Diffuse scattering. X-rays vs. neutrons

Fundamental and experimental differences between X-ray and neutron scattering methods are outlined and the advantages of either radiation for the study of disorder in crystals are described. Special emphasis is laid on their complementarity, e.g. to identify the atomic species involved in the disorder or to decide between the elastic or inelastic nature of the diffuse scattering (static or dynamic disorder). This is illustrated by three examples: LiNbO₃ (dynamic nature of chain-like disorder), doped Zr0₂ (identification of cationic and anionic disorder components) and decagonal quasicrystals (distinction between quasi-isoelectronic elements).     

Personal neutron dosimetry in the space station MIR and the Space Shuttle

A passive neutron dosemeter based on nuclear track detectors and TLD's was used in 1995 and 1997 on the MIR station and in Space Shuttle flights to MIR. As it is equipped with neutron converters and shieldings of different types the track detector system allows the neutron dose equivalent to be determined in rough energy intervals. The results of the measurements on the MIR station and in the Space Shuttle flights are presented and the influence of charged particles in the complex mixed radiation field in space is discussed. Improvements are possible by means of a new active neutron dosemeter which is under development at the PTB. First measurements with a prototype in the high-energy reference fields at CERN are presented and discussed.     

The application of neutron powder diffraction techniques for crystallographic studies at high pressures

Abstract

For phase transition studies, neutron powder diffraction offers a number of important advantages over x-ray based techniques, for example ab-initio structural determination. There are two distinct methods using either monochromatic angular dispersive geometry on a reactor cold source or time-of-flight energy-dispersive techniques requiring a pulsed neutron source. Both techniques offer comparable resolution but have differing advantages for high pressure studies. Recent studies illustrate the benefits of the two methods and the application of these to solve unknown crystal structures.     

Beschleunigungsreaktionen von Neutronen

Acceleration of neutrons by isomeric nuclei. The inelastic neutron acceleration (INNA) by long living isomeric nuclei is studied. The theoretical estimates of the INNA cross-sections for thermal neutrons for a number of isomers give the values in the range 0.1 - 10²b. Inelastic acceleration of thermal neutrons was observed by ^152mEu and ^180mHf, and confirmed the high value of the INNA cross-sections. As a result of the high probability of acceleration some isomeric nuclei ^113mIn, ^115mIn, are not usual moderators but accelerators of neutrons. The use of the INNA reaction for the study of high excited levels and the nature of various nuclear hindrances are discussed.     

Nano-structured Fabry–Pérot resonators in neutron optics & tunneling of neutron wave-particles

Correlated to the quantum mechanics wave-particle duality, the optical analogy between electromagnetic waves and cold neutrons manifests itself through several interference phenomena particularly the so called Frustrated Total Reflection i.e., the tunneling process in Fabry–Pérot nano-structured cavities. Prominent resonant situations offered by this configuration allow the attainment of numerous fundamental investigations and surface-interface studies as well as to devise new kinds of neutron optics devices. This review contribution reports such possibilities in addition to the recently observed peculiar Goos–Hänchen longitudinal shift of neutron wave-particles which was predicted by Sir Isaac Newton as early as 1730.     

Damage track detectors in radioprotection dosimetry: a novel approach

Limited sensitivity and unpredictable background are the major drawbacks of damage track detectors in the assessment of low neutron doses and low concentrations of alpha emitters in biological and environmental samples. The simplest way to increase the sensitivity of the damage track detectors is to increase both the exposure time and the detector area. However, the strong variability of the background may make this task often impossible. This background problem has been finally solved by a new registration method based on counting coincidence spots in geometrically matched pair of detectors. By using spark counting and electrochemical etching, both of which produce spots visible at low magnification, coincidences induced in two matched detector-surfaces by a few-microns-long tracks can be easily seen. This novel counting approach can be considered just the converse of those used in the past with Bi-fission detectors and cosmic ray stacks.     

