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Figure 22. UCN Upscattering Rates in Solid hcp Deuterium at low Temperatures (Maxwell Flux Density at Tn = 30 K) In Fig. 23 the absorption and upscattering rates of ucn are compared for the pure o-D2 moderator and for a mixture with 0.2 wt% p-H2 . For the deuterium sample, the upscattering rate is slightly lower than the absorption rate. Taking the mixture, the upscattering rate remain nearly unchanged whereas the absorption rate is increased by a factor of 4 cause of the large absorption cross-section of hydrogen. Figure 23. UCN reaction rates in solid hcp o-deuterium at 5 K (Maxwell flux density at Tn = 30 K) 30
4. SOLID METHANE AS A COLD MODERATOR As a moderator for cold neutron sources methane essentially is discussed to be used in the solid phase with temperatures around 20 to 30K. In this temperature range methane crystallises in a face-centered cubic (fcc) lattice. The methane molecule CH4 is built up as a tetrahedral structure of the hydrogen atoms with the carbon atom at the centre of mass. As for the solid hydrogen containing moderators like polyethylene or light water ice the neutron interaction with the bound H nuclei is treated in incoherent approximation. This can be done without significant loss of accuracy due to the strong dominance of the incoherent neutron scattering. Therefore, for simplification the coherent neutron scattering in the regular lattice can be omitted. The consequence of this approximation is that the neutron scattering below the Bragg edge will be slightly overestimated. The methane molecule has N=5 atoms. Taking 3N degrees of freedom in the solid phase they split into 3 (hindered) translational modes, 3 strongly hindered rotations and 9 relatively free oscillations. Omitting the translational modes weight and assuming the optical modes being equivalently weighted, the part of the hindered rotational band in the frequency distribution for the bound hydrogen should be weighted approximately by 1/4, the part of the discrete oscillators should be weighted by 3/4. Following the ideas of MacFarlane [46] the 9 oscillators are reduced to 4 with corresponding parameters listed in Table 5. Table 5. Optical modes data for H bound in solid methane mode ω1 ω2 ω3 ω4 Vib./meV 162 190 361 374 Meff/amu 3.247 5.376 23.81 6.944 Rot./meV Smeared Band Maximum 8.0 Meff/amu - - - 3.125 The smeared band of the hindered rotations in the frequency distribution is shown in Fig. 24 together with a proposal of Morishima et al [47]. The distribution given by MacFarlane is derived from the measurements of Harker, Brugger [48] It is handled in harmonic approximation. Compared to the older ρ(ω) the new one has a lower weight which seems to be not adequate compared with measured total neutron cross sections of solid methane (Fig. 25). For the Morishima model the authors reduced the number of discrete oscillators to two (170 and 387 meV). Generating a Scattering Law data file with this model we have taken the same relative weight ratio as for the four oscillators. In Table 6 an overview is given for the temperature grid and the MCNP data sets. To validate the neutron transport model and the scattering dynamics some thermalisation moderator experiments [45, 49] with solid methane as moderator were recalculated. Compared to the measured neutron flux densities the calculated results agree well within the published accuracy (Fig. 26 and Fig. 27). Table 6. Temperature grid of the generated data sets for H bound in solid methane T in K 20 22 31 57 77 89 MCNP set s-meth.00t s-meth.01t s-meth.02t s-meth.03t s-meth.04t s-meth.05t 31
Page 1 and 2: ACoM - 6 6th 6 International Worksh
Page 3 and 4: Forschungszentrum Jülich GmbH Inst
Page 5: Preface These are the Proceedings o
Page 8 and 9: List of participants, cont’d. Kü
Page 10 and 11: Pepe M. 43 Petriw S. 43 Picton D.J.
Page 12 and 13: Experimental background, results an
Page 14 and 15: • Light water ice as H(H2O) at T
Page 16 and 17: In Table 2 a survey is given about
Page 18 and 19: sigma-T in barns/molecule Figure 4.
Page 20 and 21: 2.3 Liquid water Compared to the so
Page 22 and 23: presented. The corresponding experi
Page 24 and 25: 3.2 Gaseous hydrogen and deuterium
Page 26 and 27: Rho(Omega) normalised 160 140 120 1
Page 28 and 29: elatively free and can also suffer
Page 30 and 31: Figure 16. Heat Capacity for Solid
Page 32 and 33: o-D2 and p-H2 at 5K for an ucn ener
Page 36 and 37: ho(omega) normalised 40 35 30 25 20
Page 38 and 39: derived from Walker [52]. The neutr
Page 40 and 41: Since there is only a small gain of
Page 42 and 43: 7. CALCULATION OF SCATTERING LAW DA
Page 44 and 45: For the validation of these data se
Page 46 and 47: [36] Nielsen, M.: Phonons in Solid
Page 48 and 49: 2. THE CASE OF MOLECULAR SOLIDS An
Page 50 and 51: γ r ( 0) ≅ 2 2 2 T erf e ( ) / 2
Page 52 and 53: 4. PRELIMINARY APPLICATION Solid Me
Page 54 and 55: [5] Meeting on Moderator Concepts a
Page 56 and 57: state. Conversely, the monatomic hy
Page 58 and 59: Note that the equilibrium trihydrog
Page 60 and 61: para hydrogen system under irradiat
Page 62 and 63: [12] T. E. Fessler and J. W. Blue,
Page 64 and 65: tion of neutron intensities, partic
Page 66 and 67: varied between calculations but the
Page 68 and 69: 5. THE OPTIMISATION OF MULTIPLY GRO
Page 70 and 71: Figure 7. A comparison of time dist
Page 72 and 73: 4.8 cm hydrogen slab in front of a
Page 75 and 76: ACoM - 6 6 th Meeting of the Collab
Page 77 and 78: the moderator is annealed every 12
Page 79 and 80: The method requires two additional
Page 81 and 82: ABSTRACT ACoM - 6 6 th Meeting of t
Page 83 and 84: Heat transfer processes across phas
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3. TWO-PHASE FLOW PATTERN IN THE MO
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continuous phase dominating, and a
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Also the velocity pattern has chang
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Figure 8. Methane pellets concentra
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ACoM - 6 6 th Meeting of the Collab
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sample volume and the nominal densi
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The results presented in this paper
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Table 10. Summary of energy transfe
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2. EXPERIMENTAL Methane hydrate was
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Figure 4. Energy levels of methane
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102
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phase transitions from phase II to
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After melting phase I in the cryost
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4. SOLID PHASES OF MESITYLENE-D0 AN
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G(ν) [a.u.] 8 6 4 2 Phase II Phase
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112
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A not quite as perfect cold spectru
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2.2 Inelastic neutron scattering Th
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Figure 4. Inelastic spectra at the
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3.2 Moderator performance The resul
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122
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Figure 2. The methane pelletizer, m
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Figure 5. The cryogenic hopper fill
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128
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ture
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The charging device scheme could be
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Table 1. Data of irradiation of met
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5. METHODS OF PROCESSING OF RAW EXP
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of radicals, that is, before a burp
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6.2 Condition for thermally stimula
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Table 4. Estimated values of an ene
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• Nine spontaneous burps were obs
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APPENDIX Some useful relations and
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APPENDIX. Graphs of burps. 148
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150
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152
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154
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2. EXPERIMENTAL SETUP Fig. 1 will g
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All these parameters are kept const
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4. CONCLUSIONS We demonstrated that
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JESSICA (Juelich Experimental Spall
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The moderator system consists of th
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to change the temperatures and to p
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168
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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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174
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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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182
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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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200
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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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Druck-Materie 20b.qxd - JUWEL - Forschungszentrum Jülich

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