# The `FRMII_SPODI` Instrument

*McStas: Simple monochromator diffractometer for powders*

## Identification

- **Site:** FRMII
- **Author:** V. Kochetov, C. Hauf, M. Hoelzel, and A. Senyshyn
- **Origin:** MLZ, Garching, Germany
- **Date:** August 24, 2026

## Description

```text
SPODI is a high-resolution thermal neutron diffractometer at the research
reactor Heinz Maier-Leibnitz (FRM II) especially dedicated to structural
studies of complex systems. Unique features like a very large monochromator
take-off angle of 155 degrees and a 5 m monochromator-sample distance in its
standard configuration achieve both high-resolution and a good profile shape
for a broad scattering angle range of 160 degrees. Two-dimensional data are
collected by an array of 80 vertical position sensitive 3He detectors typically
during 40 resolution steps of 0.05 degrees.

List of available wavelengths in A delivered by germanium monochromator:
Reflection    Take-off 155 degrees, L2=5m      Take-off 135 degrees, L2=2.8m
Ge 331            2.536 A                           2.396 A
Ge 551            1.549 A                           1.463 A
Ge 771            1.111 A                           1.050 A

The implemented model corresponds to the standard setup with the take-off angle
of 155 degrees, Ge 551 reflection giving rise to the wavelength of ~1.5482 AA,
and L2 of 5 m. A simplified detector system is used instead of 80 linear Soller
collimators plus PSD tube detectors. This includes radial collimator with the same
divergence of 10 minutes and banana detector with 3200 bins to simulate
the 2D data that are collected on the real detectors.
```

## Input parameters

Parameters in **boldface** are required; the others are optional.

| Name | Unit | Description | Default |
|------|------|-------------|---------|
| lambda | A | Wavelength; if 0, calculated from HKLmono and TOA | 1.5482 |
| HKLmono | str | Miller indices of the Ge-monochromator reflection | "551" |
| TOA | deg | Take-Off Angle; if 0, calculated from HKLmono and lambda | 0 |
| L2 | m | Distance from the monochromator to the sample | 5.0 |
| Dsample | m | Sample diameter | 8e-3 |
| Hsample | m | Sample height | 40e-3 |
| Powder | str | LAZ/HKL/PCR/CIF file for powder description | "Na2Ca3Al2F14.laz" |
| Holder |  | Flag to enable (1) / disable (0) the vanadium sample holder | 0 |
| StartTheta | deg | Start angle of the detector relative to the beam | 0 |

## Links

- [Source code](FRMII_SPODI.instr) for `FRMII_SPODI.instr`.
- M. Hoelzel, A. Senyshyn, N. Juenke, H. Boysen, W. Schmahl and H. Fuess, Nucl. Instr. and Meth. in Ph A 667 (2012) 32-37

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