Because multiple scattering degrades the performance of a spectrometer, it is important that the spectrometer volume be evacuated and that the vacuum entrance and exit windows be as low mass as possible. However, catastrophic window failure would generate a significant shock wave as air rushed to fill the vacuum volume. It would also cause a loud noise which could cause hearing damage to anyone in the immediate vicinity. The material chosen for the vacuum windows, then, must be both light enough to have a minimum effect on the beam and strong enough to operate reliably and safely.
To achieve the above goals,
composite Mylar/Kevlar vacuum windows have been constructed for the
Hall C spectrometers. The density of the Kevlar used
is 
gm/cm
with a radiation
length of 
cm [1]. The radiation length
of Mylar is 
cm. The Kevlar has a tensile strength of 900 lbs/inch.
The HMS spectrometer vacuum can has a volume
of approximately 
m, representing a stored energy of 
Joules. A drawing of the exit flange to which the vacuum
window is attached is shown in Figure 2.18. It is a
circle with a center-to-center bolt hole diameter of 
inches
and a 
inch diameter vacuum opening. This is the largest vacuum window
required for Hall C.
Under vacuum, this window must support 16,785
lbs (74,425 N). It is located in the HMS detector hut. The HMS
entrance window is located near the pivot and has a center-to-center
bolt hole diameter of 
inches.
The SOS spectrometer vacuum can has a volume of approximately 
m,
representing a stored energy of 
Joules. The entrance
window (near the pivot) is round and has a diameter of 
inches.
The SOS exit window is the second largest window in Hall C. This
window is located in the SOS detector hut. It is rectangular.
The opening has a length of 
inches and a width of 
inches.
The SOS exit
window must support a load of 
lbs (
N) under vacuum.
One of the responsible personnel must be present for any work directly affecting a Hall C vacuum window.
