SOME RESULTS OF THE AIRBORNE GAMMA-SPECTROMETRY RESEARCH OF THE USSR EUROPEAN TERRITORY RADIATION SITUATION DURING THE ELIMINATION OF THE 1986 CHERNOBYL ACCIDENT CONSEQUENCES

The operating research of the radiation situation of the European territory of the USSR (ETU) in 1986 caused by the Chernobyl accident was executed jointly the scientists of Moscow Engineering Physical Institute (MEPhI) and the USSR Ministry of Defence in the period between 28-th of April and 26-th of September 1986.

The objective of the research was determining the fallout radionuclides contamination characteristics of the ETU. For the correct evaluation of the population dose burden and radioecological consequences of the accident and as well as effective planning of preventive and protective measures all radionuclides injected in an environment should be taken into account. So particular emphasis has been given to determination of the fallout field contamination characteristics for the greatest possible set of the radionuclides.
Using the remote high energy resolution gamma-spectrometry method (airborne-gamma-spectrometry) the following characteristics were determined: radionuclide composition, activity density for individual radionuclides, space distribution for the reasonably large set of the radionuclides.
Generally the essence of the airborne gamma-spectrometry is the gamma-radiation energy distribution measurements (during the flight) with subsequent determination of the radiation source characteristics in accordance with the model accepted. For the determination of the radioactive contamination spatial distribution the consecutive series of spectra measurements should be replicated over the territory of interest. So every individual measured spectrum appears to be related to some area. Given T_i and T_f as the initial and final time of individual spectrum measurements respectively, the length of this area is L=t·V, where t=T_i-T_f and V is velocity of the aircraft. The L parameter defines the spatial resolution of the airborne gamma-spectrometry method. With the parameters of the spectrometer fixed and minimum safety velocity T the only way to improve the spatial resolution is the reduction of the measurement time ensuring at the same time that t is large enough to obtain statistically significant spectrum.
The contamination radionuclide combination was a rather complicated one in summer 1986 for all ETU and varied significantly from region to region. Thus it was impossible to measure the surface activity density for individual nuclide by using the spectrometry based on the scintillation detector because of its poor energy resolution. Therefore we used an airborne gamma-spectrometry system (AGSS) with two types of the detector units: a Ge detector unit with high energy resolution and a scintillation unit with high sensitivity. Using both spectrometers’ spectra measured simultaneously over the same territory we can obtain the values of individual nuclide surface activity density (using Ge detector spectra) and fine spatial resolution (using scintillation detectors) for practically any radionuclide combination. During the realization of this approach the t parameter value was varied from 30 s to 10 min for Ge detector spectra and from 2 to 30 for Nal spectra depending on level of territory of interest contamination. These spectra were constructed by consecutive summing the initial spectra with constant measuring time set obviously short enough. The summing went on until the acceptable statistic for an individual nuclide content measurement was reached. Thus the final t values may be different for different radionuclides.

To solve the task set the MEPhI airborne gamma-spectrometry system (AGSS) with the following characteristics was installed onboard the An-24RR radiation surveyor airplane:

1.Full energy absorption spectrometer with a collimated semiconductor Ge(Li) detector and microprocessor based pulse height analyzer MCA NOKIA LP4900Â as the device for visualization, processing and accumulation of the information:

sensitivity in full absorption peak for 1333 keV

about 100 mm²

energy range

50-2500 keV

energy resolution for 1333 keV in flight conditions

3 keV

number of channels

2048

2.Full energy absorption spectrometer based on the multi detector scintillation NaI(Tl) detector unit and the MCA NOKIA LP4900B as the device for visualization, processing and accumulation of the information:

sensitive volume

about 20 l

energy range

100-2500 keV

energy resolution for 662 keV

about 10%

number of channels

256

3.The software and methodical maintenance for realization of airborne gamma-spectrometric measurements and determinations of the gamma-radiation sources characteristics.

The flights were organized by an irregular tacking at height 200 m with a flight speed of 360 km/h.

The calculations of the surface activity densities S(E) were carried out using the model of flat thin infinitive extent source at constant distance Hfrom the detector. For each dot (that a center of an elemental area corresponding to measurement moment is) the value of S(E) for individual radionuclides was calculated by the following formula:

where:

For the nuclide with several gamma lines the average value of S was determined by least squares method.

At the beginning of August 1986 the significantly contaminated ETU regions were revealed and the suitable maps of surface activity density spatial distribution for ¹³¹I, ¹³²Te, ⁹⁵Zr+⁹⁵Nb, ¹⁰³Ru, ¹⁰⁶Ru, ¹⁴⁰Ba+¹⁴⁰La, ¹⁴¹Ce, ¹³⁴Cs, ¹³⁷Cs radionuclides were constructed. The values of surface activity density were recalculated to 20 May 1986. The absence of area fill in some zones of these maps indicates the lack of information for the radionuclide spatial distribution reconstruction in these zones. It must be emphasized that this work took only 3 months. This fact supports to the above method efficiency.

Click picture to enlarge.

⁹⁵Zr+⁹⁵Nb (21 kB) distribution

¹⁰³Ru (21 kB) distribution

¹³⁴Cs (24 kB) distribution

¹⁴¹Ce (36 kB) distribution

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