Air Shower Observation by a Simple Structured Fresnel lens Telescope with Single Pixel for the Next Generation of Ultra-High Energy Cosmic Ray Observatory
Yuichiro Tameda, Takayuki Tomida, Mashu Yamamoto, Hirokazu Iwakura, Daisuke Ikeda, Katsuya Yamazaki
aa r X i v : . [ a s t r o - ph . I M ] M a r Prog. Theor. Exp. Phys. , 00000 (10 pages)DOI: 10.1093 / ptep/0000000000 Air Shower Observation by a SimpleStructured Fresnel lens Telescope with SinglePixel for the Next Generation of Ultra-HighEnergy Cosmic Ray Observatory
Yuichiro Tameda , Takayuki Tomida , Mashu Yamamoto , Hirokazu Iwakura ,Daisuke Ikeda , and Katsuya Yamazaki Osaka Electro Communication University, Neyagawa, Osaka, Japan ∗ E-mail: [email protected] Shinshu University, Nagano, Nagano, Japan Institute for Cosmic Ray Research, The University of Tokyo, Kashiwa, Chiba, Japan Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo, Japan Kanagawa University, Yokohama, Kanagawa, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Improved statistics and mass-composition-sensitive observation are required to clarifythe origin of ultra-high energy cosmic rays (UHECRs). Inevitably in the future, theUHECR observatories will have to be expanded due to the small flux; however, theupgrade will be expensive with the detectors currently in use. Hence, we are developinga new fluorescence detector for UHECR observation. The proposed fluorescence detector,called cosmic ray air fluorescence Fresnel-lens telescope (CRAFFT), has an extremelysimple structure and can observe the longitudinal development of an air shower. Fur-thermore, CRAFFT has the potential to significantly reduce costs for the realization ofa huge observatory for UHECR research. We deployed four CRAFFT detectors at theTelescope Array site and conducted the test observation. We have successfully observedten air-shower events using CRAFFT. Thus, CRAFFT can be a solution to realize thenext generation of UHECR observatories. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Subject Index F00, F03
1. Introduction
Since the discovery of cosmic rays in 1912 by V.F. Hess, cosmic rays over a wide range ofenergies have been observed. However, the origin of cosmic rays, especially those with energiesabove 10 eV, has not yet been clarified. In this regard, understanding the UHECR energyspectrum, mass composition, and anisotropy is crucially important. AGASA [1] reported thatthe energy spectrum of cosmic rays extends beyond 10 eV without the Greisen-Zatsepin-Kuzmin (GZK) limit [2], even though the energy spectrum of HiRes [3] is consistent withthe GZK cut-off. In recent years, the Pierre Auger Observatory (Auger) [4], and TelescopeArray (TA) [5] confirmed this energy spectrum, demonstrating a flux suppression consistentwith the GZK limit. TA also reported that an intermediate-scale of anisotropy of cosmicrays with energies greater than 57 EeV exists in the northern sky, called as the hotspot [6].In addition, Auger reported that the direction of arrival of cosmic rays with energies above c (cid:13) The Author(s) 2012. Published by Oxford University Press on behalf of the Physical Society of Japan.This is an Open Access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by-nc/3.0), which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited.
