An experiment with a tethered baloon at Baikal lake
By means of an experimental facility SPHERE-2 developed by scientists of SINP MSU and Lebedev Physical Institute an experiment on the studies of super-high energy (1016 - 1018 eV) cosmic rays was carried out successfully. In particular, it concerned measurements of energy spectrum and composition analysis of cosmic rays. This research is of particular importance for understanding of the origin of galactic sources of cosmic rays.
The experiment is carried out in the following way. The experimental facility SPHERE-2 is rised over the snow-covered Earth surface with a balloon. It is made at Moonless night, when it becomes possible to detect photon flux reflected from the snow surface, which is called in honor of the scientists who had discovered this prenomenon - Cherenkov radiation (in Russian - Vavilov and Cherenkov radiation), or Cherenkov light. This light is produced by giant air showers (or Auger showers) - a cascade produced by interaction of cosmic rays and the Earth's atmosphere. Image of Cherenkov light at snow surface is rojected to mosaic of SPERE-2 photomultipliers by means of a sperical mirror of 1.5 m diameter.
One of the participants of the experiment PhD in Technical Sciences, Senior researcher of the Department of Radiation and Calculation Methods Dmitry Chernov have told us about the studies.
- Dmitry, please, tell us operational principles of SPHERE-2? Can you explain it, as they say, in lay terms.
Operational principle of the experimental facility is close to customary photo-camera, or, rather to video-camera. SPHERE-2 differs from consumer electronics by high light sensitivity and fast response. Even 100 photons coming to the Earth are safely detected by the facility. Fast response of detection is so high that during time period when one splash of Cherenkov light of Auger showers is detected at te snow, a bullet fired from small arms flyes a distance less than 1 mm.
- What is the difference between SPHERE-2 and SPHERE-1?
SPHERE-1 (1993-2000) was a prototype for a technically more complicated SPHERE-2 (started in 2005). The basic differences between these facilities are detector's optical resolution and electronics. SPHERE-1 contained 19 photomultipliers, while SPHERE-2 contains 109 photomultipliers. Electronics of SPHERE-2 provides an opportunity for detection of impulse shape from each of 109 photomultipliers with minimum resolution of 12.5 nanoseconds during 12 microseconds, while SPHERE-1 could detec only charge of photomultipliers' anodes during approximately 2 microseconds. There are basic differences. Besides, SPHERE-2 contains GPS, temperature and pressure sensors, inclinimeter for measurements of the facility's inclination, electronic compass for evaluation of its orientation, LED system for evaluation of relative sensistivity of each photomultiplier. New instruments and software allows to improve accuracy of the measurements significantly. Nevertheless, the merit of SPHERE-1 is that we succeeded in registration of cosmic rays spectrum in the range of 10-100 PeV by this method for the first time. And 10-15 yars ago some experimenters called down even possibility for such measuremetns!
- What is the weight of SPHERE-2?
Total weight of the facility with accumulators is approximately 80 kg. A rope of 1000 m length is additonally about 40 kg.
- What balloon (I mean air volume) was used for the rising of SPHERE-2?
Initially we planned to use a military barrier balloon AZ-55 with air volume of approximately 750 m3. Such a balloon can rise the facility up to the altitude of 3 km. But later we understood, that this equipment is very complicated and costly to operate. So we applied to the specialists of the Scientific and Production Association "Avgur - RosAeroSystems" for development of a small and simple in operation flying vehicle. Especially for our studies a balloon BAPA-1 (Russian abbreviation for BAikal Tethered Balloon) was developed and manufactured. Maximum volume of theballoon is about 250 m3. Working fluid is Helium. For initial filling of the bag it is required 44 balloons of compressed Helium, each of 40 liters volume. Unfortunately, buoyancy of the balloon is enough only for rising of the facility to the altitude of 1000 m, and it caused decreasing if the square of the observed snow surface, and therefore, to decreasig maximum detected energy down to 500 PeV instead of expected 1000 PeV.
- At what altitude SPHERE-2 was rised in March 2013?
Maximum lift of the balloon BAPA-1 with SPHERE-2 is 1000 m. As a rule, maximum lift is determined by the total mass of the equipment and rope and wind effect. During the measurements conducting altitude of the facility allows to control energy range for detected events and optical resolution (degree of detail) of the images of Cherenkov light splashes on the snow surface. I.e. the higher we rise the facility, the more higher initial energy we register, but the less the photomultipliers see this splash. Calculations made by our scientists have shown that ideal operating altitude is about 600-700 m over the snow surface, where a balance among different requirements is achieved: possibly low energy treshold, high spatial resolution and essentially wide energy range. Therefore, in 2012 and 2013 we basically carried out measurements at this altitude. But, when wind became more stronger we had to move the balloon down and several sessions were made at the altitude of 400 m.
