Hofverberg, P

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29th International Cosmic Ray Conference Pune (2005) 8, 271274First results from the Stockholm Educational Air Shower Array(SEASA)P. Hofverberg , H. Johansson , M. Pearce and S. Rydstro¨m (a) Royal Institute of Technology (KTH), Department of Physics, Stockholm, SwedenPresenter: P. Hofverberg (petter@particle.kth.se), swe-pearce-M-abs1-he15-oralThe ’Stockholm Educational Air Shower Array’ (SEASA) project is establishing a network of time- syn-chronised scintillator detector stations at high-schools in the Stockholm region. High school students arecontributing to the construction, installation, testing and running of the detector station placed on the roof oftheir school. This initiative aims to increase the students’ interest in science and technology subjects by expos-ing them to modern research. Each station is equipped with three plastic scintillator detectors (each 0.3 m)arranged in a triangular formation. Signals from GPS satellites are used to time-synchronise signals from thewidely separated detector stations, allowing cosmic ray air showers to be identified and studied. A low-costand highly scalable data acquisition system has been produced using embedded Linux processors which com-municate station trigger and monitoring data to a central database. Air shower data and the performance ofeach detector station can be visualised in real-time via a web browser. The status of the project is presentedalong with first results from the observation of air showers over Stockholm.1. IntroductionThe ’Stockholm Educational Air Shower Array’ (SEASA) [1] project is establishing a network of cosmicray detector stations over the Stockholm region. Each station consists of 3 large plastic scintillator detectorsarranged in a triangular formation. Cosmic ray activity at stations separated by arbitrary distances is correlatedusing timing signals from GPS navigation satellites. A primary aim of the project is to give high-schoolstudents, aged between 16 and 18 years, the possibility to gain insight and work on a modern research projectand so the detector stations are located at the high-schools (on roofs or in attics). The students themselveswill participate in the construction, testing, installation, commissioning and running of their detector station.As well as these ’outreach’ aims, scientific studies are also foreseen. A particularly interesting prospect isto study correlations between stations separated by large distances (e.g.: between towns) in order to identifycosmic rays sharing a common history. Such correlations could arise from the solar photo disintegration ofheavy nuclei [2] or transient cosmic gamma-ray activity [3]. Another possible application is the forecasting ofgeomagnetic storms [4]. During the first phase of the project in 2005, the network will consist of 7 detectorstations located in Stockholm at AlbaNova University Centre and four local high-schools. Additional stationsare expected to be installed at other high-schools both within and outside of Stockholm in subsequent years.2. System DesignFig. 1 shows an overview of the SEASA system. A station comprises of three plastic scintillator detectorswith photomultiplier read out, a data acquisition system based on a programmable logic array (PLA) and anembedded Linux processor, a programmable high voltage supply and a GPS receiver system.The design of the scintillator detectors for the first two detector stations was motivated by the availability ofmaterials left over from other research projects. The results presented in this paper are derived from thesedetector stations, which are located at the AlbaNova University Centre in Stockholm. A wavelength shiftingbar (Bicron BC-482A) was used to transfer light from four rectangular pieces of plastic scintillators (Bicron272 P. Hofverberg et alBC-408), approximately 150 cm x 13 cm x 1.2 cm, to a compact photomultiplier (Hamamatsu R5900U). Thescintillators were placed either side of the wavelength shifting bar to maximise the light collection efficiency.One drawback with this setup is the large amount of expensive wavelength shifting bar which is needed. Futuredetectors will therefore use single pieces of scintillator, approximately 1.5 cm x 30 cm x 100 cm, read out bya large diameter (76 mm) photomultiplier (Photonis XP3314B) glued to the centre of the scintillator’s largestsurface. For both designs, the scintillator assemblies were wrapped in Dupont Tyvek and placed in a light-tightbox. This was in turn put in a car roof box which prevents the scintillator assembly from being directly exposedto rain and snow.The detectors have a typical separation of 10 m to 15 m and operate in a coincident mode, requiring all detectorsto be hit within a predefined time window (up to 1  s) for the station to trigger. Air showers with a primaryparticle energy above 	 -  eV are detectable by a station depending on the separation of the scintillatordetectors [5].An Altera Cyclone PLA is the key digital component. It processes data arriving from the detectors and envi-ronmental sensors (atmospheric, pressure, temperature), forming events which are sent to an embedded Linuxsystem (Axis 82 Developer board [6]). The Linux system provides cold start configuration of the GPS receiverand PLA, as well as working as an interface to the central server. A GPS receiver card (Motorola M12+) and an-tenna (Motorola Timing 2000) [7] are used to provide a time tag for each coincidence event. The performanceof the GPS time tagging is