1. Central detector for CLAS12: CTOF and Neutron detector
Where we stand
Objectives
Requirements
Constraints
Possible solutions
S. Niccolai, IPN Orsay

2. Central (CD), 40o<q<135o Forward (FD), 5o<q<40o
CLAS12: the current design
Full detector:
Central (CD), 40o<q<135o
Forward (FD), 5o<q<40o
CD + exploded view of one sector of FD
12/03/08 - S. Niccolai – IPNO

3. and/or neutron detector
Central Detector
Main coil
Central tracker
CTOF
Space for calorimeter
and/or neutron detector
(12 cm radial thickness)
Cryostat vacuum jacket
Magnetic field in the center = 5 T
12/03/08 - S. Niccolai – IPNO

4. Central TOF CTOF: Measuring TOF for particle ID of charged particles
Goal: s = 50 ps to allow:
p/K separation up to 0.6 GeV/c
p/p separation up to 1.25 GeV/c
Challenge: light collection in the
strong magnetic field (5 T)
12/03/08 - S. Niccolai – IPNO

5. 1) Long bent light guides Prototype under construction
Solutions studied at
Kyoungpook and JLab
1) Long bent light guides
and ordinary PMTs
mTesla
Prototype under construction
at JLab
(V. Baturine)

6. Solutions studied at Kyoungpook and JLab
2) Straight light guides and
magnetic-field-resistant PMTs
(micro channel, fine mesh)
In-field tests planned
at Kyoungpook
(S. Kouznetsov)
0.6-2 Tesla

7. JLab’s design (option 1)
50 scintillators 66x2x3 (cm)
100 Light guides l= m
100 Photo-multipliers
σ = 72 ps (?)
Measurements have been done
without light guides
and with straight LG

8. JLab’s prototype H8500/R2083 What can we do?
Studying alternative solutions
SiPM as photodetectors?
Can we achieve 50 ps of time
resolution with SiPM?

9. We need photodetectors that are insensitive to magnetic field
Neutron detector
Goal: detection of neutrons with pn = GeV, with PID (n or g?) and measurement of angles q, f
Challenges:
detection efficiency (15 cm of
material available, including CTOF)
g/n separation: TOF resolution (or…?)
strong magnetic field (5 T)
no space for light guides
We need photodetectors that are
insensitive to magnetic field
12/03/08 - S. Niccolai – IPNO

10. GEANT4 simulation
estimate and maximize detection efficiency for neutrons
resolution on TOF to separate neutrons and g
or find an alternative way to separate neutrons and g x z y l R r
Realistic geometry,
following design
for CTOF: solid
composed by
trapezoids
Segmentation:
Radial, to determine interaction point
TOF  p (5 layers in r, 3 cm thick)
Azimuthal, to determine f (30 slices)
q can be obtained via z=1/2veff(t1-t2)
Dimensions
R= 39 cm
r = 24 cm
l = 50 cm
12/03/08 - S. Niccolai – IPNO

11. Light quenching effect taken into account by reducing Edep for
Neutron efficiency
Detector material: scintillator
Generated neutrons with pn= GeV/c, q=90°, f=13.5° (center of 6th f slice)
Efficiency: Nrec/Ngen
Nrec= number of events
in 6th f slice having
Edep>Ethreshold
(r: first good hit only)
2 MeV
5 MeV
10 MeV
Light quenching effect taken
into account by reducing Edep for
protons by a factor 5
Efficiency increases
decreasing the threshold
Eff ≈ 15% for thr. = 2 MeV
and pn=500 MeV/c
In agreement with « thumb rule »:
1% efficiency for 1 cm of scintillator

12. TOF resolution Slava’s measurement Simulation For each f, r slice:
TOF = (t1+t2)/2
t1 = tofGEANT+ tsmear+ (l/2-z)/veff
t2 = tofGEANT+ tsmear+ (l/2+z)/veff
veff=16 cm/ns (value used in GSIM)
tofGEANT= average of times of all «steps»
z = average of z positions from all steps
tsmear = smearing factor: Gaussian centered at 0 with s= t0/√Edep (MeV),
t0 = 92 ps (deduced from Slava’s measurement at 6 MeV) for 1st r layer,
for other layers t0 = 200 ps
Simulate time distribution of
the scintillator light
Introduce spread due to light transmission
in the bar
Account for transmission from the
scintillator to the pmt photocathode
(different size, lightguide...)
Account for conversion to photoelectrons
(q.e. =20%)
Include additional time spread due to PMT transit time and amplification
Slava’s measurement
Simulation

13. TOF resolution: results
s = 70 ps
s = 117 ps
s = 117 ps
s = 118 ps
Neutrons, pn= 1 GeV/c
s = 80 ps
Threshold = 2 MeV
Resolution is worse with smaller threshold

14. TOF resolution: results
s = 48 ps
s = 87 ps
s = 83 ps
s = 81 ps
Photons, pg=1 GeV/c
s = 80 ps
At 3s no overlap between n
and g in the first layer
starting to overlap in other layers
n/g separation
possible up to
pn≤1 GeV/c

15. Hits multiplicity: another PID method?
Photon, no lead
Photon, lead (0.625 cm)
Added lead layers, and studied the number of hits per events
with Edep>threshold, for neutrons and photons
12/03/08 - S. Niccolai – IPNO

16. Very different multiplicities
Neutron, no lead
Neutron, lead cm
Very different multiplicities
between n and g
with the lead layers
BUT
Eff(g) = 90%
Eff(n) = 15%
Tested thinner lead layer (1mm)
 lower photon efficiency (50%)
but photon multiplicity gets lower
(Eff(g) = 20% no lead)
Photon, no lead
Photon, lead cm
12/03/08 - S. Niccolai – IPNO

17. The spaghetti option: KLOE
Active material:
1.0 mm diameter scintillating fiber
Core: polystyrene, r=1.050 g/cm3, n=1.6
High sampling structure:
200 layers of 0.5 mm grooved lead foils (95% Pb and 5% Bi).
Lead:Fiber:Glue volume ratio = 42:48:10
Conceived as an electromagnetic calorimeter,
it turned out to be very efficient for neutrons
More than twice the efficiency/cm
of a scintillator, measured with neutron beam (Uppsala) and reproduced by simulation (FLUKA)
Could this solution be viable for us?
Coud it work also for measurement of TOF?
Timing resolution with fibers + SiPM?
12/03/08 - S. Niccolai – IPNO

18. Summary Can SiPM be the solution? We need photodetectors insensitive
Whichever solution will be chosen for the neutron detector (layers of scintillators, sandwich lead-scintillator, spaghetti, etc.), there are the following issues:
limited space upstream and downstream, due to the presence of the light guides for CTOF
→ no space for additional light guides to “escape” from the high magnetic field
light collection in the high magnetic field
BUT, compared to CTOF, the requirement on TOF resolution is less stringent:
from preliminary simulations, a time resolution twice as bad
as the one currently achieved in KNU and Jlab measurements can still be good enough to
separate photons from neutrons for neutron momentums up to 1 GeV
Can SiPM be the solution?
We need photodetectors insensitive
to magnetic field, providing decent
timing resolution
12/03/08 - S. Niccolai – IPNO