Monochromatic Neutron and X-ray Reflectometry

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This proposed instrument definition adds a new class (NXspin) and also incorporates a few additions to existing base classes.

Additions

New background NXcharacterization type:

specular_offset_background

The two existing backgrounds are isotropic_scatter and empty_container. Physically we are moving the detector/sample slightly away from the specular reflection condition and measuring the strength of the signal rather than putting an isotropic scatter at the sample position or leaving it empty.

Note: this simply identifies the location of the background scan, but does not specify how the background was measured.

Need to know the sample angle.

Propose adding the following to NXsample:

   <polar_angle units="degrees" type="NX_FLOAT[np]">
   {Polar angle of the sample with respect to the beam incident on
    the monochromator}
   </polar_angle>
   <azimuthal_angle units="degress" type="NX_FLOAT[np]">
   {Azimuthal angle of the sample with respect to the beam incident on
    the monochromator}
   </azimuthal_angle>
   <rotation_angle units="degrees" type="NX_FLOAT[np]">
   {Rotation angle of the sample}
   </rotation_angle>

These fields are already used in the ratified NXmonotas, so there should be no problem ratifying them in the base class.

Record Spin State

For polarized neutron reflectometry the control software allows the user to select and tune a particular spin state. Expressed in terms of the individual filters and flipper currents, it is difficult to determine which spin state is desired. Instead the instrument component should record the target spin state.

Propose NXspin class to record the target spin at the sample or at the detector:

<?xml version="1.0" encoding="UTF-8"?>
<!--
URL:    http://definitions.nexusformat.org/NXspin.xml
Editor: NIAC
$Id$
    
This is a description of the neutron spin state at a point in the
beam path, as set by the combination of filters and flippers.
-->
<NXspin name="{presample_spin|predetector_spin}">
        <azimuthal_angle type="NX_FLOAT[np]" units="degrees">
        {90 for spin up, -90 for spin down}
        </azimuthal_angle>
        <polar_angle type="NX_FLOAT[np]" units="degrees">
        {Constant usually ignored. 0 in the direction of the beam path.}
        </polar_angle>
</NXspin>

Accurate record of the start and stop of every scan

We need an accurate record of the start and stop of every measurement in a scan (e.g., to normalize by measurement time, and to correct for He3 polarizer efficiency decay).

NXmonitor already stores count duration. Propose adding count start_time as well:

     <start_time type="NX_FLOAT[n]" units="seconds">
        {Start time for each scan point}
     </start_time>

The alternative is to use NXlog and assume a particularly named log corresponds to the NXmonitor, with the same length as the measured points:

   <NXlog name="count_time">
      <time start="{ISO8601}" type="NX_FLOAT[np]" units="seconds">
        {Start of the measurement of point np}
      </time>
      <value type="NX_FLOAT[np]" units="seconds">
        {Durations of the measurement point np}
       </value>
   </NXlog>

The first solution is cleaner.