sPHENIX EMCal discussion

[Mickey Chiu <chiu AT bnl.gov>]

Hi John, et al,

I think this idea isn't as crazy as it sounds.  The energy resolution 
would be much better, 8% vs 12%, which buys back some of the e/pi 
separation (through E/p and residually through shower shape cuts).  The 
caveat in the E-resolution argument is that we'd need to understand 
whether the constant term can be reduced to the version we had in the 
test beam, or the one we had in PHENIX.

I think if you go this route you would want to optically split the PbSc 
longitudinally, which then would provide equal or superior e/p 
separation to the dual EMC/iHCal that is the default now.  Just reading 
out the front and back wouldn't help very much, though in the 
longitudinal split you certainly would want to collect the light from 
the front and back.  I think you could start working on the 
"construction" almost right away, with just a short R&D period to figure 
out how to best split it longitudinally.  Longitudinal segmentation also 
gives you a crude sort of projectivity along eta.

One down-side that I see is the segmentation is much coarser, but this 
can also be solved rather easily.  The PbSc Moliere radius is actually 
3.0 cm, a fact that is not very well known.  The fibers could be 
rearranged to buy back some of that segmentation.

The only other down-side I can think of are the the triangular gaps 
between sectors.  (Well, there's also the mental down-side that this 
won't have that new car smell...)  I'm not sure what you want to do with 
those gaps, but I'm betting you will need to fill them somehow. However, 
I don't agree with Craig that re-using the PbSc is going to be as 
expensive as building brand new... I'd expect it to be _significantly_ 
cheaper, and with a better schedule.  As I mentioned above, you could 
probably start working on the "construction" of this as soon as we get 
the PbSc off the East Carriage around late August/September.  My guess 
right now is that it would be some simple mods to the existing 
calorimeter.  We have a prototype in the basement which we can get Sean 
cranking on today if desired.

With the money I'd expect we could save, we could consider more optimal 
configurations of the tracking.  In addition, based on the EIC R&D I 
have been working on, I'm currently 90% confident that I can build a 
mRPC TOF system capable of 20 ps, which would give PID capabilities 
similar to what we have in PHENIX, but over the whole 2pi acceptance. 
This would cost just $4M (not counting contingency but otherwise is the 
whole project cost).  This estimate is based on 4 times larger STAR TOF 
cost, which was $7M, so it might be considered a conservative estimate. 
 The remaining 10% uncertainty in my confidency level above is due to 
whether we could create electronics capable of handling the 4 us trigger 
latency.  I had thought I could retire that risk by this coming winter, 
but current labor changes at BNL make that time-scale more uncertain. 
In any case, we have a plan in place for how to make the electronics for 
this.  Over the next year I'm hoping we can demonstrate 10 ps resolution 
(and incidentally reduce the cost further).  If we can do that, we 
should be able to surpass the PHENIX (and RHIC-wide) momentum reach for 
PID by \sqrt{2}.  Of course it should be obvious that full acceptance 
PID would significantly enhance the sPHENIX physics program...

Cheers,
Mickey

On 06/02/2016 09:23 AM, Gabor David wrote:
> 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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