Speculative first look at neutron detection by n

Speculative first look at neutron detection by (n, p) charge exchange in the central detector Dan Watts – University of Edinburgh

Neutron detector/polarimeter: CB at MAMI p(g, p 0)p Eg=650 Me. V MAID Cx SAID MAID Cx Scatter point (and therefore qn) extrapolated from MWPC track. D. Watts et. al. , Chin. Phys. C 33: 1183 (2009) SAID p(g, h)p q. CM=120 15 Photon energy (Me. V)

(n, p) in CLAS 12 ? ? Central detector - excellent proton detection capabilities (micromegas/SVT) Convert a fraction of neutron flux to protons - utilise existing detectors for neutron detection? Simple G 4 simulation to take first look: Convertor material Neutrons T=200 Me. V 100 k thrown

12 C conversion prob. ~2. 3% with 2 cm G 4 simulation: 100 k incident neutrons 12 C conversion prob. ~4% with 4 cm Proton energy (Me. V) 56 Fe conversion prob. 56 Fe conversion prob. ~3% per with 2 cm ~3% Proton energy (Me. V) Pb. WO 4 conversionprob. ~2. 2% 2 cm ~2. 2%per with 2 cm Proton energy (Me. V)

Convertor placement options - outside Convertor placement “options” Would need convertor and tracker in limited space → Not favourable!

Convertor placement options - inside Micromegas : ~4 cm thick 12 C before first MM cylinder (or replace 1 st cylinder? ) SVT : additional convertor material between detector planes? Large acceptance neutron/proton polarimeter for free?

Summary Convertor could be a feasible fall back option for neutron detection Potential to add neutron (and proton) polarimetry to the suite of possibilities with CLAS 12 Of course - many issues still to address. . !

¡ To hit polarimeter TN>100 Me. V in (p, p)N above the D Proton energy loss <10 Me. V for Tp>100 Me. V. ¡ Multiple scattering <1 o FWHM for Tp>100 Me. V ¡ 0. 37 radiation lengths conversion ~ 30% Tp after graphite Energy loss Tp incident proton (Me. V) FWHM scattering angle (deg) ¡ Tp exit proton (Me. V) Detrimental side-effects of scatterer material Coulomb scattering Proton energy (Me. V)

Convertor placement “options” - inside In micromegas array - replace inner cylinder with ~4 cm cylinder of graphite? Additional convertor material between Si detectors (~4 cm gap)? Large acceptance neutron/proton polarimeter for free?

Convertor option for neutron detector/polarimeter CTOF CND Central Tracker

The neutron counter for the Central Detector of CLAS 12 Workshop, Genova, 2/27/08 S. Niccolai, IPN Orsay

The neutron counter for the Central Detector of CLAS 12 • GPDs and n. DVCS • Neutron kinematics for n. DVCS • Central Neutron Detector for CLAS 12 • Simulations: expected performances of CND • Ongoing and planned R&D: Si. PM, APDs, MCP-PMTs INFN Frascati, INFN Genova, IPN Orsay, LPSC Grenoble, SPh. N Saclay, University of Glasgow CLAS 12 Workshop, Genova, 2/27/08 S. Niccolai, IPN Orsay

SVT BST

JJ-Slice

BST Support / Cooling Fixture Downstream Side Upstream Side Internal Cooling Channel

Deeply Virtual Compton Scattering and GPDs • Q 2= - (e-e’)2 • x. B = Q 2/2 M =Ee-Ee’ e’ e g t • x+ξ, x-ξ longitudinal momentum fractions • t = (p-p’)2 • x x. B/(2 -x. B) g. L*(Q 2) x+ξ x-ξ ~ ~ H, H, E, E (x, ξ, t) p’ p conserve nucleon helicity Vector: H (x, ξ, t) Tensor: E (x, ξ, t) « Handbag » factorization valid in the Bjorken regime: high Q 2 , (fixed x. B), t<<Q 2 flip nucleon helicity H(x, 0, 0) = q(x) ~ H(x, 0, 0) = Δq(x) Quark angular momentum (Ji’s sum rule) 1 [ 1 1 J = - JG = ò xdx H q ( x, x, 0) + E q ( x, x, 0) 2 2 -1 q X. Ji, Phy. Rev. Lett. 78, 610(1997) ~ Axial-Vector: H (x, ξ, t) ~ Pseudoscalar: E (x, ξ, t) ] « 3 D» quark/gluon image of the nucleon

