Re: Straw Problems?

["Fernando J. Barbosa" <barbosa@jlab.org>]

Hi Yves,

Just one more point. I mentioned the wire sag in the middle (half 
length) of the straw but the same would apply for a sagging straw. Any 
offset of the wire from the center of the straw would cause the same 
problem.

If you have a chance, I suggest you simulate this in Garfield for 
verification. It would be really nice to show these effects and that 
would certainly guide the design, procurement and testing of the straws.

Regards,
Fernando




Fernando J. Barbosa wrote:
> 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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