Variability of fast neutron yield in underground environment

¹ Laboratorio per l'Energia Nucleare Applicata (LENA), Università di Pavia - Pavia, Italy
² Dipartimento di Fisica Nucleare e Teorica dell'Università and INFN Sezione di Pavia - Via Bassi 6, 2700 Pavia, Italy
³ Politecnico di Milano, Dipartimento di Ingegneria Nucleare and INFN Sezione di Milano - via Ponzio 34/3, 20133 Milano, Italy
⁴ Dipartimento di Fisica e Matematica dell'Università dell'Insubria via Valleggio 11, Como, Italy

Received: 12 March 2004
Accepted: 7 July 2004

Abstract

One topical question in today's particle physics and astrophysics concerns the search for low-energy rare events. Spurious counts, resulting from environmental radiation, in particular neutrons, can easily overcome the signals of interest in this kind of research. Hence a quantitative evaluation of the neutron background and a full understanding of the parameters, which conditionate its intensity and energy spectrum, is particularly important. In this letter we will show and discuss the evidence of long-term fluctuations (in months scale) appearing in the measurements of fast neutron rate. The data were collected in the Baradello underground laboratory (Como, Northern Italy, 300 depth) from May 2002 until December 2003. We find a strong correlation between fast neutron rate and rain precipitation at the surface, but only when we introduce a phase delay of about one month between the timing of the rain flux and that of the neutron rate. We suggest that the fast neutron flux intensity is related to the moisture contained in the ground, which surrounds the laboratory walls, and we assume that the moisture depends on the rain quantity at the surface. The delay we find is consistent with the percolation time of the water from the surface to the underground laboratory. This mechanism should be taken into account as a possible systematic effect in any experiment which uses the signal rate modulation as a signature for a new physical phenomenon.