Artyomovsk scintillation detector

The Artyomovsk scintillation detector (ASD) is a 100-ton scintillation underground detector that was created in the Department of High-Energy Leptons and Neutrino Astrophysics of the Institute of Nuclear Research of the USSR Academy of Sciences in 1969 to study:

1) antineutrino streams from collapsing stars in the Galaxy;

2) the spectrum and interactions of cosmic ray muons with energies up to 10^13 eV.

In addition to these two main tasks, the research program includes:

3) study of hadron fluxes contained in the nuclear cascades generated by muons underground;

4) study of the characteristics of decays of transuranium elements;

5) search for galactic variations of cosmic rays;

6) Study of the angular characteristics of cosmic ray muons;

7) search for atmospheric neutrinos coming from the lower hemisphere.

The experimental conditions require continuous operation of the installation.

The events registered in problems 1), 7) are rare: the estimated frequency of the event is about 0.02-0.2 per year, atmospheric neutrinos are expected to be about 8-10 per year. The counting rate in other problems is higher: the number of muons passing through the installation is 5 sec-1, showers with an energy release of> 80 GeV – 3 h-1, of which about 15% are cascades generated in the inelastic interaction of muons with soil nuclei. The detector is located in a salt mine in the city of Soledar in the municipality of Bakhmut, located in the Donbass, Ukraine. The municipality of Bakhmut was formerly known as Artyomovsk or Artemovsk or Arteomovsk (hence the name of the detector); the municipality changed its name in 2016. The detector is located at a depth of more than 100 m (570 m.w.e.). It is cylindrical in shape, diameter (556 ± 3) cm, height 547 cm. The detector contains 105 tons of liquid scintillator (ZhS) based on white spirit.

The height of the scintillator column is 540 cm, the density is 0.78 g / cm3. The size and location of the setup are determined by the objectives of the experiments. Registration of antineutrinos from a collapsing star is based on the Reines-Cowen reaction of the interaction of an antineutrino with a proton (hydrogen nucleus), which is part of the scintillator:

n(e) + p -> e + n

The contribution of interactions with protons of the 12C nucleus is small, because the interaction threshold is ~ 19 MeV, and the energy spectrum of antineutrino has a maximum in the region of ~ 10 MeV and covers the interval from 3 to 30 MeV. The installation detects the appearance of both particles – a positron and a neutron, while the energy of the positron is determined.

The pulse from the ionization losses of the positron is the starting one for the registration of neutrons from capture gamma quanta on hydrogen with a characteristic capture time of t = 170 μsec.

n + p -> d * -> d + g, E = 2.23 MeV

For the reliable detection of collapse, a sufficiently large mass of the registering substance is required, as well as to increase the efficiency of registration of g-quanta from the reaction. An increase in the size of the detector was also desirable from the point of view of studying muon interactions.

This reduces the undersized energy release of the cascade due to its exit from the setup and, as already noted, leads to an increase in the detection efficiency of neutrons contained in the cascades and used for their separation depending on the nature of their origin.

The mass of the detector was chosen ~ 100 tons. It was supposed to be the prototype of a huge 1000-ton mono detector. On the 100-ton model, it was planned to check the details of the design solution and work out the experimental technique.

To this we can add that the problems of creating a 100-ton monodetector, as well as a detector, by an order of magnitude larger in mass, are qualitatively different from the problems that had to be faced when designing liquid scintillation counters with a mass of 0.3 tons. A 100-ton detector has dimensions on the order of the length of an electromagnetic shower with an initial energy of 100 GeV.

The cylindrical shape of the detector with almost the same height and diameter was chosen, on the one hand, for reasons of simplification of estimation calculations and manufacturability of the design, on the other.

Placing the installation in a salt mine reduced the background of natural radioactivity by about 300 times compared to a room in ordinary soil. The counting rate was 1.1 * 103 imp / s in the range (1-3) MeV of registration of energy release from gamma quanta np capture. The depth and large dimensions of the detector reduced the background associated with cosmic ray muons to 0.4 pulses / sec in the range of 5-50 MeV, where a positron from reaction (I.I) is recorded. This background is created mainly by muons crossing the detector along short paths, and by gamma quanta of cascades generated by muons in salt.

