Solar neutrons

One way to create unstable nuclides is by neutron capture. Because neutrons have no electrical charge, they readily penetrate any nucleus and may be captured as they pass through a nucleus. The sun emits neutrons, so a continuous stream of solar neutrons bathes the Earth s atmosphere. The most abundant nuclide in the atmosphere,... 

A logical explanation for the global nature of these correlations is that they are all related to variations of the sun, which cause variations in the temperature of the sea surface, thus causing variations in the isotopic composition of water vapor which distills off the sea and is stored as wood in trees and also forms the annual layers of the ice cap. The variations of the sun are furthermore related to the flux of solar neutrons in the earth s atmosphere and so cause small variations in the carbon-14 content of the bristle cones. During times of a quiet sun the average carbon-14 production is about 25 percent larger than when solar activity is high [43]. 

Some years later, a Japanese-built detector (Kamiokaride II), designed to delect the more energetic solar-emitted neutrinos ( -5 mil electron volts), came up about 50% short of the expected counts, thus reconfirming a shortage of solar neutrons. 

SOLAR NEUTRON OBSERVATION AT GROUND LEVEL AND FROM SPACE... 

Abstract The study of particle acceleration, particle transport and interaction mechanisms associated with high-energy phenomena at the Sun is of general astrophysical relevance. In this context an exemplary overview is given of ground-based and spaceborne instruments to measure the energy spectrum of solar neutrons, and of the significance of such observations. 

Figure 1. Schematic representation of solar neutron and 7-ray production proton energy spectrum (A), neutron production (B), 4.438 MeV 7-ray line production (C), 2.223 MeV 7-ray line production (D), and pion production (E). [Lockwood et al., 1997]...

The first identification of solar neutrons at Earth took place on June 3,1982 (Fig. 2a), by neutron monitor measurements at Jungfraujoch, Lomnicky Stit, and Rome [Debrunner et al., 1983 Chupp et al., 1987], This was two years after the discovery of solar neutrons in near-Earth space by the Gamma Ray Spectrometer (GRS) aboard the Solar Maximum Mission (SMM) satellite [ Chupp et al., 1982], Thereafter, standardized neutron monitors were set up at favorable observational locations at Earth, such as Haleakala, Hawaii [Pyle and Simpson, 1991], Additionally, new ground-based detectors with enhanced sensitivity to solar neutrons were developed [e.g. Shibata et al., 1991 Muraki et al., 1993]. 

Figure 2. June 3, 1982, solar neutron event (a) Count rate enhancement of the Jungfraujoch 18-IGY neutron monitor, (b) Simulated omnidirectional spectra of secondary particles at 700 g cm-2 induced by 1068 MeV solar neutrons at onset time [Moser et al., 2003].

Solar Neutron Observation at Ground Level and from Space... 

Initiated by the Solar-Terrestrial Environment Laboratory of the Nagoya University in Japan, a new global network of solar neutron telescopes was set up. Seven locations well distributed in longitude enable the observation of solar neutrons during 24 hours a day. The detectors are capable to determine the incident neutrons energies and directions. Observations based on this network are reported e.g. by Sako et al., 2003. 

The Compton Telescope (COMPTEL) on the Compton Gamma Ray Observatory (CGRO) was the first instrument built to detect 7-rays and solar neutrons in space [Ryan et al., 1993a], COMPTEL observations of the June 15, 1991, solar neutron event were the first that allowed to generate an image of the Sun in neutrons [Ryan et al., 1993b Nieminen, 1997],... 

Since the CGRO mission ended in 2000, leaving a lack of spaceborne neutron detectors, we will present two new projects of solar neutron detectors to be operated on future space missions. 

Fast Neutron Imaging Telescope (FNIT). The FNIT detector [Moser et al., 2004a] is sensitive to solar neutrons in the energy range 3-100 MeV, and is intended to be operated on inner heliosphere missions. 

Solar Neutron Tracking and Imaging Spectrometer (SONTRAC). The... 

Some properties of silver are summarized in Table 1. The solar energy transmittance and neutron-absorption characteristics of silver are shown in Eigures 1 and 2, respectively. Thermal properties are given in Table 2. Other properties are given in References 1,3, and 4. 