- Why the experiment was carried out in March?
Currently we carry out measurements at Baikal Lake. Covered with snow ice surface of Baikal is an ideal screen for Chrenkov light splashes registration. In the end of February and in March Baikal ice is abequately strong to withstand launch platform of the balloon. Core role in carrying out of this experiment is played by the Head and scientists of Baikal Neutrino Station of the Institute of Nuclear Reasearch RAS, who simultaneously participate in development of neutrino telescope NT-1000.
- How many time SPHERE-2 was rised in March 2013 and earlier?
It was rised 5 times in March 2013, and 23 times - totally during the period 2008-2013.
- For how long period SPHERE-2 was rised at an average?
Exposure time for each rising is calculated according to the duration of dark period of day and Moon phases. At an average each measurement period is approximately 8-9 hours.
- What results have you obtained?
In 2012 we reconstructed spectrum of all nuclei of cosmic rays. Used method allowed to study in details systematic and methodical errors which increase uncertainties of measured spectrum. It must be noted that declared measurements errors often several times less than real because in many experiments systematic effects are not under due consideration.
In 2012 we also made the first attempt to separate spectrum by nuclei groups. Mean part of light nuclei within the range of energies of 30-150 PeV (3*1016-1.5*1017 eV) was evaluated at the level of 21+-11 %. In general, obtained results agree with world ones. Currently studies of systematic results are being continued (it is of particular importance concerning the problem of mass composition analysis) and therefore all results are only preliminary. New experimental data for 2013 will allow to diminish measurements errors. We put especial hopes on the improvement of data analysis procedure. In fact, in the best up-to-date experiments measuring equipment and programs for data analysis produce single measuring and calculating system, and the second part (analysis) is of the same importance, as the first (measurements).
- What difficulties have you met?
We have typical for Russian science problems: severe shortage of high-qualified staff and lack of financing. It is a reason for extension of project realization terms and decreasing of research efficiency. As I've already mentioned, using of cheaper balloon equipment resulted in decreasing of maximum detected energy. Even more problems appeared when we had to use cheaper, but old-fashioned and providing less sensitivity photomultipliers instead of ones up-to-date Hamamatsu R3886 which were planned to be used for photo-receiving mosaic production. It caused 6 times less number of detected events! Correspondingly, statistic errors of measuremnts increased over than twice. For instance, last year we presented preliminary results of cosmic rays chemical composition studies at the European Symposium ECRS-2012. But low statistical reliability of the measurements did not allow us to obtain firm results. Adding experimental data of 2013 we'll be able to improve our results, but unfortunately we'll not be the first else - analogous results were published in April 2013 by KASCADE-Grande collaboration, which has used another method. This example shows that it is necessary to carry out experiments using the best equipment at the highest technical level, and that savings on science results in chronic delay of fundmamental studies.
- Where SPHERE-2 is kept when it is not operated? How is it kept for many years?
SPHERE-2 is kept diassembled. After finishing of measurements the facility is disassembled. Photomultipliers mosaic and electronics we bring to the Institute for testing, service and improvement. Metallic frame with mirror and thermostatic box for electronics are stayed at Baikal. All equipment is storaged in a customary case at the base of the Institute of Nuclear Rrsearch RAS on Baikal bank. Surely, change of temperature and high humidity influence negatively on the safe-keeping of the reflecting layer of the mirror. Therefore, in 2010 and 2013 we changed all segments of the mirror before beginning the measurements.
- What is operational life of SPHERE-2?
At appropriate service operational life of the facility is 10-15 years. But it is rather determined by xpediency of data collection. During first 3 of 6 past expeditions we have completed balloons launches and tested equipment. During last three expeditions we have collected basic statistic material. Further data collection becomes low-effective, because even if we increase statistic material twice, it will result in very negligeable decreasing of statistic errors of the results. So we do not plan to continue operation of SPHERE-2. Currently our basic efforts are focused at processing of obtained data and a new project development. Its koperating name is HD - analogous to HD TV, and a new facility must provide high definition of Cherenkov light splashes images on the snow. Comparing with SPHERE-2 we expect that SPHERE HD will provide 100 times more statistical reliability, wider energy ranges from 3 up to 1000 PeV and better sensitivity to the type of primary particle.
There is a 30 minutes long movie about the expedition to Baikal and studies of cosmic rays by means of SPHERE-2, done by participants of the experiment. You can find it in Section "Video" or following hyperlink http://sphere.sinp.msu.ru/video/Sphere2_640_ver2_5.avi