C9 was upgraded with a new rear window to accomodate the new hodoscope H9 for the E852 1997 run. The window consists of a sandwich of Tedlar, Mylar, Roha-Cell, Mylar, and Kevlar as shown in the figure below:
The material densities are given in the table below.
| Material | Thickness | Density |
| (cm) | (g/cm^2) | |
| --------------------- | --------- | ------- |
| Tedlar | 0.0059 | |
| 3M "90" Adhesive (each layer) | 0.0016 | |
| Mylar (each layer) | 0.008 | 0.0106 |
| Shell Epon (each layer) | 0.0185 | |
| Roha-Cell | 0.4 | 0.0208 |
| Shell Epon (each layer) | 0.0185 | |
| Mylar (each layer) | 0.008 | 0.0106 |
| 3M "90" Adhesive (each layer) | 0.0016 | |
| Kevlar | 0.009 | 0.0063 |
| --------------------- | ------- | |
| Total window material | 0.0944 |
The window material is supported by an angle-aluminum frame just outside the perimeter of the Roha-Cell window body.