Atmospheric Fluorescence Research Articles

Comparison of air fluorescence and ionization measurements of EM shower depth profiles: Test of a UHECR detector technique

Measurements are reported on the fluorescence of air as a function of depth in electromagnetic showers initiated by bunches of 28.5 GeV electrons. The light yield is compared with the expected and observed depth profiles of ionization in the showers. It validates the use of atmospheric fluorescence profiles in measuring ultra high energy cosmic rays.

THE FOCAL SURFACE OF THE EUSO TELESCOPE

The Extreme Universe Space Observatory (EUSO) is a space mission to study extremely high-energy cosmic rays. The EUSO instrument is a wide-angle refractive telescope in near-ultraviolet wavelength region to observe time-resolved atmospheric fluorescence images of the extensive air showers from the International Space Station. The Focal surface is an aspherical curved surface, and its area amounts to about 4.5 m2. The focal surface detector is designed as a mosaic of multianode photomultipliers (MAPMT) for the single photoelectron counting capability. The strongest requirement for the focal surface detector is the maximization of the photon detection efficiency together with the uniformity over the focal surface. We have developed a new type of MAPMT. It is modified from the ordinary one and has a grid between the photocathode and the first dynode to electrostatically demagnify the photoelectron image on the dynode. We are also developing the HV supply system for a great number of MAPMTs. EUSO experiments the day-time and night-time every 90 minutes. The heat flow must be considered to stabilize the PMT characteristics, in parallel with the heat dissipation of the electronics attached on the focal surface supporting structure.

MEMS based Space Telescope for Extreme Energy Cosmic Rays Experiments

We propose a new idea of tracking mirror telescope that observes in space atmospheric fluorescence lights of EAS (Extensive Air Shower) generated by axtreme energy cosmic rays. The concept of the tracking mirror is based on recent development on semiconductor MEMS (MicroElectroMechanical Systems) technology. The tracking mirror consists of a large number of micromirror arrays each of which is controlled independently by addressing signals from the active silicon matrix. The array system is programmed and configured at a given instance to be a mirror-concentrator with axis direction and focal distance both under control in real time. When an EAS event is detected the mirror is rearranged to track the event by minimizing aberrations. The detector resolution can be also controlled during the operation by correcting positions of micromirror arrays. In this paper the conceptual design of the tracking mirror is described.

Photon yields from nitrogen gas and dry air excited by electrons

In order to detect ultrahigh-energy cosmic rays (UHECR), atmospheric fluorescence light from the trajectory of the extensive air shower may be measured by mirror-photosensor systems. In this type of experiment the photon yield from electrons exciting air of various densities and temperatures is most fundamental information for estimating the primary energy of UHECR. An experiment has been undertaken using a 90Sr β source to study the pressure dependence of photon yields, and the life times of the excited states, for radiation in nitrogen and dry air. The photon yield between 300 and 406 nm in air excited by 0.85 MeV electrons is 3.73 ± 0.15 (±14% systematic) photons per meter at 1000 hPa and 20 °C. The air density and temperature dependence is given for application to UHECR observations.

Evaluation method of optical filters used in fluorescence detectors

A calculation method for evaluating UV optical filters, suitable for fluorescence detectors (FD), such as the one from the Auger Project and other similar cosmic ray observatory setups, is presented. We have considered the adopted geometry of a telescope based on a spherical mirror and a camera consisting of a photomultiplier pixel array. In the calculations, an angular weighting factor is introduced, which affects the results of the optical filter performance. Two optical filters, of the interference and absorption type, are evaluated using this method. Experimental measurements of the spectral transmittance of the filters for various angles of incidence are made by a double beam spectrophotometer. To calculate an overall performance, the most dominant spectral lines of the atmospheric fluorescence are used, modified by the atmospheric transmission coefficients. This performance is determined by the detection efficiency of the pixel detector (photomultiplier) with the optical filters used.

The fluorescence detector prototype for the Auger project: mechanical structure, optical system, and filter

The Pierre Auger Observatory will study cosmic rays with energy exceeding 10/sup 19/ eV. The experiment will measure properties of extensive air showers with a hybrid detector consisting of a surface array and an atmospheric fluorescence telescope. This paper presents the characteristics and performance of the optical system for the fluorescence detector prototype. The system adopts a Schmidt camera design with a diaphragm to remove coma aberration. A large spherical mirror (approximately 3.6/spl times/3.6 m, with radius of curvature 3.47 m), segmented in 49 trapezoidal elements, is supported by a mechanical structure, where segments are positioned on the meridians of the spherical cap. At the diaphragm, an absorption filter is installed to preferentially transmit the nitrogen fluorescence light. The construction technique, image characteristics, and results of tests performed on the system are discussed.

