1 IDM2004 Edinburgh, 9 september 2004 Helenia Menghetti Bologna University and INFN Study of the muon-induced neutron background with the LVD detector

1 IDM2004Edinburgh, 9 september

2004

Helenia MenghettiBologna University and INFN

Study of the muon-induced Study of the muon-induced neutron background with neutron background with the LVD detector at LNGSthe LVD detector at LNGS


2004


Large Volume Detector

The Large Volume Detector (LVD) in the INFN Gran Sasso National Laboratory (4000 m.w.e.), Italy, consists of an array of 840 scintillator counters, 1.5 m3 each. These are interleaved by

streamer tubes, and arranged in a compact and modular way to maximize the livetime of the experiment .

The active scintillator mass is M=1000 t.

LVD

LNGS


2004


Schematic view: three tower with 260 counters each

1 32

Each counter is seen from 3 PMT

…The detail of the tracking system


2004


The detector is optimized for the observation of Supernova electron antineutrinos through the inverse beta decay:

e+ p n + e+

n + p D +

Positron spectrum

Inverse beta decay

Which originates in the liquid scintillator to 2 subsequent pulses:

the prompt one, due to the positron, and the second one due to the gamma of 2.2 MeV from the neutron capture, delayed of about

180s.


2004


External: more background…External: more background…

Internal: better shielded

Two different discrimination channel:1) High Energy Threshold operated at HET = 7 MeV for the

external counter (43%), and at HET = 4 MeV for the inner ones (57%) better shielded from rock radioactivity

2) All counters are equipped with an additional discrimination channel, set at a lower threshold, LET = 1 MeV, which is active for 1 ms after the HET pulse, for the detection


2004


Neutrons in liquid scintillator have the same signature of the inverse beta decay, therefore LVD is very sensitive to their detection.

Their interactions on proton originate: a prompt signal due to the proton recoil a 2.2 MeV gamma from the neutron capture delayed with respect to the prompt one of about 180 s

Neutron candidates in LVD are then selected as high energy threshold events followed by almost a low energy threshold event within 1 ms in the same counter. Taking into account the energy transfer in the interaction between neutron and proton, the proton quenching and the value of the high energy threshold of the detector, the neutrons selected in this way have energies more than about 20 MeV.

Neutron signal in LVD


2004


How we can discriminate the accidental coincidence of an high energy threshold with a following low one from a true coincidence due to neutron

interaction? If we look at the time delay

distribution between the HET signal (or the time of the

muon) and the LET one we expect an exponential shape due to neutron capture with a

mean lifetime of 180s on the top of a flat behaviour due to

accidental coincidences.

Number of Neutron detected

A = B =C =


2004


Analysis

Selecting a group of conuters as clean as possibile we perform the following measurement:

Neutron production as a function of the distance from the muon track

Neutron production as a function of the energy

Neutron production as a function of the muon path lenght in scintillator

Study of the neutron production in the LVD detector in association with single muons events


2004


Single muon event

Selection cuts:

Only one reconstructed track per event

Almost three points in each projection

Almost two high energy threshold signal from two different counters within 250 ns

LVD data analysis from 1994 to 2002

7661673 SINGLE MUON EVENTS


2004


The number of neutrons per counter per event has been evaluated for various distances from the muon track.

For single muons we use the distance between the reconstructed muon track and the center of the counter where the neutron is detected.

Neutron flux measured up to 22 m!

Neutron production vs distance from muon track


2004


Number of neutrons detected as a function of the energy released in the scintillator from the recoiling proton.

The data are well fitted by a power law spectrum:

Y=A*E-

where

= (1.18 ± 0.05)

Neutron production vs proton energy release


2004


The mean number of neutrons per event has been evaluated as a function of the muon track lenght inside the liquid scintillator.

y=p1+p2*x

p1=0,18*10-2 neutron production in the rock

p2=0,7*10-3 increase in the neutron production with the muon path lenght in the scintillator

Comparing the two areas we can evaluate the contribution of neutron production in rock and in scintillator:

Area rock=34% Area scint=66%

Neutron production vs muon track lenght

“The production problem”


2004


Multiple muon event

Selection cuts:

Almost two reconstructed tracks with three points in each projection

Almost two high energy threshold signal from two different counters within 250 ns

Space angle between tracks less than 10º

... Work in progress...


2004


All neutrons in LVD

If we look at the time distribution between all the low energy threashold and the time of the muon track without requiring the presence of the high energy threashold due to the recoiling proton, we can extend the measurement at the all neutron produced.

PRELIMINARY


2004


Summary

We have studied the neutrons of energy greater than about 20 MeV in association to single muon events in LVD and we have

measured:

The neutron flux up to 22 m from the muon track

The power law energy distribution as E-1.18

The contribution of about 34% to the neutron production from muon interactions in rock and of about 66% from muon interactions with scintillator nuclei with respect to the total.

Work in progress:

Neutrons from multiple muon event

All energy neutrons production

Documents

1 IDM2004 Edinburgh, 9 september 2004 Helenia Menghetti Bologna University and INFN Study of the muon-induced neutron background with the LVD detector