MEASUREMENT OF FAST NEUTRON BACKGROUND IN SAGE

J.N. Abdurashitov, V.N. Gavrin, A.V. Kalikhov, V.L. Matushko,A.A. Shikhin, V.E. Yants and O.S. Zaborskaia

Institute for Nuclear Research, Russian Academy of Sciences, Moscow, Russia

International Workshop Topics in Astroparticle & Underground

Physics

8 – 12 September 2001LN Gran Sasso (L’Aquila), Italy

CONTENTS

• High sensitive spectrometer – brief description• Main performance data• Operation principle• Design• Data acquisition system• Calibration of the spectrometer• Base properties of the spectrometer• The results of fast neutrons flux measurements

in SAGE facilities

Main performance data

• Energy range: 1-15 MeV

• Sensitivity: 10-610-7 n·cm-2·s-1

• Detection efficiency: 0.110.01 (En>1 MeV)

• Energy resolution: ~60%

• Scintillator volume: 30 l

• Sizes: 3636 cm3

• Masse: 50 kg.

Detector structure

•FEATURES:•Liquid scintillator (V=30l):

•CnH2n, =0.84 g/cm3

•L.Y.=40% of anthracene•Counters (19):

•Mixture: 3He+4%Ar•Pressure: 400 kPa

•Geometrical cross section:•6267,5 cm2

Operation principle

• Calorimeter• Combined detector:

– Organic scintillator-thermalizer

– 3He proportional counters

• Delay coincidence technique

• Pulse shape record

Light Yield for NE-213 scintillator

Dependences for:

1. Electrons

2. Single proton

3. Neutron (effective Light Yield)

The main problem:

nonlinear light yield

rough resolution

42.1i

piEA

General view of the detector

Spectrometer of fast neutrons

Spectrometer of fast neutrons

Typical passive shield(one half of lead brick thickness)

Spectrometer of fast neutrons

Data acquisition system

Data acquisition system

Typical correlated event

Calibration of the PMT channel(60Co source, -lines of 1.17&1.33 MeV)

Calibration of the NC channel(Pu-Be neutron source, 2000 ns-1)

3He + n p + t + 760 keV, Ep=570 keV, Et=190 keV

Time delay distribution for neutron events (Pu-Be source)

Time delay distribution for background events (H2O+BPE shield)

Dependence detector response function on neutron energy (MC simulation)

Response function of the detector (experimental)

14 MeV neutrons source:

D + t + n + 17.6 MeV

• Peak – 87 ch. (5.8 MeV of electron scale)

• Threshold – 4 ch

• Left – scattered neutrons

• Right - saturations

Efficiency dependence on neutron energy(MC simulation)

E= tot(En)=thr(En)(1-out(En))

Dependence between electron and neutron energy scales

Fast neutrons background flux measurements

ConditionsR, s-1

RNC, h-1 RNW, h-1 RTot, h-1 RRand, h-1

RCor, h-1 RCor/E h-1

H2O 2196.4 0.8

29.5 0.5

1.25 0.4

0.070.001

1.25 0.40

11.36 3.78

Mine Rock1400.4

106.4 0.3

46.6 0.2

2.96 0.13

0.29 0.17

1.42 0.45

12.91 4.29

SAGE512 4

74.2 0.2

25.8 0.1

1.93 0.12

1.11 0.17

-0.43 0.45

-3.91 4.11

RTot=RN+RRand+RBkg, RCor=RTot-RRand=RN+RBkg, Rrand=rrwnT

Fast neutrons amplitude

distributions –

mine rock (electron scale)

Fast neutrons amplitude

distributions –

SAGE main room

(electron scale)

THE RESULTS OF FAST NEUTRON BACKGROUND FLUX MEASUREMENT AT SAGE

Neutron flux, 10-7cm-2s-1

(1.0–11.0 Energy range, for E=0.110.01)

Internal background of the detector (H2O + Borated Polyethylene shield)

6.52.1

Surrounding mine rock (at 4800 m.w.e.)

7.32.4

SAGE main room 2.3

CONCLUSIONS

• The fast neutron spectrometer created with:• high efficiency 11%, 1-11 MeV;

• low internal background high sensitivity 10-7 cm-2·s-1

• n/ discrimination (/n107) without any special technique such as PSD

• Pulse Shape registration.

• Measurements:• n-background for SAGE

• internal background of the detector.

• Possible improvements:• new fast electronics (PMT)

• new NC

• PSD

• new spectrometer (sectioned)!