1. Update on Central Neutron Detector for CLAS12 S. Niccolai, IPN Orsay European collaboration: INFN Frascati, INFN Genova, IPN Orsay, LPSC Grenoble CLAS12 Meeting, Orsay, 15 September 2008 …and maybe Glasgow? Physics goal: nDVCS Requirements & constraints Monte Carlo studies Work plan

2. 9/15/08 - S. Niccolai – IPNO Neutron detector: physics motivation Main goal: detection of recoil neutron in nDVCS H, H, E, E (x,ξ,t) ~ x-ξ t (Q 2 ) e e’ L*L* x+ξ n n’  nDVCS: strongest sensitivity to E Ji’s sum rule → access to quark orbital momentum 2J q =  x(H+E)(x,ξ,0)dx ed→e’n  (p) Detected in forward CLAS Detected in FEC, IC Not detected PID (n or  ?) + angles to identify the final state

3. 9/15/08 - S. Niccolai – IPNO nDVCS: neutron kinematics Average neutron momentum 0.3-0.5 GeV/c More than 80% of the neutrons have  >40° CD DVCS-BH event generator with Fermi motion E e = 11 GeV -t=1.1 GeV 2 Cuts applied: W 2 >4 GeV 2, Q 2 >1 GeV 2, –t<1.1 GeV 2 5°<  e <35°, 5°<   <40°

4. 9/15/08 - S. Niccolai – IPNO CND CTOF limited space available (~10 cm thickness) → limited neutron detection efficiency → no space for light guides → compact readout needed strong magnetic field → magnetic field insensitive photodetectors (SiPMs or Microchannel PMTs)  CToF can also be used for neutron detection  Central Tracker can work as a veto for charged particles Central Tracker CND: constraints & possible designs Two detector designs are under study: scintillator barrel spaghetti calorimeter Comparison of:  efficiency  PID  resolution

5. 9/15/08 - S. Niccolai – IPNO Simulation: scintillator barrel Geometry: Simulation done with Gemc Includes the full CD 4 radial layers (each 2.4 cm thick) 30 azimuthal layers (to be optimized) each bar is a trapezoid (matches CTOF) inner r = 28.5 cm, outer R = 38.1 cm Reconstruction:  Good hit: first with E dep > threshold  TOF = (t 1 +t 2 )/2, with t 2(1) = tof GEANT + t smear + (l/2 ± z)/v eff t smear = Gaussian with  =  0 /√Edep (MeV)  0 = 200 ps·MeV ½ (~2 times worse than what obtained from Slava’s TOF measurement)  β = L/T·c, L = √h 2 +z 2, h = distance between vertex and hit position, assuming it at mid-layer  θ = acos (z/L), z = ½ v eff (t 1 -t 2 )  Birks effect not included (should be added in Gemc) z y x

6. 9/15/08 - S. Niccolai – IPNO Simulation: spaghetti calorimeter Reconstruction:  Good hit: highest energy deposited  the time in each hit is weighted by energy deposited  TOF = (t 1 +t 2 )/2, with t 1(2) = tof 1(2)FLUKA + t smear t smear = Gaussian with  =  0 /√Edep (MeV)  0 = 250 ps·MeV ½ (obtained calibrating the simulation to GLUEX time measurement with  beam and PMTs)  β = L/T·c, L = √h 2 +z 2, h = distance between vertex and hit position, assuming it at the center of the cell  θ = acos (z/L), z = ½ v eff (t 1 -t 2 ) LEAD FIBERS Geometry: Simulation done with FLUKA (KLOE) Only calorimeter, no other CD elements Parallelepiped shape (12.15 x 60 x 9.6) cm Beam perpendicular 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 Active material: 1.0 mm diameter scintillating fiber Core: polystyrene,  =1.050 g/cm 3, n=1.6 95% Pb and 5% Bi. beam 60 cm y z x 9.6 cm 12.15 cm

7. Can we live with 10%-15% efficiency? CLAS12: 10 times higher luminosity than CLAS (10 35 cm -2 s -1 )  (BH p ) ~ 4  (BH n ) Scintillator barrel p n = 0.1 - 1.0 GeV/c  = 90°,  = 0 ° Efficiency: N rec /N gen N rec = number of events having E dep >E threshold Spaghetti calorimeter Threshold = 10 MeV Predictions (VGG) for 80 days of run time,  =30°, Δt=0.15 GeV 2,  = 90°

