IceCube Collaboration

Renschler, M.

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PoS(ICRC2019)401First measurements with prototype radio antennasfor the IceTop detector arrayThe IceCube Collaboration∗http://icecube.wisc.edu/collaboration/authors/icrc19_icecubeE-mail: max.renschler@icecube.wisc.eduExtending large-scale air-shower arrays with radio antennas can increase the detector’s perfor-mance, as the radio emission by cosmic-ray air showers provides an additional measurement ofthe electromagnetic component. Instrumenting the IceCube surface detector IceTop with radio de-tectors as well as with new particle detectors in a hybrid approach will enhance the measurementand reconstruction accuracy and allow for the characterization of highly inclined air showers. Thiswill enable a better understanding of the atmospheric background for the in-ice neutrino measure-ments. It also opens the opportunity for new science cases, e.g. the search for PeV gamma raysfrom the Galactic Center, which is visible from the IceCube site year-round at an inclination of61◦. Adding to several scintillator particle detectors already running at the South Pole, two proto-type radio antennas have been deployed at the IceCube site in January 2019 using the same DAQsystem as the scintillators. The antennas serve as a test setup for a future deployment of radioantennas extending the scintillator array planned inside the IceTop footprint. In this proceeding,the antennas considered for deployment and the hybrid DAQ system processing the signals of theparticle and radio detectors will be introduced. First measurement results at the South Pole willbe presented and future plans for a full hybrid particle and radio detector array inside the IceTopfootprint will be shown.Corresponding authors: Max Renschler†11 Karlsruhe Institute of Technology, IKP36th International Cosmic Ray Conference -ICRC2019-July 24th - August 1st, 2019Madison, WI, U.S.A.∗For collaboration list, see PoS(ICRC2019) 1177.†Speaker.c© Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/PoS(ICRC2019)401Prototype radio antennas for IceTop Max Renschler1. IntroductionIceCube is a cubic-kilometer neutrino detector installed in the ice at the geographic SouthPole with the aim to measure and quantify the high-energy neutrino flux [1]. The planned upgradeof the IceCube experiment to IceCube-Gen2 foresees an increase of the in-ice detector volume.With this upgrade, an extension of the surface detector IceTop [2] is needed that measures themain background of the IceCube detector consisting of high-energy atmospheric muons inducedby cosmic-ray air showers. The current IceTop detector array consists of ice-Cherenkov detectorscovering the footprint of the IceCube in-ice detectors of about 1 km2. As a first step towards a Gen2surface array, the existing IceTop array is enhanced by adding new detectors consisting of scintilla-tion particle detectors [3], air-Cherenkov telescopes [4] and radio antennas [5]. This increases thescientific capabilities of the IceTop array and serves as a test setup for a future Gen2 surface array.Adding hybrid particle and radio detectors to IceTop comes with several benefits for the precisemeasurement of cosmic-ray air showers and also enables new science cases reachable only with ahybrid detector array. Radio emission by air showers has its origin in the separation of charged par-ticles as the air shower propagates through Earth’s atmosphere [6]. The radio emission originatespurely from the electromagnetic shower component and is therefore ideal for improving mass com-position measurements and the precise tests of hadronic interaction models when measurements areperformed together with muon detectors [7]. Additionally, the measurement of the radio emissionof air showers allows for a cross-calibration between different experiments [8].Installing surface radio antennas in IceTop enables new science cases, e.g. the search for PeVgamma rays coming from the Galactic Center (GC). Since the Galactic Center is visible from theIceTop site year-round at an inclination of 61◦, the site is ideal for searching for inclined air show-ers generated by PeV gamma rays that might be produced in the Galactic Center [9]. The energythreshold of radio to detect PeV gammas can be lowered by using a higher frequency band like100 MHz to 190 MHz than the traditional frequency band of 30 MHz to 80 MHz [10]. Radio anten-nas increase the measurement accuracy of inclined showers significantly since the radio emission isnot significantly attenuated during its way through the atmosphere. As radio is sensitive purely tothe electromagnetic component, a radio array helps to differentiate between hadronic- and gamma-generated air showers when combined with particle detectors.In January 