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Selective Detectors for Nuclear Radiation

#G-646


Investigation of Thermo-Stimulated Fluorescence and Spectrums of Optical and Para-Magnetic Absorption of MgO Crystalls as Perspective Materials for Selective Detectors of Nucleus Irradiation

Tech Area / Field

  • FIR-INS/Nuclear Instrumentation/Fission Reactors
  • INS-DET/Detection Devices/Instrumentation

Status
8 Project completed

Registration date
29.11.2000

Completion date
03.08.2006

Senior Project Manager
Pradas-Poveda J I

Leading Institute
Scientific-Research Institute of Automatic Systems "Skhivi", Georgia, Tbilisi

Collaborators

  • Duke University, USA, NH, Durham\nArgonne National Laboratory (ANL) / West, USA, ID, Idaho Falls\nPacific Northwest National Laboratory, USA, WA, Richland

Project summary

The method of thermal luminescence dosimetry (TLD) occupies the distinguished place among the methods of defining radioactive irradiation dose. The distinguished feature of this method is independence of dosimetric readings from the power of the dose. From the point of view of its metrology characteristics, the TLD Method exceeds ionization and photographic film methods. Its wide application in different spheres of the science and technique is particularly promoted by means of simple construction, reliability and small size of dosimetric detectors themselves.
Out of large amount of materials proposed for TLD by the present moment, only lithium fluoride, and calcium fluoride and luminescence glass have found application. This can be explained by the fact that thermoluminescence dosimetry needs luminophor meeting certain requirements, particularly:
high sensitivity;
wide enough range of measuring;
high stability of features.
Necessity of fulfillment of the above mentioned requirements changes the problems of selection of luminophor for thermoluminescence dosimetry into complex development. That’s why, despite of the success achieved in thermoluminescence dosimetry, there are lots of unsettled problems blocking further development of the method. This applies to luminophor and measurement methods and measuring devices. However, selection of thermoluminophor material having optimum dosimetric features still remains to be the major problem. The issues of small thermoluminescence quantum output of the materials pointed out above, complexity of tribo - and chemiluminescence background removal hindering dosimetric measurements, etc. still are not resolved. However, we shall distinguish two problems out of the unsettled TLD problems:
repeated application of a detector;
detection of heavy particles.
One of the most significant characteristics of thermoluminophors being applied in dosimetry is the possibility of its repeated application. All the above mentioned luminophor materials provide this opportunity but only after special thermal processing or highlighting in inert atmosphere. This often results in reducing of detectors’ sensitivity, i.e. there is violated one of the major requirements – high convergence of detector readings when its multiple application. Besides, it is necessary to take into consideration that the regime of heating of a detector has significant meaning for practical dosimetry and measurement technique. When quick heating or cooling of a material luminophor quenching, This may result in creating new structural defects in crystalophosphore and, consequently, led to significant changing of its dosimetric features.
The most complicated and unsettled problem of TLD is the problem of detection of heavy particles, mostly of fast neutrons. This is related to the fact that all real neutron sources have either gamma-irradiation or background irradiation of charged particles. This makes it difficult in significant degree to distinguish dosimetric information on neutron flow intensity. For instance, in the case of detectors produced on the basis of lithium fluoride monocrystals, different components of neutron irradiation cause almost the same radiation effects that actually exclude the possibility of selective reading of dosimetric information. In order to provide detector selectivity towards irradiation neutron component and suppression of gamma- and/or other backgrounds there are generally used various screens (lead, cadmium, etc.). The mentioned difficulties do not give opportunity to resolve the problem of selective registration of radiation irradiation (gamma-irradiation and neutrons, gamma-irradiation and charged particles) flow. This fact is a serious obstacle for wide application of thermoluminescence dosimetry method.
Application of magnesium oxide monocrystals in the shape of dosimetric materials can turn out to be prospective for resolving the above mentioned problems and improving several characteristics of TLD (quantum output of thermofluoroscent, relative sensitivity, etc.). Due to extremely high level of thermal resistance of magnesium oxide the level of “endurance” should be lower in the suggested detectors (reducing radiation sensitivity of the detector when increasing multiplicity of recording-deletion process of dosimetric information) when regeneration requiring application of special, quite time consuming and complicated regimes of thermal processing. Expected significant reduction of the mentioned unpleasant event will contribute to resolving the problem of a detector’s repeated application.