EeV has a dipole structure [7], indicating that the sources of such high-energy cosmicrays should be outside our galaxy. Ultra-high energy cosmic rays (UHECRs) with energiesabove 10 eV and a light nucleus (e.g. a proton) can propagate with only a few degreesof deflection in the galactic magnetic field, which is a few micro gauss. As a result, it isexpected that UHECR sources can be identified. Currently, HiRes and TA have reportedthat the composition of UHECRs above ∼ EeV is dominated by or consistent with lightcomponents such as proton [8][9]. On the other hand, Auger has indicated transition in thecomposition, from a light to heavy component at energies above 10 . eV [10]. However,TA and Auger X max data are in good agreement within the systematic uncertainties at themoment [11]. To understand UHECR composition and identify its sources, higher statisticsand composition-sensitive observation are indispensable.In the future, the expansion of UHECR observatories to obtain better statistics will beinevitable, because the flux of UHECR with energies above 10 eV is as low as a few eventsper 1000 km per year. To realize these inevitable upgrades with the detectors currentlyin use, the cost will inflate in proportion to the effective area’s extension. Therefore, costreduction of detectors will be required. Hence, to help realize the required cost reduction,we have developed a cosmic ray air fluorescence Fresnel lens telescope (CRAFFT) basedon the concept of a simple air fluorescence detector [12]. This concept is highly suitable forcost reduction to realize a huge ground array composed of fluorescence detectors which canobserve air shower longitudinal developments. Other solutions, such as a simple fluorescencedetector, to achieve a large effective area have also been proposed and are being studied. Forexample, the FAST project [13] is developing telescopes comprising a composite mirror and 8-inch Photomultiplier Tubes (PMTs). Other candidates including fluorescence detectors fromspace [14][15][16] are also under development. Herein, we report the development and trialresults of CRAFFT, a novel fluorescence detector that is extremely simple and inexpensivefor the next generation of large-scale UHECR observatories.
2. Cosmic Ray Air Fluorescence Fresnel lens Telescope (CRAFFT)
The fluorescence technique adopted by CRAFFT was originally studied in the 1960s. At thattime, various fluorescence detectors were developed, then the detection of the fluorescencelight from air showers succeeded by using Fresnel lens [17]. After that, not Fresnel lensbut mirrors have been more adopted as light collectors for fluorescence detectors, of whichFlys Eye was one of the first successful example [18]. Currently, technological advancementshave enabled easy achievement of large Fresnel lenses with high UV transmittance, high-sensitivity photon sensors, and Flash ADC (FADC). JEM-EUSO is one of the challengingproject for UHECR observation developing a high resolution Fresnel lens telescope with largeaperture and large field of view (F.O.V.) from space [14]. On the other hand, we are tryingto develop a simple structure fluorescence detector with a Fresnel lens for a huge groundarray to observe UHECRs.
An air fluorescence detector measures scintillation light emitted from N molecules in theatmosphere excited by energetic particles in extensive air showers that are induced by high-energy cosmic rays. The wavelength band of the scintillation light ranges from 300 to 400 nmwith the peaks mainly at 337 . . . s attenuated via Rayleigh and Mie scattering while passing through the atmosphere. Thesefaint UV photons are gathered and concentrated at the focal point of a fluorescence detector.A modern fluorescence detector can observe an air shower track using multi pixels suchthat TA fluorescence detector (FD) has 256 PMTs [19]; this allows the shower detectorplane (SDP) to be determined. The geometry of air showers can be determined by thearrival-timing information of each pixel. When an air shower is stereoscopically observed bymore than two fluorescence detectors at different sites, its geometry can be determined as theline of SDP intersection. In the case of a hybrid detector, such as TA or Auger, comprisingfluorescence and surface detectors, the additional information of arrival timing or shower coreposition provided by the surface detectors can allow the geometry to be determined withan accuracy that is better than 1 ◦ . Once the geometry of an air shower is determined, theamount of light emitted onto the shower axis, which is proportional to the energy loss of theshower particles, can be calculated by considering the atmospheric attenuation. Followingthis calculation, the primary energy of cosmic rays can be estimated calorimetrically.In order to determine the air shower geometry, a single-pixel Fresnel lens telescope suchas CRAFFT detector records time profile of fluorescence photons from the air shower usingFADC together with at least two more detectors deployed separately with a spacing of ∼
20 km. Without the information of scintillation detector or water Cherenkov detectorarray, the geometry can be determined by arrival time information of photons. Once thegeometry is determined, the longitudinal development of energy deposit as a function ofatmospheric depth along the shower track can be reconstructed. Then, the time profile ofFADC considering the reconstructed longitudinal development can be simulated. Using thesimulated FADC time profile, the shower geometry is fitted again to reproduce the recordedFADC time profile. By the repetition of the above procedure, the accuracy of the geometryand the longitudinal development will be improved as a method of the profile constrainedgeometry fit technique adopted by the Fly’s Eye experiment [20]. Then, the energy and the X max can be determined.The effective distance of a single-pixel Fresnel lens fluorescence detector with 1 m apertureand 16 ◦ × ◦ F.O.V. in which air showers of 10 eV can be triggered is estimated to beup to 25 km by the simulation [21]. Therefore, single-pixel FDs will be deployed as a hugearray of a triangle lattice with 20 km spacing [12]. The structure of the CRAFFT detector is very simple, which makes it cost-effective and easyto deploy. Table 1 lists the main components of the detector. Figure 1 shows the structuraldesign of the CRAFFT detector. The CRAFFT detector’s light collector is a Fresnel lens,which is made of UV-transmitting acrylic plastic, with a focal length of 1 . φ .
