1. A Neutron Counter for the CLAS12 Central Detector
Detector Layout
Geant4 simulations
Background Studies
Summary and work plan
European
collaboration:
INFN Frascati,
INFN Genova,
IPN Orsay,
LPSC Grenoble

2. Detector Layout
Detector located between the central time of flight and the superconducting solenoid
Available space:
cylinder with
R= 39 cm
r = 27 cm
l = 66 cm
Strong constraints on readout system from limited space and high magnetic field
 Silicon PMs

3. Plastic scintillator detector
Plastic scintillator paddles with trapezoidal shape
4 layers in the radial direction with 30 paddles in j
Use of central time of flight as additional layer
Light readout on the two ends of each paddle to allow determination of z coordinate from time difference
Neutron/photon separation through time of flight measurement y l R x r z
4 layers along r
30 paddles in j

4. Simulation with Geant 4
Full simulation of neutron detector with Geant 4
Material:
standard plastic scintillator (BC408)
density =1.032 g/cm3
ratio H:C atoms = 1.104
bulk att. length = 380 cm
light output = 64% Anthracene
Elastic scattering on proton is the dominant process for neutron detection
Effects of light propagation, attenuation and detection included using effective parameters (l,veff, st, ...)
Light quenching effect taken
into account by reducing Edep for
protons by a factor 5

5. Neutron efficiency Efficiency increases decreasing the threshold
Generated neutrons with pn= GeV/c, q=90°, j=13.5° (center of 6th j slice)
Efficiency: Nrec/Ngen
Nrec= number of events
in 6th j slice having
Edep>Ethreshold
q=90°
q=65°
Can we live with 15% efficiency?
CLAS12 will have
10 times higher luminosity
than CLAS (1035 cm-2 s-1)
→ roughly the same statistics as
what was obtained for proton DVCS
(assuming equal cross sections)
Efficiency increases
decreasing the threshold
Eff ≈ 15% for thr. = 5 MeV
and pn=500 MeV/c
In agreement with « thumb rule »:
1% efficiency for 1 cm of scintillator

6. Particle Identification
Neutron / gamma separation by measuring the particle time of flight
Expected time resolution:
s0~50 ps for MIPs for CTOF layer with pmt readout
s0100 ps for MIPs for CND layers with SiPMs (to be measured, see Michel’s talk)
Time resolution rescaled at actual deposited energy as
s=s0√EMIPs(MeV)/√Edep(MeV)
n/g separation
possible up to
p<1 GeV/c

7. Alternative Option: spaghetti calorimeter
1.2 mm
1.35 mm
1.0 mm
Lead
Lead-fibers calorimeter developed for the KLOE experiment
Active material:
1.0 mm diameter scintillating fiber
(Kuraray SCSF-81 and Pol.Hi.Tech 0046)
High sampling structure:
99 layers of 0.5 mm grooved lead foils
Lead:Fiber:Glue volume ratio = 42:48:10
M. Anelli et al., NIM A581 (2007) 368
Calorimeter Thickness ~18 cm
Conceived as an electromagnetic calorimeter, it turned out to be
very efficient for neutrons:
50% more efficient than equal volume of scintillator, measured with neutron beam (Uppsala) and reproduced by simulation (FLUKA)

8. First simulations of detector response
Detailed simulation of the calorimeter structure with FLUKA Code
Evaluation of energy deposited in each fiber
Light propagation to the end of the fiber taking into account attenuation effects.
Central_th=0
Central_th=10 MeV
Simulation results for the neutron efficiency are consistent with the efficiency measured for the KLOE prototype on neutron beam

9. Neutron / Gamma separation
Good timing resolution for photons:
t=54 ps/E(GeV)147 ps
measured at KLOE
No measurement available for neutrons
Hit multiplicity could be an alternative method for n/g identification
Hit multiplicity for photons
E= 500 MeV
# fibers g n
< 80
0.3%
78 %
< 100
0.5 %
93 %
Hit multiplicity for neutron
E= 500 MeV

10. Pro & Con
9% increase of neutron detection efficiency at 500 MeV with Spaghetti Calorimeter based on existing measurement and confirmed by simulation
What are the ToF performances of the spaghetti option? Can the multiplicity be an alternative method?
Detection efficiency for neutron and gamma are comparable for plastic scintillator: 13% vs. 17% (th=2 MeV)
High efficiency for gamma of spaghetti calorimeter (e=100%) may result in higher contamination of neutron sample

