Module Neutron Refraction Lens

# VITESS Neutron Refraction Lens

The module lens simulates one or a stack of refractive neutron lenses. Two kinds of lenses are included in the module: a lens with spherical geometry and one with parabolic geometry. The geometry has to be chosen as the first step. (See radio button 'Lens surface geometry'). The second step is to describe the geometry of a lens numerically. Four parameters are included in the module 'Cur_radius1', 'Cur_radius2', 'RadiusMain' and 'Thickness' (see figure 1).
Please note, that, if the parameters 'Cur_radius1' and/or 'Cur_radius2' are given as zero, one of the sides of the lens will be a straight vertical line (or plane). The parameters 'Cur_radius1' and/or 'Cur_radius2' can have negative values; in this case the surfaces (parabolic or spherical) will be of convex shape - in contrast to figure 1, where concave shapes are shown.

Figure 1a: spherical lens

Figure 1b: parabolic lens
The next step is to choose the position of the lens. The paramters in 'Position main' and 'Output frame' have to be given. The section 'Position Main' contains three parameters describing where the center point of the lens is situated (see Figure 2). The section 'Output frame' contains as well three parameters and describes where the output plane and its center are situated. This is the origin for the following module. Please note that the parameters 'Position center Y', 'Position center Z', 'Output frame Y' and 'Output frame Z' usually have to be set to zero. Otherwise the lens center and/or the center point in the output plane will be shifted along axis OY and OZ. If the 'Position center Y', 'Position center Z' are not zero, (only) the lens will be shifted along axis OY and/or OZ. If the 'Output frame Y' and 'Output frame Z' are not zero, a coordinate transformation will take place and all components in the instrument after the lens will be shifted. (as with module frame).

Figure 2: positioning of the lenses, co-ordinate system and output frame
The next step is to choose the material of the lens. One has two possibilities: a) choose the material from a list, see radio button 'Material of lenses'. The data of the material included in the module are given in Table 1. b) choose the attenuation of the neutron flux in the lens (or disable the attenuation).
Please note that only the (wavelength dependent) absorption part of the attenuation is simulated by the module, but not the scattering part. This can be adjusted by the user, see option 'Scattering part'. Please give the appropriate value if it is necessary. If one does not find the material for you lens, one can give the refraction coefficient in the module. Set the radio button 'Material of a lens' to 'input' and give the refraction coefficient in the option 'refraction input' AND the wavelength in the option 'Refract wave'. Due to the (quadratic) wavelength dependence of refraction and its (linear) dependence of the attenuation coefficients, one has to know at which wavelength the refraction and attenuation coefficients are measured or calculated, usually it is 1.8 Ang, but you can give another input value, if required.
The next option is the 'Number of lenses'. This option is very important due to the very small refractive index of all materials for neutron waves. In order to have appropriate focal distance (10-20 m) you have to use a stack of 5-20 lens usually. If this module simulates only one lens, you have to create a long pipe with a lot of modules 'lens'. This is very uncomfortable and slows the simulations. To avoid that, you can simply give the number of lenses you like. The lenses will be situated as given in Figure 2. All neutrons leave the stack via the final output frame (after the last lens).

Table: properties of different lens materials

Figure 3: diaphragma after lens

### Parameters for module 'quadr_field'

Parameter Unit | Description | Range or Values | Command Option
Cur_Radius1 [cm] | Spherical lens: Curvature radius of the first surface of the lense (where the neutrons enter) - see Figure 1a Parabolic lens: distance between parabola at y=z=0 and y=z=RadiusMain for the first surface of the lense (where the neutrons enter) - see Figure 1b | any | -a
Cur_Radius2 [cm] | Spherical lens: Curvature radius of the second surface of the lense (where the neutrons leave) - see Figure 1a Parabolic lens: distance between parabola at y=z=0 and y=z=RadiusMain for the second surface of the lense (where the neutrons leave) - see Figure 1b | any | -b
RadiusMain [cm] | Both lenses: radius of the lens - see Figure 1a and 1b | >0 | -c
Thickness [cm] | Both lenses: Thickness of the lens along the central axis | >0 | -A
Lense surface geometry | geometry of the lens | 'spherical' 'parabolic' | -K
position center X, Y, Z [cm] | one lens: position of the lens several lenses: center position of the stack of lenses | any | -d -e -k
output frame X, Y, Z [cm] | x-, y- and z-position of the output frame (in the input frame) (see figure 2) | any | -s -t -w
Refract input | Refractive index given by the user used if 'material of the lens'='input' (cf. text) | >0 | -R
Refract wave [Å] | wavelength for the refractive index given by the user used if the refractive index is given by the user (cf. text) | >0 | -C
Absorption part [1/cm] | macroscopic absorption cross-section given by the user used if 'material of the lens'='input' (cf. text) | >=0 | -D
Scattering part [1/cm] | macroscopic scattering cross-section given by a user | >=0 | -Q
surface roughness [deg] | amplitude of waviness of a rectangular distribution this value is the maximal angle of deviation of the surface normal from the ideal normal. | >=0 | -q
material of a lens | material of the lens | 'O','CO2','C' 'Be','F','Bi' 'MgO','Pb','MgF' 'SiO2''ZrO2','Mg' ,'Si','Zr','Al' | -i
attenuation activation | yes: attenuation inside the lens no: no attenuation inside the lens | 'yes' 'no' | -H
Number of lenses | number of lenses in the stack | >=1 | -I
Lens number | Lens number for visualisation (0 - means all lenses) | >=0 | -E
Number of trajectories | Number of trajectories for the visualisation after the lens | >0 | -x
Max X | maximal x value at the ray-tracing picture, 0.0 means auto-calculation | >=0.0 | -S
Visual ray-tracing after lense | no: no visualisation XZ: visualisation using a projection to the XZ plane XY: visualisation using a projection to the XY plane | 'no' 'XZ' 'XY' | -W
Inner radius [cm] | Inner radius of the diaphragm at the exit of the lenses (see Figure 3) | >=0 | -m
Outer radius [cm] | Outer radius of the diaphragm at the exit of the lenses (see Figure 3) | >0 | -M
file name | name of the output file, into which the entry and exit positions of the trajectories through all lenses are written i.e. color and x,y,z position of entry into lens 1, of exit from lens 1, of entry into lens 2, of exit from lens 2, ... | - | -p
Lens number | Lens number for the output (0 - means all lenses) | >=0 | -v
activate output | yes: activate output of the coordinates for the lens no: no output written | 'yes' 'no' | -z
wavelength [Å] | wavelength for focal distance calculation | >0 | -r
choose formula | formulae for focal distance calculation: thick: exact formula thin: approximation for thin lenses | 'thick' 'thin' | -V
activate flight | yes: propagation to the focal distance no: neutrons remain at the lens surface | 'yes' 'no' | -Y

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Last modified: Tue May 8 17:08:06 MET DST 2001