Neutron Detector: INFN plans
Patrizia Rossi for the INFN groups:
Genova, Laboratori Nazionali di Frascati, Roma Tor Vergata
Tracker
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Simulations
Timing Tests
Tracker
Tracker
CLAS12 Central Detector Meeting - Saclay 2-3 December 2009
Detector Simulations
A Spaghetti Calorimeter option has been extensively studied by the INFN group
with Monte Carlo simulations in order to determine:
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Neutron detection efficiency
TOF resolution (for n- separation in the momentum range of interest)
Angular resolution (for the definition of the neutron direction)
Simulation done with FLUKA (KLOE)
Parallelepiped shape (12.15 x 60 x 9.6) cm
Beam  to the longer side, and to fibers
20 cells (5 x 4), each 2.43 x 2.4 cm (x,z)
each cell contains 360 fibers
LEAD
FIBERS
Compared to Scintillator Barrel :
y
Neutron Efficiency: spaghetti calorimeter ~30-40% more efficient
Angular resolution: the two options give comparable results
TOF resolution: the two options give comparable results
But spaghetti calorimeter:
1) Too efficient for 
2) Energy loss localized in few fibers
 limitation for signal read-out
x
d=1mm
12.15 cm
1)
2)
3)
60 cm
beam
Option discarded
z
Background Simulations_1
Simulation of the background were done with gemc
1 event @ L=1033cm-2s-1
ELECTROMAGNETIC BACKGROUND
We want to understand:
• the actual rates seen by the CND, their energy distribution etc.
• the probability of such background to be reconstructed as a
“good neutron” event
Results:
• The background consists of photons
• The overall rate is 2 GHz at luminosity of 1035
• The maximum rate on a single paddle is 22 MHz (1.5 for
Edep>100KeV)
• This background can be reconstructed as a neutron:
- using a 5 MeV energy threshold the resulting rate is few KHz
- the  of this “fake”neutrons is <0.1-0.2
- the actual contamination depends on the hadronic rate in the
forward part of CLAS12 (@ 1 KHz the rate of fake events is 0.4 Hz)
We can handle it
0
0,2 0,4 0,5 0,6
0,7 0,8
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Background Simulations_2
PHYSICS BACKGROUND
First estimate of hadronic background based on
clasDIS event generator (pythia)
All event rate
Background events that could mimic a DVCS event are
defined as:
 Q2>1 GeV2
 W>2 GeV
 one energetic photon (E>1 GeV) in forward direction
 one photon in the central detector
 MM(e) < 1.1 GeV
Estimated rate at full luminosity (1035 cm-2 s-1)
~ 5 Hz (with one photon in CD)
We need to finalize nDVCS
event generator
to estimate neutron rates
e missing mass
Timing Tests
 TOF resolution required to separate  from n for neutron momentums up
1 GeV/c ~100-120 ps
 Constraints on photodetectors:
- Light collection in high magnetic field
- Limited space for signal read-out
No space for light guides due to the presence of the CTOF light guides
Timing tests wil be performed in 2010 by
the INFN groups using different setup
Timing Tests
Scintillator type
Readout
BC-408
PMT H2431-50
BC-408
• SiPM 1x1 mm2/ 3x3 mm2/matrix 12x12
Fermilab estruded
scintillators +1 WLS
• SiPM 1x1 mm2/ 3x3 mm2/matrix 12x12
Fermilab estruded
scintillators > 1 WLS
• SiPM 1x1 mm2/ 3x3 mm2/matrix 12x12
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BC-408:  =380 cm ; decay constant=2.1 ns
PMT H2431-50: rise time = 0.7 ns; transit time spread = 0.37 ns
Acquisition: Full electronic chain  discriminator+TDC
Electronics, redout, scintillators in Genova
Estruded scintillator + WLS
Redout
Timing Tests
Cosmic ray
PMT1
PMT2
X1
PMT3
SiPM
Fermilab estruded scintillator
PMT4
SiPM
X3
PMT5
PMT6
X5
t1=t0+x/v+c1
t2=t0+(L-x)/v+c2
(t1-t2)=(2x1-L)/v+c1-c2
(t3-t4)=(2x3-L)/v+c3-c4
(t5-t6)=(2x5-L)/v+c5-c6
(t1+t2)=2*t01+L/v+c1+c2
(t3+t4)=2*t03+L/v+c3+c4
(t5+t6)=2*t05+L/v+c5+c6
taking into account that: (x1+x5)=2*x3 or (t01+t05)=2*t03
(t1-t2)+(t5-t6)-2(t3-t4)=costant
(t1+t2)+(t5+t6)-2(t3+t4)=costante
Spread of these quantities is a measurement of the timing resolution
Conclusions
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INFN groups of Genova, Laboratori di Frascati, Roma Tor Vergata are
involved in the development of the central neutron detector
Simulations have been done to determine its characteristics (neutron
detection efficiency, angular resolution, timing resolution..) as well as
the e.m. and physics background
Timing tests are planned for 2010 using different scintillators and
redout systems