1 | # rectangular_prism 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 | Definition |
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5 | ---------- |
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6 | |
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7 | This model provides the form factor, $P(q)$, for a hollow rectangular |
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8 | parallelepiped with a wall of thickness $\Delta$. The 1D scattering intensity |
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9 | for this model is calculated by forming the difference of the amplitudes of two |
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10 | massive parallelepipeds differing in their outermost dimensions in each |
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11 | direction by the same length increment $2\Delta$ (\ [#Nayuk2012]_ Nayuk, 2012). |
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12 | |
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13 | As in the case of the massive parallelepiped model (:ref:`rectangular-prism`), |
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14 | the scattering amplitude is computed for a particular orientation of the |
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15 | parallelepiped with respect to the scattering vector and then averaged over all |
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16 | possible orientations, giving |
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17 | |
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18 | .. math:: |
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19 | P(q) = \frac{1}{V^2} \frac{2}{\pi} \times \, \int_0^{\frac{\pi}{2}} \, |
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20 | \int_0^{\frac{\pi}{2}} A_{P\Delta}^2(q) \, \sin\theta \, d\theta \, d\phi |
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21 | |
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22 | where $\theta$ is the angle between the $z$ axis and the longest axis |
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23 | of the parallelepiped, $\phi$ is the angle between the scattering vector |
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24 | (lying in the $xy$ plane) and the $y$ axis, and |
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25 | |
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26 | .. math:: |
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27 | :nowrap: |
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28 | |
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29 | \begin{align*} |
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30 | A_{P\Delta}(q) & = A B C |
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31 | \left[\frac{\sin \bigl( q \frac{C}{2} \cos\theta \bigr)} |
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32 | {\left( q \frac{C}{2} \cos\theta \right)} \right] |
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33 | \left[\frac{\sin \bigl( q \frac{A}{2} \sin\theta \sin\phi \bigr)} |
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34 | {\left( q \frac{A}{2} \sin\theta \sin\phi \right)}\right] |
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35 | \left[\frac{\sin \bigl( q \frac{B}{2} \sin\theta \cos\phi \bigr)} |
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36 | {\left( q \frac{B}{2} \sin\theta \cos\phi \right)}\right] \\ |
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37 | & - 8 |
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38 | \left(\frac{A}{2}-\Delta\right) \left(\frac{B}{2}-\Delta\right) \left(\frac{C}{2}-\Delta\right) |
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39 | \left[ \frac{\sin \bigl[ q \bigl(\frac{C}{2}-\Delta\bigr) \cos\theta \bigr]} |
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40 | {q \bigl(\frac{C}{2}-\Delta\bigr) \cos\theta} \right] |
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41 | \left[ \frac{\sin \bigl[ q \bigl(\frac{A}{2}-\Delta\bigr) \sin\theta \sin\phi \bigr]} |
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42 | {q \bigl(\frac{A}{2}-\Delta\bigr) \sin\theta \sin\phi} \right] |
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43 | \left[ \frac{\sin \bigl[ q \bigl(\frac{B}{2}-\Delta\bigr) \sin\theta \cos\phi \bigr]} |
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44 | {q \bigl(\frac{B}{2}-\Delta\bigr) \sin\theta \cos\phi} \right] |
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45 | \end{align*} |
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46 | |
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47 | where $A$, $B$ and $C$ are the external sides of the parallelepiped fulfilling |
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48 | $A \le B \le C$, and the volume $V$ of the parallelepiped is |
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49 | |
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50 | .. math:: |
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51 | V = A B C \, - \, (A - 2\Delta) (B - 2\Delta) (C - 2\Delta) |
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52 | |
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53 | The 1D scattering intensity is then calculated as |
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54 | |
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55 | .. math:: |
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56 | I(q) = \text{scale} \times V \times (\rho_\text{p} - |
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57 | \rho_\text{solvent})^2 \times P(q) + \text{background} |
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58 | |
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59 | where $\rho_\text{p}$ is the scattering length density of the parallelepiped, |
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60 | $\rho_\text{solvent}$ is the scattering length density of the solvent, |
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61 | and (if the data are in absolute units) *scale* represents the volume fraction |
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62 | (which is unitless) of the rectangular shell of material (i.e. not including |
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63 | the volume of the solvent filled core). |
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64 | |
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65 | For 2d data the orientation of the particle is required, described using |
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66 | angles $\theta$, $\phi$ and $\Psi$ as in the diagrams below, for further details |
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67 | of the calculation and angular dispersions see :ref:`orientation` . |
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68 | The angle $\Psi$ is the rotational angle around the long *C* axis. For example, |
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69 | $\Psi = 0$ when the *B* axis is parallel to the *x*-axis of the detector. |
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70 | |
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71 | For 2d, constraints must be applied during fitting to ensure that the inequality |
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72 | $A < B < C$ is not violated, and hence the correct definition of angles is |
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73 | preserved. The calculation will not report an error if the inequality is *not* |
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74 | preserved, but the results may be not correct. |
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75 | |
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76 | .. figure:: img/parallelepiped_angle_definition.png |
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77 | |
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78 | Definition of the angles for oriented hollow rectangular prism. |
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79 | Note that rotation $\theta$, initially in the $xz$ plane, is carried out first, then |
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80 | rotation $\phi$ about the $z$ axis, finally rotation $\Psi$ is now around the axis of the prism. |
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81 | The neutron or X-ray beam is along the $z$ axis. |
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82 | |
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83 | .. figure:: img/parallelepiped_angle_projection.png |
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84 | |
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85 | Examples of the angles for oriented hollow rectangular prisms against the |
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86 | detector plane. |
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87 | |
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88 | |
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89 | Validation |
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90 | ---------- |
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91 | |
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92 | Validation of the code was conducted by qualitatively comparing the output |
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93 | of the 1D model to the curves shown in (Nayuk, 2012). |
