Neutron imaging of an operational dilution refrigerator.

The invention of the 3He/4He dilution refrigerator opened a new chapter in experimental ultra-low temperature physics. Dilution refrigerators became essential for providing ultra-low temperature environments for nuclear demagnetisation experiments, superconducting-qubit quantum processors and highly sensitive bolometers used in fundamental physics experiments. Development of dilution refrigeration technology requires thorough understanding of the quantum mechanical processes that take place in liquid helium at ultra-low temperatures. For decades the quantum fluids research community provided valuable information to engineers and designers involved in the development of advanced dilution refrigerators. However, the lack of methods that allow the measurement of physical parameters of liquid helium during the operation of a dilution refrigerator was hindering development of the technology. Here we show direct imaging of an operational dilution refrigerator using neutron radiography. This allows direct observation of the dilution process in 3He/4He mixtures and opens an opportunity for direct measurement of the 3He concentration. We observe the refrigerator behaviour in different regimes, such as continuous circulation and single shot, and show that our method allows investigation of various failure modes. Our results demonstrate that neutron imaging applied to the study of dilution refrigeration processes can provide essential information for developers of ultra-low temperature systems. We expect that neutron imaging will become instrumental in the research and development of advanced dilution refrigerators.

Profiling of human burned bones: oxidising versus reducing conditions.

Complementary optical and neutron-based vibrational spectroscopy techniques (Infrared, Raman and inelastic neutron scattering) were applied to the study of human bones (femur and humerus) burned simultaneously under either aerobic or anaerobic conditions, in a wide range of temperatures (400 to 1000 °C). This is the first INS study of human skeletal remains heated in an oxygen-deprived atmosphere. Clear differences were observed between both types of samples, namely the absence of hydroxyapatite's OH vibrational bands in bone burned anaerobically (in unsealed containers), coupled to the presence of cyanamide (NCNH2) and portlandite (Ca(OH)2) in these reductive conditions. These results are expected to allow a better understanding of the heat effect on bone´s constituents in distinct environmental settings, thus contributing for an accurate characterisation of both forensic and archaeological human skeletal remains found in distinct scenarios regarding oxygen availability.

Human bone probed by neutron diffraction: the burning process.

The first neutron diffraction study of human burned bone is reported, aiming at a comprehensive elucidation of the heat-induced bone diagenesis process. Chemical and crystallinity changes were probed in different types of bone (femur, humerus and tibia) upon heating to different maximum temperatures (from 400 to 1000 °C, under aerobic conditions). Fourier transform infrared spectroscopy has provided valuable complementary information. Noticeable crystallographic and domain size variations were detected, mainly between 700 and 900 °C, the high temperature interval (>700 °C) corresponding to an organized, highly symmetric inorganic bone matrix, virtually devoid of carbonates and organic constituents, while the lower range (<700 °C) revealed a considerably lower crystallinity associated with the presence of carbonates, lipids and collagen. This work contributes to a better understanding of heat-induced changes in bone and is therefore relevant for archaeology, biomaterials and forensic science.

Magnetic structure and field-dependent magnetic phase diagram of Ni2In-type PrCuSi.

The magnetic structure of the ternary equiatomic intermetallic compound PrCuSi is investigated using neutron powder diffraction experiments in 0 T as well as in external magnetic fields up to 2 T. The PrCuSi compound crystallizes in the hexagonal Ni2In-type structure, in the space group P63/mmc. In this structure, cationic ordering of Cu and Si takes place. The antiferromagnetic phase transition in the Pr sublattice takes place at [Formula: see text] K in 0 T. Under an external magnetic field of 2 T, a field-induced ferromagnetic phase is observed. Magnetoelastic coupling is evidenced by an increase in the unit cell volume. Clear signatures of a mixed antiferromagnetic and ferromagnetic phase in weak, intermediate fields, 0.4-0.8 T, are obtained from the present study. Using the present set of experimental data, we construct the H - T phase diagram of PrCuSi.

Design and Characterisation of Metallic Glassy Alloys of High Neutron Shielding Capability.

This paper reports the design, making and characterisation of a series of Fe-based bulk metallic glass alloys with the aim of achieving the combined properties of high neutron absorption capability and sufficient glass forming ability. Synchrotron X-ray diffraction and pair distribution function methods were used to characterise the crystalline or amorphous states of the samples. Neutron transmission and macroscopic attenuation coefficients of the designed alloys were measured using energy resolved neutron imaging method and the very recently developed microchannel plate detector. The study found that the newly designed alloy (Fe48Cr15Mo14C15B6Gd2 with a glass forming ability of Ø5.8 mm) has the highest neutron absorption capability among all Fe-based bulk metallic glasses so far reported. It is a promising material for neutron shielding applications.

Neutron diffraction study of the non-Fermi liquid compound CeNiGa2: magnetic behaviour as a function of pressure and temperature.

The magnetic symmetry and structure of the non-Fermi liquid heavy fermion compound CeNiGa2 has been determined by neutron powder diffraction. The orthorhombic CeNiGa2 compound orders antiferromagnetically below 4.4(2) K at ambient pressure with a magnetic moment magnitude of µCe = 0.80(4) µB for moments aligned along the c-axis. The magnetic (Shubnikov) space group is C2cm'm'm. The nature of the magnetic order of CeNiGa2 is further elucidated by neutron diffraction at elevated pressures up to 4.5 kbar, allowing for the confirmation of a critical pressure PC of about 4.2(2) kbar above which the magnetic moment ordering is suppressed.

Complex magnetic behavior in the novel Kondo lattice compound CeRhSn3.

