EMMA - An Underground Cosmic Ray Experiment

EMMA (Experiment with MultiMuon Array) aims to study the composition of cosmic rays around the knee region. It is an underground experiment situating 75 metres underground at the Pyhäsalmi Mine, Pyhäjärvi, Finland. The 75 metres overburden (corresponding to 240 m.w.e.) produces an energy cut-off of approximately 45 GeV for vertical muons.

The array is capable of measuring the multiplicity and the lateral distribution of underground muons, and arrival direction of the air shower with an accurace better than 1 degree. The array consists of nine detector stations of an area of 15 square meters each (see Fig. 1) Three of the stations (C, F, G) are so called tracking stations with three layers of position sensitive drift chambers. In the rest of the stations there are one layer of similar drift chambers. In addition, the tracking stations will also be filled with small-size plastic scintillation detectors.

The drift chambers are the former muon barrel detectors (MUB) of the DELPHI experiment at CERN. The scintillation detectors are designed especially for the EMMA experiment to improve the multiplicity measurement. They are manufactured by RAS/INR, Moscow.

EMMA layout

Figure 1. Schematic layout (top view) of the array. It consist of nine detector stations (called A, ..., I), from which three (shown by green) are so called tracking stations with are three layers of drift chambers and one layers of small-size plastic scintillation detectors. One-layer stations (shown by red) occupy one layer of drift chambers. Detector stations from A to E have been finished and are taking data (situation at July 2010).

Due to the energy cut-off of approximately 45 GeV, only high-energy muons of an air shower are detected (the low-energy ones being filtered out by the rock overburden). Since these high-energy muons are created in the upper part of an air shower close(r) to the primary collision, the experiment is also sensitive to the interactions in the upper part and not disturbed by more numerous low-energy particles. This is expected to suppress the uncertainty in the reconstruction of the primary cosmic-ray particle.

the first cottage

Figure 2. A photograph from the cavern at the depth of 75 metres. At the front is station B and at the back station C.

Figure 2 shows a photograph from the underground location.

tracking example

Figure 3. An example of particle tracks detected by the first tracking unit (station C in Fig. 1).

Approximately two months have now been measured with the first tracking unit. This is still mainly for testing purposes. As an example, a set of 12 identified particle tracks is shown in Fig. 3.

One scientific aim is also the confirmation of the high muon-multiplicity measurements performed at LEP. With the full array an information of the composition is obtained which may give some more information about the origin of the muon bundles.

University of Oulu, Finland:
University of Jyväskylä, Finland:
Russian Academy of Sciences, Moscow, Russia:
University of Aarhus, Denmark:


Last modified: Monday, July 5, 2010 by Timo