Add dataset card

README.md

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+---
+license: apache-2.0
+pretty_name: BEvAn — COSI activation simulations (β⁺ / 511 keV)
+tags:
+  - astrophysics
+  - gamma-ray
+  - compton-telescope
+  - megalib
+  - cosi
+  - simulation
+size_categories:
+  - 1K<n<10K
+---
+
+# BEvAn — COSI activation simulations
+
+MEGAlib simulation files backing [`cosi-betadecay/BEvAn`](https://github.com/cosi-betadecay/BEvAn),
+a physics-based likelihood classifier that tags each simulated COSI event as a
+β⁺ / positron-annihilation signal (back-to-back 511 keV photons) or background
+from its Compton kinematics.
+
+All events come from a **cosima activation simulation** (`BeamType Activation`,
+`DecayMode ActivationBuildup`): cosmic-ray protons irradiate the instrument, the
+activated isotopes decay, and the β⁺ emitters among them produce the 511 keV
+annihilation signal the classifier is trained to find. The three-step cosima
+source files that generate this are in the GitHub repo under
+[`data/cosima/`](https://github.com/cosi-betadecay/BEvAn/tree/main/data/cosima)
+and are **not** duplicated here. Produced with MEGAlib 4.00.00.
+
+## Datasets
+
+One folder per detector geometry. The geometries ship with MEGAlib under
+`$MEGALIB/resource/examples/geomega/`:
+
+| Folder | Geometry (`.geo.setup`) | Size |
+|---|---|---|
+| `SPILike/` | `special/SPILike` | 382 MB |
+| `NCT/` | `special/Simple_Ge_NCT_try1_040721` | 848 MB |
+| `Max/` | `special/Max` | 1.2 GB |
+| `GeACT/` | `special/GeACT` | 3.8 GB |
+| `COSIBalloon_9det/` | `cosiballoon/COSIBalloon.9Detector` | 1.0 GB |
+| `COSIBalloon_10det/` | `cosiballoon/COSIBalloon.10Detector` | 980 MB |
+| `COSIBalloon_12det/` | `cosiballoon/COSIBalloon.12Detector` | 1.1 GB |
+
+## Files in each folder
+
+For a dataset `{name}`:
+
+| File | What it is |
+|---|---|
+| `{name}.sim` | **Concatenation file written by `mcosima` — it holds no events.** It is a short header plus `IN {name}.incN.id1.sim.gz` lines pointing at the chunks below. This is the path you pass to the pipeline; MEGAlib follows the pointers. |
+| `{name}.incN.id1.sim.gz` | The actual simulated events, one chunk per parallel `mcosima` thread (N = 1…4). Required — `{name}.sim` is useless without them. |
+| `{name}.tra` | Compton-reconstructed events (`revan` output) for `{name}.sim`. |
+| `{name}_P1.*`, `{name}_P2.*` | Two **dedicated prior simulations** against the same geometry, each a full `.sim` (+ `.tra`) set of its own. Their ANNI-labeled events, counted per hit-multiplicity bucket, form the classifier's class priors. The training split is never used for the prior, which is why these are separate runs. Passed via `--prior-sim`. `_P1` files are self-contained `.sim` files from an earlier run; `_P2` files use the same `mcosima` concatenation layout as above. |
+
+The `COSIBalloon_*` folders additionally carry a `crossections/` directory —
+MEGAlib-generated cross-section tables. They are regenerated on demand by
+MEGAlib and are included only so the folders are a byte-exact mirror.
+
+## Download
+
+```bash
+hf download aryaraeesi/BEvAn-data --repo-type dataset --local-dir data/
+```
+
+Or a single dataset (remember to take its `.inc*` chunks along with the `.sim`):
+
+```bash
+hf download aryaraeesi/BEvAn-data --repo-type dataset \
+  --include "SPILike/*" --local-dir data/
+```
+
+## Gotcha: the geometry path is absolute
+
+The `Geometry` line inside every `.sim` header is an **absolute path from the
+machine that ran the simulation** (`/home/arya/Documents/megalib/...`). MEGAlib
+will not find the geometry on your machine unless you either point that line at
+your own `$MEGALIB` checkout or pass the geometry explicitly — which is what the
+BEvAn entry scripts do via `--geo-file`:
+
+```bash
+python src/BEvAn/analysis.py \
+  --geo-file $MEGALIB/resource/examples/geomega/special/SPILike.geo.setup \
+  --sim-file data/SPILike/SPILike.sim \
+  --tra-file data/SPILike/SPILike.tra \
+  --prior-sim data/SPILike/SPILike_P1.sim data/SPILike/SPILike_P2.sim
+```
+
+## License
+
+Apache-2.0, matching the BEvAn repository.