Central Neutron Central Neutron Detector Detector

March ’11 UpdateMarch ’11 Update

Daria SokhanDaria Sokhan

IPN OrsayIPN Orsay

CLAS 12 GeV Workshop Paris, France – 9th March 2011

Neutron DVCS Neutron DVCS (see Silvia Niccolai’s talk)(see Silvia Niccolai’s talk)

Generalised Parton Distributions (GPDs) provide correlation between longitudinal momentum and transverse position of partons inside a nucleon and can give access to the contribution of orbital momentum of quarks to nucleon spin.

Four GPDs accessible in DVCS at large Q2:

,x E, ,H H E and

which are functions of and .t

Neutron DVCS: needed for flavour separation gives access to GPD E through the amplitude of the beam-spin asymmetry.

GPD E is currently least well known.

Effective way of studying GPDs is through Deep Virtual Compton Scattering (DVCS):

x longitudinal momentum transfer

Important as it features in Ji’s Sum Rule, which relates E and H to the total angular momentum carried by each quark.

t

(Q2)

eL*

x+ξ x-ξ

H, H, E, E (x,ξ,t)~~

p p’

e’

2

'2 ( )Be e

Qx

M E E

2 2( ')t p p

2B

B

x

x

2 2( ')Q e e

~

For exclusive reconstruction of the DVCS process

require detection and measurement of all three final state particles.

The recoil neutronsThe recoil neutrons Proposed experimental programme on neutron DVCS is complementary to proton-target experiments at JLab aimed at accessing , , ,H H E E

' 'en e n

Scattered electron and photon are typically produced at low forward angles (into forward detector of CLAS 12).

Over 80% of neutrons recoil at θlab > 40° with peak momentum at ~ 0.4 GeV/c.

Requires central neutron detector sensitive to 0.2 < pn < 1.2 GeV/c.

← Simulation at Ee= 11 GeV (Baptiste Guegan, Orsay).

Neutron Detector in CLAS 12Neutron Detector in CLAS 12

Available:

10 cm of radial space

in a high magnetic field (~ 5 T)

between the CTOF and the solenoid magnet.

Detector proposal approved @ PAC 37:

Barrel geometry

Plastic scintillator bars

Trapezoid cross-section

Long light-guides

PMT read-out upstream

CND

CTOF CentralTracker

z

y

x

The Central Neutron DetectorThe Central Neutron Detector

3 layers

48 paddles (azimuthal segments)

Inner radius 28.5 cm, outer 38.1 cm

Length ~ 70 cm

The U-Turn GeometryThe U-Turn Geometry

Each couple of scintillator paddles connected at the downstream end with a semi-circular light-guide

At the upstream end, curved light-guides take the signal from each paddle to PMTs out of the magnetic field

Three-layer assembly, all PMTs on upstream end

Tests in the LabTests in the Lab

Tests @ OrsayTests @ Orsay (Giulia Hull)

Measurements with cosmic rays, two short scintillator segments, above and below the test paddle, used for the trigger.

Last year:

Different “U-turn” light-guide geometries:

Semicircular provides ~ 10% better time resolution!

Reminder…Reminder…

Triangular

Semicircular

Different wrapping materials:

Al foil chosen based on charge-collection and timing tests, cost and ease of mechanical wrapping.

Mylar

Aluminium foil

VM 2000

Current test set-upCurrent test set-up

Scintillator BC408 (70 cm long) coupled to two R2083 PMTs by means of 150 cm long light guides, wrapping in Al foil, semicircular light guide at the “U turn”

Which PMTs?Which PMTs?

Previous tests with PMT R2083. A new PMT made by Hamamatsu, R9779, has recently become available at ~ 1/3 of the cost of R2083.

Timing resolution of new R9779 ~ 10% worse than old R2083.

Acceptable!

PMT-N

PMT-D

NEW

SimulationsSimulations

SimulationsSimulations

PMT-N

PMT-D

Performance of CND simulated using GEMC.

Energy deposited by the neutron at each step propagated to the two PMTs, smeared, integrated and converted into ADC and TDC channels.

Thresholds applied to mimic ADC / TDC response.

Reconstruct hits using TDC signals from coupled pairs of scintillator paddles.

Require: Reconstructed position within paddle length Minimum reconstructed energy of hit 2 MeV Maximum time threshold 8 ns

Contamination from mis-reconstructed events, after all time and energy cuts have been applied, is 1 - 3 %.

Neutron / Photon SeparationNeutron / Photon Separation

β β

β

Neutrons up to ~ 0.9 GeV/c can be well separated from photons on the basis of the measured β

Error bars on the β - axis represent 3 σ

θ = 60° in all plots

EfficiencyEfficiency

neutrons

Photon efficiency is around 9 – 12%

Neutron efficiency is mostly in the range 8 – 9.5 %, depending on momentum

photons, θ = 60°

Momentum ResolutionMomentum Resolution

Momentum resolution in the range 4 – 10 %

θ = 60°

θθ Resolution Resolution

θ-resolution in the range 2° – 3.5°

Pn = 0.4 GeV/c

φ-resolution determined by the azimuthal segmentation into 48 paddles, so 3.75°

MechanicsMechanics

CND – position within solenoidCND – position within solenoid

Julien Bettane, Orsay

Problem of SpaceProblem of Space

In the initial design, CND overlapped with CTOF! Solution: modify the magnet, extend CTOF.

Solenoid

CTOF

CND

Solenoid modifications – to be Solenoid modifications – to be confirmed!confirmed!

Solenoid modifications – to be Solenoid modifications – to be confirmed!confirmed!

Modifications:

Upstream magnet opening angle from 30° to 41°.

Reduce length of straight section (and therefore CND paddles) by a few cm.

Move cryogenic supply pipe to the top of the magnet.

Pending agreement from magnet construction team…

Back-up planBack-up plan

If the magnet cryogenic pipe cannot be moved, one section of the CND can be removed (1/24th of the total)

CND and CTOFCND and CTOF

CTOF in blue

CND in grey

CTOF paddles need to be longer to accommodate CND light-guides.

Pending agreement…

Construction PlanConstruction Plan Imminent: decision of the magnet construction team – within a number of weeks.

By the Summer:

Design of the mechanical support structure

Study of the magnetic field shielding around the PMTs

Development of bases with amplifiers for the PMTs

By early Autumn:

Completed detector segment of 3 layers of a coupled pair of bars each, with mechanical support, light-guides, PMTs and electronics – to be used in cosmic ray measurements:

Define electronic configuration for real experimental conditions

Compare with cosmic ray tests made with CTOF prototype