The composition of the Earth was determined both by the chemical composition of the solar nebula, from which the sun and planets formed, and by the nature of the physical processes that concentrated materials to form planets. The bulk elemental and isotopic composition of the nebula is believed, or usually assumed to be identical to that of the sun. The few exceptions to this include elements and isotopes such as lithium and deuterium that are destroyed in the bulk of the sun s interior by nuclear reactions. The composition of the sun as determined by optical spectroscopy is similar to the majority of stars in our galaxy, and accordingly the relative abundances of the elements in the sun are referred to as "cosmic abundances." Although the cosmic abundance pattern is commonly seen in other stars there are dramatic exceptions, such as stars composed of iron or solid nuclear matter, as in the case with neutron stars. The... 

The silver white, shiny, metal-like semiconductor is considered a semimetal. The atomic weight is greater than that of the following neighbor (iodine), because tellurium isotopes are neutron-rich (compare Ar/K). Its main use is in alloys, as the addition of small amounts considerably improves properties such as hardness and corrosion resistance. New applications of tellurium include optoelectronics (lasers), electrical resistors, thermoelectric elements (a current gives rise to a temperature gradient), photocopier drums, infrared cameras, and solar cells. Tellurium accelerates the vulcanization of rubber. 

The chemical analysis has revealed that rather low C/O ratios are found in metal-poor extragalactic carbon stars, as found for galactic carbon stars of the solar vicinity. Furthermore, the three analyzed stars show similar s-elements enhancements [ls/Fe]=0.8-1.3 and [hs/Fe]=l.l-1.7. This leads to new constraints for evolutionary models. For instance, the derived C/O and 13C/12C ratios are lower than model predictions at low metallicity. On the contrary, theoretical predictions of neutrons exposures for the production of the s-elements are compatible with observations (see Fig. 1). Finally, from their known distances, we have estimated the luminosities and masses of the three stars. It results that SMC-B30 and Sgr-C3 are most probably intrinsic carbon stars while Sgr-Cl could be extrinsic. 

This is an extremely small quantity, which combined with the also extremely small interaction of gravitational waves (GWs) with matter makes it impossible to generate and detect GW on earth. Fast conversions of solar-size masses are required to produce signals with amplitudes that could be detectable. Astrophysical sources are for instance supernova explosions or a collision of two neutron stars or black holes. 

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy nuclei. 

Low (<1 solar mass) Middle (5-10 solar masses) High (>20 solar masses) Protostar — pre-main sequence — main sequence — red giant — planetary nebula — white dwarf — black dwarf Protostar - main sequence — red giant — planetary nebula or supernova —> white dwarf or neutron star Protostar — main sequence —> supergiant — supernova — neutron star... 

The result of all these processes is that the Sun was bom 4.6 Gyr ago with mass fractions X 0.70, Y 0.28, Z 0.02. These abundances (with perhaps a slightly lower value of Z) are also characteristic of the local ISM and young stars. The material in the solar neighbourhood is about 15 per cent gas (including dust which is about 1 per cent by mass of the gas) and about 85 per cent stars or compact remnants thereof these are white dwarfs (mainly), neutron stars and black holes. 

Fig. 6.3. Product aN of abundance and neutron capture cross-section for s-only nuclides in the Solar System. The main and weak s-process components are shown by the heavy and light curves respectively. Units are mb per 106 Si atoms. After Kappeler, Beer and Wisshak (1989). Copyright by IOP Publishing Ltd. Courtesy Franz Kappeler.

Table 6.1 shows some other best-fit parameters to Solar-System s-process abundances. The seed nucleus is basically 56Fe light nuclei have low cross-sections (but can act as neutron poisons , e.g. 14N for the 13C(a, n) neutron source), whereas heavier nuclei are not abundant enough to have a major influence. Certain nuclidic ratios, e.g. 37Cl/36Ar and 41K/40Ca, indicate that under 1 per cent of Solar-System material has been s-processed. 

Fig. 6.5. Development of the convective region, neutron density from 13C and 22Ne sources and maximum temperature as functions of time during a thermal pulse in a low-mass star with Z Z0/3, which seems to give the best fit to Solar-System abundances from the main s-process. However, more recent models imply that 13C is all used up in the radiative phases. After Kappeler et al. (1990). Courtesy Maurizio Busso and Claudia Raiteri.

Thus the 13 C neutron source (with a little assistance from 22Ne) in thermally pulsing low- and intermediate-mass stars is well established as the chief source of the main component of s-process nuclides in the Solar System. It is not quite clear, however, whether the r0 parameter is something unique, or just the average over a more-or-less broad distribution of values nor is it clear why a similar s-process pattern is seen in stars that are metal-deficient by factors of up to 100 (see Pagel Tautvaisiene 1997). 

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