2019, two prototype antennas were deployed at the South Pole within an already ex-isting scintillation-detector array consisting of seven detectors. Together with the antennas, a firstversion of a hybrid DAQ system was installed that processes and digitizes the signals of the sevenscintillation detectors and the signals of the two radio antennas. This hybrid detector station servesas a test setup for future deployment plans. In addition, measurements of the radio background nearthe IceCube Laboratory (ICL) were performed to estimate the radio background near the prototypeantenna positions.The final detector layout of the enhanced IceTop array will consist of 32 detector stations witheight scintillation detectors and three radio antennas per station. This adds up to 256 scintillationdetectors and 96 radio antennas in an area of 1 km2 in addition to the original 162 ice-Cherenkovdetectors of IceTop [11].2PoS(ICRC2019)401Prototype radio antennas for IceTop Max Renschler2. The SKALA prototype antennaFigure 1: SKALA-2 antenna of the South-Pole design with a height of 0.3m from ground to the lowest lobe.The left picture shows a test setup at the Karlsruhe Institute of Technology (KIT). The right picture showsthe deployed antenna at the South Pole.A prototype version of an antenna developed by the Square Kilometer Array (SKA) [12] isused. Desirable characteristics of the antenna include the gain of the antenna for inclined showersdown to an inclination of 61◦, the low internal electronic noise of about 30 K and the wide frequencybandwidth of 70 MHz to 350 MHz.The antenna structure and the antenna itself have to withstand the harsh environmental conditionsat the South Pole. For this, a new antenna structure made out of glass-fiber reinforced plastic(GFK) material and plywood has been designed, and critical antenna parts like plastic mountingshave been stress-tested at low temperatures.Pictures of the antenna are shown in Fig. 1. The antenna arms are extended with GFK poles leadingto a small metal foot that is attached to a plywood mounting structure. The mounting structure isattached to the ground with four GFK snow spikes with a length of 1 m each. A middle pole servesas an additional stabilization and as a cable guide from the low-noise amplifier (LNA) on the topof the antenna. Future designs of the antenna structure will enable the possibility to lift the antennawhen the snow level rises during years of operation.The antenna structure has been designed in three different heights of 0.3 m (shown in Fig. 1), 0.7 mand 1.7 m from ground level to the lowest antenna lobe resulting in a total height of the antenna of1.8 m, 2.3 m and 3.3 m. The shortest and tallest antenna designs were deployed at the South Polein January 2019.Prior to the deployment at the South Pole, the LNA of the antenna was tested and characterizeddown to temperatures of -70◦C. For this, the LNA response to an input signal generated by acalibration source was measured while temperature cycling the LNA from 20◦C to -70◦C. In Fig. 2the results of the LNA gain measurement in a temperature range of 20◦C to -70◦C and a frequency3PoS(ICRC2019)401Prototype radio antennas for IceTop Max RenschlerFigure 2: Measurement of the LNA gain. The gain is measured in a temperature range of 20◦C to -70◦Cand a frequency range of 50 MHz to 600 MHz. As input, a 50 Ω calibration source has been used.range up to 600 MHz are shown. The LNA gain stays constant within about 2 dB throughoutthe whole temperature range, allowing for stable operation in the cold and temperature-dynamicenvironment of the South Pole.3. Hybrid particle and radio detector DAQThe scintillation detectors and the radio antennas share a common DAQ system which is basedon the Transportable Array for eXtremely large area Instrumentation studies (TAXI) [13]. To beable to digitize scintillation and radio detector signals with TAXI, the original design of TAXI hasbeen adapted. To add essential functions for the radio-signal processing, a radio front-end elec-tronic board has been designed and embedded into the system.In Fig. 3 a block diagram of the hybrid DAQ system is shown. The TAXI DAQ samples theincoming analog signal with DRS4 sampling chips. Each DRS4 chip has eight input channels with1024 sampling cells each. The sampling frequency of the DRS4 chips is adjustable from 0.7 GHzto 5 GHz. A sampling rate of 1.0 GHz is used. After a trigger event, all channels of each DRS4 areread out through an eight-channel LTM9007IY-14 analog-to-digital converter (ADC) with 14-bitsampling depth. The trigger is realized with a comparator as a signal-over-threshold trigger withadjustable threshold voltage. This trigger is based on the scintillation detectors and requires the co-incidence of a variable number of