Application of magnesium oxide monocrystals can provide resolution of the problem of simultaneous recording and further separate reading in one and the same crystal of dosimetric information on neutron flow and it’s accompanying gamma background. Reading can be carried out by means of different methods, by thermoactivated spectroscophy method among them. The following two moments partially mentioned above serve as the basis of such assumption:
Radiation effects in MgO monocrystals, irradiated by “pure” г-irradiation, and in a reactor’s mixed field, are not similar. In (n, г) - irradiated crystal there were noticed the same changes as during г-irradiated crystal, plus F-type coloring centers (basically F+ - centers) and F – aggregate centers;
Radiation effects in MgO monocrystals stipulated by г- and (n, г)-irradiation have different thermal stability.
Besides, after annealing at the temperature about 900K, only F+ - centers are left in MgO crystals irradiated in the reactor’s mixed field. The mentioned centers intensively change into F – centers during the further crystal annealing (~ 1000K). In the result F+ - centers can act as the center-carriers of dosimetric information about neutron flows.
Thus, for MgO crystal irradiated by mixed (n, г) irradiation there can be distinguished two groups of thermo-stimulated luminescence peaks that significantly differ from each other by their temperature state and nature (their different sensibility towards different types of irradiation). This will allow us to carry out selective detection of the components mixed according to irradiation composition. In another words, there is the possibility to allocate the dosimetric information related to the ingredients of mixed gamma-neutron irradiation in “memory cell” of one and the same irradiated crystal and its selective reading by means of the relevant devices. It can be added here, that its radiospectroscopic variant and method of thermo-stimulated bleaching in the regime of gradual heating using the means of electronic absorption and vibration spectroscopy can be used in the shape of the later except luminescence variant of thermoactive spectroscopy.
Realization of the proposed approach will allow us to expand scope of thermoactivated spectroscopic dosimetry due to application of new class detectors being more efficient than on the basis of activated lithium fluoride crystals.
However, on the basis of the information being known at present (from the works of the Project participants, among them), it can be only assumed that magnesium oxide crystals are perspective active elements for selective detectors of nuclear irradiation. In order to resolve this issue finally it is necessary to investigate completely their luminescent and dosimetric characteristics, develop, develop relevant methods of measuring dosimetric information being accumulated in monocrystals, find acceptable means of their regeneration.
The purpose of this Project is to investigate dosimetric features of MgO crystals as of perspective environments for nuclear dosimetry. The plan of activities envisages: studying of thermo-stimulated luminescence; irradiated crystals optical and paramagnetic absorption spectrum in stationary state as well as in the regime of gradual annealing; development of the methodology of measuring dosimetric information and crystals regeneration for their multiple application.
Thus, the volume of the planned activities can be pided into four basic problems, particularly:
Investigation of radiation effects caused by pure gamma-irradiation;
Investigation of radiation effects caused by pure neutron irradiation
Investigation of radiation effects caused by mixed gamma-neutron irradiation;
Investigation of dosimetric characteristic of thermoactivated spectroscophy peaks and corresponding relaxation processes.
The majority of the scientists participating in the Project are highly qualified specialists in the field of physics and chemistry of solid state, radiation physics, low temperatures physics and quantum electronics.
Realization of the Project will allow us to follow on the fundamental investigations in the field of radiation physics of solid state investigations, which have been carried out with the specialists from the SRI AS “Skhivi” on the basis of close co-operation with the leading scientists of Andronikashvili Institute of Physics of Georgian Academy of Sciences. At the same time, the Project will give us the opportunity to employ 13 scientists being working on military issues earlier.
The Project envisages participation of foreign scientists as of collaborators that will foster integration of “weapon” specialists of the former USSR into the world’s scientific community. The forms of co-operation with foreign collaborators can have perse characteristics. Particularly, exchange of scientific information on the progress of the project activities, comments on the results obtained, joint discussion of the results obtained, joint and agreed experimental and theoretical investigations, etc.


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