1 | #barbell model |
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2 | # Note: model title and parameter table are inserted automatically |
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3 | r""" |
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4 | Calculates the scattering from a barbell-shaped cylinder. Like |
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5 | :ref:`capped-cylinder`, this is a sphereocylinder with spherical end |
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6 | caps that have a radius larger than that of the cylinder, but with the center |
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7 | of the end cap radius lying outside of the cylinder. See the diagram for |
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8 | the details of the geometry and restrictions on parameter values. |
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9 | |
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10 | Definition |
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11 | ---------- |
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12 | |
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13 | .. figure:: img/barbell_geometry.jpg |
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14 | |
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15 | Barbell geometry, where $r$ is *radius*, $R$ is *bell_radius* and |
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16 | $L$ is *length*. Since the end cap radius $R \geq r$ and by definition |
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17 | for this geometry $h < 0$, $h$ is then defined by $r$ and $R$ as |
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18 | $h = - \sqrt{R^2 - r^2}$ |
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19 | |
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20 | The scattered intensity $I(q)$ is calculated as |
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21 | |
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22 | .. math:: |
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23 | |
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24 | I(q) = \frac{\Delta \rho^2}{V} \left<A^2(q)\right> |
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25 | |
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26 | where the amplitude $A(q)$ is given as |
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27 | |
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28 | .. math:: |
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29 | |
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30 | A(q) =&\ \pi r^2L |
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31 | \frac{\sin\left(\tfrac12 qL\cos\theta\right)} |
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32 | {\tfrac12 qL\cos\theta} |
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33 | \frac{2 J_1(qr\sin\theta)}{qr\sin\theta} \\ |
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34 | &\ + 4 \pi R^3 \int_{-h/R}^1 dt |
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35 | \cos\left[ q\cos\theta |
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36 | \left(Rt + h + {\tfrac12} L\right)\right] |
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37 | \times (1-t^2) |
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38 | \frac{J_1\left[qR\sin\theta \left(1-t^2\right)^{1/2}\right]} |
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39 | {qR\sin\theta \left(1-t^2\right)^{1/2}} |
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40 | |
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41 | The $\left<\ldots\right>$ brackets denote an average of the structure over |
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42 | all orientations. $\left<A^2(q)\right>$ is then the form factor, $P(q)$. |
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43 | The scale factor is equivalent to the volume fraction of cylinders, each of |
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44 | volume, $V$. Contrast $\Delta\rho$ is the difference of scattering length |
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45 | densities of the cylinder and the surrounding solvent. |
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46 | |
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47 | The volume of the barbell is |
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48 | |
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49 | .. math:: |
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50 | |
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51 | V = \pi r_c^2 L + 2\pi\left(\tfrac23R^3 + R^2h-\tfrac13h^3\right) |
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52 | |
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53 | |
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54 | and its radius of gyration is |
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55 | |
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56 | .. math:: |
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57 | |
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58 | R_g^2 =&\ \left[ \tfrac{12}{5}R^5 |
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59 | + R^4\left(6h+\tfrac32 L\right) |
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60 | + R^2\left(4h^2 + L^2 + 4Lh\right) |
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61 | + R^2\left(3Lh^2 + \tfrac32 L^2h\right) \right. \\ |
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62 | &\ \left. + \tfrac25 h^5 - \tfrac12 Lh^4 - \tfrac12 L^2h^3 |
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63 | + \tfrac14 L^3r^2 + \tfrac32 Lr^4 \right] |
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64 | \left( 4R^3 6R^2h - 2h^3 + 3r^2L \right)^{-1} |
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65 | |
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66 | .. note:: |
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67 | The requirement that $R \geq r$ is not enforced in the model! It is |
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68 | up to you to restrict this during analysis. |
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69 | |
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70 | .. figure:: img/barbell_1d.jpg |
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71 | |
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72 | 1D plot using the default values (w/256 data point). |
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73 | |
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74 | For 2D data, the scattering intensity is calculated similar to the 2D |
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75 | cylinder model. |
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76 | |
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77 | .. figure:: img/barbell_2d.jpg |
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78 | |
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79 | 2D plot (w/(256X265) data points) for $\theta = 45^\circ$ and |
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80 | $\phi = 0^\circ$ with default values for the remaining parameters. |
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81 | |
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82 | .. figure:: img/orientation.jpg |
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83 | |
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84 | Definition of the angles for oriented 2D barbells. |
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85 | |
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86 | .. figure:: img/orientation2.jpg |
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87 | |
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88 | Examples of the angles for oriented pp against the detector plane. |
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89 | |
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90 | References |
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91 | ---------- |
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92 | |
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93 | H Kaya, *J. Appl. Cryst.*, 37 (2004) 37 223-230 |
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94 | |
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95 | H Kaya and N R deSouza, *J. Appl. Cryst.*, 37 (2004) 508-509 (addenda and errata) |
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96 | """ |
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97 | from numpy import inf |
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98 | |
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99 | name = "barbell" |
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100 | title = "Cylinder with spherical end caps" |
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101 | description = """ |
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102 | Calculates the scattering from a barbell-shaped cylinder. That is a sphereocylinder with spherical end caps that have a radius larger than that of the cylinder and the center of the end cap |
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103 | radius lies outside of the cylinder. |
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104 | Note: As the length of cylinder(bar) -->0,it becomes a dumbbell. And when rad_bar = rad_bell, it is a spherocylinder. |
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105 | It must be that rad_bar <(=) rad_bell. |
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106 | """ |
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107 | category = "shape:cylinder" |
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108 | |
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109 | # ["name", "units", default, [lower, upper], "type","description"], |
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110 | parameters = [["sld", "4e-6/Ang^2", 4, [-inf, inf], "", "Barbell scattering length density"], |
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111 | ["solvent_sld", "1e-6/Ang^2", 1, [-inf, inf], "", "Solvent scattering length density"], |
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112 | ["bell_radius", "Ang", 40, [0, inf], "volume", "Spherical bell radius"], |
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113 | ["radius", "Ang", 20, [0, inf], "volume", "Cylindrical bar radius"], |
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114 | ["length", "Ang", 400, [0, inf], "volume", "Cylinder bar length"], |
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115 | ["theta", "degrees", 60, [-inf, inf], "orientation", "In plane angle"], |
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116 | ["phi", "degrees", 60, [-inf, inf], "orientation", "Out of plane angle"], |
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117 | ] |
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118 | |
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119 | source = ["lib/J1.c", "lib/gauss76.c", "barbell.c"] |
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120 | |
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121 | # parameters for demo |
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122 | demo = dict(scale=1, background=0, |
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123 | sld=6, solvent_sld=1, |
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124 | bell_radius=40, radius=20, length=400, |
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125 | theta=60, phi=60, |
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126 | radius_pd=.2, radius_pd_n=5, |
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127 | length_pd=.2, length_pd_n=5, |
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128 | theta_pd=15, theta_pd_n=0, |
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129 | phi_pd=15, phi_pd_n=0, |
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130 | ) |
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131 | |
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132 | # For testing against the old sasview models, include the converted parameter |
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133 | # names and the target sasview model name. |
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134 | oldname = 'BarBellModel' |
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135 | oldpars = dict(sld='sld_barbell', |
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136 | solvent_sld='sld_solv', bell_radius='rad_bell', |
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137 | radius='rad_bar', length='len_bar') |
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