33 mm, respectively.The material of the Fresnel lens is acrylic plastic (Mitsubishi Chemical, ACRYLITE 000)and its transparencies is shown in Fig. 2 on the left. The scattering loss caused by the fine ig. 1
The structure of the CRAFFT detector made of anodized T-slotted aluminumextrusions. Left: At the aperture, two frames are attached to support the plastic lens tomaintain the flatness. The elevation angle can be adjusted. Right: PMT mount installed atthe focus of the Fresnel lens.
Table 1
Component list of the CRAFFT detectorComponent ProductFresnel lens NTKJ, CF1200-BUV-transmitting filter Hoya, U330Photomultiplier tube Hamamatsu, R5912HV power supply CAEN, N1470ARHV divider (special order)FADC board TokushuDenshiKairo, Cosmo-ZLow pass filter Mini Circuit, BLP-15+Amplifier LeCroy, MODEL 612AMStructure YUKI, anodized T-slotted aluminum extrusionspitch of Fresnel lens is estimated 1.5% for the incident light with the angle of 4 ◦ by raytracing simulation.To reduce night sky background light in the visible range that does not contribute to thesignal, a UV-transmitting filter of the range of 300 −
400 nm is used. The filter is flat, andits dimensions are width 130 mm, length 130 mm and thickness 2 . ± ◦ to reduce the ambiguity of detection efficiency at the periphery of thephoton entrance windows (Fig. 4). The quantum efficiency at 390 nm and the typical gainwith 1500 V of the PMT are 25 % and 1 . × , respectively. ig. 2 Transparency of PMMA which is the material of the Fresnel lens (Left) and theUV-transmitting filter (Right) measured by a spectrometer (USB4000, Ocean Optics) witha mini deuterium halogen light source (DT-MINI-2-GS, Ocean Optics).We use a high-voltage power supply that is controlled via a network to ensure that theData acquisition (DAQ) system of CRAFFT can be controlled remotely.The signal from air showers is so faint that we use amplifier to amplify the signal and lowpass filter to reduce the high frequency night sky background. The signal from the PMTis digitized and recorded by the FADC board on which Linux is available with Zynq. Zynqis a FPGA on which We can implement our own trigger algorithm now we are developing.The sampling rate and resolution of the Cosmo-Z are 80 MHz and 12 bits, respectively.We can time events with precise time stamps using a GPS module (Linx Technologies Inc.EVM-GPS-FM), which provides 1 pulse per second as well as time information.For the frame of the detector, T-slotted aluminum extrusions, that are black anodizedto reduce light reflection, are used. Aluminum extrusions are easy-to-build and modify;hence, this a reasonable choice for the prototype test. To shield stray light and protect thecomponents inside, the detector is covered by an 0 . ± ◦ , limited by the spatial filter (Fig. 4). The elevation angles of thethree detectors were 28 ◦ and that of the other detector was 20 ◦ . In this observation, weoptimized the arrangement of the detectors to observe relatively low-energy air showersbecause the priority was to detect as many air shower events as possible. The F.O.V. ofthe four detectors corresponded to the 6th fluorescence detector of TA, as shown in Fig 5.The upper center and lower detectors observe the vertical laser from the TA Central LaserFacility (CLF) [23]. The remaining upper-viewing two detectors observe at an angle 8 ◦ eastor west from the center detector.
