Druck-Materie 20b.qxd - JUWEL - Forschungszentrum Jülich

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2.2 Inelastic neutron scattering The experiments have been conducted at the time-of-flight spectrometer for thermal neutrons, SV29 at the FRJ-2 reactor. In order to find the respective optimum range for energy transfer with simultaneously obtaining the required energy resolution, the measurements have been performed with three different incoming energies, 7 meV, 26 meV and 41 meV and various scattering angles. Samples have been measured with flat aluminium vessels, 3 cm by 7 cm wide and 0.1 and 0.2 cm thick, the latter used for the granular methane clathrate. Measurements have been performed between 2 K and 70 K using a closed cycle crystat. A distinct separation from the elastic line of the expected lowest energy levels of the order of meV (e.g. for methane clathrate, ±1 meV, +2 meV and +3 meV) was possible with 7 meV, as can be seen in Fig. 2. 2.3 Cold neutron leakage current measurements In order to assess the different performances of methane clathrate, ice and mesitylene moderators, respectively, we have performed neutron leakage current measurements at the <strong>Jülich</strong> spallation source mock-up JESSICA. The measurement of the leakage spectrum of THF clathrate has not been included in these studies so far. We have used the same moderator vessel for the three media quoted above. Therefore the results do not necessarily provide information on the optimum performance of the investigated materials. In the experiments 1 µs long pulses of 1.3 GeV protons are deposited into a 70 liter mercury target and the fast neutrons generated in and emitted from this target are partly intercepted and slowed down in the adjacent moderator materials. The volume of the moderator vessel is of the order of one liter with the dimensions 15×12×5 cm³ (width×height×thickness). The largest face of the vessel is viewed by the neutron detector located about 5 m from the moderator. The media are cooled down to 20 K by a closed cycle refrigerator. The energy distribution of the slow neutrons leaking from the moderator is measured by a time-of-flight technique and transformed to the energy afterwards. Background is measured by placing a boron carbide (B4C) absorber in the neutron flight path and these data subtracted from the leakage current data. The JESSICA and cryogenic installations, respectively, are described in more detail elsewhere [9], [10]. 3. RESULTS 3.1 Inelastic neutron scattering 3.1.1 Synthetic methane clathrate The inelastic spectra from synthetic methane clathrate taken at five different temperatures between5 K and 70 K are shown in Fig. 2. With an incoming neutron energy of 7 meV a clear separation of the expected lowest rotational levels for methane clathrate (±1 meV, +2 meV and +3 meV) from the elastic line was possible with. It should be noted that at the highest temperature of 70 K the discrete lines of the free rotations are smeared out to a quasi-elastic line due to diffusional motions. The energies of the observed lines at the lowest temperature corresponds perfectly to the lines found with natural deep sea methane clathrate [4]. 116
�� 5 K � 20 K �� 30 K � 50 K �� 70 K Figure 2. Inelastic spectra from synthetic methane clathrate at 5 different temperatures (E0 = 7 meV, δE = 0.4 meV; Qo = 2.9 Å -1 ). 3.1.2 Tetrahydro-furane (THF) clathrate The spectrum from THF clathrate is shown in Fig. 3. It exhibits a feature at about ∆E = 10 meV not present in pure ice (compare Fig. 5 below) and therefore attributed either to the THF molecule embedded in the ice or to the different crystalline structure of the inclusion compound. With improved resolution due to the lower incoming neutron energy of 7 meV we investigated the lowest energy transfer regime. The results are shown in Fig. 4. The observed peaks at energy transfers of about 1.8 meV and 2.8 meV, respectively, we have observed in ice as well. Figure 3. Inelastic spectra up to 20 meV from THF clathrate at T = 2 K and for three different scattering angles (E0 = 26 meV, δE = 2.1 meV; Qo = 3.0; 3.9 and 5.8 Å -1 resp.). 117
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ACoM - 6 6th 6 International Worksh
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Forschungszentrum Jülich GmbH Inst
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Preface These are the Proceedings o
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List of participants, cont’d. Kü
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Pepe M. 43 Petriw S. 43 Picton D.J.
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Experimental background, results an
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• Light water ice as H(H2O) at T
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In Table 2 a survey is given about
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sigma-T in barns/molecule Figure 4.
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2.3 Liquid water Compared to the so
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presented. The corresponding experi
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3.2 Gaseous hydrogen and deuterium
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Rho(Omega) normalised 160 140 120 1
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elatively free and can also suffer
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Figure 16. Heat Capacity for Solid
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o-D2 and p-H2 at 5K for an ucn ener
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Figure 22. UCN Upscattering Rates i
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ho(omega) normalised 40 35 30 25 20
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derived from Walker [52]. The neutr
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Since there is only a small gain of
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7. CALCULATION OF SCATTERING LAW DA
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For the validation of these data se
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[36] Nielsen, M.: Phonons in Solid
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2. THE CASE OF MOLECULAR SOLIDS An
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γ r ( 0) ≅ 2 2 2 T erf e ( ) / 2
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4. PRELIMINARY APPLICATION Solid Me
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[5] Meeting on Moderator Concepts a
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state. Conversely, the monatomic hy
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Note that the equilibrium trihydrog
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para hydrogen system under irradiat
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[12] T. E. Fessler and J. W. Blue,
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tion of neutron intensities, partic
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varied between calculations but the
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5. THE OPTIMISATION OF MULTIPLY GRO
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Page 81 and 82: ABSTRACT ACoM - 6 6 th Meeting of t
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Page 97 and 98: The results presented in this paper
Page 99: Table 10. Summary of energy transfe
Page 102 and 103: 2. EXPERIMENTAL Methane hydrate was
Page 104 and 105: Figure 4. Energy levels of methane
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Page 128 and 129: Figure 2. The methane pelletizer, m
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Page 138 and 139: Table 1. Data of irradiation of met
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Page 150 and 151: APPENDIX Some useful relations and
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Page 164 and 165: 4. CONCLUSIONS We demonstrated that
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to change the temperatures and to p
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supplied from the high pressure bom
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10 9 10 8 10 7 10 6 Para35%[Experim
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2. TIME OF FLIGHT SPECTRA AND ENERG
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When applying this transformation t
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data were analyzed in more detail.
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First of all, we should conclude th
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The equation (a) may also describe
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Even at h=0.01 mm which is too smal
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model and some consequences of its
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methane and 0.7-1.5% for water ice
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case: the sample of infinite size.
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Phase Diagram of the CH4-H20 system
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Reaction Chamber for Hydrate Produc
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Analysis: X-ray diffraction pattern
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Comparison with non-hydrate forming
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Mechanical Tests: Apparatus 220
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Morphology of Indium Jacket 222
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Schriften des Forschungszentrums J
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Schriften des Forschungszentrums J
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