Space experiment TUS on board the Lomonosov satellite as pathfinder of JEM-EUSO
Space-based detectors for the study of extreme energy cosmic rays (EECR) are being prepared as a promising new method for detecting highest energy cosmic rays. A pioneering space device – the “tracking ultraviolet set-up” (TUS) – is in the last stage of its construction and testing. The TUS detector will collect preliminary data on EECR in the conditions of a space environment, which will be extremely useful for planning the major JEM-EUSO detector operation.
KeywordsAir-shower fluorescence telescope JEM-EUSO Pathfinder
Existing ground-based experimental arrays do not collect statistically significant data on EECR – especially beyond the energy of GZK suppression. New methods of EECR observation, with two orders of magnitudes larger exposure, are needed for the solution of this problem. The method of EAS fluorescence observation from satellites, as proposed by Linsley & Benson , promises to become such a technique. Alternative optical designs of space detectors were suggested from the beginning: First, wide field-of-view lens optics (first the OWL project , then the EUSO project  and now the JEM-EUSO project ), and second, mirror optics (KLYPVE project) based on the experience of producing large area concentrators for solar generators (Russian initiative of SINP MSU and RSC Energia [5, 6] which now is realized as Tracking Ultraviolet Set-up (TUS) prototype of KLYPVE detector by SINP MSU, JINR-Dubna with participation of Universities in Korea and Mexico [7, 8]). The TUS detector will be launched on board the Lomonosov satellite as part of the instrumentation aimed for studies of Extreme Phenomena in the Universe.
Space detectors have advantages in several aspects. EAS fluoresent tracks generated by EECR particles can be observed on a huge area of the Atmosphere owing to great distance from the detector to the atmosphere. For example, at the orbit height of the ISS (350–400 km), the JEM-EUSO detector will survey 2·105 km2 area of the Atmosphere.
In one (or more) year of its in-orbit operation, the detector observes the entire celestial sphere. This will allow the distribution of EECR sources to be studied, despite possible inaccuracy in the determination of the primary particle energy. An unavoidable difference in the absolute value of the energy measured by different ground-based arrays causes a difference in EECR intensity in different sky regions covered by different arrays.
At the same time, detectors on board the satellites encounter a variety of difficulties. Observation of EAS from a large distance requires higher sensitivity and resolution of the optical system, including the photon detector. Desirable resolution of one detector pixel should be equal to the diameter of the lateral electron distribution in a shower. For a satellite orbit height of 500 km, the angular resolution of the orbital detector should be 0.4–2 mrad. Nighttime atmospheric noise in the UV wavelength band (300–400 nm) used for EAS detection varies on the path of the satellite. The data from the Universitetsky-Tatiana satellites [9, 10] gave a scale of such variations: from 3·107-2·108 photons cm−2 sr−1 s−1 on moonless nights (lower values are above oceans, higher values are above aurora zones and cities) to 2·109 photons cm−2 sr−1 s−1 on nights of full moon. Ground-based arrays operate on moonless nights at specially chosen locations with a noise level lower than 5·107 photons cm−2 sr−1 s−1. Impulsive noise from lightning and accompanying high altitude discharges will add to the average noise level.
Technology of an orbital fluorescence detector should satisfy complex conditions of space operation. Bearing these difficulties in mind, a program for the gradual conversion of a UV fluorescence detector that operated at ground level to a space version was started . The TUS detector is the first, comparatively “simple” instrument that will verify the reliability and stability of an optical system and photo detector design for an operation in space.
2 The space detector TUS
The mirror-concentrator is designed as a combination of a central parabolic mirror and 11 parabolic rings focusing a parallel beam on one focal point. In this design, the thickness of the mirror construction is small (3 cm), which is important for the mirror implementation into the satellite construction. The mirror focal distance is 1.5 m. The mirror is cut into hexagonal segments with a diameter of 66 cm. Mirror segments are made of carbon plastic strengthened by honey comb aluminum plate so that the mirror construction is temperature stable in a wide range of temperatures. The mirror surface is obtained as plastic replicas of aluminum press forms (one for the central mirror part and one for the 6 lateral parts).
The plastic mirror surface is covered by aluminum film and protected by a MgF2 coat in a vacuum evaporation process. Reflectivity of the mirror surface at wavelengths 300–400 nm (operation range for measuring the atmosphere fluorescence) is 85 %. TUS mirror passed various space qualification and optical tests. Those tests show the stability of the optical quality of the mirror in space conditions.
Photo detector pixels are photomultiplier tubes PMT R1463 of Hamamatsu with multi-alcali cathode of 13 mm diameter. The quantum efficiency of the PMT cathode is 20 % for wavelengths 300–400 nm. A PMT’s multi-alcali cathode (instead of bi-alcali one usually used in ground-based fluorescence detectors) was chosen for operation in a wider range of temperatures, in which the cathode operates in the linear regime. To make the detector field of view (FOV) uniformly filled with pixels, light guides, with square entrance (15 × 15 mm), and circle output, adjusted to PMT cathode, were used. TUS electronics [11, 12] measure the integral number of PMT photoelectrons digitized by ADC every interval of 0.8 µs in every pixel.