3. Observation
In 2017, we conducted a test observation for ten days from November 9 to November 23,when the TA FD was operating. The total observation time was 63.4 h. ig. 3
Left: The location of CRAFFT next to the TA FD station. Right: Four deployedCRAFFT detectors.
Fig. 4
Spatial filter to limit the F.O.V. of the CRAFFT detector, which in turn reducesthe effect of the nonuniform photo detection efficiency due to the spherical surface of thePMT.We operated CRAFFT remotely from the control room of the TA FD at the Black RockMesa site. Figure 6 shows the block diagram of the electronics and DAQ system of CRAFFT.TA FDs recognize air shower events every 12 . µ s and generate trigger timing pulses thatare distributed from the front panel of trigger electronics [22]. We used a trigger timingsignal from TA FD for data collection. We measured the relative gain of the telescopes,including the transparency effect, for both the lens and the UV filter, using a UV LEDmounted on the lens surface. We regulate the applied voltage of PMT to adjust gain whilemeasuring the LED light. As a result, we adjusted the gain of each telescopes within 20%.In this observation, we acquired 456,727 events. We searched air shower candidate eventsfrom all 456,727 data points recorded by CRAFFT. Most of the data did not include asignal because CRAFFT recorded the data using the trigger pulse from TA FD, the F.O.V.of which is about ten times larger than that of the CRAFFT detector. First, we selectedthe data with significant signals against the background. In addition, we excluded the CLFevents that could easily identified from the time stamps. Figure 7 shows an example of aCLF event. Moreover, air plane events were also removed using a pulse width that is muchwider than that of typical air shower events. Next, we selected the events with significant − − − − − − E l e v a ti on a ng l e [ d e g ] FD00FD01 FD02FD03 FD04FD05 FD06FD07 FD08FD09 FD10FD11
Fig. 5
The F.O.V. of each CRAFFT detector is shown as circle drawn over the F.O.V.of the TA FDs edge of which is shown as a solid line.
Fig. 6
Block diagram of electronics and DAQ system of CRAFFT.signals registered by more than two CRAFFT detectors. After this selection process, weobtained 6,600 event candidates. Finally, we extracted signals like air shower event via eyescanning while considering pulse width and height. We found ten apparent air shower eventsas a result of having compared those signals with corresponding TA Signals.
Table 2
The number of events that applied the selection.Number of acquired data 456,727Number of event candidate 6,600Number of clear air shower event 10Figure 8 shows some typical events that we successfully observed. The left panels of Fig.8 show waveforms of an air shower fluorescence signal recorded by CRAFFT. The rightpanels show the corresponding event displays of TA FD. As shown in Fig. 8, we successfullyobserved air shower events that were also identified as air shower events by the TA FD.
10 20 30 40 50 60s] µ Time [10001100120013001400150016001700180019002000 AD C c oun t LowerUpper center µ Time [10001100120013001400150016001700180019002000 AD C c oun t LowerUpper center
Fig. 7
Waveforms of CLF detected by CRAFFT. Left: Typical single CLF event. Right:Averaged waveform of 133 CLF events.