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94 | |
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95 | |
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96 | References |
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97 | ---------- |
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98 | |
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99 | .. [#Nayuk2012] R Nayuk and K Huber, *Z. Phys. Chem.*, 226 (2012) 837-854 |
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100 | .. [#] L. Onsager, *Ann. New York Acad. Sci.*, 51 (1949) 627-659 |
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101 | |
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102 | Source |
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103 | ------ |
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104 | |
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105 | `hollow_rectangular_prism.py <https://github.com/SasView/sasmodels/blob/master/sasmodels/models/hollow_rectangular_prism.py>`_ |
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106 | |
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107 | `hollow_rectangular_prism.c <https://github.com/SasView/sasmodels/blob/master/sasmodels/models/hollow_rectangular_prism.c>`_ |
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108 | |
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109 | Authorship and Verification |
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110 | ---------------------------- |
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111 | |
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112 | * **Author:** Miguel Gonzales **Date:** February 26, 2016 |
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113 | * **Last Modified by:** Paul Kienzle **Date:** December 14, 2017 |
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114 | * **Last Reviewed by:** Paul Butler **Date:** September 06, 2018 |
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115 | * **Source added by :** Steve King **Date:** March 25, 2019 |
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116 | """ |
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117 | |
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118 | import numpy as np |
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119 | from numpy import inf |
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120 | |
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121 | name = "hollow_rectangular_prism" |
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122 | title = "Hollow rectangular parallelepiped with uniform scattering length density." |
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123 | description = """ |
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124 | I(q)= scale*V*(sld - sld_solvent)^2*P(q,theta,phi)+background |
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125 | P(q,theta,phi) = (2/pi/V^2) * double integral from 0 to pi/2 of ... |
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126 | (AP1-AP2)^2(q)*sin(theta)*dtheta*dphi |
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127 | AP1 = S(q*C*cos(theta)/2) * S(q*A*sin(theta)*sin(phi)/2) * S(q*B*sin(theta)*cos(phi)/2) |
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128 | AP2 = S(q*C'*cos(theta)) * S(q*A'*sin(theta)*sin(phi)) * S(q*B'*sin(theta)*cos(phi)) |
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129 | C' = (C/2-thickness) |
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130 | B' = (B/2-thickness) |
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131 | A' = (A/2-thickness) |
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132 | S(x) = sin(x)/x |
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133 | """ |
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134 | category = "shape:parallelepiped" |
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135 | |
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136 | # ["name", "units", default, [lower, upper], "type","description"], |
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137 | parameters = [["sld", "1e-6/Ang^2", 6.3, [-inf, inf], "sld", |
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138 | "Parallelepiped scattering length density"], |
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139 | ["sld_solvent", "1e-6/Ang^2", 1, [-inf, inf], "sld", |
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140 | "Solvent scattering length density"], |
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141 | ["length_a", "Ang", 35, [0, inf], "volume", |
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142 | "Shortest, external, size of the parallelepiped"], |
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143 | ["b2a_ratio", "Ang", 1, [0, inf], "volume", |
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144 | "Ratio sides b/a"], |
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145 | ["c2a_ratio", "Ang", 1, [0, inf], "volume", |
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146 | "Ratio sides c/a"], |
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147 | ["thickness", "Ang", 1, [0, inf], "volume", |
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148 | "Thickness of parallelepiped"], |
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149 | ["theta", "degrees", 0, [-360, 360], "orientation", |
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150 | "c axis to beam angle"], |
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151 | ["phi", "degrees", 0, [-360, 360], "orientation", |
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152 | "rotation about beam"], |
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153 | ["psi", "degrees", 0, [-360, 360], "orientation", |
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154 | "rotation about c axis"], |
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155 | ] |
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156 | |
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157 | source = ["lib/gauss76.c", "hollow_rectangular_prism.c"] |
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158 | have_Fq = True |
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159 | effective_radius_type = [ |
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160 | "equivalent cylinder excluded volume", "equivalent outer volume sphere", |
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161 | "half length_a", "half length_b", "half length_c", |
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162 | "equivalent outer circular cross-section", |
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163 | "half ab diagonal", "half diagonal", |
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164 | ] |
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165 | |
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166 | def random(): |
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167 | """Return a random parameter set for the model.""" |
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168 | a, b, c = 10**np.random.uniform(1, 4.7, size=3) |
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169 | # Thickness is limited to 1/2 the smallest dimension |
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170 | # Use a distribution with a preference for thin shell or thin core |
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171 | # Avoid core,shell radii < 1 |
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172 | min_dim = 0.5*min(a, b, c) |
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173 | thickness = np.random.beta(0.5, 0.5)*(min_dim-2) + 1 |
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174 | #print(a, b, c, thickness, thickness/min_dim) |
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175 | pars = dict( |
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176 | length_a=a, |
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177 | b2a_ratio=b/a, |
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178 | c2a_ratio=c/a, |
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179 | thickness=thickness, |
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180 | ) |
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181 | return pars |
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182 | |
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183 | |
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184 | # parameters for demo |
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185 | demo = dict(scale=1, background=0, |
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186 | sld=6.3, sld_solvent=1.0, |
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187 | length_a=35, b2a_ratio=1, c2a_ratio=1, thickness=1, |
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188 | length_a_pd=0.1, length_a_pd_n=10, |
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189 | b2a_ratio_pd=0.1, b2a_ratio_pd_n=1, |
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190 | c2a_ratio_pd=0.1, c2a_ratio_pd_n=1) |
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191 | |
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192 | tests = [[{}, 0.2, 0.76687283098], |
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193 | [{}, [0.2], [0.76687283098]], |
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194 | ] |
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