We report the magnetic and transport properties of a new ternary intermetallic compound, CeRhSn3, using magnetic susceptibility, magnetization, specific heat, electrical resistivity, muon-spin relaxation (µSR) and neutron diffraction investigations. The dc magnetic susceptibility data reveal two magnetic phase transitions at 0.9 and 4 K. The overall behavior of dc susceptibility and magnetization indicates a ferrimagnetic-type phase transition near 4 K. The specific heat data also exhibit sharp λ-type anomalies at 1 and 4 K. The behavior of the specific heat anomaly under the application of a magnetic field suggests that the 1 K transition is probably related to a transition from a ferri- to a ferromagnetic state. The low temperature specific heat exhibits an enhanced Sommerfeld coefficient γ (~100 mJ mol⁻¹ K⁻²) due to the formation of a moderate heavy fermion state. The resistivity of CeRhSn3 demonstrates an interplay between the RKKY and Kondo interactions which is further modified by the presence of the crystal electric field. Interestingly, the resistivity of the nonmagnetic reference compound, LaRhSn3, is found to increase with decreasing temperature. Further, the onset of long-range magnetic order below 1 K is confirmed from our µSR study on CeRhSn3. However, the 4 K transition is not detected in the µSR and low temperature neutron diffraction data. Analysis of the dc magnetic susceptibility data within the framework of a two-sublattice model of ferrimagnetism supports the ferrimagnetic-type transition at 4 K in CeRhSn3. We have observed an unusual frequency dependence of the peak near 4 K in the ac susceptibility, which shows that the transition temperature shifts toward the lower temperature side with increasing frequency.

Neutron scattering and crystal field studies of the rare earth double perovskite Ba2ErSbO6.

The rare earth double perovskite Ba(2)ErSbO(6) contains an ordered face-centred cubic lattice of Er(3+) ions, suggesting that this material is a candidate for showing the effects of geometric magnetic frustration. Crystal field effects have also been shown to be important in this series. We report a systematic experimental study involving neutron scattering and bulk measurements that show no evidence of long ranged magnetic order or spin glass freezing down to 70 mK. A description of the system in terms of a crystal field scheme is established from inelastic neutron scattering. These measurements rule out significant magnetic coupling and show that all observed properties are fully explained by a model of uncoupled magnetic Er(3+) ions.

Structural transitions in RNi(10)Si(2) intermetallics.

Intermetallic compounds of the type RFe(10)Si(2) and RCo(10)Si(2) crystallize in the ThMn(12) structure (space group I4/mmm) whilst the heavy rare earth series RNi(10)Si(2) crystallize in a maximal subgroup of I4/mmm, P4/nmm. Reported here are neutron powder diffraction investigations for TbNi(10)Si(2) and ErNi(10)Si(2) which show that the P4/nmm structure undergoes a high temperature order-disorder phase transition at approximately 930 °C above which the ordered Ni and Si fractions revert to a random distribution on 4d and 4e sites. The volume expansion has been tracked in detail via the temperature dependence of the lattice parameters, whilst the temperature dependence of the thermal expansion coefficients α(11), α(33) and α(volume) has been determined from the lattice parameters. Associated with the order-disorder transition is a transition associated with a displacement of the R ion along the c-axis. Both transitions are of second order and the critical exponent associated with the order-disorder and displacive transitions, ß = 0.31, is in excellent agreement with the exponent determined for the three-dimensional Ising model.

Magnetic ground state of an experimental S=1/2 kagome antiferromagnet.

We present a detailed analysis of the heat capacity of a near-perfect S=1/2 kagome antiferromagnet, zinc paratacamite Zn(x)Cu(4-x)(OH)(6)Cl(2), as a function of stoichiometry x-->1 and for fields of up to 9 T. We obtain the heat capacity intrinsic to the kagome layers by accounting for the weak Cu2+/Zn2+ exchange between the Cu and the Zn sites, which was measured independently for x=1 using neutron diffraction. The evolution of the heat capacity for x=0.8...1 is then related to the hysteresis in the magnetic susceptibility. We conclude that for x>0.8 zinc paratacamite is a spin liquid without a spin gap, in which unpaired spins give rise to a macroscopically degenerate ground state manifold with increasingly glassy dynamics as x is lowered.

Ternary alkali metal transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt).

Ternary transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt) can be synthesised by reaction of the respective alkali metal acetylide A2C2 with palladium or platinum in an inert atmosphere at about 350 degrees C. The crystal structures are characterised by (infinity)1[M(C2)(2/2)2-] chains, which are separated by the alkali metals (P3m1, Z = 1). The refinement of neutron powder diffraction data gave C-C = 1.263(3) A for Na2PdC2 (Na2PtC2: 1.289(4) A), which is distinctively longer than the expected value for a C-C triple bond (1.20 A). On the basis of band-structure calculations this can be attributed to a strong back-bonding from the metal into the anti-bonding orbitals of the C2 unit. This was further confirmed by Raman spectroscopic investigations, which showed that the wavenumbers of the C-C stretching vibrations in Na2PdC2 and Na2PtC2 are about 100 cm(-1) smaller than in acetylene. 13C MAS-NMR spectra demonstrated that the acetylenic C2 units in the title compounds are very different from those in acetylene. Electrical conductivity measurements and band-structure calculations showed that the black title compounds are semiconductors with a small indirect band gap (approximately 0.2 eV).

Novel Ternary Alkali Metal Silver Acetylides M(I)AgC(2) (M(I)=Li, Na, K, Rb, Cs).

Three different packings of (1)(infinity)[Ag(C(2))(2/2)(-)] chains (represented by rods in the picture) have been found in the crystal structures of the first ternary alkali metal silver acetylides, which were obtained by the reaction of M(I)C(2)H (M(I)=Li-Cs) with AgI in liquid ammonia and subsequent heating of the remaining residue to 120 degrees C.