CosMO Experiment

      DESY in Zeuthen has developed the CosMO experiment (Cosmic Muon Observer) within the Cosmic Project of Netzwerk Teilchenwelt. The experiment offers high school and university students an independent study of cosmic particles. CosMO consists of components also used in large experiments, which gives a direct insight into the scientific endeavors of astroparticle physics.

      DESY and the other partner institutes of Netzwerk Teilchenwelt provide a large number of CosMO Experiments for student projects performed in institutes or in schools.


        The experiment consists of:

        • 3 detector boxes containing a (20cm x 20cm x 1cm) EJ-200 plastic scintillator where 9 wave length shifter WLS fibers of 1mm diameter are glued into the plate to guide the light flash to the 3mm x 3mm surface of the multi pixel photon counter MPPC. A small electronics card containing the power supply for the MPPC (5V input, adjustable 65-75V output) and the connection of the output signal via a LEMO jack.
        • a data acquisition board DAQ card (developed by Fermilab for the Quarknet experiment)
        • a notebook to operate the DAQ card, as well as store and analyse the data
        • the measurement and analysis program muonic running on the notebook
        • a GPS device (optional), which can be connected to the DAQ card.

        Cosmic particles reaching the Earth's surface are mainly muons. If they cross a scintillator, a very short and weak light signal is generated. This scintillation light is guided via light conducting fibers to a MPPC, which, like a PMT, is sensitive enough to allow the detection of even very few photons. The MPPC generates a short electrical pulse which is transferred to the DAQ card. There the input signals are amplified, filtered and digitised. Then the card sends a data string with the relevant information to the notebook.

        Preparation and Experimentation

          As with all experiments, the three CosMO detectors have to be calibrated before the measurement can be started. First, the particle rate variable, which is dependent on the threshold voltage, is measured several times using varying threshold voltages. Using the reference value for the rate of muons at sea level: 1 particle per cm2 and per minute, the expected rate can be calculated for the CosMO detector. Then, one has to choose for this expected rate the corresponding threshold voltage.

          Onr can undertake various experiments with calibrated detectors, some of which are proposed below. The data, stored in the notebook, can be analysed using the software muonic. Alternatively, it can also be evaluated using the platform Cosmic Ray e-Lab.

          Further information about the experimental set-up and the experimentation can be found in the CosMO manual and in the manual of the analysing program muonic. (German only)

          Possible Student Exercises

            Various interesting experiments with cosmic particles can be performed with the CosMO detectors:

            • Measurement of particle rates depending on the distance between the detectors and on different zenith angles.

              • Measurement of particle showers by arranging the detectors in a triangular position on a large table or on the floor. The shower rate is dependant on the distance between detectors, and should be measured several times using varying distances. the detectors.

                • Measurement of the mean lifetime of muons by searching the recorded time for events for which one of the detectors has registered two independent signals with a time difference of 1-20 µs. The first signal is caused by a muon and the second one by one of its decay products – either an electron or a positron. The other decay particles, two neutrinos, cannot be detected. Displaying the number of events versus the time difference between the two signals allows one to estimate the muon's mean lifetime.

                  • Measurement of the muon velocity. Since muons have velocities close to the velocity of light, the distances between the first and the second detector need to be larger than 2m. A third detector could voluntarily be used to correct faulty signals, it would be placed between the two velocity measuring detectors. To reduce systematic errors, the distance between the detectors should be varied (>2m) and the detectors should be switched.

                  The steering and analysis program muonic provides different measurement options (rate, muon lifetime, muon velocity). The rate measurements proposed here can be performed within hours. For lifetime and velocity studies one requires up to several days for sufficient statistics to be compiled.
                  All CosMO components fit into a larger notebook bag. With a power bank charger for the DAQ card which provides the 5V power for the 3 detector boxes, an outdoor operation of the CosMO experiment is possible for several hours. For long-term outdoor measurements one can install the muonic program on a Raspberry Pi single-board computer. It has a much lower power consumption and can be remotely controlled by a notebook via an ethernet or wifi connection.