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. 2016 Mar 3;531(7592):70-3.
doi: 10.1038/nature16976.

A large light-mass component of cosmic rays at 10(17)-10(17.5) electronvolts from radio observations

S Buitink  1   2 A Corstanje  2 H Falcke  2   3   4   5 J R Hörandel  2   4 T Huege  6 A Nelles  2   7 J P Rachen  2 L Rossetto  2 P Schellart  2 O Scholten  8   9 S ter Veen  3 S Thoudam  2 T N G Trinh  8 J Anderson  10 A Asgekar  3   11 I M Avruch  12   13 M E Bell  14 M J Bentum  3   15 G Bernardi  16   17 P Best  18 A Bonafede  19 F Breitling  20 J W Broderick  21 W N Brouw  3   13 M Brüggen  19 H R Butcher  22 D Carbone  23 B Ciardi  24 J E Conway  25 F de Gasperin  19 E de Geus  3   26 A Deller  3 R-J Dettmar  27 G van Diepen  3 S Duscha  3 J Eislöffel  28 D Engels  29 J E Enriquez  3 R A Fallows  3 R Fender  30 C Ferrari  31 W Frieswijk  3 M A Garrett  3   32 J M Grießmeier  33   34 A W Gunst  3 M P van Haarlem  3 T E Hassall  21 G Heald  3   13 J W T Hessels  3   23 M Hoeft  28 A Horneffer  5 M Iacobelli  3 H Intema  32   35 E Juette  27 A Karastergiou  30 V I Kondratiev  3   36 M Kramer  5   37 M Kuniyoshi  38 G Kuper  3 J van Leeuwen  3   23 G M Loose  3 P Maat  3 G Mann  20 S Markoff  23 R McFadden  3 D McKay-Bukowski  39   40 J P McKean  3   13 M Mevius  3   13 D D Mulcahy  21 H Munk  3 M J Norden  3 E Orru  3 H Paas  41 M Pandey-Pommier  42 V N Pandey  3 M Pietka  30 R Pizzo  3 A G Polatidis  3 W Reich  5 H J A Röttgering  32 A M M Scaife  21 D J Schwarz  43 M Serylak  30 J Sluman  3 O Smirnov  17   44 B W Stappers  37 M Steinmetz  20 A Stewart  30 J Swinbank  23   45 M Tagger  33 Y Tang  3 C Tasse  44   46 M C Toribio  3   32 R Vermeulen  3 C Vocks  20 C Vogt  3 R J van Weeren  16 R A M J Wijers  23 S J Wijnholds  3 M W Wise  3   23 O Wucknitz  5 S Yatawatta  3 P Zarka  47 J A Zensus  5
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A large light-mass component of cosmic rays at 10(17)-10(17.5) electronvolts from radio observations

S Buitink et al. Nature. .

Abstract

Cosmic rays are the highest-energy particles found in nature. Measurements of the mass composition of cosmic rays with energies of 10(17)-10(18) electronvolts are essential to understanding whether they have galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal comes from accelerators capable of producing cosmic rays of these energies. Cosmic rays initiate air showers--cascades of secondary particles in the atmosphere-and their masses can be inferred from measurements of the atmospheric depth of the shower maximum (Xmax; the depth of the air shower when it contains the most particles) or of the composition of shower particles reaching the ground. Current measurements have either high uncertainty, or a low duty cycle and a high energy threshold. Radio detection of cosmic rays is a rapidly developing technique for determining Xmax (refs 10, 11) with a duty cycle of, in principle, nearly 100 per cent. The radiation is generated by the separation of relativistic electrons and positrons in the geomagnetic field and a negative charge excess in the shower front. Here we report radio measurements of Xmax with a mean uncertainty of 16 grams per square centimetre for air showers initiated by cosmic rays with energies of 10(17)-10(17.5) electronvolts. This high resolution in Xmax enables us to determine the mass spectrum of the cosmic rays: we find a mixed composition, with a light-mass fraction (protons and helium nuclei) of about 80 per cent. Unless, contrary to current expectations, the extragalactic component of cosmic rays contributes substantially to the total flux below 10(17.5) electronvolts, our measurements indicate the existence of an additional galactic component, to account for the light composition that we measured in the 10(17)-10(17.5) electronvolt range.

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