8. 9/15/08 - S. Niccolai – IPNO Scintillator barrelSpaghetti calorimeter Angular resolution   Neutron momentum (GeV/c)  resolution depend on timing resolution → measurements with actual photodetectors needed! Also needed event generator for signal & background to determine resolution requirements

9. 9/15/08 - S. Niccolai – IPNO Scintillator barrelSpaghetti calorimeter PID:  /n separation n, p n = 0.5 GeV/c n, p n = 1.0 GeV/c , E  = 0.1 GeV/c Same number of generated for n and  number of generated neutrons is 5 times the number of generated   resolution depend on timing resolution → measurements needed! Very important to estimate the expected neutron and photon rates in the CND

10. 9/15/08 - S. Niccolai – IPNO Electromagnetic background Photon Energy (GeV) Photon Angle L=10 32 cm -2 s -1 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 The majority of the photons have very low energy Rate of photons reaching the neutron detector is being estimated with gemc in actual configuration within the central detector

11. Physics background First estimate of hadronic background based on clasDIS event generator (pythia) Background events that could mimic a DVCS event are defined as: Q 2 >1 GeV 2 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 (10 35 cm -2 s -1 )  5 Hz (with one photon in CD) All event rate e  missing mass We need to finalize nDVCS event generator to estimate neutron rates

12. CND: constraints on photodetectors Whichever solution will be chosen for the neutron detector (barrel of scintillators or spaghetti), 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 We need photodetectors insensitive to magnetic field, providing decent timing resolution Can SiPM be the solution?

13. 9/15/08 - S. Niccolai – IPNO General view Photo-sensitive side 25 µm Al ARC -V bias Back contact p n+n+n+n+ p n+n+n+n+  R quenching h p + silicon wafer Front contact SiPM (proposed by Sadygov and Golovin in the ’90) –matrix of tiny microcells in parallel / each micro-cell = Geiger Mode-APD + R quench –output signal is proportional to the number of triggered microcells SiPM: characteristics PROS: Insensitive to magnetic field High gain (10 6 ) Good intrinsic timing resolution (30 ps/pixel) Good single photoelectron resolution CONS: Very small active surface (1mm 2 ) → small amount of light collected Noise Cost (many SiPMs needed) Solution considered at Orsay/LAL is a matrix of SiPMs: larger area covered reduce noise by requiring time coincidence of several SiPMs within the matrix

14. 9/15/08 - S. Niccolai – IPNO Tests on photodetectors with cosmic rays at Orsay Scintillator bar (NE102) 80cm (length) x 4 cm (width) x 3 cm (thickness) “Trigger” PMTs (Photonis XP2020) “Trigger” scintillators (BC408) 1cm thick Drawing by A. Maroni

15. 9/15/08 - S. Niccolai – IPNO Photonis XP20D0 The current setup Plan: Measuring TOF resolution with reference PMTs (ongoing) Substituting PMT at one end with SiPMs (matrix of SiPMs to be constructed, collaboration with neighbors at LAL for electronics)

16. Conclusions and to-do list Detection of DVCS recoil neutrons with ~10% of efficiency and n/  separation for p < 1 GeV/c seems possible from simulations, using scintillator as detector material A spaghetti calorimeter could give 50% higher neutron efficiency, but it increases photon background a lot TOF measurements with SiPMs (or whichever will be the chosen photodetector) needed for for both detector designs To do list: use complete CLAS12 simulation and realistic event generators for signal (nDVCS) and backgrounds (ed→en  0 (p)) to define needed resolutions (  ) evaluate count rate for signal (n) and background (  ), to understand if high photon detection efficiency is a problem or not CTOF and Neutron detector could coexist in one detector, whose first layer can be used as TOF for charged particles when there’s a track in the central tracker, while the full system can be used as neutron detector when there are no tracks in the tracker. Neutron detection is necessary for the measurement of nDVCS (Ji’s sum rule) Hardware: tests on timing with SiPM planned for the spring at Orsay, tests with ordinary PMTs already underway

17. 9/15/08 - S. Niccolai – IPNO