F/T - Measurements with respect to the reflecting material

I used the standard setup for F/T - measurements (HIX-Tomography) to compare the ability of Teflon, Millipore and Tyvek to reflect UV light for to maximize the F/T - ratio. In fact the best F/T ratio will be achieved without any wrapping, since nearly every material absorbs rather UV light than visible light. On the other hand light loss by transmission is related to the transparency of the reflecting material, which for example is fairly high for a single layer of Teflon and not to be seen for Millipore.
With two rectangular CsI crystals of 4cm2 x 20cm I started to look for differences in the F/T -values.
But it will be of more importance to know, how much light of the fast component can be collected to achieve a good energy resolution.
Therefore I fixed the EMI 9822QKB #59 tube (which was set at a voltage of 1850V - as usual) in a light tight box and attached the crystal without any grease but slight pressure to it. Doing so I checked the reproducibility by rearranging the crystal to be within 2% . Further systematically errors (like differences in wrapping, gain instabilities of the PMT) are estimated to contribute another 2% .Finally I looked for the statistical error. To do so I generated a bunch of spectra with the same setup for several changes in the numbers of event. In addition I looked for the change of the muon response by producing more than 10 spectra with identical settings during the weekend. The statistical error with at least 2500 received events is about 1%.
So the total error appears to be 5% meanwhile the F/T-value is not affected by positioning uncertainties and can considered to be precise within 1% (relative). I did not bother to correct the numbers for the temperature change, which has been within 2°C and therfore less than the assumed error.
To be insensitive to random - non (single) muon related - pulses, I fitted the spectra with a gauss-exponential curve as to be seen in the following picture.

So far for setup and processing; now it's time to report results. I examined mainly the properties of PTFE-foil ('Teflon'), Millipore-membrane and Tyvek-paper, which I tried to improve with Kodak's White Paint. The two rectangular crystals have already been coated with Teflon AF, which I removed after the first set of measurements.
Tab:  Results of using different white reflecting materials together
with rectangular CsI crystals. For the upper values also an 30
 micron Teflon coating was applied. The newer results on the right
side refer to measurements, which has been taken out  with one
layer of Millipore (.22mm poresize) and PTFE - wrapping of comparable
thickness ( 100mm); in fact it looks  completely different now
looking at a 150um of Teflon and a 120 um layer of Millipore.

Now I simply took the center of gravity, so the absolute numbers are higher.
So finally two layers of properly wrapped Teflon (PTFE) -foil provides a F/T value close to the one of the bare crystal, and - more important - appears to provide higher light output in the UV-region as one layer of Millipore. Therefore - since both materials are chemically similar - it seems to me, that one layer of Millipore is not sufficient to provide good reflectivity, especially since it is (nearly) impossible to get a close wrapping.
Even more dramatic is this effect on the Hexagon Crystals, because this affects also the uniformity.
So both, the light yield and the uniformity claim that Teflon should be used.
(More detailed results are following soon.)

Th. Flügel, 20 Mar 1995