First results from the High Resolution Fly's Eye experiment

The High Resolution Fly's Eye Experiment (HiRes) is the only operational experiment of it's kind in the world. HiRes is the direct descendent of the original Fly's Eye experiment. HiRes is designed to make stereo observations of Ultra High Energy Cosmic Rays (UHECR) via atmospheric fluorescence. HiRes will measure the energy spectrum, composition, and anisotropy of UHECR. The first of two sites, HiRes-1, has been in operation since June, 1997. Over 1000 hours of good weather data has been taken, monocularly, and analyzed. A preliminary energy spectrum is presented based on this data set and compared to the results from the original Fly's Eye experiment. The second site, HiRes-2, began limited data-taking in April, 1999. The installation of the HiRes-2 detector is now complete and commissioning is underway. Some high energy events have already been observed in stereo.

The pierre auger project: an observatory for the highest energy cosmic rays

Recent data from the Akeno [1] Giant Air Shower Array seem to statistically support the idea that the cosmic Ray spectrum does extend beyond the GZK limit. The challenge is set for finding the mechanisms that produce these UHECR as well as their origin and composition. The Pierre Auger Observatories will consist of two hybrid systems employing water Cherenkov tank arrays overlooked by atmospheric fluorescence detectors covering the entire celestial sphere. A description follows.

Results and current status of the water cherenkov detector development for the auger surface array

The Auger Observatory is a hybrid detector in which 3000 water Cherenkov detectors (WCD) operate in coincidence with atmospheric fluorescence detectors. Due to the limited duty cycle of the later the bulk of the data is provided by the surface detector array. Hence, the capacity of WCDs to discriminate the muonic from the electromagnetic component within air showers, and thereby provide a measure of the composition, is of great interest. This work describes performance studies of the capacity of WCDs to separate the muonic from the electromagnetic component of simulated air shower signals as they will be recorded in the Auger detectors. In addition recent results from the water Cherenkov prototype detectors triggered by the Akeno Giant Air Shower Array (AGASA) upon detection of extended air showers with energies above 10 18 eV will be presented.

A laser induced fluorescence instrument for measuring tropospheric NO2

Nitrogen dioxide (NO2), an essential gas in atmospheric chemistry, is almost always measured indirectly with just one technique: photolysis of NO2 to NO, followed by detection of resulting NO. However, the photolysis introduces artifact signals that complicate the extraction of the NO2 signal at low NO2 volume mixing ratios. A new prototype instrument uses laser induced fluorescence to measure tropospheric NO2 directly. The atmospheric fluorescence spectra measured with this instrument match the fluorescence spectra of NO2 in a reference cell over the wavelength range 565.1–565.2 nm. This match indicates that fluorescence from other chemical species is not interfering with the measurements. The prototype’s current detection limit is 280 parts per trillion (pptv) in 2.5 min with an absolute uncertainty of ±15% (2σ confidence). This detection limit, which can be improved with better optical filtering and more laser power, is currently adequate for urban air conditions.

Capabilities of a giant hybrid air shower detector

Abstract Recent reports of superhigh energy cosmic rays beyond the expected spectral cutoff have intensified interest in the unknown origin of the highest energy cosmic rays. There is a need for a much larger data base of more precisely measured air showers. This requires new sensitive detectors of enormous aperture. Combining a ground array of particle counters with an optical detector of atmospheric fluorescence yields a detector of outstanding capability. Such a hybrid detector provides far more accurate measurements of energies, arrival directions, and primary particle atomic masses than can be achieved by either type of detector separately.

The calibration of the absolute sensitivity of photomultiplier tubes in the high resolution Fly's eye detector

The High Resolution Fly's Eye Experiment (HiRes) measures cosmic rays from 10 17 eV to beyond 10 20 eV by the atmospheric fluorescence light produced in their cascade showers. The light is detected with photomultiplier tubes (PMTs). The full HiRes project proposes to use about 30000 PMTs in two sites. More than four thousand tubes are already in operation. Accurate calibrations of the absolute sensitivity of the PMTs are essential to this experiment. We have developed at the University of Utah a facility to calibrate the absolute gain and quantum efficiency of the tubes to the accuracy of better than three percent. This facility has proved to be capable of testing a large number of photomultiplier tubes efficiently and economically. In addition, it can be used to map the gain and the quantum efficiency of a PMT as a function of where the incident photons hit the tube's window. Although this facility is developed for HiRes, it is applicable to other experiments that utilise PMTs. We shall describe in this report the operation principles, the apparatus and some typical results.

