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

Scatter point (and therefore n) extrapolated from MWPC track.

MAID

SAID

p(0)pE=650 MeV

MAID

SAID

p()pCM=12015

Photon energy (MeV)

Cx

Cx

D. Watts et. al., Chin. Phys. C 33:1183 (2009)

(n,p) in CLAS12 ??

Central detector - excellent proton detection capabilities (micromegas/SVT)

Convert a fraction of neutron flux to protons - utilise existing detectors for neutron detection?

NeutronsT=200 MeV100k thrown

Simple G4 simulation to take first look:

Convertor material

12C conversion prob.~2.3% with 2cm

56Fe conversion prob.~3% per 2cm

PbWO4 conversion prob.~2.2% per 2cm

12C conversion prob.~4% with 4cm

Proton energy (MeV)

Proton energy (MeV) Proton energy (MeV)

G4 simulation:100k incidentneutrons

56Fe conversion prob.~3% with 2cm

PbWO4 conversion prob.~2.2% with 2cm

Convertor placement “options”

Would need convertor and tracker in limited space

→ Not favourable!

Convertor placement options - outside

Micromegas : ~4cm thick 12C before first MM cylinder (or replace 1st cylinder?)

SVT : additional convertor material between detector planes?

Large acceptance neutron/proton polarimeter for free?

Convertor placement options - inside

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 CLAS12

Of course - many issues still to address..!

Detrimental side-effects of scatterer material

To hit polarimeter TN>100 MeV in (p,)N

above the

Proton energy loss

<10 MeV for Tp>100 MeV.

Multiple scattering

<1o FWHM for Tp>100 MeV

0.37 radiation lengths conversion ~ 30%

Tp incident proton (MeV)

Tp e

xit

pro

ton

(M

eV

)

Tp after graphiteEnergy loss

0

0.5

1

1.5

2

2.5

3

0 200 400 600 800 1000 1200

Series1

Coulomb scattering

Proton energy (MeV)FW

HM

scatt

eri

ng

an

gle

(d

eg

)

In micromegas array - replace inner cylinder with ~4cm cylinder of graphite?

Additional convertor material between Si detectors (~4cm gap)?

Large acceptance neutron/proton polarimeter for free?

Convertor placement “options” - inside

CND

CTOF CentralTracker

Convertor option for neutron detector/polarimeter

S. Niccolai, IPN Orsay

The neutron counter for the Central Detector of CLAS12

CLAS12 Workshop, Genova, 2/27/08

INFN Frascati, INFN Genova,

IPN Orsay,LPSC Grenoble,

SPhN Saclay, University of Glasgow

• GPDs and nDVCS

• Neutron kinematics for nDVCS

• Central Neutron Detector for CLAS12

• Simulations: expected performances of CND

• Ongoing and planned R&D: SiPM, APDs, MCP-PMTs

S. Niccolai, IPN Orsay

The neutron counter for the Central Detector of CLAS12

CLAS12 Workshop, Genova, 2/27/08

SVT BST

JJ-Slice

BST Support / Cooling FixtureDownstream Side Upstream Side

Internal Cooling

Channel

Deeply Virtual Compton Scattering and GPDs

e’t

(Q2)

eL*

x+ξ x-ξ

H, H, E, E (x,ξ,t)~~

p p’

« Handbag » factorization validin the Bjorken regime:

high Q2 , (fixed xB), t<<Q2

• Q2= - (e-e’)2

• xB = Q2/2M=Ee-Ee’

• x+ξ, x-ξ longitudinal momentum fractions• t = (p-p’)2

• xB/(2-xB)

0,x ),( Ex q21 Hxdx qJG =

21J q

1

1)0 ,, (

Quark angular momentum (Ji’s sum rule)

X. Ji, Phy.Rev.Lett.78,610(1997)

Vector: H (x,ξ,t)

Tensor: E (x,ξ,t)

Axial-Vector: H (x,ξ,t)

Pseudoscalar: E (x,ξ,t)

~

~

conserve nucleon helicity

flip nucleon helicity

«3D» quark/gluonimage of

the nucleon

H(x,0,0) = q(x)

H(x,0,0) = Δq(x) ~

Extracting GPDs from DVCS spin observables

LU ~ sin Im{F1H + (F1+F2)H +kF2E}d~

Polarized beam, unpolarized proton target:

Unpolarized beam, longitudinal proton target:

UL ~ sinIm{F1H+(F1+F2)(H + … }d~

= xB/(2-xB)k=-t/4M2

Hn, Hn, En

Kinematically suppressed

Hp, Hp

~

A =

=

~

leptonic planehadronic

planep’

e’

e

LU ~ sin Im{F1H + (F1+F2)H - kF2E}d~Polarized beam, unpolarized neutron target:

Suppressed because F1(t) is small

Suppressed because of cancellation between PPD’s of u and d quarks

Hp, Hp, Ep

~

nDVCS gives access to E, the least known and

least constrained GPD that appears in Ji’s sum ruleHp(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)

Unpolarized beam, transverse proton target:

UT ~ sinIm{k(F2H – F1E) + ….. }d Hp, Ep

Ju=.3, Jd=.1

Ju=.8, Jd=.1

Ju=.5, Jd=.1

= 60°xB = 0.2Q2 = 2 GeV2

t = -0.2 GeV2

Beam-spin asymmetry for DVCS: sensitivity to Ju,d

VGG Model(calculations by M. Guidal)

DVCS on the proton

Ju=.3, Jd=.8

Ju=.3, Jd=-.5

Ee = 11 GeV

= 60°xB = 0.17Q2 = 2 GeV2

t = -0.4 GeV2

Beam-spin asymmetry for DVCS: sensitivity to Ju,d

The asymmetry for nDVCS is:• very sensitive to Ju, Jd • can be as big as for the protondepending on the kinematics and on Ju, Jd

→ wide coverage needed

VGG Model(calculations by M. Guidal)

DVCS on the neutron Ju=.3, Jd=.1

Ju=.8, Jd=.1

Ju=.5, Jd=.1

Ju=.3, Jd=.8

Ju=.3, Jd=-.5

Ee = 11 GeV

First measurement of nDVCS: Hall A

Ee= 5.75 GeV/c Pe = 75 %L = 4 ·1037 cm-2 · s-1/nucleon

Q2 = 1.9 GeV2

xB = 0.360.1 GeV2 < -t < 0.5 GeV2

HRS

Electromagnetic Calorimeter (PbF2)

LH2 / LD2 target

e’

e

deedneenpeepXeeD ),(),(),(),(

Subtraction of quasi-elastic proton contribution deduced from H2 data convoluted with initial motion of the nucleon

Analysis done in the impulse approximation:Active nucleon identified

via missing mass

Twist-2

M. Mazouz et al., PRL 99 (2007) 242501

nDVCS in Hall A: results

S. Ahmad et al., PR D75 (2007) 094003

VGG, PR D60 (1999) 094017

M. Mazouz et al., PRL 99 (2007) 242501

Q2 = 1.9 GeV2 - xB = 0.36

Im(CIn) compatible with zero (→ too high xB?)

Strong correlation between Im[CId] and Im[CI

n]Big statistical and systematic uncertainties

(mostly coming from H2 and 0 subtraction)

Model dependentextraction of Ju and Jd

F. Cano, B. Pire, Eur. Phys. J. A19 (2004) 423

nDVCS with CLAS12: kinematics

More than 80% of the neutrons have >40°→ Neutron detector in the CD is needed!

DVCS/Bethe-Heitler event generatorwith Fermi motion, Ee = 11 GeV (Grenoble)

Physics and CLAS12 acceptance cuts applied:

W > 2 GeV2, Q2 >1 GeV2, –t < 1.2 GeV2

5° < e < 40°, 5° < < 40°

<pn>~ 0.4 GeV/c

ed→e’n(p)

Detected in forward CLAS

Detected inFEC, IC

Not detected

PID (n or ?) + angles to identify the final state

CD

In the hypothesis of absence of FSI:pμ

p = pμp’ → kinematics are complete

detecting e’, n (p,,),

pμe + pμ

n + pμp = pμ

e′ + pμn′ + pμ

p′ + pμ

FSI effects can be estimated measuringen, ep, edon deuteron in CLAS12(same experiment)

• 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 Micro-channel plate PMTs)

CTOF can also be used for neutron detection Central Tracker can work as a veto for charged particles

CND

CTOF CentralTracker

CND: constraints & design

Detector design under study:scintillator barrel

MC simulations underway for: efficiency PID angular resolutions reconstruction algorithms background studies

Simulation of the CNDGeometry:• Simulation done with Gemc (GEANT4)• 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 Edep > threshold

TOF = (t1+t2)/2, with

t2(1) = tofGEANT+ tsmear+ (l/2 ± z)/veff

tsmear = Gaussian with = 0/√Edep (MeV)

0 = 200 ps·MeV ½ (~2 times worse than

what obtained from KNU’s TOF measurement) β = L/T·c, L = √h2+z2 , h = distance betweenvertex and hit position, assuming it at mid-layer θ = acos (z/L), z = ½ veff (t1-t2) Birks effect not included (should be added in Gemc) Cut on TOF>5ns to remove events produced in the magnet and rescattering back in the CND

z

y

x

CND: efficiency, PID, resolution

pn= 0.1 - 1.0 GeV/c= 50°-90°, = 0°

Efficiency: Nrec/Ngen

Nrec= # events with Edep>Ethr.

