[ Identification | Description | Input parameters | Links ]

# The `Refractor` Component

A refractor material/shape, which can be used to model e.g. lenses and prisms.

## Identification

- Site:
- Author: E. Farhi, B. Cubitt
- Origin: ILL
- Date: Oct 2014

## Description

    Single bulk material shape that can be used as a prism or lens.

    NEUTRON INTERACTION PROCESSES:
    The bulk material can reflect, refract, scatter and absorb neutrons, depending
    on the material cross sections and incident angles.

    The refracting material is specified from its molar weight, density, coherent
    scattering cross section. The refractive index is computed as:
    n   = sqrt(1-(lambda*lambda*rho*bc/PI)) is the refraction index

    The surface can be coated when specifying a critical wavevector Qc, with e.g.
    Qc=m*0.0219 for a super mirror coating. The mirror coating can be suppressed
    by setting Qc=0. The critical wavevector is then set to
    Qc = 4*sqrt(PI*rho*bc);                                             with
    rho= density*6.02214179*1e23*1e-24/weight;
    bc = sqrt(fabs(sigma_coh)*100/4/PI)*1e-5;

    COMPONENT GEOMETRY:
    The component shape can be a sphere, box, cylinder, biconcave spherical lens
    or any other shape defined from an external OFF/PLY file.
    sphere:      radius
    cylinder:    radius, yheight
    box:         xwidth, yheight, zdepth
    OFF/PLY:     geometry="filename.off or ply", xwidth, yheight, zdepth (bounding box)
    lens_sphere:   geometry="lens_sphere", radius, zdepth (thickness)

    The lens_sphere geometry is composed of two concave half spheres of same radius,
    separated with a minimal thickness zdepth along the Z axis.

    Optionally, you can specify the 'geometry' parameter as a OFF/PLY file name.
    The complex geometry option handles any closed non-convex polyhedra.
    It computes the intersection points of the neutron ray with the object
    transparently, so that it can be used like a regular sample object.
    It supports the PLY, OFF and NOFF file format but not COFF (colored faces).
    Such files may be generated from XYZ data using:
    qhull < coordinates.xyz Qx Qv Tv o > geomview.off
    or
    powercrust coordinates.xyz
    and viewed with geomview or java -jar jroff.jar (see below).

    All geometries are centred. The bulk material fills the shape, but can be
    set 'outside' when density is given a negative value. In this case, the material
    outside the bulk is void (vacuum).

    Usually, you should stack more than one of these to get a significant effect
    on the neutron beam, so-called 'compound refractive lens'.
    The focal length for N lenses with focal 'f' is f/N, where f=R/(1-n)
    and R = r/2   for a spherical lens with curvature radius 'r'

    COMMON MATERIALS:
    Should have high coherent, and low incoherent and absorption cross sections
    Be:            density=1.85,  weight=9.0121, sigma_coh=7.63,  sigma_inc=0.0018,sigma_abs=0.0076
    Pb:            density=11.115,weight=207.2,  sigma_coh=11.115,sigma_inc=0.003, sigma_abs=0.171
    Pb206:                                     sigma_coh=10.68, sigma_inc=0    , sigma_abs=0.03
    Pb208:                                     sigma_coh=11.34, sigma_inc=0    , sigma_abs=0.00048
    Zr:            density=6.52,  weight=91.224, sigma_coh=6.44,  sigma_inc=0.02,  sigma_abs=0.185
    Zr90:                                      sigma_coh=5.1,   sigma_inc=0    , sigma_abs=0.011
    Zr94:                                      sigma_coh=8.4,   sigma_inc=0    , sigma_abs=0.05
    Bi:            density=9.78,  weight=208.98, sigma_coh=9.148, sigma_inc=0.0084,sigma_abs=0.0338
    Mg:            density=1.738, weight=24.3,   sigma_coh=3.631, sigma_inc=0.08,  sigma_abs=0.063
    MgF2:          density=3.148, weight=62.3018,sigma_coh=11.74, sigma_inc=0.0816,sigma_abs=0.0822
    diamond:       density=3.52,  weight=12.01,  sigma_coh=5.551, sigma_inc=0.001, sigma_abs=0.0035
    Quartz/silica: density=2.53,  weight=60.08,  sigma_coh=10.625,sigma_inc=0.0056,sigma_abs=0.1714
    Si:            density=2.329, weight=28.0855,sigma_coh=2.1633,sigma_inc=0.004, sigma_abs=0.171
    Al:            density=2.7,   weight=26.98,  sigma_coh=1.495, sigma_inc=0.0082,sigma_abs=0.231
    Ni:            density=8.908, weight=58.69,  sigma_coh=13.3,  sigma_inc=5.2,   sigma_abs=4.49
    Mn: (bc < 0)   density=7.21,  weight=54.94,  sigma_coh=-1.75, sigma_inc=0.4,   sigma_abs=13.3
    perfluoropolymer(PTFE/Teflon/CF2):
    density=2.2,   weight=50.007, sigma_coh=13.584,sigma_inc=0.0026,sigma_abs=0.0227
    Organic molecules with C,O,H,F

