Hi John,
Thanks for this excellent summary. I agree that in the current
situation we find ourselves in that we are probably OK with moderate
pressure on the tiles - this is, after all, and R&D exercise. I
am a bit concerned about the tiles in the center of the outer HCAL
which are already very snug and how that will change as the outer
HCAL is assembled and rigged into place.
What we really should do, however, is treat this additional
complication as part of the R&D exercise and not leave anything
to chance or leave it undocumented. What do I mean by this? We
should compare light yields for the tiles prior to assembly with
post-assembly (cosmics) and we should be wiling to "sacrifice" and
unwrap any tiles that we suspect might have suffered physical damage
once the test beam is completed. In other words, this should be
another key aspect of the HCAL design that we are seeking to
quantify with the prototype.
Regards,
John
On 1/6/2016 8:08 AM, John Haggerty
wrote:
I
corresponded with Edward quite a bit yesterday about stressing the
scintillator, and he's busy preparing a talk for the conference
this week, so I'll try and summarize what he and I arrived at
about how much we could safely stress the scintillator tiles in
the HCAL. (I'll write a second description of why he thinks they
are thicker than we specified, but I might do that later today.)
I found a paper which studied this a little in extruded
polystyrene scintillator, and I attached a figure showing the loss
of light over a period where a tile was stressed at about 10% of
breaking over three months. The paper is here:
http://deepblue.lib.umich.edu/handle/2027.42/31503
The PHENIX EMCAL was assembled under stress; Edward says: "PHENIX
PbSc was assembled with stacking pressure ~5-6kg/cm2 - no signs of
crazing after nearly 20 years (checked today).
Extruded plastic has much better load bearing capability. It has
much lower density of frozen high elasticity deformations which
are the crazing source under pressure. 15-20kg/cm2 should be OK."
(20 kg/cm^2 is 280 PSI.)
He also said, "it is basically impossible to force this plastic
(especially with 0.1mm foam like coating on both sides) to craze
under any practical load (I kind of thought of suggesting to load
the tiles while assembling the prototype - should be no problem at
all). I would suggest to make a small test - take 10x10 cm2 piece
of scintillator and place it into the press with pressure sensor.
Do it in steps - bring and release and inspect then repeat at
increased pressure. I may try to do it here at USM.
My experience is with injection molded scintillator used in all
kinds of shashlik’s. We always compressed the structure to the
state when 10x10x50cm2 module will hold its weight cantilevered
without visible bending. I inspected number of layears old modules
from GEPARD - no crazing but ~1% a year light loss - ask me why.
I’ll check with Uniplast if they kept the record of pressure
cycling which they did."
Also, it seems to me that crazing is usually degradation of the
surface finish, reducing transmission from total internal
reflection, and the surface of our tiles is etched, so it only
diffusely reflects anyway, so probably it doesn't make it much
different. I suppose as the crazing extends a finite depth into
the volume, we could reduce transmission. We could do a test
ourselves if we have a tile (and a control tile) where we measure
the light output over the next few months, where one is stressed
and one is not. I suspect doing a test over a short period will
show nothing, but that's what experiments are for.
The outer HCAL prototype has 4 tiles with an area of about 6829
cm^2 according to my arithmetic (someone could check that), so if
the entire 400 kg plate was supported by the tiles, the pressure
is pretty small. Of course, effectively clamping them down could
increase the stress to be arbitrarily high, and there are high
spots and low points.
So I'd conclude from this that if we can get the tiles into the
gaps, they will probably be ok. We would not take tiles out in
the field anyway, although it may make it necessary to either
unstack it at Fermilab or bring it back after the beam test to
install the high rapidity tiles.
For the inner HCAL, I think it means we can stack it up like a
Dagwood sandwich.
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