Extracting GPDs from DVCS spin observables s+ s- Ds A = s+ + s- = 2 s x= x. B/(2 -x. B) k=-t/4 M 2 Polarized beam, unpolarized proton target: ~ Ds. LU ~ sinf Im{F 1 H + x(F 1+F 2)H +k. F 2 E}df Kinematically suppressed Unpolarized beam, longitudinal proton target: ~ Ds. UL ~ sinf. Im{F 1 H+x(F 1+F 2)(H + … }df Unpolarized beam, transverse proton target: Ds. UT ~ sinf. Im{k(F 2 H – F 1 E) + …. . }df Polarized beam, unpolarized neutron target: ~ ~ sinf Im{F H + x(F +F )H - k. F E}df Ds. LU 1 g 1 2 2 Suppressed because F 1(t) is small Suppressed because of cancellation between PPD’s of u and d quarks Hp(ξ, ξ, t) = 4/9 Hu(ξ, ξ, t) + 1/9 Hd(ξ, ξ, t) Hn(ξ, ξ, t) = 1/9 Hu(ξ, ξ, t) + 4/9 Hd(ξ, ξ, t) f e’ e ~ leptonic plane hadronic plane p’ Hp, Ep ~ H p, H p Hp, Ep ~ Hn, En n. DVCS gives access to E, the least known and least constrained GPD that appears in Ji’s sum rule

Beam-spin asymmetry for DVCS: sensitivity to Ju, d DVCS on the proton Ju=. 3, Jd=. 1 Ju=. 8, Jd=. 1 Ju=. 5, Jd=. 1 Ju=. 3, Jd=. 8 Ju=. 3, Jd=-. 5 f= 60° x. B = 0. 2 Q 2 = 2 Ge. V 2 t = -0. 2 Ge. V 2 Ee = 11 Ge. V VGG Model (calculations by M. Guidal)

Beam-spin asymmetry for DVCS: sensitivity to Ju, d DVCS on the neutron Ju=. 3, Jd=. 1 Ju=. 8, Jd=. 1 Ju=. 5, Jd=. 1 Ju=. 3, Jd=. 8 Ju=. 3, Jd=-. 5 The asymmetry for n. DVCS is: • very sensitive to Ju, Jd • can be as big as for the proton depending on the kinematics and on Ju, Jd → wide coverage needed Ee = 11 Ge. V f= 60° x. B = 0. 17 Q 2 = 2 Ge. V 2 t = -0. 4 Ge. V 2 VGG Model (calculations by M. Guidal)

First measurement of n. DVCS: Hall A M. Mazouz et al. , PRL 99 (2007) 242501 Ee= 5. 75 Ge. V/c Pe = 75 % L = 4 · 1037 cm-2 · s-1/nucleon e’ HRS e LH 2 / LD 2 target Electromagnetic Calorimeter (Pb. F 2) Active nucleon identified via missing mass Q 2 = 1. 9 Ge. V 2 x. B = 0. 36 0. 1 Ge. V 2 < -t < 0. 5 Ge. V 2 Analysis done in the impulse approximation: Subtraction of quasi-elastic proton contribution deduced from H 2 data convoluted with initial motion of the nucleon Twist-2

n. DVCS in Hall A: results M. Mazouz et al. , PRL 99 (2007) 242501 F. Cano, B. Pire, Eur. Phys. J. A 19 (2004) 423 Q 2 = 1. 9 Ge. V 2 - x. B = 0. 36 Model dependent extraction of Ju and Jd S. Ahmad et al. , PR D 75 (2007) 094003 VGG, PR D 60 (1999) 094017 Im(CIn) compatible with zero (→ too high x. B? ) Strong correlation between Im[CId] and Im[CIn] Big statistical and systematic uncertainties (mostly coming from H 2 and p 0 subtraction)

n. DVCS with CLAS 12: kinematics DVCS/Bethe-Heitler event generator with Fermi motion, Ee = 11 Ge. V (Grenoble) <pn>~ 0. 4 Ge. V/c Physics and CLAS 12 acceptance cuts applied: W > 2 Ge. V 2, Q 2 >1 Ge. V 2, –t < 1. 2 Ge. V 2 5° < qe < 40°, 5° < qg < 40° More than 80% of the neutrons have q>40° → Neutron detector in the CD is needed! Detected in forward CLAS Not detected ed→e’n (p) CD PID (n or g? ) + angles to identify the final state Detected in FEC, IC pμe + pμn + pμp = pμe′ + pμn′ + pμp′ + pμg In the hypothesis of absence of FSI: pμp = pμp’ → kinematics are complete detecting e’, n (p, q, f), g FSI effects can be estimated measuring eng, epg, edg on deuteron in CLAS 12 (same experiment)

