sPHENIX EMCal discussion

["Sickles, Anne M" <sickles AT illinois.edu>]

Hi everyone,

Thanks John for kicking off an interesting discussion.  

I think if people would like to seriously consider this route, we need a real model of the geometry for a reused PbSc and simulations.  The segmentation that John quotes is significantly worse than even the ganged 2x2 towers that we are considering and while the improved resolution would help with the e/h separation the lack of projectivity in both eta and phi will be a drawback.  In addition, the gaps between the sectors would result in a decreased acceptance for upsilons and photon and non-uniformities in the jets. It’s a quantitative question as to what the performance actually would be and since we have real simulations for the SPACAL, we would need real simulations for a reused PbSc as well. There seems to be a factual disagreement between John and Mickey as to what the Moliere radius of the PbSc actually is and I couldn’t find documentation of that in either the web page John linked to or the NIM paper:

http://www.phenix.bnl.gov/phenix/WWW/pub/phenixnim/d_emcal/nim_4d_emcal.pdf

It is really not clear to me what the impact to the schedule is of a reused EMCal.  It does exist, but it would take a few months to implement in simulations and convince ourselves that it could meet our physics requirements.  Given the physics aims of sPHENIX (jets and upsilons), I don’t see the logic in trading the EMCal performance for a barrel time of flight detector.

The 2D projective shashlik that Craig mentions obviously deals with some of these issues (it could be designed to be 2D projective for example) and I think we had a MC model for it at some point (?).  However given that it is orthogonal to the last few years of R&D we have done, I agree with Craig’s assessment of a very long delay to the project.

This is something we could certainly discuss at the EMCal meeting next week.

Best,

Anne

On Jun 2, 2016, at 9:41 AM, Mickey Chiu <chiu AT bnl.gov> wrote:

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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Anne Sickles

Assistant Professor, Department of Physics

University of Illinois at Urbana-Champaign

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