VITESS Module Reflectometer Sample

The reflectivity data of a sample are taken from a file that contains the reflectivity R as a function of momentum transfer Q. To be able to get a simulated function R(Q), it is possible to simulate a reference measurement. The reference sample has a reflectivity of 1 for all Q-values.

The reflection of neutrons at several angles can be done in one simulation. In this case, a minimal value, a maximal value, and a step size for the reflection angle must be given (see chapter C). If only one angle shall be considered, the step size must be set to zero or minimal and maximal value must be identical.
 

A. Options

The options are:
 

1. Sample:	Reflectivity data from the reflectivity file
2. Reference:	Reflectivity R=1 for all angles.

 

B. Parameter and file descriptions
 

File	Format	Examples Attached
Parameter File	Includes FILE INPUT PARAMETERS. This file can be read or created/modified by the VITESS shell. Values are read from separate rows i.e. 1 value/row for scalar and 3 values/row for vector type variables.	refl_probe.rpb
Reflectivity File	File that contains the reflectivity of  the sample as a function of momentum transfer 1. column: momentum transfer [1/A] 2. column: reflectivity It is only needed for the option 'Sample'.	refl_probe.dat

- MAIN PARAMETERS
 

Parameter	Physical Symbol, Description	Range, Examples
Axis of rotation	Axis around which the sample is rotated	'Y' or 'Z'
reflection angle [deg]	q q=0 means: no reflection, sample parallel to incoming beam positive q.value means: reflection towards the positive z- or positive y-direction resp.	-90° .....90° typical. +1°....+4°

- FILE INPUT PARAMETERS
 

Parameter	Physical Symbol, Description	Range, Examples
main position  [cm]	x, y, z Generally this position defines the reference point (origin) of the sample in the frame provided by the former module.	x = distance to CE y = z = 0.0
thickness, width, height of CE [cm]	t, w, h Thickness, width and length give the sizes of the (rectangular) sample: for reflection angle 0, the reflecting surface of the sample is in the x-y-plane. The thickness is along the z-axis, width along the y-axis and length along the x-axis. Thickness is used to determine a distance from the surface (by MC choice) where the reflection is supposed to take place.	t = 0.00002 cm w = h = 2.0 cm
frame generation [-]	'user defined frame' sets the following coordinate system according to the values given below. 'standard frame generation' leaves the coordiate system as it is, i.e. origin at sample centre, x-axis along the incoming beam.
output angle horizontal, vertical [deg]	F, Q In case of "user defined output frame", a F rotation about the Z axis and then a Qrotation about the (new)Y axis defines a new reference orientation. According to this and the output frame translation vector (see below) the neutrons are written to the output file. If "standard frame generation" is activated, the output frame is not rotated relatively to the original frame.	F = 180°, Q= - 2q
output frame [cm]	x, y, z The position of the output frame origin (O) in the original frame. (x, y, z) represents the translation vector applied to shift the origin of the original (input) frame (O) to the new (output) position. Default setting is: (x, y, z) = (x, y, z) i.e. main position of the sample.

 

C. Data Evaluation

If several reflection angles are considered simultaneously, one incoming neutron can cause up to
    N_max = (q_max - q_min) / Dq + 1
outgoing neutrons. (This number is reached, if the sample is hit by the neutron at all angles.)

So the overall neutron flux at the detector is no longer comparable to the flux before the sample. Thus it is senseful to evaluate the data (using the module EVAL_ELAST) to separate the neutrons reflected at the different angles.
An evaluation in scattering angle is possible, if the angle difference Dq is larger than the fluctuation in the orientation of the incoming neutrons. For an evaluation in the momentum transfer Q, the variation in Q of the incoming neutrons (caused by fluctuation in orientation and wavelength) must be smaller than the difference in Q caused by the angle difference Dq.
 

Last modified: Tue Oct 5 16:09:32 MET DST 1999