sPHENIX EMCal discussion

[Gabor David <david AT bnl.gov>]

On Thu, 2 Jun 2016, Craig Woody wrote:

        Hi, Craig and All,

> 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

        In fact, ideally dual readout (front and back), this would
buy back some of the lost e/p separation

> actually consider using the existing PHENIX shashlik, since it's
> completely the wrong geometry and I would be the cost to rework it would

        Fully agree

> 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.

        Aye, there's the rub...  (more later, I got an appointment)

        Gabor

> 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.
>
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--
Gabor David  (david AT bnl.gov)
Brookhaven National Laboratory
Physics Department, Bldg 510/c
UPTON NY 11973
Tel: (631)344-3016
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