The CLAS Cerenkov Detector

Window attachment

After all the mirrors are in place, the windows are attached. We have recently changed the design of the windows; the new design, created by Yuri Sharabian, is discussed here.

The window material is a sandwich of 10 mil mylar surrounded on either side by 2 mil Tedlar. Tedlar is a trademark of the DuPont corporation. It is a black material that has extremely good light tightness, and is very strong. However, because the gas we are using in the Cerenkov detector is very heavy (approximately ten times the weight of air), it is not strong enough. The addition of the mylar helps a great deal. First, all the rough edges of the sector must be smoothed out so that the window will not be torn on them. Next, the window is glued on in a very conventional fashion. The curved side of the detector presents a challenge for this procedure, but the careful work on the part of the Jefferson Lab technical staff led by Tom Carstens was sufficient to attach the windows cleanly.

After both windows have been attached, the sector is tested for gas leaks. This is not a simple task, since the volume of the sector depends on the pressure differential between the inside and the outside of the sector. With a simple assumption, however, the problem becomes tractable. Begin with the ideal gas law:

pV=nRT,

where the variables are as follows:

p	Pressure within the sector
V	Volume of the sector
n	moles of gas within the sector
R	Ideal gas constant
T	Temperature within the sector

From this, we get dn/n = dp/p + dV/V - dT/T. To measure the pressure within the sector, we make two separate measurements: the first is the barometric pressure of the atmosphere (p_atm), and the second is the differential pressure between the inside and the outside of the detector (p_dif). This gives us dn/n = dp_atm/p + dp_dif/p + dV/V - dT/T. If we then make the assumption that the volume of the detector depends only on p_dif, V=V(p_dif), we get dV/V = (1/V) x (dV/dp_dif) x dp_dif
dn/n = dp_atm/p + dp_dif/p x (1 + (dV/dp_dif)(p/V)) - dT/T

dn/n = dp_atm/p + dp_dif/p x (1 + C) - dT/T,

where we assume that p and V are roughly constant. Note that for a rigid box, C=0, and for an infinitely flexible box, C=&inf;.