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Re: Straw Problems?



Hi Fernando,

It is a possibility. That's why it would be very useful to have a new 
prototype. I believe that I once did a Garfield simulation with a wire 
that had an offset from the center in order to see what would happen with 
the drift time (we also observe too long drift times). I can look more 
into this.

Cheers,
 	Yves

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                          /   Yves Van Haarlem
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On Fri, 20 Jun 2008, Fernando J. Barbosa wrote:

> 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
>