[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
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
begin:vcard
fn:Fernando J. Barbosa
n:Barbosa;Fernando J.
org:Jefferson Lab
adr:Suite #10, 12B3;;12000 Jefferson Ave.;Newport News;VA;23606;USA
tel;work:757-269-7433
version:2.1
end:vcard