High resolution measurement of bragg cut-off in beryllium

Bragg cut-off for plane of polycrystalline beryllium of various lengths of 300 and 116 K has been measured with an energy resolution of 5 μeV. The natural width of the cut-off is 12.5±1.5 μeV, independent of temperature and length of beryllium and also of physical characteristics and certain metallurgical treatments of the powder. Such blocks of beryllium would be suitable for designing a ΔT-window spectrometer with resolution ⩾20 μeV. Bragg cut-offs corresponding to (0002) and planes of beryllium have been separated for the first time. These can also be used for producing additional energy windows in a ΔT-window spectrometer, thus increasing its efficiency. Paper entitled ‘ΔT-window spectrometer’ will appear in the November issue of Pramana.     

New process of preparation, structure, and physicochemical investigations of the new titanyl phosphate Ti2O(H2O)(PO4)2

New titanyl phosphate Ti₂O(H₂O)(PO₄)₂ has been prepared and characterized by X-ray and neutron diffraction, nuclear magnetic resonance, infrared and Raman spectroscopies and thermogravimetric analysis. The crystal structure has been solved from neutron powder diffraction data at 300 K by Rietveld method in P2₁ space group. The refinement led to satisfactory profile factors (R_p=2.7%, R_wp=3.2%) and crystal structure model indicators (R_B=5.8%, R_F=3.2%). The cell is monoclinic with a=7.3735 Å, b=7.0405 Å, c=7.6609 Å and β=121.48°, Z=4. The structure can be described as a three-dimensional framework built up by chains of [TiO₅(OH₂)] octahedra with alternative short bonds [Ti₍₁₎-O₍₁₂₎; Ti₍₂₎-O₍₁₂₎, 1.88-1.84 Å] and long ones [Ti₍₁₎-O_W; Ti₍₂₎-O_W, 2.25-2.23 Å] along c-axis and connected via [PO₄] tetrahedra. Oxygen atom denoted O₍₁₂₎ is only linked to two titanium atoms and Oxygen atom denoted O_W is linked to two titanium atoms and two hydrogen atoms. O₍₁₂₎ and O_W are not linked to P atoms and justify the titanyl phosphate formulation Ti₂O(H₂O)(PO₄)₂. The infrared and Raman spectra presents peaks due to vibrations of Ti-O, P-O and O-H bonds. The ³¹P MAS NMR spectrum reveals two ³¹P resonance lines, in agreement with the structure which showed two crystallographic sites for phosphorus. The thermogravimetric analysis show that Ti₂O(H₂O)(PO₄)₂ is thermally stable until 400 °C. Above this temperature, it losses water and decomposes to Ti₅O₄(PO₄)₄ and TiP₂O₇.     

The determination of the neutron component of cosmic radiation fields in spacecraft

Poly allyl diglycol carbonate (PADC or CR-39^®) etched track detectors may be used to estimate the neutron component of the cosmic radiation in spacecraft using simple techniques developed for neutron personal dosimetry. Electrochemically etched pits are identified and counted using fully automated read-out procedures. The neutron component of the radiation field at the location of the dosimeter will produce electrochemically etchable tracks, as will the proton and energetic heavy charged particle components, depending on particle type, energy and angle of incidence. The response to incident charged particles which produce tracks and are counted as if produced by a neutron, will lead to an over-estimate of the neutron component. A correction can be applied to take account of this, or an additional chemical etch carried out which allows discrimination. Recent results for exposures in low-Earth orbit are reported.     

Monoenergetic neutron sources below 100 MeV

Monoenergetic neutron sources are essential for fundamental studies in radiobiology and dosimetry, for measurement of cross sections and kerma coefficients, for calibration of detectors, for activation analysis and for fusion research. Monoenergetic neutrons below energies of 20 MeV are most conveniently produced by reactions between the hydrogen isotopes or between protons and ⁷Li. By proper choice of reaction type monoenergetic neutrons up to 20 MeV can be produced with negligible secondary background radiation. These reactions cannot provide monoenergetic beams between about 8 and 14 MeV and in this “gap” region inverse reactions are most favourable. The most practical way of producing quasi-monoenergetic neutron beams in the energy range from 20 to 100 MeV is by the bombardment of light elements with protons. Because of the relative simplicity of manufacturing suitable isotopically-pure targets and the large 0° cross section, the ⁷Li(p,n)⁷Be reaction is a convenient source of quasi-monoenergetic neutrons over this range of energies, although the ⁹Be(p,n)⁹B reaction is also used.