Neutron thermalization

Neutron source Neutron sources Neutrons, thermal Neutropenia... 

Irradiation time min Total dosages3 Fast neutrons Thermal neutrons n/cm2 n/cm2 Gamma R Weight loss % 10% Wt loss on TGA °C Vst 200°C Avg gas evol cc/g/hr DTA Exotherm 20°C/min °C DTA 5-sec Endotherm Expl 20°C/min temp °C °C Strand Burning rate in/sec Melting point °C... 

We are using a plastic scintillator in our spectrometer. This has the advantage not to get neutron activated, to be a good neutron thermalizer and to yield fast timing signals. It is also relatively cheap. An inorganic scintillator would certainly also perform satisfactorily. 

Shortages of oil and coal will be followed by one of uranium. The nuclear industry knows that the fuel of today s thermal nuclear reactors (U235) is exhaustible and therefore in a few decades they plan to shift to breeder reactors. They say little to the public, except that this conversion would make nuclear power inexhaustible. This is true, because the conventional "slow neutron" thermal reactors are "once through" (in the sense that they consume their uranium fuel), while fast neutron breeder reactors make more fuel than they use. 

O Reactor core Hot neutrons Thermal neutrons Cold neutrons... 

Nexkin, M. Slow-neutron inelastic scattering and neutron thermalization. 

Nuclear and pion related 7-rays provide important information about the spectra of protons and ions accelerated in solar flares [e.g. Hua and Lingen-felter, 1987 Murphy et al., 1987 Lockwood et al., 1997 Hua et al., 2002], However, nuclear 7-ray lines probe the proton spectrum only up to 40 MeV, while 7-rays from pion decays are only observed in the most intense flares. In addition, any spectral break in the proton spectrum is likely to he below the pion production threshold. Neutrons produced at the solar surface over a wide range of energies may provide important information from the 50-300 MeV regime, complementing 7-ray observations. Due to the long neutron thermal-ization time ( 100 s) the 2.223 MeV neutron capture line is only a limited measure of neutron production. The spectrum of accelerated and interacting protons can be deduced more reliably from direct neutron measurements. 

Slow neutrons ( thermal neutrons) are produced when fast neutrons collide with moderators such as hydrogen, deuterium, oxygen, or the carbon atoms in paraffin. These neutrons are more likely to be captured by target nuclei. Bombardments with slow neutrons can cause neutron-capture ( , y) reactions. 

Coherent cross section acoh Neutrons, thermal in diffraction in reflectivity X 9 ... 

R. H. Blessing, On the differences between X-ray and neutron thermal vibration parameters, Acta Cryst. B51, 816-823 (1995). 

In the case of a fast reactor, neutron thermalization is not desirable, and the reflector will consist of a dense element of high mass number. 

It was clearly demonstrated that the composite BN semiconductor polycrystalline bulk detectors with BN grains embedded in a polymer matrix operate as an effective detector of thermal neutrons even if they contain natural boron only (Uher et al. 2007). A reasonable signal-to-noise ratio was achieved with detector thickness of about 1 mm. A Monte Carlo simulation of neutron thermal reactions in the BN detector was done to estimate the detection efficiency and compare with widely used He-based detectors to prove advantages of BN detectors. They are found to be promising for neutron imaging and for large area sensors. 

Every neutron. . . thermal neutrons quoted in ibid., p. 85. 

These measurements have shown that It may be feasible to make a neutron monitoring system vhlch would be sensitive to the fission product delayed neutrons on an operating reactor s rear face yet Insensitive to the background photo-neutrons, thermal neutrons, and gamma radiation. 

The effect of small perturbations (localized impurities, etc.) on the pile is important not only because of the theory of danger coefficient measurements but also because the effect of certain influences cannot be taken into account easily by any other than the perturbation method. The present report endeavors to go beyond Fermi s simple theory which applies only to a uniform bare pile and recognizes only one kind of neutrons (thermal). 

Although the pebble bed neutronics/thermal hydraulics code VSOP of KFA Jfllich is available through the NEA Databank now, ECN decided to develop it s own code system. The dynamic neutronics code PANTHER has been acquired from AEA, UK. This code is being coupled to the thermal hydraulics code THERMIX-DIREKT, kindly delivered to ECN by KFA Jfllich in a cooperation framework. 

During the late 1930s, physicists around the world had found that a slow moving neutron (thermal energies) could fission uranium and they became convinced... 

An adequate theoretical basis for the calculation of slow neutron scattering from chemically bound systems exists in the pseudo-potential approximation introduced by Fermi in 1937 [1]. The fundamental cross section of interest for neutron thermalization is the differential cross section g(Eo,E,6) for energy transfer Eq- E with scattering through an angle 0 in the laboratory system. The calculation of this cross section, even in the pseudo-potential approximation, depends on the detailed dynamics of the atomic motion in the moderator. The dynamics of atomic motion in crystals and liquids is complicated and not as yet known in detail. The direction of most fundamental interest, therefore, is to determine these dynamical properties from experimental measurements of slow neutron scattering. 

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