arXiv subjects

O. Kirichek

Publications and source records attributed to O. Kirichek.

7 recordsLinked to original sources

arXiv2017

Recombination of radiation defects in solid methane - neutron sources and cryo-volcanism on celestial bodies

Physicochemical properties of solid methane exposed to ionizing radiation have attracted significant interest in recent years. Here we present new trends in the study of radiation effects in solid methane. We particularly focus on relaxation phenomena in solid methane pre-irradiated by energetic neutrons and electron beam. We compare experimental results obtained in the temperature range from 10K to 100K with a model based on the assumption that radiolysis defect recombinations happen in two stages, at two different temperatures. In the case of slow heating up of the solid methane sample, irradiated at 10K, the first wave of recombination occurs around 20K with a further second wave taking place between 50 and 60K. We also discuss the role of the recombination mechanisms in burp phenomenon discovered by J. Carpenter in the late 1980s. An understanding of these mechanisms is vital for the designing and operation of solid methane moderators used in advanced neutron sources and could also be a possible explanation for the driving forces behind cryo-volcanism on celestial bodies.

cond-mat.mtrl-sci↗

arXiv2013

Phonon-roton modes of liquid 4He beyond the roton in MCM-41

We present neutron scattering measurements of the phonon-roton (P-R) mode of superfluid 4He confined in 47 ÅMCM-41 at T = 0.5 K at wave vectors, Q, beyond the roton wave vector (Q_R =1.92 Å$^{-1}$). Measurements beyond the roton require access to high wave vectors (up to Q = 4 Å$^{-1}$) with excellent energy resolution and high statistical precision. The present results show for the first time that at T = 0.5 K the P-R mode in MCM-41 extends out to wave-vector Q$\simeq$3.6 Å$^-1$ with the same energy and zero width (within precision) as observed in bulk superfluid 4He. Layer modes in the roton region are also observed. Specifically, the P-R mode energy, $ω_Q$, increases with Q for Q > QR and reaches a plateau at a maximum energy Q = 2Δ where Δ is the roton energy, Δ = 0.74 $\pm$ 0.01 meV in MCM-41. This upper limit means the P-R mode decays to two rotons when its energy exceeds 2Δ. It also means that the P-R mode does not decay to two layers modes. If the P-R could decay to two layer modes, $ω_Q$ would plateau at a lower energy, $ω_Q$ = 2ΔL where ΔL = 0.60 meV is the energy of the roton like minimum of the layer mode. The observation of the P-R mode with energy up to 2Δ shows that the P-R mode and the layer modes are independent modes with apparently little interaction between them.

cond-mat.other↗

arXiv2010

High pressure gas vessels for neutron scattering experiments

The combination of high pressure techniques with neutron scattering proves to be a powerful tool for studying the phase transitions and physical properties of solids in terms of inter-atomic distances. In our report we are going to review a high pressure technique based on a gas medium compression. This technique covers the pressure range up to ~0.7GPa (in special cases 1.4GPa) and typically uses compressed helium gas as the pressure medium. We are going to look briefly at scientific areas where high pressure gas vessels are intensively used in neutron scattering experiments. After that we are going to describe the current situation in high pressure gas technology; specifically looking at materials of construction, designs of seals and pressure vessels and the equipment used for generating high pressure gas.

physics.ins-det↗

arXiv2008

Neutron Reflection from the Surface of Liquid 4He with and without a Layer of 3He

We report and discuss the first neutron reflection measurements from the free surface of normal and superfluid 4He and of liquid 3He-4He mixture. In case of liquid 4He the surface roughness is different above and below the lambda transition, being smoother in the superfluid state. For the superfluid, we also observe the formation of a surface layer ~200 A thick which has a subtly different neutron scattering cross-section. The results can be interpreted as an enhancement of Bose-Einstein condensate fraction close to the helium surface. We find that the addition of 3He isotopic impurities leads to the formation of Andreev levels at low temperatures.

cond-mat.other↗

arXiv2007

Neutron reflection from the surface of normal and superfluid 4He

The reflection of neutrons from a helium surface has been observed for the first time. The 4He surface is smoother in the superfluid state at 1.54 K than in the case of the normal liquid at 2.3 K. In the superfluid state we also observe a surface layer ~200 Angstroms thick which has a subtly different neutron scattering cross-section, which may be explained by an enhanced Bose-Einstein condensate fraction close to the helium surface.

cond-mat.soft↗

arXiv2007

Absence of low temperature anomaly in the Debye-Waller factor of solid He-4

The mean square atomic displacement in hcp-phase solid He-4 has been measured in crystals with a molar volume of 21.3 cm^3. It is temperature independent from 1 K to 140 mK, with no evidence for an anomaly in the vicinity of the proposed supersolid transition. The mean square displacement is different for in-plane motions (0.122 +/- 0.001 A^(2)) and out-of-plane motions (0.150 +/- 0.001 A ^(2)).

cond-mat.other↗

arXiv2007

Bose-Einstein Condensate in Solid Helium

We present neutron scattering measurements of the atomic momentum distribution, n(k), in solid helium under a pressure p = 41 bars and at temperatures between 80 mK and 500 mK. The aim is to determine whether there is Bose-Einstein condensation (BEC) below the critical temperature, T_c = 200 mK where a superfluid density has been observed. Assuming BEC appears as a macroscopic occupation of the k = 0 state below T_c, we find a condensate fraction of n_0 = (-0.10 \pm 1.20)% at T = 80 mK and n_0 = (0.08\pm0.78)% at T = 120 mK, consistent with zero. The shape of n(k) also does not change on crossing T_c within measurement precision.

cond-mat.stat-mech↗