sPHENIX EMCal discussion

[Craig Woody <woody AT bnl.gov>]

Hi John,
  I have to say that I was thinking the same thing. When we talk about 
eliminating the Inner HCAL (which is clearly like having a vital 
appendage removed in my opinion...), one does have to ask, why is there 
this large gap between the EMCAL and the wall of the magnet ? I agree 
the main justification for going to a tungsten absorber was to keep the 
EMCAL compact, thus allowing the HCAL to be kept smaller in order to 
reduce the cost. However, if the entire radial space from ~ 90-140 cm is 
available for the EMCAL, keeping the Outer HCAL the same, then it would 
make sense to reconsider lead as an absorber again. If I recall Tom 
Cormier's design, it essentially just fit into this space with ~ 18 Xo. 
One could then consider a projective (even 2D projective) shashlik 
design with SiPM readout. However, I don't think it makes sense to 
actually consider using the existing PHENIX shashlik, since it's 
completely the wrong geometry and I would be the cost to rework it would 
be more than building a new one. Nevertheless, if we have the radial 
space, it would be worth considering a lead absorber again in terms of 
cost and performance. The only problem with doing that is that it would 
set us back about two years in terms of schedule.

Craig

On 6/2/2016 12:10 AM, John Haggerty wrote:
> 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.