Neutron background measurements at LNGS

Gian Luca RaselliINFN - Pavia

JRA1 meeting, Paris 14 Feb. 2006

Purposes• New measurement of the spectrum and intensity

of the neutron flux (fast component) in the LNGS underground laboratory.

• This activity is mainly focused on the evaluation of possible fluctuations with time of the neutron intensity.

• Identification of the physical correlation with other environmental parameters.

Pulse Shape Discrimination

The response of BC501A liquid scintillator has different time components which depend on the ionising particle (p recoils for n; e- for )

n

The electronics combines a double pulse shape discrimination to select highly-ionising events from gamma background

CAEN V76512 bit ADC with independent gatesallows the acquisition of both Total and Delayed Charge

CAEN N715Pulse Shape Discriminatorallows anevent identification at trigger level

Data selection and analysis(neutron source)

OverflowsSpurious events

e- ()

p (n)

Data selection and analysis• The energy distribution of selected data (p

recoils) is then obtained using a detector calibration carried out by means of -ray sources (eee Electron Equivalent Energy) and conversion factors (pee Proton Equivalent Energy).

• The energy distribution of neutrons is computed by means of a deconvolution program once the response function of the detector are known.

Overview of detector installation

• February 2005 received first 2 litres BICRON cell• April 2005 installed in Baradello-Como underground

lab.• June 2005 received second 2 litres BICRON cell• July 2005 calibration with neutron beams at LNL• July 2005 installation at LNGS hall A of

electronics 1 litre cell 2 x 0.3 litres cells

• August 2005 installation of the second 2 litres cell• September 2005 start of the regular data

acquisition• November 2005 installation at LNGS of a dedicated DAQ (0.2 l cell) for gamma background evaluation

The BICRONdetector

CELL 2MAB-F6BC501A

5”diameter x 6”long2 litres of BC501A

Photomultiplier tubesXP4512B (Ø = 5”)

Set up in Hall A(neutron detector)

0.3 litrecells

2 litrescells

Electronics

DAQ

Set up(gammas measurement)

0.2 litrecells

Electronics

DAQ

Distribution of gamma events1 day, 0.2 litres

0

500

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3000

0 1000 2000 3000 4000 5000

Co

un

ts p

er

da

y

ADC Channel

K40 (1460 keV)

Tl208 (2615 keV)

Electronics flow chart

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100 1000

De

laye

d C

ha

rge

(A

DC

Co

un

ts)

Total Charge (ADC Counts)

1 day scatter plot2 litres cell

e-

p

Interactions 1x108 d-1

PSD 1 sel. events 1x103 d-1

PSD 2 sel. events 7x102 d-1

Selected events: threshold ~ 4 MeV (pee) 3 – 4 d-1

This frequency is compatible with a background neutron field flux (> 4 MeV) of about10-7 cm-2 s-1

Spurious events

0.1

1

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104

0 2 4 6 8 10

CELL_ACELL_BCELL_CCELL_D

Co

unts

Energy (MeV)

Distribution of selected events

p

Very preliminary results

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200Rate ARate CRate tot

Co

unts

Month

Sep/05 Oct/05 Nov/05 Dec/05 Jan/06

Conclusions• A neutron detector is now operating in the Hall A of

the LNGS.

• The sensitivity is enough to perform, in few months of data taking, an evaluation of possible fluctuations with time of the neutron intensity.

• The measurement of the absolute neutron flux requires a detailed knowledge of the detector response functions. This can be achieved by:

Calibration with quasi-monoenergetic neutron beams; Monte Carlo simulation of the detector (FLUKA).

Identification of the physical correlation with other environmental parameters, such as the rock moisture, ground water level and pressure, … .What is available?