CND: constraints & design CTOF Central Tracker CND Detector design under study: scintillator barrel • 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 (Si. PMs or Micro-channel plate PMTs) Ø CTOF can also be used for neutron detection Ø Central Tracker can work as a veto for charged particles MC simulations underway for: Ø efficiency Ø PID Ø angular resolutions Ø reconstruction algorithms Ø background studies

Simulation of the CND Geometry: • Simulation done with Gemc (GEANT 4) • 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 y x z Reconstruction: Ø Good hit: first with Edep > threshold Ø TOF = (t 1+t 2)/2, with t 2(1) = tof. GEANT+ tsmear+ (l/2 ± z)/veff tsmear = Gaussian with s= s 0/√Edep (Me. V) s 0 = 200 ps·Me. V ½ (~2 times worse than what obtained from KNU’s TOF measurement) Ø β = L/T·c, L = √h 2+z 2 , h = distance between vertex and hit position, assuming it at mid-layer Ø θ = acos (z/L), z = ½ veff (t 1 -t 2) Ø Birks effect not included (should be added in Gemc) Ø Cut on TOF>5 ns to remove events produced in the magnet and rescattering back in the CND

CND: efficiency, PID, resolution pn= 0. 1 - 1. 0 Ge. V/c q = 50°-90°, f = 0° Efficiency: Nrec/Ngen Nrec= # events with Edep>Ethr. Layer 1 Efficiency ~ 10 -16% for a threshold of 5 Me. V and pn = 0. 2 - 1 Ge. V/c Layer 2 “Spectator” cut Layer 3 Layer 4 Dp/p ~ 5% Dq ~ 1. 5° b distributions (for each layer) for: • neutrons with pn = 0. 4 Ge. V/c • neutrons with pn = 0. 6 Ge. V/c n/g misidentification • neutrons with pn = 1 Ge. V/c for pn ≥ 1 Ge. V/c • photons with E = 1 Ge. V/c (assuming equal yields for n and )

n. DVCS with CLAS 12 + CND: expected count rates N = ∆t ∆Q 2 ∆x ∆f L Time Racc Eeff <f (°)> σ(nb Ge. V 4) N 16 0. 01794 5354 • L = 1035 cm-2 s-1 42 0. 00627 1873 • Time = 80 days 74 0. 00276 824 • Racc= bin-by-bin acceptance 104 0. 00174 520 134 0. 00137 410 165 0. 00127 379 195 0. 00126 377 225 0. 00140 417 256 0. 00172 513 286 0. 00279 835 317 0. 00616 1838 347 0. 0182 5432 → DN = 1%- 5% • Eeff = 15% neutron detector efficiency (CND+CTOF+FD) <t> ≈ -0. 4 Ge. V 2 <Q 2> ≈ 2 Ge. V 2 <x> ≈ 0. 17 Dt = 0. 2 Ge. V 2 DQ 2 =0. 55 Ge. V 2 Dx. B = 0. 05 Df = 30° Count rates computed with n. DVCS+BH event generator + CLAS 12 acceptance (LPSC Grenoble)

Electromagnetic background rates and spectra for the CND have been studied with Gemc (R. De Vita): • The background on the CND produced by the beam through electromagnetic interaction in the target consists of neutrals (most likely photons) • Total rate ~2 GHz at luminosity of 1035 cm-2·s-1 • Maximum rate on a single paddle ~ 22 MHz (1. 5 MHz for Edep>100 Ke. V) This background can be reconstructed as a neutron: with a 5 Me. V energy threshold the rate is ~ 3 KHz For these “fake” neutrons b<0. 1 -0. 2 → pn < 0. 2 Ge. V/c The actual contamination will depend on the hadronic rate in the forward part of CLAS 12 (at 1 KHz, the rate of fake events is 0. 4 Hz) b, for Edep>5 Me. V

