sPHENIX EMCal discussion

[John Haggerty <haggerty AT bnl.gov>]

Hello,

This is not for Friday's descoping document, but I think that given some 
of the ideas going around now, we might have to go back to ideas that 
were dismissed many workfests ago, since they might not be as bad as 
some of the descoping options we're now talking about.

The one in particular that's been bothering me the last few days is 
going to a single calorimeter inside the solenoid with lead absorber. 
If we multiplex the EMCAL to 2x2, one could ask why you need expensive 
tungsten to make a short radiation length.  Maybe lead would be good 
enough.  (I should note that Edward often asks this question, but when 
we had 1 Moliere radius towers, it was easy to say that only tungsten 
could achieve the short radiation length without going to hugely 
expensive crystals, and only tungsten would allow us to cram ~1.5 
interaction lengths inside the solenoid.)

Once you think lead, you may as well think the PHENIX EMCAL, it's as 
good as any lead-scintillator calorimeter.  It's about 17.5 radiation 
lengths, about 0.85 interaction lengths in depth, which is about 375 mm, 
so it would fill up about the same volume as the EMCAL and Inner HCAL 
(1000 to 1400 mm in radius), and one could read it out at the inner 
radius with SiPM's.

My facts com from Edward's web page
> http://www.phenix.bnl.gov/phenix/WWW/emcal/computing/online/EmcDoc112602/Introduction.html
and the NIM paper, by the way.

A tower is 55.35 mm square, I think, so there are something like 100 
towers in phi and 60 in eta and natural numerology would make it 0.058 
(phi) x 0.037 (eta).  I think the Moliere radius of the EMCAL is about 
55 mm, so it is matched, as is common, to the segmentation, and the 
108*60=6480 towers takes 3 PHENIX sectors.

The natural segmentation leaving the (12x12) supermodules alone would 
lead to a nine-sided calorimeter (9*12=108), odd, but not impossible. 
(If one can take apart the supermodules, one could contemplate a seven 
sided structure with 16x16 supermodules, I don't know if that's feasible 
or not).

There are many bad features of this idea, which is why we dismissed it 
in the first place.  It's not naturally projective; there are likely to 
be sizable gaps for support; you can afford less photodetector coverage 
than you could with PMT's; the segmentation is just barely adequate. 
Some of these problems could conceivably be overcome with ingenuity and 
labor, but the segmentation and the radiation length are what they are.

You can say the rework still costs a lot, but the difference in cost is 
more in design, mechanics, and modifying the existing modules, and some 
of that is common, and much of it is labor, which is not the immediate 
problem.  You lose the Inner HCAL, but the total interaction lengths is 
less than one less, from nearly 6 at zero rapidity to 5.  Part of this 
package would be to leave the Outer HCAL alone, or perhaps even augment 
it by splitting the tiles in half and reading both inner and outer 
radius.  We may lose in e-pi, but if the calorimeter's resolution 
survives intact (to be show with reduced photodetector coverage), maybe 
E/p is enough better to make up for that loss.

Bad idea?  Many of us thought so, but depending on the results of the 
budget exercise, it may be worth kicking around again.

--
John Haggerty
email: haggerty AT bnl.gov
cell: 631 741 3358