Low energy thresholds

[George Lolos <George.Lolos@uregina.ca>]

Hi all:

I believe that by now we all have been convinced that detection of low 
energy photons is very important due to the physics interest in that 
region of phase space for reactions of interest.   The main problem here 
is the number of photons reaching the sensors, the number of pe's we 
collect, and the noise we allow to leak in that imposes higher 
thresholds.  From Elton's plots, the main culprit is the limitations due 
to sampling fraction so whatever attempts we make to improve the sensor 
side of the problem is guided by the sampling fraction contribution to 
the energy resolution.  So, the way I see it, we have two avenues to 
push the limits:

1.  Push SensL for as high a PDE and as low a DR as possible.  We will 
do this, however, I believe that we will reach a stage where we will hit 
the ceiling of technology and will need substantial cooling of the 
sensors to keep high PDE and low DR.  This has budgetary and technical 
challenges we may not wish to face.  In addition, after pulsing becomes 
more severe with lower temperatures and this imposes an additional R&D 
to reduce this.  What I am saying is that we can only push DR down so 
far without cooling and we should have other options.

2.  Increase the number of photons reaching the SiPM's by increasing the 
number of SciFi's read out by them.  This, in practical terms means 
increasing the SciFi to Pb ratio, so increasing the sampling fraction.  
This has two benefits by improving both the pe statistics and improving 
the contributions due to the sampling fraction fluctuations.  However, 
the down side may be more leakage out the back and many more SciFi's 
than we can afford or want, for that matter, if we go for, say, 3 mm 
instead of the present 5 mm of Pb thickness.

One now has to ask, what region of the BCAL is really the one that needs 
the increased photo-statistics and higher sampling fraction?  This is 
clear, the critical region of the BCAL is that of the inner layers.   
Using 3 mm Pb for the inner layers will increase the sampling fraction 
by almost a factor of 5/3 over the present one.  This will also increase 
the number of photons by almost the same amount while keeping the DR 
constant.  The outer layers of ~ 12 cm, on the other hand, have no need 
for such improvements and we are better off to keep the Pb thickness to 
5 mm to contain as much of the shower energy as possible.

I am proposing then that we pursue a MC study of the "hybrid" 3 mm and 5 
mm Pb thickness and investigate how many layers do we need for the 
former and how many for the latter to reach an optimum figure of merit.

Any comments/shoot-downs/new ideas?

George