Solid State Radiation Physics Group (240/3) Researches

During several decades, basic investigations in the field of condensed state physics were devoted to the crystals with extreme properties such as wide-band gap oxide single crystals (laser crystals: corundum (a-Al2O3), ruby (a-Al2O3:Cr3+), garnet (Y3Al5O12:TR3+)), diamonds, semiconductors, high-temperature superconductors, natural zeolites, basalts which are of both scientific and applied interest. This is due to significant extension of the range of their application in various areas.

Due to their high irradiation stability, they can be used in conditions with increased radiation, for example in the space environment. It is especially important to have fast time response for the detectors operating at high load, for example, in electromagnetic calorimeters, in experiments on high energy physics at modern super-colliders.

The increase of irradiation stability of crystals promoted the increase of reliability and durability of devices, in which the above-stated crystals are used.

Of high interest is also investigation of properties of natural Armenian zeolites, which can be used to treat radioactive wastes at nuclear power plants.

1. It is experimentally revealed that at irradiation by high energy electrons and neutrons, defects are induced in the lattice of single-crystal sapphire and ruby. Stable structural defects are formed in anionic sub-lattice corresponding to F, F+ and АiF+ color centers.
2. Processes of accumulation of anionic and cationic centers in corundum at irradiation by fast electrons (50 MeV) are presented experimentally and theoretically. Values of oscillator strength are determined for transitions between ground and excited states of color centers at maximum absorption frequencies.
3. In corundum crystals irradiated by high-energy particles, strong increase of reflect capacity is revealed in wide energy range (5-30 eV, 70-200 eV and 530-550 eV) owing to radiation stimulated desorbtion leading to the formation of Аl  on the surface – effect  of “mirror reflection”.
4. The effect of “radiation memory” corundum crystals irradiated by fast electrons after heating at 1000^оС and irradiation by SR-quanta with energy hν~12 keV, when concentration of color centers does not reach the levels before heating.
5. Changes of critical parameters were studied depending on the amplitude of applied the enclosed magnetic field h₀ and frequency f for the samples on the basis of Y and Bi with various doping substitutional elements before and after their irradiation by electrons with energy 4 MeV and 8 MeV.
6. It is revealed that with increase of irradiation dose, J_c, in general, decreases, however at a dose of 8·10¹⁶ e/cm² strongly pronounced local minimum is detected coinciding with the same minimum and for the parameters Т_сon and Т_mJ. It is supposed that this minimum is caused by annealing of instable Fraenkel defects heating at T_irrad = 100°C and formation of more stable secondary radiation defects containing both native and doped atoms. Introduction of doped Fe and Ni atoms results in small displacement of this minimum to the area of low doses and simultaneous deceleration of J_c at higher doses (up to 10¹⁸ e/cm²).
7. The influence of 8 MeV electron irradiation on the properties of doped high-temperature superconductor YBa₂Cu₃-xM_xO_y (M=Fe, Ni; x=O; x=0,1) ceramics has been studied. It is established that with the increase of irradiation, onset temperature of transition to superconducting state (T_con) and the intergranular coupling temperature (T_mj) exhibit an oscillation behavior around their initial values of 2K. A critical irradiation dose is obtained (2«10¹⁸el/cm²), after which all HTSC parameters strongly decrease, i. e. superconductivity of HTSC is destroyed.
8. It is revealed, that after irradiation of high-temperature superconductor samples by high energy electrons (4 MeV and 8 MeV), the formed radiation defects migrate after the termination of the irradiation and cause changes of critical parameters over a long period. This effect is named “ageing” or “memory” effect and was observed by other authors. The observable effect depends on initial structure of samples, irradiation conditions as well as temperature and time of after radiation.
9. Dynamics of Josephson in superconducting ceramics YBa₂Cu₃O_y was studied in a frequency interval 0.01-0.5 Hz in mode of incomplete penetration of magnetic field. It was determined that real χ’ and imaginary χ” parts of magnetic susceptibility have complex frequency dependence. It was revealed that dynamic magnetic susceptibility, depending on frequency and amplitude of magnetic field, shows two important features: with increase of frequency growth (starting with 0.01 Hz), constant and variable component ∆χ’and χ‘’ of magnetic susceptibility appear; in the first half-period of sinusoidal field, two asymmetric peaks of х peak of ∆χ’and χ‘’ susceptibility components appear.
10. In-situ measurements of electro-conductivity (s) of silicon single crystals (n-Si) were carried out in a vacuum simulating chamber (VSC). The experimental conditions were as follows: vacuum of 10^-5 Torr, temperature of 177 K, electron irradiation energy of 8MeV, solar UV radiation corresponding to near Earth space conditions. The experiments were performed step-by-step with and without UV radiation. After preliminary measurements, the UV source was switched on, electron beam was switched on, and s was measured as a function of the electron beam dose. The irradiation intensity was 1.6·10¹⁰el/sec·cm² that is more than 100 times higher than on near Earth space satellite orbit (LEO). It is obvious that the initial value of s decreases by more than 40% after switching on UV for 5 min. It increases again and reaches an equilibrium value which is lower than the initial value (by about 30%). Additional decrease of s by a factor of 3 is observed after electron irradiation at a dose of 5.76·10¹²el/cm² in the presence of UV irradiation.
11. The observed behavior of s is explained by the fact that the UV excitation stimulates and the temperature increase suppresses the formation of structural radiation defects (RD) in n-Si, while in p-Si just the contrary is the case. This effect leads to the localization of electrons around the RD and hence, to the decrease of carrier mobility, i.e. to s During this process both new RD are formed and their redistribution takes place. As a result of these interactions, RD complexes are formed that are stable at room temperature and determine s of the silicon sample.
12. Measurements of silicon single crystal conductivity were carried out directly in the course of electron irradiation (in-situ). It was shown that the specific conductivity of silicon samples measured in the course of irradiation was much higher than that after irradiation. The higher value of σ in the course of irradiation was due to ionization mechanisms (Auger or other irradiation induced process), which resulted in the formation of non-equilibrium carriers (hole-electron pairs).
13. It is shown by the performed measurements that in the case of electron irradiation at room temperature in air, specific resistance (ρ) of p- and n- Si type samples changes at different rate depending on radiation dose. Besides, in p- Si samples the strong increase of ρ (compared to the n- Si type ones) is observed at significantly higher irradiation doses, i.e. p- Si  type samples are more stable in view of irradiation. It is revealed that in n- Si type samples the change of conductivity varies significantly depending on irradiation dose and temperature. Besides, conductivity depends on the fact, if UV excitation is combined with electron irradiation. Preliminary results show that simultaneous influence of UV irradiation supports the conductivity variation process in n- Si type sample.

