Re: CDC design (fwd)

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

Hi Gerard,

Gerard Visser wrote:
> Indeed, I have one small comment -->
>
>     - Gerard
>
> Elton Smith wrote:
>> Hi all,
>>
>> I think this discussion should go to the listserver, both to inform a
>> larger audience, as well as for archiving.
>>
>> Cheers, Elton.
> ...
>>>>> 4. How many channels need to be dead before a wire replacement
>>>>> attempt is made?
>>>> If one wire breaks and stays shorted to ground, 24 channels will be
>>>> inoperative (minimum segmentation). Under the best of hypotheses,
>>>> obviously other wires breaking within this same group won't impact
>>>> the detector any further. So, it is important to select which wires
>>>> are connected to one preamp card to get a minimal impact on tracking.
>>>> Ultimately, the tracking efficiency and re-work logistics will
>>>> dictate the maintenance period.
>
> Well, if a wire breaks and stays shorted to ground, you can at some 
> small access opportunity pull the HVB/preamp assembly, remove the one 
> bias resistor, and reinstall, restoring the other 23 straws without 
> opening the detector itself. Now, I may be overoptimistic in thinking 
> how easy or often such an access will come... But, still, I believe we 
> should be very careful before defining any "minimal failure impact" 
> mapping of straws to HVB's, if the tradeoff is longer connecting wire 
> lengths. Fernando, I think you share my concern about any more wire 
> length than we can barely get away with. It raises the crosstalk and 
> especially the EMI or feedback issues. Of course if there are choices 
> like HVB orientation that affect the "failure impact" while leaving 
> the connecting wire length minimized, that is great.
Removing a resistor is also a good option and an alternative to having 
jumpers, for example. I think access is not going to be easy: there is 
the target, start counter, cables and a myriad of other devices and 
structures in this area.

Yes, we share the same concern about wire length  - I guess we have had 
our share of long hours trying to get these type of detectors to work 
right. I have been emphasizing for some time that the connections from 
the straws to the preamp need to be short. With reference to the 
tracking efficiency and straw groups, I was thinking more in terms of a 
defined radius of minimal cable length from a connector to select the 
straws. Grouping straws radially with reference to the target is not 
good but choosing those perpendicular to the tracks, at the same radius 
say, is a better approach. Generally speaking, there are preferred 
orientations for selecting a group of 24 straws. There are also 
orientations to avoid.

Regards,
Fernando

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