Editing Super-Kamiokande (section)

===PMTs and associate structure===
The basic unit for the ID PMTs is a "supermodule", a frame which supports a 3×4 array of PMTs. Supermodule frames are 2.1 m in height, 2.8 m in width, and 0.55 m in thickness. These frames are connected to each other in both the vertical and horizontal directions. Then the whole support structure is connected to the bottom of the tank and to the top structure. In addition to serving as rigid structural elements, supermodules simplified the initial assembly of the ID.

Each supermodule was assembled on the tank floor and then hoisted into its final position. Thus the ID is in effect tiled with supermodules. During installation, ID PMTs were pre-assembled in units of three for easy installation. Each supermodule has two OD PMTs attached on its back side. The support structure for the bottom PMTs is attached to the bottom of the stainless-steel tank by one vertical beam per supermodule frame. The support structure for the top of the tank is also used as the support structure for the top PMTs.

Cables from each group of three PMTs are bundled together. All cables run up the outer surface of the PMT support structure, ''i.e.'', on the OD PMT plane, pass through cable ports at the top of the tank, and are then routed into the electronics huts.

The thickness of the OD varies slightly, but is on average about 2.6 m on top and bottom, and 2.7 m on the barrel wall, giving the OD a total mass of 18 kilotons. OD PMTs were distributed with 302 on the top layer, 308 on the bottom, and 1275 on the barrel wall.

To protect against low-energy background radiation from radon decay products in the air, the roof of the cavity and the access tunnels were sealed with a coating called Mineguard. Mineguard is a spray-applied polyurethane membrane developed for use as a rock support system and radon gas barrier in the mining industry.<ref name="auto1" />

The average geomagnetic field is about 450 mG and is inclined by about 45° with respect to the horizon at the detector site. This presents a problem for the large and very sensitive PMTs which prefer a much lower ambient field. The strength and uniform direction of the geomagnetic field could systematically bias photoelectron trajectories and timing in the PMTs. To counteract this 26 sets of horizontal and vertical Helmholtz coils are arranged around the inner surfaces of the tank. With these in operation the average field in the detector is reduced to about 50&nbsp;mG. The magnetic field at various PMT locations were measured before the tank was filled with water.<ref name="auto1" />

A standard fiducial volume of approximately 22.5 kilotonnes is defined as the region inside a surface drawn 2.00 m from the ID wall to minimize the anomalous response caused by natural radioactivity in the surrounding rock.