4. Discussion
We are developing a simple Fresnel lens FD, named CRAFFT, for UHECR observation.We deployed four CRAFFT detectors at the TA site and performed test observations. Weacquired ten air shower events. We successfully demonstrated that UHECR can be observedusing CRAFFT, which has a simple structure and an economic detector comprising onlycommercial products. This can be an attractive option for application in next generationUHECR observatories. In the case of a reflecting telescope especially for air shower obser-vation, a large phototube cluster can be an obstacle to screen incident lights. On the otherhand, all of incident lights can be focused at the focal plane with a refracting telescope.Therefore, a refracting telescope can be smaller than a reflecting telescope to achieve thesame light collection efficiency. Additionally, it is much easier to extend the field of view byenlargement of the focal plane. The structure of a refracting telescope is very simple andcompact so that all the component can be covered easily to keep detectors clean. This isgood for easy maintenance. From these points, it is expected to impart the advantage oflower cost than ever.FDs previously used for UHECR observation are expensive due to their strong structurerequired to support a large composite mirror system and the multi-channel DAQ system.Additionally, detectors must be covered by a building. In contrast, a refracting telescopeusing a Fresnel lens with a single-pixel phototube, such as the CRAFFT detector, possessesa simple structure. Thus, the cost reduces by more than ten times than that of the previousdetectors. Moreover, the CRAFFT detector is easy to deploy and can be installed withouta hut as all of its components can be covered by the structure alone due to its compactness.We successfully demonstrated that a single-pixel Fresnel lens FD can be used for UHECRdetection. For future work, we plan for stereo or multiple observations to establish a methodto reconstruct geometry using CRAFFT. We believe that the current configuration ofCRAFFT can be further optimized. We will also attempt the use of multiple pixels toimprove
S/N or double lenses to extend the F.O.V., and reduce the cost per view. CRAFFT µ Time [05001000150020002500 AD C c oun t Upper leftLowerUpper centerUpper right − − − − − − − − − Azimuth angle [deg]0510152025303540 E l e v a ti on a ng l e [ d e g ] FD04FD05 FD06FD07 [a] 2017/11/11 05:59:54.835620750 µ Time [05001000150020002500 AD C c oun t Upper leftLowerUpper centerUpper right − − − − − − − − − Azimuth angle [deg]0510152025303540 E l e v a ti on a ng l e [ d e g ] FD04FD05 FD06FD07 [b] 2017/11/15 06:16:57.922469950
Fig. 8
Typical air shower event acquired by CRAFFT. Left: The FADC count of eachchannel that can be compared within the accuracy of 20%. Each bin of waveform is summedas 100 ns. Right: TA FD event display. Solid line is the edge of TA FD F.O.V. 1 ◦ circles areviews of TA FD triggered channels. 8 ◦ circles are the F.O.V. of CRAFFT.is a promising detector that may significantly contribute to the realization of next-generationUHECR observatories, post TA × Acknowledgment
This work was supported by the JSPS KAKENHI Grant Numbers 25610051 andJP16K17710. This work was partially carried out by the joint research program of theInstitute for Cosmic Ray Research (ICRR), The University of Tokyo. This study wasalso supported by the Earthquake Research Institute The University of Tokyo JointUsage/Research Program. The Telescope Array Collaboration supported CRAFFT as anassociated experiment and allowed us to use TA equipments and FD event displays. Wewish to thank the staffs at the University of Utah, especially Prof. J.N. Matthews. eferences [1] M. Takeda et al. , Phys. Rev. Letters , 1163-1166 (1998).[2] K. Greisen, Phys. Rev. Lett. , 748 (1966); G.T. Zatsepin and V.A. Kuzmin, JETP Lett. , 78 (1966).[3] R.U. Abbasi et al. , Astropart. Phys. , 157-174 (2005).[4] The Pierre Auger Collaboration, Physics Letters B , 239246 (2010).[5] T. Abu-Zayyad et al. , The Astrophysical Journal Letters , 1 (2013).[6] R.U. Abbasi et al. , The Astrophysical Journal Letters , 2 (2014).[7] The Pierre Auger Collaboration, Science no.6537, 1266-1270 (2017).[8] R.U. Abbasi et al. , Physical Review Letters , 161101 (2010).[9] R.U. Abbasi et al. , The Astrophysical Journal , 76 (2018).[10] A. Aab et al. , Physical Review D , no.12, 122005 (2014).[11] R.U. Abbasi et al. , Proceedings of international symposium for Ultra-High Energy Cosmic Rays, 010016(2016)[12] P. Privitera et al. , Internal Symposium on Future Directions in UHECR Physics (2012).[13] D. Mandat et al. , Journal of Instrumentation T07001 (2017).[14] M. Casolino, Proceedings of the 35th International Cosmic Ray Conference, 370 (2017).[15] P. Klimov et al.
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