triggered scintillation detectors within a time window of 180 ns.In future designs a trigger based on the radio signal will also be available that enables the option ofa radio self-trigger.The TAXI DAQ is designed to house three DRS4 sampling chips of which one samples the incom-ing scintillation detector signals. The remaining two DRS4 chips digitize the signals of two radioantennas with two polarizations each. For the radio-signal sampling the DRS4 chips are operated4PoS(ICRC2019)401Prototype radio antennas for IceTop Max RenschlerFigure 3: Block diagram of the hybrid TAXI DAQ system.in channel-cascading mode which reduces the available DRS4 channels to two per DRS4 but in-creases the number of sampling cells to 4096 for each channel resulting in a radio-trace length ofabout 4 µs.Radio-specific components are placed on a radio front-end board before the TAXI inputs. Theradio front-end electronics consist of a bias tee (JEBT-4R2G+), a protection against electrostaticdischarge (ESD) reaching the electronics over the antenna signal cable, a high-pass filter (SXHP-48+) and a low-pass filter (ULP-340+) which limit the measured signal bandwidth to about 50–350 MHz. To enable the DRS4 channel cascading, the antenna signal has to be fanned out into fourDRS4 inputs. For this, the filtered radio signal is passively fanned out into the inputs of four par-allel amplifiers stages that amplify the signal by 10 dB and convert the single-ended input signal toa differential output signal. The gain of the amplifier stages is chosen to counteract the attenuationfrom the signal cable and prior electronic components. The fanned-out and amplified signals aretransferred via CAT-7 cables with RJ45 connectors to an adapter board that connects the signals tothe respective TAXI inputs.4. Deployment at the South PoleIn January 2019, the existing scintillation detector array was extended by two antennas and anew DAQ system capable of processing the scintillation and radio detector signals (Fig. 3). In Fig.4, a sketch of the detector layout is shown. The antennas are located in the middle of the array,next to the central scintillation detectors and the DAQ electronics. The antenna polarizations aimto possible sources of radio background like the ICL, the DAQ electronics and other detectors. Thegoal of this arrangement is to measure the radio background in the field and to identify strong noisesources producing radio-frequency interference (RFI) peaks in the targeted frequency bandwidth.In Fig. 5, a photo of the detector setup at the South Pole in January 2019 is shown. The pictureshows the two deployed antennas together with the central scintillation detectors and the IceAct5PoS(ICRC2019)401Prototype radio antennas for IceTop Max RenschlerFigure 4: Not-to-scale layout of the deployed scintillation and radio detectors with scintillation detectors asrectangles, IceTop tanks as circles and radio antennas as stars (January 2019).Figure 5: Detector site with scintillation detectors (1), radio antennas (2) and elevated IceAct camera (3).camera. The antenna mechanical structure was found to be fast and easy to deploy, an importantcriterion for a future deployment of 96 antennas in the footprint of IceTop. The two antennas weredeployed in a period of two days with a team of two people. The deployment may be sped up totwo to three antennas per day with an experienced team.5. First measurement resultsParallel to the deployment of the antennas and the hybrid DAQ electronics, background mea-surements near the ICL were performed to estimate the radio background prevalent near the detec-tors. For these measurements, the 2.3 m-high antenna has been used. The antenna was connectedwith a 30 m cable to a commercial DAQ system (CAEN DT5730) located inside of the ICL. Thedistance between the ICL and the antenna was roughly 25 m. The nearby scintillation detectors andthe IceAct camera were powered off.6PoS(ICRC2019)401Prototype radio antennas for IceTop Max RenschlerIn Fig. 6, the average frequency spectrum of two one-hour measurements in the direction of theFigure 6: Measurement of the background frequency spectrum near the ICL. The black dashed line showsthe expected background noise originating from the Galactic Center and other sources in the sky.ICL (blue) and perpendicular to the ICL (red) are shown. Comparing the two measurements, theexpected behaviour of an increase in noise when aiming the antenna polarization towards the ICLis visible. Both spectra show unexpectedly high RFI peaks. These peaks become visible at 50 MHzand reappear every 10 MHz. The source of this RFI noise is unknown. With available informationfrom the Antarctic Service Contractor (ASC), sources such as radio communication or beacons canbe excluded. Other experiments performing