1. A possible BGO Setup for the 2008 Beam Test Campaign
A. Cardini / INFN Cagliari

2. A. Cardini / INFN Cagliari, Italy
Outline
Motivations
2007 Setup and Results
BGO Crystals
A (few) Possible Setup(s)
Readout
Simulation Issues
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

3. A. Cardini / INFN Cagliari, Italy
Motivations
Very interesting results taken with a single BGO crystal in 2007
BGO seems an almost ideal candidate for our purposes, thanks to:
Transparency in the near-UV region extends to ~320 nm
Scintillation emission peaks at 480 nm, and very small scintillation light below 400 nm
No (or very low) scintillation self-absorption
Yes, it is an intrinsic scintillator!
Relatively long scintillation light decay time allow to easily separate it from the Cherenkov component
BGO might not be the “perfect” homogeneous material for a dual readout homogeneous calorimeter, but it represents (at least) a very important learning and evaluation tool
We definitely need to continue these studies using a small BGO electromagnetic calorimeter with “dual” readout
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

4. A. Cardini / INFN Cagliari, Italy
2007 BGO Setup
GG495 filter
L3 End-cap ECAL BGO
2 x Hamamatsu R1355 PMs
UG11 filter
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

5. A. Cardini / INFN Cagliari, Italy
Summary of 2007 results
(a) Distribution of C/S values measured in the BGO crystal for 200 GeV positive pions
(b) DREAM energy deposits for events with a small (grey) and a large (red) C/S value
Plot (b) shows a clear correlation between C/S measured in BGO and signal in DREAM, indicating that C/S is a powerful estimator for the electromagnetic fraction
… and all this was obtained with only one BGO crystal
Richard’s plot
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

6. A. Cardini / INFN Cagliari, Italy
A possible setup
We are now trying to get 50÷100 BGO crystals from the L3 electromagnetic calorimeter
We want to build a matrix with size ranging from 5x5 to 10x10 crystals (depending on how many crystals we will have)
Crystals come from the end-cap and have a trapezoidal shape of approximate size of 25x25x240 mm3, the “final” matrix will have a strange shape and non-flat ends
Readout with two at least photomultipliers, one on each matrix side, both equipped with “UV” filters (2 sides readout to correct for the attenuation of the UV light)
Light could also be collected through light guides (total internal reflection light guides, for example Plexiglas covered with mylar or directly aluminized)
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

7. A. Cardini / INFN Cagliari, Italy
A (typical) Crystal
24.7
24.7
240.0
~1º
~1º
25.2
32.4
25.6
25.2
32.8
33.5
0.45
0.45
~1º
25.6
240.0
3.95
~1º
All sizes in mm
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

8. A. Cardini / INFN Cagliari, Italy
Sizes (of the 3 we have…) a b A B L h H #1 #2 #3
All sizes in mm
Crystal a b h A B H L #1
21.5
22.4
22.0
28.9
30.1
29.5
240.0 #2
23.0
24.3 -
30.8
32.4 #3
24.7
25.6
25.2
33.5
32.8
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

9. Arranging Crystals in a Matrix
This surface is not flat, nor the opposite one: Not easy to directly attach a photodetector!
This is probably the best we could do if we are able to get crystals from the same region (sizes matches to better than ~0.5 mm, but not much better…)
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

10. Example: a 5x5 Matrix (v. 1) Side view Large/small end view
These surfaces (phototube ends) are not flat
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

11. Example: a 5x5 Matrix (v. 2) Side view Large/small end view
These surfaces (phototube ends) are not flat
This version has the advantage of being more regular, however the final design will depend on the shapes and sizes of the crystals we will have…
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

12. Where to readout the matrix?
How do we readout the matrix?
We need to have photodetectors seeing at least one side of each crystal (remember, n=2.15 and there is air between crystals: light will (usually) not pass from one crystal to the other, crystals are practically light guides)  We need to read it out from one/both “end” sides if we want to measure the energy deposit in each crystal
How do we place the matrix with respect to the beam line?
(1) is more ideal, however the beam could cross the PM
(2) Symmetric – however not completely due to shape of crystals
(3) “à la” single-crystal, for an optimized Cherenkov light collection on one side – at least for the first part of the shower (tilt 27.7º), but non-uniform calorimeter length
Beam line
27.7º
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

13. A. Cardini / INFN Cagliari, Italy
Readout
2 large PMs (with UV filters), one on each side (as for the PWO matrix)
no such PMs easily available (very expensive) (ex.: Photonis 5” XP4592B, 8 stages)
Non-uniform light response, large time spread
One PM (and one filter) on each crystal (or other type of photodetector)
Probably the best possible performance
We need small (<1”) PMs, resulting also in a small light collection
Expensive
Not enough digitizers’ (and HV) channels
As many ~2” (or larger) PMs packed together as we need
Not a 1:1 matching with crystals
No light-guides, proximity coupling
Coverage? Calibration?
One PM on each side, looking at the crystal through an air light guide
Needs to be carefully studied to optimize light collection from every crystal, but might be able to obtain interesting configurations
Mylar or aluminized light guide sides (could be done at CERN, see Alice ZDC)
Could use smaller PMs (for example 6 stages 2” fast Photonis XP2242)
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

14. Crystal / Crystal Matrix Simulation
We definitely need a ray tracing simulation program:
Trying to recover and adapt the PWO Toy MC to BGO crystal shapes to understand the strange patterns seen in position and angular scans
We might like to use a general purpose ray tracing software for issues like light guide design: Zemax, Oslo, … usually expensive and not easy to use, investigating…
OSLO (
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

15. A. Cardini / INFN Cagliari, Italy
Conclusions
Some ideas were shown, but difficult to design such a system if we do not know the number and shape of the BGO crystals we might be able to get
A simple readout (2 photodetectors, both with UV filters) is highly preferred and probably the only reasonable setup we could afford and we would be able to readout
Hope to have some clear answer on BGO for mid-January
December 18th, 2007
A. Cardini / INFN Cagliari, Italy

16. A. Cardini / INFN Cagliari, Italy
Other items
We might not need to read both sides:
CUV/SUV (measured behind an UV filter) does not depend on attenuation. To compensate for attenuation we just need to measure Syellow on the same side (but with a different PM) and correct C for a factor Syellow/SUV
December 18th, 2007
A. Cardini / INFN Cagliari, Italy