Introduction

Arrays of PMT panels present 2 distinct problems:-

• The complexity of the internal structure of a PMT (but all PMTs are the same).

• The complexity of the PMT deployment. There are $\sim$10000 PMTs in $\sim$ 750 panels of 5 standard types (3 triangles and 2 snowflakes). PMTs are missing from some panels for two reasons: to allow the belly plate ropes to pass through and, in a couple of cases, due to minor damage.

In order to decompose these problems into simpler ones and to design a systematic efficient search strategy, 3 artificial volumes are defined:-

PMT Hexagon Prism

This corresponds to the ABS at the top of an individual PMT but is extended backwards to encase the PMT and is closed with a second face that is coplanar with the top. This represents the interface between the GEO and PMT geometries.

Panel Enclosing Polygon

Panels are constructed by combining these hexagon prisms to form triangular and snowflake honeycomb planes. The inner and outer surfaces faces are simply planes but the sides, which are formed from individual hexagons, are complex. To speed the geometry, the panel is completely enclosed in an irregular polygon which is formed from the inner and outer surfaces together with planes that are normal to the lines joining the panel centre to its neighbours. The detailed shape of a panel is only checked for lines that intersect this polygon. Note that many panels are so close to their neighbours that their hexagons interlock and consequently their polygons overlap.

Panel Zone

The volume immediately outside the H2O is entirely filled with tesselating irregular polygons. Each is centred on a panel. The outer plane of the zone polygon is just the outer plane of the enclosing polygon. The sides are the bisecting planes between neighboring panel centres. This implies that a point in this volume always belongs to the nearest panel centre. There is no inner surface; the embedding H2O acts as the front surface.

The exact shapes of the enclosing and zone polygons, and the list of neighboring panels all derived from the standard PMT position tables so cannot be inconsistent with these tables.