1. Neutron background measurement at LNGS: present status Measurement carried out in collaboration between LNGS ILIAS-JRA1 and ICARUS groups

2. Purposes Measurement of the spectrum and intensity of the neutron flux (fast component) in the LNGS underground laboratory by means of liquid scintillator counters (about 4 litres) and pulse shape discrimination (PSD) techniques. This activity is mainly focused on the evaluation of possible fluctuations with time of the neutron intensity. Identification of the physical correlation with environmental parameters.

3. Overview February 2005 received first 2 litres BICRON cell April 2005 calibration in the Baradello-Como underground lab. June 2005 received second 2 litres BICRON cell July 2005 calibration with neutron beams at LNL August 2005 installation at LNGS hall A of electronics 2 x 2 litres cells 2 x 0.3 litres cells September 2005 start of the data acquisition March 2006 installation of a lead shield (8 mm) to reduce the stochastic contribution induced by gamma background 15 March 2006 is considered the starting date for regular data taking

4. Set up in Hall A 0.3 litre cells 2 litres cells Electronics DAQ

5. Data taking From 15 March 2006 data have been acquired continuously. Up to now (Feb. 2007) about 2x10 9 triggers have been recorded and about 700 neutron interactions have been identified (E>3.5 MeV). The stability of the system has been regularly monitored both by means of gamma-source calibrations and by means of internal calibrations based on measurement of alpha-lines produced by the liquid scintillator radioactive contaminants.

6. Data analysis The recorded data frequencies are: 5x 10 6 interactions per day 1x 10 3 interactions per day selected by hardware PSD 5x 10 2 interactions per day selected by software PSD Considering that the contribution to the rate due to stochastic events and radioactive impurities above 3.5 MeV (energy proton equivalent) are negligible, we have: 2 interactions (neutrons) per day above E=3.5 MeV (0.5 neutrons per day per litre of scintillator) This frequency is compatible with a background neutron field of about 10 -7 cm -2 s -1 as previously measured in the LNGS laboratory.

7. The presence of fluctuations of the counting rate beyond the statistical fluctuation is evident in the data taking. Identification of the physical correlation with environmental parameters, such as the rock moisture, ground water level and pressure, … is in progress. Example of dependence with time of the neutron counting rate (neutron interactions per month) as recorded by one cells in the LNGS lab..

8. Conclusions and Perspectives In March 2007 a year of data taking will be reached. The collected data will be enough to perform an evaluation of possible fluctuations with time of the neutron intensity within a one-year cycle. For this purpose, in March/April 2007, a polyethylene shield will be installed around the apparatus (at least one counter). A dedicated run in these conditions will allow, in addition, the evaluation of the actual internal radioactive contamination and to figure out the effective sensitivity of the detector. A cross-check with data collected in absence of the external neutron field could be useful.