Atmospheric muon flux measurements at the external site of the Gran Sasso Lab

Abstract

Measurements of the atmospheric flux of muons and other charged particles have been performed at the external site of the Gran Sasso Laboratory (LNGS, Italy, $\text{1000}\text{m}\text{a.s.l.}$), in view of the OPERA experiment. Scintillation counters and nuclear emulsion plates have been used. Rates and angular distributions at several depths of Fe shielding are presented and compared with Monte Carlo simulations.

Introduction

In the framework of the CNGS project [1], the OPERA experiment aims at the detection of ν_τs in an almost pure ν_μ beam [2]. OPERA is based on the nuclear emulsion technique, used to study cosmic rays since their discovery and in many other high-energy physics experiments. In the last few years, the technique was applied to neutrino experiments on unprecedented scale and substantially upgraded [3], [4]. Further developments are in progress.

The OPERA target is highly segmented in basic units called “bricks”, about $\text{12.8}\text{cm}\text{×10.3}\text{cm}$ wide, $\text{8.0}\text{cm}$ thick along the beam direction, each consisting of 57 double-coated emulsion films interleaved with 56 lead foils, $\text{1}\text{mm}$ thick. The physics program of the experiment requires an alignment between emulsion sheets to within a few μm, more accurate than the alignment provided by the packing procedure. This goal can be accomplished by exposing bricks, selected as containing neutrino interactions induced by the CNGS beam, to an external source of penetrating particles, namely cosmic ray muons. A few particles/mm² have to be collected.

For this purpose, a dedicated pit will be excavated at the external site of the LNGS, suitably shielded by an iron cover. Bricks will be exposed to cosmic rays at the bottom of the pit, and then extracted and disassembled. The emulsion sheets will be promptly developed and sent to scanning laboratories. The thickness of the iron cover must be such that (a) effective shielding is provided against cosmic ray e.m. showers, useless for calibration and harmful for the particle (electron) identification methods applied to neutrino events, (b) a suitable muon flux can be collected within a reasonable exposure time (1 or 2 days), (c) to some extent the energy spectrum of muons is hardened, (d) they are focussed at narrow dip (zenith) angle. In order to study these items in detail, we performed a test experiment at the external site of the LNGS.

An iron tower structure was built, allowing to host detectors in three distinct slots under different and variable iron thickness. To measure the flux of cosmic ray muons and the corresponding angular distribution, both relevant for intercalibration purposes, we made separate and independent measurements inserting emulsion sheets or scintillation counters inside the iron tower. Fluxes were measured in both cases as a function of the iron thickness. Emulsions also allowed angular measurements for passing-through particles. Scintillators allowed some study of particle correlation in the time gate. A Monte Carlo (MC) simulation was performed, whose reliability was validated and cross-checked with experimental data. Notice that the lower-energy tail of the cosmic radiation, more challenging for simulation codes, but negligible in other experimental conditions, will also play a role in the OPERA brick-calibration case, and thus was carefully taken into account.

In Section 2 the experimental setup is described. Details are provided about the scintillation counter system as well as the emulsions and their handling. The automatic scanning procedure is also described, with details about the sheet intercalibration and the raw data handling after scanning. In Section 3, the MC shower generation and the methods adopted to simulate the particle propagation and the detector response are presented. In Section 4, the experimental results are presented and compared to the expectations from the MC simulation. Conclusions are given in Section 5.

Besides the relevance for the OPERA experiment, these measurements are of some interest by themselves, considering the lack of muon data at various atmospheric depths, especially at the lower end of the energy spectrum.