Status of Projectile Spectator Detector

A.Ivashkin

INR , Moscow

I. Introduction to PSD. II. Motivations for PSD use in CBM

III. Design and readout. IV. Results of beam test of PSD prototype.

V. Summary.

Very Forward Hadron Calorimeter for

detection of projectile spectators

PSD in CBM setup.

PSD

Experimental tasks:

1. Measurement of centrality: b~ A - Nspect ;

2. Measurement of event-by-event fluctuations;

3. Reconstruction of the reaction plane;

4. Monitor of beam intensity by detecting the neutrons from electromagnetic dissociation .

Projectile Spectators are non-interacting nucleon in beam ions.

PSD geometry.

spectator spots at Z=15m Eau=15 AGeV

Beam hole

X

Z

Transverse sizes ~1x1 m2;

Distance from target - 15 m;

Number of modules – 107;

Module dimensions – 10x10x1600 cm2

1. Measurement of centrality

Impact parameter: b~Np ,

Np is number of interacting (participant) nucleons.

Np=A - Nspect=A - Es/EA ,

Es is sum of spectator energies, measured by Zero

Degree Calorimeter (ZDC) or by PSD;

EA is beam energy.

This technique is used in most heavy ion experiments at CERN (WA80, NA49, NA50, ALICE…) and RHIC.

Correlation between Ezdc and ET in NA50 experiment at CERN. (Pb+Pb at 158 AGeV)

A few methods of centrality measurement must be used to get the proper event selection

2. Event-by-event fluctuations

Total multiplicity : Ns- number of sources,

mi- multiplicity from a single source.

i

Ns

i

mN

1

s

NmN

Nm

mNs

222

mNN sGeometry of

collision

Second component is not interesting and must

be removed

physics! QGP?

Number of interacting nucleons must be known

3. Reconstruction of Reaction Plane

Input UrQMD: reaction plane at 00

MC simulation

Reconstruction from centers of modules

Reconstructed angle of reaction plane, deg.

Good accuracy is due to fine transverse PSD granulation. To be improved by taking deposited energy weights.

→ → M rk rk – position vector

Q = ∑ ----- , of the particle k

k=1 → in perpendicular

│rk│ to the beam axis

plane

M – particles in the event used for reconstruction

Design and readout

ConceptionLight readout with WLS-fibers for reliable and uniform light collection.

Signal readout with Micropixel APD (MAPD) to avoid nuclear counter

effect, detection of a few photons signal, compactness, low cost. Longitudinal segmentation – for permanent calibration of scintillators in radiation hard conditions, rejection of secondary particles.

Modular design – transverse uniformity of resolution, good reconstruction of reaction plane, flexible geometry, simplicity.

Modular Lead/Scintillator sandwich compensating calorimeter. Sampling ratio Pb:Scint=4:1.

Expectation: For thickness δPb=16 mm and δScint=4 mm σE/E ~ 50%/√E .

Structure of PSD module.

• 6 WLS-fiber/MAPD.

• 10 MAPDs/module.

• 10 Amplifiers with gain~40.

• 60 lead/scintillator sandwiches.

• 10 longitudinal sections.

MAPDs and amplifiers.

Photodetectors in PSD are MAPDs,Micropixel Avalanche Photodiodes

working in Geiger mode

Active area: - 3x3 mm2. Number of pixels: - 15000/mm2. Photon detection efficiency: ~ 20%. Gain: ~ 5x104. Single electron noise: ~ 1 MHz/mm2. Working

voltage: ~ 66.5 V. Dark current: ~ 50 nA

- deep micro-well type; -avalanche cells done by ion implantation; - number of pixels might achieve ~105/mm2.

Initially developed by Z. Sadygov at Dubna.

Now produced by Zecotek.com at Singapore.

Properties of MAPD3-A type used

in PSD prototype

Linearity of MAPD response.

)1( totalN

PDEphotonsN

eNN totalfired

pixels

Dependence of signal width (σ2) on signal amplitude Nph.el. (both

in photoelectrons).

analytical formula

Pixel density is crucial for the calorimetry!

(σ2 ~ Nph.el. in linear case )

1.5 month long-term stability test. Irradiation with LED pulses:

1 MHz, 104 ph.e./pulse

10 MAPDs in test

Perspectives of MAPDs(general comments)

• MAPD is promising photodetector for large highly segmented detectors with number of readout channels > 103.

• The cost of one piece < $100.

• Current Photon Detection Efficiency (PDE) of 20% might be improved a few times.

• Larger size – 5x5 mm2 are expected soon.

• Fast MAPDs with ~1-2 ns rise time are expected soon.

• MAPD application for very granular TOF systems is promising.

• At present TOF~100 ps is achievable for 20x20x20 mm3 plastic scintillator. Might be significantly improved for fast samples.

Radiation doses for PSD.

beam

~2 times scintillator light yield drop. No changes in uniformity and resolution.

Neutron flux affects MAPDs. Recent tests at PSI: at flux ~1013 n/cm2 MAPD gain dropped ~2

times, dark current ~10 A. But it works.

Must be compared with Fluka simulation.

UrQMD+CBMRoot:

~5 MRad/(12 months of CBM run)

Magnet ON. Proton and neutron spots are separated.

In 2007 first nine PSD modules were assembled at INR for the beam test at CERN.

assembling

readout part

transportation

at beam

Calibration of modules by 75 GeV muon beam (response of each section in module)

Deposited spectra of pion energies in

PSD prototype: simulation and

experiment

l.y~2ph.e/MeV. Nice MAPD sensitivity to a few photons signal.

Performance of calorimeter.

Energy spectrum in first section of module for mixed 30 GeV beam of pions, positrons and muons.

PID and rejection of secondary particles are possible.

muons

hadrons positrons

1

1

1

12 )(1)()()(

E

E

NNE

EN

E

E

E

E

N

ii

N

N

• The constant term in energy resolution is essential only for energy measurement of single particle.

• It is not very important in case of measurement of total energy from many particles with the same energy.

• The reason is dependence of energy resolution for N spectators as:

Summary.• R&D on PSD calorimeter is going on.

• The structure of PSD modules is well established now.

• The readout of the modules is done by novel photodetectors, MAPDs.

• These photodetectors are intensively developed as a good alternative of traditional PMTs.

• Test of first PSD prototype confirmed the expected performance.

• The energy resolution for hadrons ~54%/sqrt(E) was obtained. Depending on energy deposition in longitudinal sections the electrons, muons and hadron identification is possible.

• The test of larger size prototype of 5x5 modules is planed.

• Further R&D on PSD readout will be continued.

Thank you!