presented in detail elsewhere [8].An important issue for the project is to visualize data interactively and in an interesting way, to engage studentsin the project. Therefore, a ’live’ web-based viewer system has been developed. A server running a MySQLdatabase under Linux constitutes the core of the system, where the information from the detector nodes isstored. Data from the server is sent in real-time to “viewer clients”, on-line java-applet clients displaying liveinformation about the detector nodes, as well as trigger statistics and housekeeping “on demand”. Hardwareparameters, for example high voltage levels and the coincidence window length, can also be set from theviewer. More details about the data acquisition and viewer system can be found elsewhere [8].Figure 1. An overview of the SEASA system. The data ow is as follows: when an air shower triggers a station, a timestamp is provided by the GPS system and information about the event is sent over the internet to a central server. The serverstores the data in a database and sends it to all active viewer clients.First results from SEASA ... 2733. First observations of air showersTwo cosmic ray detector stations have been installed on the roof of the main building at the AlbaNova Uni-versity Center, approximately 200 m apart. As described above, each station has its own GPS system, andcorrelated events should be seen when large air showers hit both stations coincidently.When both detector stations register triggers within a gate of 1  they are referred to as “super triggers”, whereit is assumed that a large air shower has hit both stations. The gate was chosen with the distance between thestations and the assumed shower front thickness taken into consideration. Fig. 2 shows the correlation in triggertime between station 1 and station 2, for all triggers that coincided within the second between the stations. Outof 78 triggers, 33 coincided within  and 45 were uncorrelated. The normal trigger rate per station observedis about 35 triggers per hour. The probability of two air showers coinciding within the second is .For an acquisition time of 120 hours this gives ﬀﬁﬃﬂ ! triggers coinciding within thesecond, which is close to what is measured (45). This is a good indication that the GPS time tagging techniqueworks, and that air showers can be correlated over arbitrary distances. More detailed studies are currentlyon-going.Figure 2. The trigger time for station 2 versus station 1 for events coinciding within the second. Out of 78 triggers, 33coincided within 1 "$# . The acquisition time was 120 hours.4. Pulse length analysisSuper triggers originate, on average, from larger air showers than station triggers. Larger showers have agreater particle density and therefore deposit a larger amount of energy in the scintillator detectors. The energydeposited is not measured in a direct way, but an estimate can be made by recording the length of a discriminatorsignal which varies depending on he amplitude of the PMT pulse. Fig. 3 shows the pulse length distributionfor ordinary triggers (top) and super triggers (bottom) for one of the detector stations. Similar behaviour wasobserved for the other station. A shift to higher pulse lengths can clearly be seen for super triggers, indicatingthat single large air showers have been observed.274 P. Hofverberg et alFigure 3. Pulse length distribution for ordinary triggers and super triggers in units of 5 ns for station triggers (left) andsuper triggers (right). A shift in pulse length is evident for the super triggers compared to the pulse length for the stationtriggers, indicating that super triggers originate from larger showers than station triggers.5. ConclusionsThe SEASA project is now at the end of the first phase: two stations have been deployed at the AlbaNovaUniversity Centre, and a proof-of-principle has successfully been made. Air showers have been detected, bothby single stations, and coincidently by multiple stations, thereby proving the GPS time stamp technique. Athird station is currently being installed at AlbaNova. An especially interesting feature with three stations isthe possibility to make accurate determinations of the shower angle. This autumn, SEASA is brought to thenext phase expanding the detector network to include four high-schools in the Stockholm area.An interesting future possibility is to expand the SEASA network to towns outside Stockholm, making it pos-sible to study long range correlations between air showers. This would allow studies which are complementaryto professional arrays, such as searches for ultra-high energy bursts of % -rays [3].6. AcknowledgementsPhotonis are thanked for their generous donation of photomultiplier tubes.References[1] SEASA homepage http://www.particle.kth.se/SEASA[2] G.A. Medina-Tonco and A.A. Watson, Astroparticle Physics 10 (1999) 157.[3] N. Ochi et al., J. Phys G. 29 (2003) 1169.[4] F. Jansen et al., Proc. ESA Space Weather Workshop. ESTEC, Noordwijk, The Netherlands. Dec. 2001.[5] P. Hofverberg, Master’s Thesis, ”Air Shower Simulations for the SEASA Project”, Royal Institute ofTechnology (KTH), Stockholm 2003. http://www.particle.kth.se[6] Axis developer homepage http://developer.axis.com[7] MOTOROLA homepage http://www.motorola.com/ies/GPS/products timing.html[8] P. Hofverberg et al., ’The Data Acquisition System of the Stockholm Educational Air Shower Array’.IEEE Trans. Nucl. Sci. Submitted.

First Results from the Stockholm Educational Air Shower Array (SEASA)

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