Efficiency ~ 10-16% for a threshold of 5 MeVand pn = 0.2 - 1 GeV/c

Layer 1 Layer 2

Layer 3 Layer 4

distributions (for each layer) for:• neutrons with pn = 0.4 GeV/c• neutrons with pn = 0.6 GeV/c• neutrons with pn = 1 GeV/c• photons with E = 1 GeV/c (assuming equal yields for n and )

n/ misidentificationfor pn ≥ 1 GeV/c

“Spectator” cut

p/p ~ 5%~ 1.5°

nDVCS with CLAS12 + CND: expected count rates

< (°)> σ(nb GeV 4) N

16 0.01794 5354

42 0.00627 1873

74 0.00276 824

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

t = 0.2 GeV2 Q2 =0.55 GeV2

xB = 0.05 = 30°

• L = 1035cm-2s-1

• Time = 80 days

• Racc= bin-by-bin acceptance

• Eeff = 15% neutron detector efficiency (CND+CTOF+FD)

N = ∆t ∆Q2 ∆x ∆ L Time Racc Eeff

Count rates computed with nDVCS+BHevent generator + CLAS12 acceptance

(LPSC Grenoble)

<t> ≈ -0.4 GeV2

<Q2> ≈ 2GeV2

<x> ≈ 0.17

→ N = 1%- 5%

Electromagnetic background

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>100KeV)

This background can be reconstructed as a neutron:with a 5 MeV energy threshold the rate is ~ 3 KHzFor these “fake” neutrons <0.1-0.2 → pn < 0.2 GeV/c

The actual contamination will depend on the hadronic rate in the forward part of CLAS12 (at 1 KHz, the rate of fake events is 0.4 Hz)

, for Edep>5 MeV

Technical challenge: TOF resolution & B=5T

SiPM - PROS:

• Insensitive to magnetic field• High gain (106)• Good intrinsic timing resolution (30 ps/pixel)• Good single photoelectron resolution

SiPM - CONS:

• Very small active surface (1-3 mm2)

→ small amount of light collected (TOF~1/√Nphel)

• Noise

SiPM

APD – PROS:

• insensitive to magnetic field• bigger surface than SiPM → more light collected

APD – CONS:

• low gain at room temperature• timing resolution?

MCP-PMT – PROS:

• resistant to magnetic field ~1T• big surface• timing resolution ~ordinary PMT

MCP-PMT – CONS:

• behavior at 5T not yet studied• high cost (10K euros/PMT)

MCP-PMT

Plan: Measure TOF resolution with 2 standard PMTs

Substitute PMT at one end with one SiPM, one APD• Try with a matrix of SiPMs

• Redo the same measurements with extruded scintillator (FNAL) + WLS fiber (Kuraray) + SiPM (Stepan’s idea, used in IC hodoscope, ~ x5 more γ’s/mm2)

• Test of channel PMTs (collaboration with Glasgow)

Tests on photodetectors with cosmic rays at Orsay

“Trigger” PMTs (Photonis XP2020)

Scintillator bar (BC408)80cm x 4 cm x 3 cm

“Trigger” scintillators(BC408) 1cm thick

“Reference PMT”Photonis XP20D0

Preliminary results from Orsay’s test bench

Single peDouble pe

σ2test =1/2 (σ2

test,trig + σ2test,ref − σ2

ref,trig − 4σ2x/c2

s) σ2

ref =1/2(σ2test,ref + σ2

ref,trig − σ2test,trig − 4σ2

x/c2s)

σ2trig =1/2(σ2

ref,trig + σ2test,trig − σ2

test,ref + 2σ2x/c2

s)

TestRef

Trig

test = PMT:• TOF < 90 ps• nphe ~1600

test = 1 SiPM Hamamatsu MPPC 1x1 mm2:• TOF ~ 1.8 ns (~consistent with expectation)• rise time ~ 1 ns• nphe ~1

test = 1 SiPM Hamamatsu MPPC 3x3mm2:• rise time ~5 ns (increased capacitance)• more noise than 1x1 mm2, work in progress to get TOF…

Thanks toT. Nguyen Trung, B. Genoliniand J. Pouthas (IPN Orsay)

test = 1 APD Hamamatsu 10x10 mm2 + IC preamp:• TOF ~ 1.4 ns• high noise, high rise time

Next steps:• Complete measurement of 3×3 mm2 MPPC• Try 5×5 mm2 APDs • Extruded scintillator + WLS fibers + SiPM• Matrix of SiPM (cost?)• Glasgow: in-field tests (5T) for MCP-PMT

• Using scintillator as detector material, detection of nDVCS recoil neutrons with ~10-15% of efficiency and n/ separation for p < 1 GeV/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-fieldinsensitive and compact photodetectors: SiPM are a good candidate, but their timing performances need to be tested

• CTOF and neutron detector could coexist in one detector, whose first layer can be usedas TOF for charged particles when there’s a track in the central tracker, while the fullsystem can be used as neutron detector when there are no tracks in the tracker.

• Tests on timing with SiPM and APDs in cosmic rays are underway at Orsay• Ongoing tests for MCP-PMTs in magnetic field at Glasgow University

Conclusions and outlook• nDVCS 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 nDVCS recoil neutrons are mostly going at large angles (n>40°), therefore a neutron detector should be added to the Central Detector, using the (little) available space

LoI submitted to PAC34, encouraged to submit full proposal

Are you interested in detecting neutrons at large angles and p<1 GeV/c?

Are you interested in the photodetectors studies (useful for CTOF too)?

→ You are more than welcome to join in!