Re: FCAL test

[David Lawrence <davidl@jlab.org>]

Hi Matt,

Matthew Shepherd wrote:
>
> On Sep 16, 2007, at 11:57 AM, Elke-Caroline Aschenauer wrote:
>
>> I'm not completely sure I can exactly follow your argument.
>> If I have bigger blocks like the ones from Hermes 9*9*45cm^3 I contain
>> more of the shower in one block, I need to take this into account in 
>> my shower
>> algorithm, but that should be possible. Do I miss something.
>
> The problem with the blocks is not that they are bigger, but that they 
> are not a precise integral multiple of the current block dimension.  
> By construction this means it will introduce gaps either in the 
> boundary of the surrounding blocks or elsewhere in the array.  By 
> removing blocks from the array one only creates holes that are very 
> precise multiples of 4 cm.  What goes in those holes (other blocks, 
> supports, etc.) must match the dimensions exactly or else it will 
> create gaps in the stacking of the rest of the array.
>
This problem can be overcome by breaking your "outer" detectors into 4 
groups and shifting them like an iris so that they exactly bound the 
"inner" detectors. This was how PrimEx did it. Look here for a cartoon 
to see what I mean:

http://www.jlab.org/primex/subsystems/electronics/hycal_groups/index.php

PrimEx also did beam tests to study the boundary between lead glass 
(TF-1) and lead tungstate (PbWO4). The lead glass was 3.84x3.84 cm^2 
while the PbWO4 was 2x2 cm^2. I can't seem to find the note just now, 
but I can dig it up if you are interested.

One of the problems you'll have with a hybrid calorimeter is if you have 
materials with very different radiation lengths, the denser blocks will  
tend to be much shorter. The shorter blocks will be placed further 
downstream to optimize the energy collection. However, the z profile of 
the shower will be very different depending on whether the particle 
entered the "short" block or the "long" block which can lead not only to 
a nonlinearity, but a discontinuity in your detector response due mainly 
to attenuation. I don't know that you'd run into this with the Hermes 
blocks since they look to be the same length in z as the Rad-phi blocks. 
Clearly though, we'd need a very serious beam test of a hybrid prototype 
though before going this route because for us (PrimEx), handling the 
border region was non-trivial.


Regards,
-David

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