Technical  Infrastructure  (buildings,  premises,  energy  supply, etc.)

It should be mentioned that thanks to the financing from projects, now the laboratory owns a 3 storied building (Photo 1), of 500 m2 total area with labs, administrative offices, special facility for 2 linear accelerators (4 and 8 MeV), local library, seminar-hall, special rooms with equipment for optical and electrical-physical studies. On the ground floor, linear accelerators ELU-4 and ELU-8 are mounted in a special hall meeting the requirements of radiation safety. This hall is connected with the experimental hall, where vacuum chamber  for  imitation  of  cosmic  space  (space  environment  simulator-SES)  is  mounted. Mechanical workshop, room with a plasma spraying unit BULAT-6, luminescence spectroscopy setup room as well as various subsidiary rooms are also located on the ground floor. On  the  second  floor  there  are  rooms  with  experimental  devices  for  electro-physical measurements,  library and seminar hall.

Laboratory building of solid state radiation physics and optics groups

Photo1.

  On the third floor rooms with experimental devices for optical researches (UV and IR spectroscopy, vacuum evaporation units VUP-4) are located. The building is connected to YerPhI electric power and water supply networks and high speed internet (Electric power requirement: 10.000 kWtt Water supply: 5000m³/year, internet speed: 50/4Mbps – down/up )

Scientific Infrastructure (Experimental Facilities) and Techniques

In the laboratory there are:

• 2 linear accelerators electron (4 and 10 MeV ), equipment for optical, electrical-physical studies;
• Recently a new facility was created for near Earth environment simulation (Photos 2,3) (electron beam with 8-10 MeV energy, vacuum 10-5Torr, temperature 70-300K. It should be noted, that this it is a unique facility, allowing investigation of  simultaneous influence of 4 space factors on the properties of materials and devices;
• Installations for measurement of dielectric constants and electrical conductivity at different temperatures (78 – 370 K) and frequencies from 200 Hz to 1 MHz;

– Installation  for  measurement of photoluminescence and luminescence excitation spectra in 77-300 K temperature range;

• Photo-spectrometer with measuring range from 2 to 2.5 microns;
• Vacuum evaporation unit VUP-4.
• Equipment for Hall effect parameters measurements
• Superconduction parameter measurements device

Connection of space environment simulator (SES) with electron accelerator

Photo 2.

    Horizontal cross-section through SES Photo3. Scientific Techniques Created and Used in Laboratory

• Techniques of measurement of complex magnetic susceptibility in the range of temperatures 78-370 K for high-temperature superconductive ceramic samples and natural zeolites have been The technique allows measuring of hysteresis losses in high-temperature superconductive environment, superconductive state, density and current.
• Technique of measurement of excitation of luminescence and photoluminescence in visible/VUV spectral ranges for wide-band-gap oxide single crystals and natural zeolites has been
• Technique of measurement of doses of irradiation of materials by high-energy electrons (<10, 50 MeV) has been
• Calculation procedure of optical absorption and reflection of semi-conductor materials and natural zeolites has been