The cylindrical container of the detector is welded from 4 mm thick stainless steel. The container is hermetically sealed with a steel lid using clamping clamps and a polyethylene seal. For measurements and checks, 6 also hermetically sealed openings with a diameter of 3 cm, located along the radius, and a hatch with a diameter of D = 80 cm were made in the lid. In the center of the container there is a support, D = 10 cm, supporting the lid. The detector has a system for emergency draining of the scintillator into containers located nearby. It is installed on a platform moving on rails and is surrounded by a structure for the operation of the unit along its entire height. On the platform, the detector is pushed into a cavity selected in the salt according to its size (the average distance from the walls of the detector to the ground is 120 cm). 144 holes, D = 25 cm, are made in the side surface of the container, which are closed with 10 cm thick plexiglass windows using flanges and fluoroplastic spacers.The refractive indices of plexiglass and scintillator are the same and equal to ~ 1.5 (for a wave l = 4200 A °). The photomultipliers are rubbed against the windows, viewing the entire volume of the detector. The optical contact between the photocathode and the illuminator is made by means of a petroleum jelly-based lubricant. The inner surface of the container is pasted over with a reflective film – lavsan with a diffusely reflecting coating of titanium dioxide TiO2, kotr of the film in the region of the maximum photocathode sensitivity of the used photomultipliers (l = 4200 A) is 0.86.

The surface of the detector is made reflective in order to increase the number of photoelectrons for one PMT from small energy releases. From this point of view, it is desirable to have a high transparency of the sctillator for the indicated light length l. ZhS was prepared on the basis of white spirit CnH2n + 2 (n = 10). White spirit is a low-toxic substance with a flash point of + 38 ° C, it was purified by distillation under pressure through chromatographic aluminum oxide Al2O3 with simultaneous dissolution of the POPOP'a C24H16N202 shifter to a concentration of 0.03 g / liter.

The scintillator was poured into the container in portions of 1.2 m3, the transparency of each portion until the POPOP was dissolved was monitored on a modified SF-14 spectrophotometer with an increased measurement base of 80 cm and a FEU-110 as a recording element. The value of the transparency lsc, gradually changed depending on the purity of Al203 from ~ 35 to 20 meters. High transparency can be maintained by frequent replacement of contaminated alumina. The optimal transparency value, taking into account the desired physical properties of the detector and economy, was chosen lsc, ~ 25 m.

After filling the detector, the scintillator was sparged with argon, then the scintillation additive PPO (C15H11NO) was dissolved to a concentration of 0.7 g / liter. During bubbling, oxygen dissolved in the scintillator was replaced by argon, and the light yield increased by about two times. The bubbling was carried out under continuous control until the moment when the light output stopped growing. An argon atmosphere was created over the scintillator.

The detector is equipped with three types of photomultiplier tubes. To register low energy releases in reactions (II), (1.2) in the energy range I-IOO MeV, 128 FEU-49B (multialkaline SbKNaCs photocathode, photocathode diameter D = 15 cm) are used, the energy release of cascades (10 – 104) GeV is recorded by sixteen FEU-PO (D f.c. = 8 cm). 16 fast, with a rise front of 3 nsec, FZU-30 (D f.c. = 5 cm) are installed to determine the coordinates of the shower barrel and muon tracks from the delay times of signals from the outputs of the multipliers.

Photomultipliers are located on the lateral surface of the detector, on average, evenly. FEU-110 and FEU-30 are located on the same window. All 144 portholes with photomultipliers are hermetically sealed with metal housings, which partially act as a shield from the Earth's magnetic field.

The detector is divided into four sectors A, B, C and D by conventional vertical diametrical planes. In the sector there are thirty-two PMTs – 49B and four PMT-30. PMT-49B photomultipliers of each sector have their own high-voltage source VS-28. FEU-110 and FEU-30 are powered from a common high-current source B1-4. Distribution boards are installed in the sectors for power supply and regulation of the gain of each PMT.
References

Ashikhmin, V. V.; Enikeev, R. I.; Pokropivny, A. V.; Ryazhskaya, O. G.; Ryasny, V. G. (2013). "Search for neutrino radiation from collapsing stars with the artyomovsk scintillation detector". Bulletin of the Russian Academy of Sciences: Physics. 77 (11): 1333–1335. doi:10.3103/S1062873813110051.

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