 

As a result of slow (thermal) neutron irradiation, a sample composed of stable atoms of a variety of elements will produce several radioactive isotopes of these activated elements. For a nuclear reaction to be useful analytically in the delayed NAA mode the element of interest must be capable of undergoing a nuclear reaction of some sort, the product of which must be radioactively unstable. The daughter nucleus must have a half-life of the order of days or months (so that it can be conveniently measured), and it should emit a particle which has a characteristic energy and is free from interference from other particles which may be produced by other elements within the sample. The induced radioactivity is complex as it comprises a summation of all the active species present. Individual species are identified by computer-aided de-convolution of the data. Parry (1991 42-9) and Glascock (1998) summarize the relevant decay schemes, and Alfassi (1990 3) and Glascock (1991 Table 3) list y ray energy spectra and percentage abundances for a number of isotopes useful in NAA. 

All isotopes of plutonium are radioactive. The two isotopes that have found the most uses are Pu-238 and Pu-239. Pu-238 is produced by bombarding U-238 with deuterons in a cyclotron, creating neptunium-238 and two free neutrons. Np-238 has a half-life of about two days, and through beta decay it transmutates into plutonium-238. There are six allotropic metallic crystal forms of plutonium. They all have differing chemical and physical properties. The alpha (a) aUotrope is the only one that exists at normal room temperatures and pressures. The alpha allotrope of metallic plutonium is a silvery color that becomes yellowish as it oxidizes in air. AH the other allotropic forms exist at high temperatures. 

Californium is a synthetic radioactive transuranic element of the actinide series. The pure metal form is not found in nature and has not been artificially produced in particle accelerators. However, a few compounds consisting of cahfornium and nonmetals have been formed by nuclear reactions. The most important isotope of cahfornium is Cf-252, which fissions spontaneously while emitting free neutrons. This makes it of some use as a portable neutron source since there are few elements that produce neutrons all by themselves. Most transuranic elements must be placed in a nuclear reactor, must go through a series of decay processes, or must be mixed with other elements in order to give off neutrons. Cf-252 has a half-life of 2.65 years, and just one microgram (0.000001 grams) of the element produces over 170 mhhon neutrons per minute. 

This chain of events involved the so-called weak interaction, a puny and slow force compared with the strong and electromagnetic interactions. The weak interaction governs the conversion of protons into neutrons and vice versa, with creation of a neutrino (antineutrino). It thus determines the lifetime of free neutrons, which naturally decay into protons. In fact, neutrons have a life expectancy of around 10 minutes. However, before they disappear, they may have the opportunity to combine with protons, one which they readily accept. In that case, nuclear physics makes its appearance in the Universe. 

The abundance of each element is fixed by its binding energy, which characterises its strength as an entity, and the temperature and density of free neutrons and protons attacking the nucleus (Fig. A3.1). If, as is usually the case, nuclear equilibrium is reached before a significant number of radioactive decays have had the time to occur, an auxiliary constraint can be imposed the total number density of protons and neutrons, both free and bound, must preserve the mean n/p ratio. 

Decay. The neutron in the free state undergoes radioactive decay. Elaborate experiments by Robson were required to identify the products of the decay and to measure the half-life of the neutron. He showed that the neutron emits a / -particle and becomes a proton. The half-lite was found to be 12.8 minutes. In stable nuclei, neutrons are stable. In radioactive nuclei, decaying by -emission, the neutrons decay with a half-life characteristic of the nuclei of which they arc a part. See also Radioactivity. 

One important aspect of leptons is that their number is conserved in nuclear processes. Consider, for example, the decay of the free neutron... 

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