11. Background studies
Precise understanding of the background sources and rates is needed to finalize the detector layout
Electromagnetic background due to the interaction of the electron beam with the target may results in high rates and could affect the reconstruction of the neutron hit position and multiplicity
Physics background (photons produced in hadronic reaction) may result in significant contamination
Full simulations of neutron detector response in the actual configuration within the central detector are needed
gemc is the best tool for this purpose

12. Electromagnetic background
Electromagnetic background rates and spectra have been studied with geant3 by A. Vlassov
Photon rate in the central detector of ~ 2 GHz at full luminosity integrating over all energies
Rate of photons reaching the neutron detector is being estimated with gemc in actual configuration within the central detector
L=1032cm-2 s-1
Photon Energy
L=1032cm-2 s-1
Photon Angle

13. Physics background
First estimate of hadronic background based on clasDIS event generator (pythia)
Background events that could mimic a dvcs event defined as:
one energetic photon (>1 GeV) in forward direction + one photon in the central detector)
Estimated rate at full luminosity (1035 cm-2 s-1)  2 KHz
All event rate
Photon rate from ‘fake’ DVCS events

14. Full Simulations of Neutron Detector with gemc
Neutron detector added to gemc thanks to Maurizio

15. Summary and Work Plan
Design of neutron counter for the CLAS12 central detector is in progress
Limited space and operation in high magnetic field result in strong constraints on the detector layout and readout
Different detector configurations (plastic scintillator detector and spaghetti calorimeter) are being studied
GEANT4 simulations of the detector response indicate that 15-20% neutron efficiency can be achieved
To Do:
complete evaluation of relevant parameters (efficiency, background rejection factors, angle and momentum resolution) for both detector options under study to choose best layout
perform full simulations of neutron counter in realistic configuration
complete study of background rates and compare with DVCS rate
optimize reconstruction algorithm
determine final efficiency and resolution for detected neutrons

17. Neutron interactions in the calorimeter
Simulated neutron beam: Ekin = 180 MeV
target
Pel(%)
Pinel(%) Pb
32.6
31.4
fibers
10.4
7.0
glue
2.3
2.2
Each primary neutron has a high probability to have elastic/inelastic scattering in Pb
In average, secondaries generated in inelastic interactions are 5.4 per primary neutron, counting only neutrons above 19.6 MeV.
neutrons
above 19.MeV
62.2%
photons
26.9%
protons
6.8%
He-4
3.2%
deuteron
0.4%
triton
0.2%
He-3
Typical reactions on lead: n Pb  x n + y  + Pb n Pb  x n + y  + p + residual nucleus n Pb  x n + y  + 2p + residual nucleus
neutrons
94.2%
protons
4.7%
photons
1.1%
In addition, secondaries created in interactions of low energy neutrons (below 19.6 MeV) are - in average – particles per primary neutron.

18. A typical inelastic process
Z(cm) p n1 n2 n3 n4
X(cm)
primary vertex
En = MeV
En (p) = 126 MeV
The enhancement of the efficiency appears to be due to the huge inelastic production of neutrons on the lead planes. These secondary neutrons: - are produced isotropically; - are produced with a non negligible fraction of e.m. energy and of protons, which can be detected in the nearby fibers; - have a lower energy and then a larger probability to do new interactions in the calorimeter with neutron/proton/γ production.

19. The measurement of the neutron efficiency @ TSL
KLOE calorimeter module
( 2 cm)
Neutron beam
EKIN (MeV)
A quasi-monoenergetic neutron beam is
produced in the reaction 7Li(p,n)7Be.
Proton beam energy from 180 MeV to ~ 20 MeV
Neutron energy spectrum peaked at max energy
(at 180 MeV fp = 42% of neutrons in the peak)
Tail down to termal neutrons

20. Experimental setup (3) (2) (1)
( 1 ) Old prototype of the KLOE calorimeter
60 cm long, 3 x 5 cells (4.2 x 4.2 cm2),
read out at both ends
by Hamamatsu/Burle PMTs
(1)
(3)
(2)
( 2 ) Beam position monitor
array of 7 scintillating counters,
1 cm thick.
( 3 ) Reference counter
NE110, 10×20 cm2, 5 cm thick
A rotating frame allows for:
- vertical positions (data taking with n beam)
- horizontal positions
(calibration with cosmic rays)