background studies like ARA [14, 15] and previousfield measurements [16] did not observe these RFI signals. Since the presented measurements havebeen performed at a distance of about 25 m to the ICL, which houses a workshop with electronicequipment on the first floor, a noise source inside of the ICL is possible. Measurements with thedeployed antenna farther away from the ICL will help understand this radio background.Fig. 6 also includes the prediction of the sky radio background as a solid black line and the ex-pected amplified sky radio background as a dashed black line. The amplified sky radio backgroundincludes the gain of the antenna, the LNA, the low-pass filter and the cable together with a constantthermal noise of 30 K coming from the antenna and the LNA. As LNA response, the simulatedLNA gain from E. de Lera Acedo et al. [17] has been used that stops at a frequency of 50 MHz.The overall measured frequency spectra is higher than the expectation of the sky noise. For the skynoise, the Cane model [18] has been used which predicts an average sky noise originating fromthe whole sky including the Galactic Center. Due to this averaging over the whole sky, a slightdeviation of the expectation and the measurement is expected. The origin of the discrepancy be-tween the measurement and the prediction is unclear. The deviation could come from an additionalthermal noise contribution which has not been included in the amplified background estimation.Measurements with the deployed antennas will give additional information about the overall noisesituation.7PoS(ICRC2019)401Prototype radio antennas for IceTop Max Renschler6. Conclusion and OutlookTwo prototype antennas together with a hybrid DAQ system were deployed at the South Pole inJanuary 2019. Tests of the antenna mechanics as well as the antenna LNA demonstrate the ability ofthe antenna to withstand very low temperatures. First background measurements near the detectorsite and next to the ICL show a frequency spectrum higher than the prediction of the sky radiobackground and with a contamination of RFI signals of unknown origin. Work on investigating theradio background and improving the radio front-end electronics as well as the antenna mechanicsis ongoing, with an aim to deploy an improved test station at the South Pole in the 2019–20 season.These test deployments are preparing for a proposed final detector array consisting of in total 256scintillation detectors and 96 radio antennas. The array will be subdivided in stations of eightscintillation detectors and three radio antennas wthat share one DAQ system [19].References[1] IceCube Collaboration, M. G. Aartsen et al., JINST 12 (2017) P03012.[2] IceCube Collaboration, M. G. Aartsen et al., arXiv:1412.5106.[3] IceCube-Gen2 Collaboration, S. Kunwar, T. Huber, J. Kelley, and D. Tosi, PoS(ICRC2017)401(2018).[4] IceCube Gen2 Collaboration, J. Auffenberg, PoS(ICRC2017)1055 (2018).[5] F. G. Schröder, EPJ Web Conf. 208 (2019) 15001.[6] F. G. Schröder, Prog. Part. Nucl. Phys. 93 (2017) 1–68.[7] T. Huege, Phys. Rept. 620 (2016) 1–52.[8] Tunka-Rex, LOPES Collaboration, W. D. Apel et al., Phys. Lett. B763 (2016) 179–185.[9] HESS Collaboration, Nature 531 (2016) 476 – 479.[10] A. Balagopal V., A. Haungs, T. Huege, and F. G. Schröder, Eur. Phys. J. C78 (2018) 111.[11] IceCube Collaboration, F. G. Schröder, PoS(ICRC2019)418,these proceedings (2019).[12] E. de Lera Acedo, N. Razavi-Ghods, N. Troop, N. Drought, and A. J. Faulkner, ExperimentalAstronomy 39 (Oct, 2015) 567–594.[13] T. Karg, A. Haungs, M. Kleifges, R. Nahnhauer, and K. H. Sulanke, 6th International Workshop onAcoustic and Radio EeV Neutrino Detection Activities (ARENA 2014) Annapolis, MD, June 9-12,2014 (2014).[14] ARA Collaboration, P. Allison, J. Auffenberg, R. Bard, J. J. Beatty, D. Z. Besson, S. Böser, C. Chen,P. Chen, and A. Connolly, Astroparticle Physics 35 (Feb, 2012) 457–477.[15] ARA, IceCube Collaborations, S. Böser, AIP Conference Proceedings 1535 (2013) 116–120.[16] J. Auffenberg, T. Gaisser, K. Helbing, T. Huege, T. Karg, and A. Karle, Nucl. Instrum. Meth. A604(2009) S53–S56.[17] E. de Lera Acedo, N. Drought, B. Wakley, and A. Faulkner, 2015 International Conference onElectromagnetics in Advanced Applications (ICEAA) (Sep., 2015) 839–843.[18] H. V. Cane, Monthly Notices of the Royal Astronomical Society 189 (Nov., 1979) 465–478.[19] IceCube Collaboration, M. Kauer, PoS(ICRC2019)309,these proceedings (2019).8

First measurements with prototype radio antennasfor the IceTop detector array

First measurements with prototype radio antennas for the IceTop detector array

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First measurements with prototype radio antennasfor the IceTop detector array

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