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McStas: Refractor

[ Identification | Description | Input parameters | Links ]

The Refractor Component

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


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


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

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;

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=" 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 < Qx Qv Tv o >
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'

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
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.
xwidthmwidth of box0
yheightmheight of box/cylinder0
zdepthmdepth of box0
radiusmradius of sphere/cylinder0
geometrystrOFF/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"
R01Low-angle reflectivity0.99
sigma_cohbarncoherent cross section of refracting material. Use negative value to indicate a negative coherent scattering length11.74
densityg/cm3density of the refracting material. density < 0 means the material is outside/before the shape.3.148
weightg/molmolar mass of the refracting material62.3018
sigma_incbarnincoherent cross section0
sigma_absbarnthermal absorption cross section0
QcAngs-1critical scattering vector, e.g. Qc=0.0219 for Ni coating. Set Qc=0 to use the bulk critical grazing angles.0
p_interact1MC Probability for scattering the ray; otherwise transmit. Use 0 to compute true probability, or specify it as e.g. 0.050.05
RMSAngsroot mean square wavyness of the surface0
focus_scatterdegangle in which to scatter in bulk, with probability 'p_interact'10
verbose1flag to display detailed component behaviour0
p_scatter1flag to allow scattering in the refractor bulk1
p_reflect1flag to allow reflection (grazing angle) at the surface1
p_refract1flag to allow refraction at the refractor surface1


  • 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 ]

Generated on 2023-09-19 20:01:36

Last Modified: Tuesday, 16-Apr-2024 20:05:20 CEST
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