Cosmic Ray Test Stand
with Scintillating Cells for
Digital Hadron Calorimeter
Kurt Francis - Northern Illinois University
06/23/2003
1
Introduction
• At NICADD we studying technology for the design of a Digital
Hadron Calorimeter using scintillating cells
• As a first step we built a cosmic ray test stand with 2 layers of 7
hexagonal scintillating cells (9.4 cm2 x 5mm thick BC408 with sigma
groove, painted white)
• Light from the 14 cells is fed through optical fibers into a 16 channel
PMT
• The analog electrical output of the PMT is digitized with a VME bus
QDC and then read into a PC
2
Cosmic Ray Test Stand
• 14 Hexagonal Cells and Optical Fibers connected to 16
channel Hamamatsu H6568 Multichannel-PMT
Multi-Channel
PMT
Multi-Channel
PMT
Top view
Side view
3
COSMIC RAY TEST STAND
LIGHT PROOF BOX
TRIGGER COUNTER #1-set of
7 cells ganged to one single
channel PMT
14 cells connected to 16
channel PMT to collect comic
ray data under investigation
TRIGGER COUNTER #2
BLOCK OF PASSIVE ABSORBER
MATERIAL (STEEL, BRASS, LEAD)
NUCLEAR INTERACTION LENGTH ~
1.5
TRIGGER COUNTER #3
4
Software
• Used National Instruments LabView to create software to collect data
from 32 channel VME QDC
• Data is dumped into a flat data file
• Wrote my own software to view and analyze data
– used Microsoft Visual Basic
– displays histograms or scatter plots
– plots can be arranged by cell or by PMT channel
• Also used Excel, Origin, JAS, and PAW
5
6
SCATTER PLOTS
First Layer of Cells
Second Layer of Cells
7
Initial Observations: Data collected has two curves to right
of pedestal. What is first peak?
First Peak: Crosstalk?
8
First step isolate a channel
Channel 4 Not Isolated
Channel 4 isolated from other channels
by black paper between cells and by
covering all but ch4 photocathode with
black paper
9
Crosstalk
• Isolating a single channel from others reduces the first peak - indicates
the first peak is caused by crosstalk from the other cells / channels
• Crosstalk can be:
– Optical crosstalk between cells
– Optical crosstalk at PMT due to photocathode cover
– Electrical crosstalk at PMT
10
Crosstalk at cells evaluated by covering some
of cells with black paper.
Cell numbering scheme
8
3
Number on cell is percent of total events in first peak
6
4
2
Standard Configuration
with no black paper
7
5
Cells 5,8,6 covered by black paper
and layers separated by paper
14%
11%
15%
10%
8%
10%
11%
13%
8%
5%
11%
-->Suggests Optical crosstalk at cells can be
reduced by isolating cells with black paper
6%
5%
7%
Note: this apparently weak cell (#5)
was later found to be due
to the ADC channel 11
Crosstalk at PMT evaluated by covering all but one channel.
Scatter plot of all PMT channels with
Channel 4 stimulated by several fibers
driven by LED
-Other channels are not connected
-Cells are not connected
-->Suggests crosstalk at PMT
12
Another way to view crosstalk at PMT
Crosstalk in other channels due to channel 4 as a
percentage of channel 4
100%
80%
60%
40%
1
5
20%
0%
16 15
9
13
14
13
In this case only channel 4 is connected to a cell collecting
cosmic ray data - all other channels have crosstalk from channel 4
13
Summary of crosstalk issues
• Part of crosstalk is optical crosstalk between cells
– Initially used cells painted in white paint to reflect light back into
scintillator
– We can reduce optical crosstalk by covering cells with black paper
– Another option would be to paint cells with black paint
• A greater part of crosstalk occurs at the PMT
– Probably due to optical diffusion in the photo-cathode cover
– This crosstalk is significant only on adjacent channels
14
Finding the single photo-electron peak for a single
channel
• Use 2,3,4,5 layers of neutral density gelatin filter material
– (Kodak No.96 N.D. 0.20 = 63% transmission)
•
•
•
•
Stimulate with an LED channeled through optical fiber
Start with 2 layers of filter -> get peak far from pedestal
Increase filter layers -> pedestal appears and grows,
Position of peak moves closer to pedestal, shrinks and merges with
pedestal
• When the position of the peak stops changing as filter layers are added
then moving closer top pedestal then we have found the single photoelectron peak
15
Example using channel 4/20
16
Position of Peak in ADC
Counts
Light Attenuation versus Number of Filter Layers
(Channel 4)
350
300
250
200
150
100
50
0
0
1
2
3
4
5
6
7
8
Number of filter layers (63% transmission)
--> plots with 4 - 8 filter layers is approximately flat and linear
- expected for the single photo electron peak
17
Single Photo-Electron Map
Single Photo-electron peak
Distance from Pedestal in ADC Counts
32
29
25
30
24
26
33
17
27
31
29
23
27
25
25
28
--> Indication of ‘gain’ for that particular channel
18
Light Yield Map
The light yield for the cell/channel combination is equal to the
ADC Counts from the pedestal of the MIP peak (determined
from cosmic ray data sets) divided by the ADC Counts from
pedestal of the S.P.E peak (determined from the LED with filter
layer tests as described on the previous slides)
Light Yield by Channel (P.E.)
--12.8
10.2
14.6
16.2
--11.1
16.6
8.3
11.8
14.4
12.8
11.1
10.7
8.4
11.0
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Number of Channels
Distribution of Light Yield
5
4
3
2
1
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Light Yield in P.E. (rounded)
Range of a factor of 2
20
Summary
• NICADD has developed a cosmic ray test stand using scintillating
cells to prepare for DHC
• The test stand reveals crosstalk
– Optical crosstalk between cells - can be reduced by isolating cells
with black paper or paint = 30 to 40% of total crosstalk
– Crosstalk at PMT (probably optical) = 60 to 70 % of total
• Can use LED driven by pulse generator and layers of filter material to
find Single photo-electron peak (S.P.E.)
• S.P.E. and M.I.P. average can be used to determine light yield cells
• The light yield has a range of 8 to 17 P.E.
21