Technical challenge: TOF resolution & B=5 T Si. PM - PROS: Si. PM • Insensitive to magnetic field • High gain (106) • Good intrinsic timing resolution (30 ps/pixel) • Good single photoelectron resolution Si. PM - CONS: • Very small active surface (1 -3 mm 2) → small amount of light collected (s. TOF~1/√Nphel) • Noise APD – PROS: • insensitive to magnetic field • bigger surface than Si. PM → more light collected APD – CONS: • low gain at room temperature • timing resolution? MCP-PMT – PROS: • resistant to magnetic field ~1 T • big surface • timing resolution ~ordinary PMT MCP-PMT – CONS: • behavior at 5 T not yet studied • high cost (10 K euros/PMT)

Tests on photodetectors with cosmic rays at Orsay “Trigger” PMTs (Photonis XP 2020) “Reference PMT” Photonis XP 20 D 0 Scintillator bar (BC 408) 80 cm x 4 cm x 3 cm “Trigger” scintillators (BC 408) 1 cm thick Plan: ü Measure TOF resolution with 2 standard PMTs ü Substitute PMT at one end with one Si. PM, one APD • Try with a matrix of Si. PMs • Redo the same measurements with extruded scintillator (FNAL) + WLS fiber (Kuraray) + Si. PM (Stepan’s idea, used in IC hodoscope, ~ x 5 more γ’s/mm 2) • Test of mchannel PMTs (collaboration with Glasgow)

Preliminary results from Orsay’s test bench Test Ref Trig σ2 test =1/2 (σ2 test, trig + σ2 test, ref − σ2 ref, trig − 4σ2 x/c 2 s) σ2 ref =1/2(σ2 test, ref + σ2 ref, trig − σ2 test, trig − 4σ2 x/c 2 s) σ2 trig =1/2(σ2 ref, trig + σ2 test, trig − σ2 test, ref + 2σ2 x/c 2 s) Single pe Double pe Next steps: • Complete measurement of 3× 3 mm 2 MPPC • Try 5× 5 mm 2 APDs • Extruded scintillator + WLS fibers + Si. PM • Matrix of Si. PM (cost? ) 2: test = 1 Si. PM Hamamatsu MPPC 1 x 1 mm • Glasgow: in-field tests (5 T) for MCP-PMT • s. TOF ~ 1. 8 ns (~consistent with expectation) • rise time ~ 1 ns • nphe ~1 test = PMT: • s. TOF < 90 ps • nphe ~1600 Thanks to. T. Nguyen Trung, B. Genolini and J. Pouthas (IPN Orsay) test = 1 Si. PM Hamamatsu MPPC 3 x 3 mm 2: • rise time ~5 ns (increased capacitance) • more noise than 1 x 1 mm 2, work in progress to get s. TOF… test = 1 APD Hamamatsu 10 x 10 mm 2 + IC preamp: • s. TOF ~ 1. 4 ns • high noise, high rise time

Conclusions and outlook • n. DVCS is a key reaction for the GPD experimental program: measuring its beam-spin asymmetry can give access to E and therefore to the quark orbital angular momentum (via the Ji’s sum rule) • A large kinematical coverage is necessary to sample the phase-space, as the BSA is expected to vary strongly • The detection of the recoil neutron is very important to ensure exclusivity, reduce background and keep systematic uncertainties under control • The n. DVCS recoil neutrons are mostly going at large angles (qn>40°), therefore a Lo. I submitted to PAC 34, encouraged to submit full proposal neutron detector should be added to the Central Detector, using the (little) available space Are you interested in detecting neutrons at large angles and p<1 Ge. V/c? • CTOF and neutron detector could coexist in one detector, whose first layer can be used Are you interested in the photodetectors studies (useful for CTOF too)? as TOF for charged particles when there’s a track in the central tracker, while the full → You are more than welcome to join in! system can be used as neutron detector when there are no tracks in the tracker. • Using scintillator as detector material, detection of n. DVCS recoil neutrons with ~10 -15% of efficiency and n/g separation for p < 1 Ge. V/c seems possible from simulations, provided to have ~120 ps of TOF resolution, • The strong magnetic field of the CD and the limited space available call for magnetic-field insensitive and compact photodetectors: Si. PM are a good candidate, but their timing performances need to be tested • Tests on timing with Si. PM and APDs in cosmic rays are underway at Orsay • Ongoing tests for MCP-PMTs in magnetic field at Glasgow University