Summary of the workshop

There are two rather well developed techniques to obtain a coverage of 5000 km 2 for the detection of cosmic rays with energies ⩾ 10 19 eV. One is an array of surfaces detectors. The other is an optical arrangement such as the Fly's Eye to observe showers by their atmospheric fluorescence. The pros and cons of these techniques are discussed. The energy spectrum and anisotropy of the highest energy cosmic rays can be measured by both techniques. What is more problematic is the identification of the primary composition. The galactic magnetic field and the 2.7 0 K cosmic background radiation can help in understanding the observations even if the composition is not directly determined. It is concluded that two identical installations, located respectively in the northern and southern hemispheres, are required to give complete coverage of the sky.

Design considerations for a super Fly's Eye experiment

We propose a detector utilizing current atmospheric fluorescence techniques to explore the cosmic ray regime above 10 EeV. Its design is similar to that of the High Resolution Eye, but in order to maximize event rate, modifications to the phototube aperture and geometrical configuration have been made. With a time-integrated trigger aperture of 4200 km2-sr at 10 EeV, the proposed design is potentially competitive with a 5000 km2 extensive air shower array. Finally, we examine the factors affecting experimental site selection.

Measurements of cosmic-ray air shower development at energies above 10 to the 17th eV

Measurements are presented of the depth of maximum for extensive air showers (EAS) produced by cosmic-ray nuclei with energies above 10 to the 17th eV. The air showers were observed using the University of Utah's Fly's Eye detectors operating in stereo mode. Measurements of the atmospheric nitrogen fluorescence light generated by EAS have been used to obtain longitudinal development curves of individual showers and, in particular, the depth of maximum of each shower. The data imply an elongation rate of 69.4 + or - 5.0 g/sq cm per decade above 10 to the 17th eV without correction for triggering and resolution effects. These effects contribute approximately +5 g/sq cm per decade to the apparent elongation rate. The distribution of depths of shower maximum has a mean value of 690 + or - 3 g/sq cm (with an estimated systematic uncertainty of + or - 20 g/sq cm) and a width (standard deviation) of 85 + or - 2 g/sq cm for showers with energies above 3 x 10 to the 17th eV. The tail of the distribution has a logarithmic decrement of lambda = 70 + or - 14 g/sq cm. Comparison with predictions for a pure proton and amore » pure iron flux indicates that the data are not consistent with either and are instead consistent with a mixture. 14 refs.« less

Response of ‘atmospheric’ lightning to systematic changes in the global electric field

A wide-angle photomultiplier system has detected optical pulses of lightning origin while searching for atmospheric fluorescence emissions from cosmic X-ray and gamma ray bursts. A broad-band spectral analysis of these events has helped identify lightning discharges which preferentially occur in the atmosphere and are similar in morphology to the ‘A-events’ seen by previous fluorescence experiments elsewhere. We show here that these events are facilitated by increases in the global fair-weather potential gradient, occurring as a result of its diurnal variation or brought about by a decrease in the flux of the galactic cosmic radiation that may accompany moderate changes in the level of geomagnetic activity.

The Utah Fly's Eye detector

We report the details of the design, operation and performance of the University of Utah Fly's Eye detector which was built to record the passage of ultra-high energy cosmic rays through the atmosphere via atmospheric fluorescence. Emphasized in the presentation are (1) light production by charged particles in the atmosphere, (2) kinematics of an EAS as seen by the Fly's Eye, (3) signal to noise considerations and its impact on detector design, (4) details of detector hardware and software, (5) detector calibration, (6) techniques employed in measurement of shower longitudinal development profiles and primary particle energy, and (7) assessment of detector performance by a comparison of Monte Carlo and real data distributions.

Evidence for a nonmagnetospheric origin of fast atmospheric pulsations

An atmospheric fluorescence detection system has been in operation at Gulmarg (India), during 1972–1978, to search for short‐duration (≤ 1 ms) optical pulses produced following the absorption of cosmic X ray and gamma ray photon bursts in the terrestrial atmosphere. A systematic examination of the records reveals no evidence of events with the characteristics of fast atmospheric pulsations (FAP), the millisecond tine scale optical pulses detected during similar studies carried out elsewhere. The complete absence of FAP activity at Gulmarg raises serious doubts about the proposed magnetospheric origin of these events.

Evidence for a high-energy cosmic-ray spectrum cutoff.

We report a measurement of the ultrahigh-energy cosmic-ray spectrum using an atmospheric fluorescence technique for extensive-air-shower detection. The differential spectrum between 0.1 and 10 EeV (1 EeV = ${10}^{18}$ eV) is well fitted by a power law with slope 2.94\ifmmode\pm\else\textpm\fi{}0.02. Above 10 EeV evidence is presented for the development of a spectral "bump" followed by a cutoff at 70 EeV.

The fly’s eye detector

A description is given of the atmospheric fluorescence technique to study extensive air showers.(AIP)