PMTs gain follows intensity of the UV received by pixel in a time interval of 100 ms so that digitized photoelectrons number is coded by 2 numbers: code N of ADC and code M of PMT gain (high voltage). In this mode, pixel PMT operates night and day keeping sensitivity for years. Two years of operation with 10 % stability of gain was proved in measurements of Tatiana satellite pixels. It should be mentioned that TUS electronics is different to JEM-EUSO electronics, currently under testing in other JEM-EUSO prototypes. The qualification testing of phototubes was done with the hardware and software experimental setup (test bench), used successfully by JINR group for PMTs test of ATLAS Tile Calorimeter at LHC. After testing, PMTs with similar gain were grouped in 16 tube clusters. Data from each tube in the cluster are digitized by one ADC and then analyzed and memorized by the cluster FPGA. Final detector triggering and memorizing of all data is done by TUS central FPGA. The information volume of one EAS data is ≈100 Kbytes. The expected volume of the EAS data from one day transmitted to the mission center is 250 Mbytes.
3 Detector performance
This result confirms the TUS EECR 2-level trigger proposed in the earlier work . In the first level, pixels with a signal A-times larger than noise RMS are selected. In the second level, events with B neighbour pixels having signal ≥ A and line up in space and time, are selected. The final trigger begins recording data from all pixels. Numbers A and B will be controlled from the mission center in a compromise between trigger rate, limited by the upper volume of information to be transmitted per day, and the TUS detector energy threshold.
4 Present data on background effects in measuring EECR from space
Taking into account these data on the atmosphere glow, the exposure of TUS detector with FOV of 9° was estimated . Efficiency of EECR event selection is close to 100 % for E > 300 EeV and those events will be collected with exposure of 12000 km2 sr yr in 3 years of in-orbit operation. Events with lower energies will be detected with less efficiency (exposure). For steep energy spectrum of EECR after the GZK limit (integral spectrum exponent ≈ -4 for energies E > 50 EeV), the statistics with minimum threshold energy (∼ 70 EeV) will be determined by real exposure at the darkest regions of the Earth: above the Pacific Ocean, deserts and part of Siberia (Fig. 5). With such limited exposure, the TUS detector will not make a breakthrough in the problem of EECR origin. Its aim is to check the EAS fluorescence detector performance in a space environment. In orbital flight, the TUS detector will operate above some more intense sources of background glow: aurora lights, city lights, some unknown sporadic lights. Experimental results of Tatiana-1 satellites showed that these higher intensity glow sources are not large in size of atmosphere area and do not greatly affect the exposure of the detector. Other sources of background in orbital EECR measurements are short UV flashes (duration of 1–100 ms), the origin of which is related to electrical discharges in the atmosphere. The last achievement in studying this background is data from Tatiana-2 satellite. The UV-detector of this satellite operated in conditions close to the orbital EECR detector, measuring the temporal structure of flashes in the atmosphere area of thousands of square km in nadir direction. Measurements were done for a wide range of photon number Qa in the atmosphere UV flash: from Qa=1021 up to Qa˜1025 where tens of events were registered. The main features of flashes with Qa > 1023 - their duration of 10–100 ms, their global distribution concentrated in equatorial region above continents - suggested that those flashes are either lightning itself or transient luminous events (TLE) generically related to lightning. Those bright flashes will be easily separated from EAS fluorescent signals due to their long duration and enormous number of photons (to compare with EAS parameters: duration of not more than 0.1 ms and number of UV photons Qa˜1016 for E=100 EeV).
Bright flashes are concentrated in the equatorial region above continents in agreement with lightning activity. Their rate in these regions is of the order of 10−3 km−2hr−1. Dim flashes are distributed more uniformly, with the rate of ≈ 10−4 km−2hr−1. Rates of both kinds of flashes are much higher than the expected rate of EECR events: ≈ 10−6 km−2hr−1 . Thus, a problem of distinguishing EAS flashes from atmospheric flashes could be complicated.
Operation of the TUS orbital detector will be an important stage of EECR study by a new method of fluorescent tracking of EAS from space. Its main aim is check the EAS fluorescence detector performance in a space environment. Experience of the TUS operation in space will be used in future work on space EECR detectors (KLYPVE, JEM-EUSO) construction.
TUS operation above different regions of the Earth will clarify the role of UV background in measuring the fluorescence signal of EECR events and test the possibility of reliable distinguishing of EAS signal from the bright TLE and dim flashes in the atmosphere. The main criteria are the photon number, duration of the event and spatial distribution of the signal. There is a great difference in the photon number of EECR flashes (Qa ≈ 1016 for E = 100 EeV) and in the number of photons in atmospheric flashes, even for dim ones (Qa ≈ 1016−1022). The duration of EECR flashes is much shorter than that of atmospheric flashes (0.1 ms for EECR flash and 1 ms for atmospheric dim flashes). Usual TLE should provide a big spot on the focal surface while the image of EAS is a rapidly moving along a straight line. But in spite of this the imitation of EECR events by early stage atmospheric flashes is possible. The TUS operation will give important data on the background effect of atmospheric flashes.
This work is supported partially by the grant of Russian Foundation for Basic Research No. 13-02-12175 ofi-m.
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