1. Neutron background measurements at LNGS Gian Luca Raselli INFN - Pavia JRA1 meeting, Paris 14 Feb. 2006

2. 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.

3. 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 V765 12 bit ADC with independent gates allows the acquisition of both Total and Delayed Charge CAEN N715 Pulse Shape Discriminator allows an event identification at trigger level

4. Data selection and analysis (neutron source) Overflows Spurious events e - (  ) p (n)

5. 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.

6. 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

7. The BICRON detector CELL 2MAB-F6BC501A 5”diameter x 6”long 2 litres of BC501A Photomultiplier tubes XP4512B ( Ø = 5”)

8. Set up in Hall A (neutron detector) 0.3 litre cells 2 litres cells Electronics DAQ

9. Set up (gammas measurement) 0.2 litre cells Electronics DAQ

10. Distribution of gamma events 1 day, 0.2 litres K 40 (1460 keV) Tl 208 (2615 keV)

11. Electronics flow chart

12. 1 day scatter plot 2 litres cell e-e-  p Interactions 1x10 8 d -1 PSD 1 sel. events 1x10 3 d -1 PSD 2 sel. events 7x10 2 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 about 10 -7 cm -2 s -1 Spurious events

13. Distribution of selected events  p

14. Very preliminary results

15. 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?