neutron background measurements at lngs gian luca raselli infn - pavia jra1 meeting, paris 14 feb....
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![Page 1: Neutron background measurements at LNGS Gian Luca Raselli INFN - Pavia JRA1 meeting, Paris 14 Feb. 2006](https://reader036.vdocument.in/reader036/viewer/2022062515/56649d225503460f949f883d/html5/thumbnails/1.jpg)
Neutron background measurements at LNGS
Gian Luca RaselliINFN - Pavia
JRA1 meeting, Paris 14 Feb. 2006
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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.
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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
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Data selection and analysis(neutron source)
OverflowsSpurious events
e- ()
p (n)
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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.
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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
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The BICRONdetector
CELL 2MAB-F6BC501A
5”diameter x 6”long2 litres of BC501A
Photomultiplier tubesXP4512B (Ø = 5”)
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Set up in Hall A(neutron detector)
0.3 litrecells
2 litrescells
Electronics
DAQ
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Set up(gammas measurement)
0.2 litrecells
Electronics
DAQ
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Distribution of gamma events1 day, 0.2 litres
0
500
1000
1500
2000
2500
3000
0 1000 2000 3000 4000 5000
Co
un
ts p
er
da
y
ADC Channel
K40 (1460 keV)
Tl208 (2615 keV)
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Electronics flow chart
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10
100
1000
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
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0.1
1
10
100
1000
104
0 2 4 6 8 10
CELL_ACELL_BCELL_CCELL_D
Co
unts
Energy (MeV)
Distribution of selected events
p
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Very preliminary results
0
50
100
150
200Rate ARate CRate tot
Co
unts
Month
Sep/05 Oct/05 Nov/05 Dec/05 Jan/06
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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?