Status of SAMURAI Neutron Array(NEBULA)

Yosuke KondoTokyo Institute of Technology

Invariant-mass spectroscopy of neutron-rich nuclei

• Physics subjects– Coulomb breakup

• as a spectroscopic tool– Giant resonance

• soft dipole resonance• pigmy resonance

– Low-lying unbound states of neutron-rich nuclei near the dripline– Unbound exotic systems beyond the dripline

• 10He, 4n, 28O– etc…

np

n

corecp

coren

AZ*

target

n

AZcore

iiirel MpEE 22 AZ

A-1Z+nSn

Ex Erel

neutron decaythreshold

excitation

neutrondecay

AZ*

measureEx (Erel) can be reconstructed

neutron

charged fragment

Pb targetRI beamfrom BigRIPS

Typical setup (Coulomb breakup)

NEBULA

NEBULA(Neutron-detection system for Breakup of Unstable-Nuclei with Large

Acceptance)

• 240 Neutron counters (60 modules x 4 stacks)• 48 VETO counters

• Neutron counter– scintillator

• BC408• 12cm x 12cm x 180cm

– PMT• R7724ASSY (both ends)• vertical position is determined by time difference

• VETO counter– 1cm-thick scintillator– used to identify the charged particles

NEBULA(Neutron-detection system for Breakup of Unstable-Nuclei with Large Acceptance)

NEBULA(Neutron-detection system for Breakup of Unstable-Nuclei with Large

Acceptance)

• 240 Neutron counters (60 modules x 4 stacks)• 48 VETO counters

• High efficiency– ~70% for 1n detection

• Large acceptance– effective area : 3.6m (H) x 1.8m (V)-10o<qH<10o, -5o<qV<5o

• Multi-neutron detection– aiming at 4n detection

Multi-neutron detection(example:2n case)

• crosstalk … more than 2 hits caused by 1n– should be eliminated for multi-neutron detectionkinematical condition is used to reject the crosstalk

– Same wall event position information• 2 hits are clustered and are regarded as 1hit if positions

are close• lose efficiency for small Erel

– Different wall event velocity information• the event is true if b12 >b1

– because crosstalk neutron must be slow

• can measure up to Erel~0

Same Wall event

Stack-2Stack-1

12b1b

Stack-2Stack-1

Different Wall event

hit detector

neutron counter

before mounting the VETO counters

Photo

VETO counter(1cm thickness)

after mounting the VETO counters

4.2m

3.6m

not yet

Status• Half of neutron counters are

purchased– the other half are not funded– Efficiency ~40%

• Mounting – finished• It was very hard…

• Test 1 stack using cosmic ray – This summer

• we are now connecting many many cables …

Summary

• NEBULA is designed for neutron measurements in SAMURAI project– half of volume will be ready in 2011– Test will be done soon