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Sraw Problems?



Hi Yves,

I reviewed all the recent plots you have posted and I believe there a 
few issues with the straws.

The X-rays from Fe55 are fully converted within the straw Ar, a point 
source, and that's one of the reasons it is so widely used in tests of 
gas detectors. Therefore, I strongly believe we should be able to see 
the escape peak of Ar. So, I believe the we are observing that peak. For 
instance, your first plot under cross talk shows the main peak at 5.9 
keV around channel 2700 and the 3.2 keV (escape peak of Ar) at about 
channel 1900. If we take the ratio of these peak amplitudes (580 and 
300, by eye) and subtract the "offset" (250 on the valley between the 
two peaks), we get a factor of about 7. This is about right - 10% to 20% 
- from what I can recall.

Now to the amplitude distribution plots. If I look at the three plots 
under "Different positions (straw supported, 1800V)", I observe that the 
escape peak is visible at the positions labeled begin and end but it is 
almost indistinguishable in the middle position. I also see that the 
main peak is on channel 2500 for begin and end but it is shifted up to 
channel 2800 in the middle. This is a symptom of a non-uniform electric 
field inside the straw. In the middle, the wire sags and there is a 
region of higher field (wire closer to straw). For point ionizations in 
this region, I would expect higher amplitude signals. Therefore, if the 
Fe55 illuminates the whole straw diameter, there will be varying 
amplitude distributions for point ionizations for the same radial 
positions but where the electric fields are different. This 
intrinsically implies that the gas gain changes even for the same 
position Z along the straw. A similar effect will be observed for straws 
that are out-of-round or for mechanical problems (crimp pins to 
perfectly centered, etc.)

I suggest you look at a number of straws and get plots at various 
positions, as you did before, say 1 cm from the end close to the preamp, 
middle and 1 cm from the other end of the straw. Maybe this should be a 
gallery of straw responses.

I will look more carefully for references in the next few days. However, 
if this is the case, we will need to be very careful about checking the 
straws and assembly details - a QC/QA plan will definitely be needed.

Regards,
Fernando
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