    Among the most commonly available and machinable materials are MgF2, SiO2, Si, and Al.

## Input parameters

Parameters in boldface are required; the others are optional.

Name | Unit | Description | Default |
xwidth | m | width of box | 0 |
yheight | m | height of box/cylinder | 0 |
zdepth | m | depth of box | 0 |
radius | m | radius of sphere/cylinder | 0 |
geometry | str | OFF/PLY geometry file name, or NULL to use simple shape A spherical bi-concave lens can be obtained with geometry="lens_sphere" and given radius and zdepth | "NULL" |
R0 | 1 | Low-angle reflectivity | 0.99 |
sigma_coh | barn | coherent cross section of refracting material. Use negative value to indicate a negative coherent scattering length | 11.74 |
density | g/cm3 | density of the refracting material. density < 0 means the material is outside/before the shape. | 3.148 |
weight | g/mol | molar mass of the refracting material | 62.3018 |
sigma_inc | barn | incoherent cross section | 0 |
sigma_abs | barn | thermal absorption cross section | 0 |
Qc | Angs-1 | critical scattering vector, e.g. Qc=0.0219 for Ni coating. Set Qc=0 to use the bulk critical grazing angles. | 0 |
p_interact | 1 | MC Probability for scattering the ray; otherwise transmit. Use 0 to compute true probability, or specify it as e.g. 0.05 | 0.05 |
RMS | Angs | root mean square wavyness of the surface | 0 |
focus_scatter | deg | angle in which to scatter in bulk, with probability 'p_interact' | 10 |
verbose | 1 | flag to display detailed component behaviour | 0 |
p_scatter | 1 | flag to allow scattering in the refractor bulk | 1 |
p_reflect | 1 | flag to allow reflection (grazing angle) at the surface | 1 |
p_refract | 1 | flag to allow refraction at the refractor surface | 1 |

AT ( | , | , |  | ) RELATIVE |  |
ROTATED ( | , | , |  | ) RELATIVE |

## Links

- Source code for `Refractor.comp`.
- M. L. Goldberger et al, Phys. Rev. 71, 294 - 310 (1947)
- Sears V.F. Neutron optics. An introduction to the theory of neutron optical phenomena and their applications. Oxford University Press, 1989.
- H. Park et al. Measured operational neutron energies of compound refractive lenses. Nuclear Instruments and Methods B, 251:507-511, 2006.
- J. Feinstein and R. H. Pantell. Characteristics of the thick, compound refractive lens. Applied Optics, 42 No. 4:719-723, 2001.
- Geomview and Object File Format (OFF)
- Java version of Geomview (display only) jroff.jar
- Meshlab can view OFF and PLY files
- qhull for points to OFF conversion
- powercrust for points to OFF conversion

[ Identification | Description | Input parameters | Links ]