Gamma Shooting of the Territory of Municipal Landfill of the Dushanbe City

The rearrangement of Municipal landfill of Dushanbe city has begun; in this connection the gamma and neutron shooting of territory of landfill has been made. The landfill territory has been divided into 128 routes, and it has been conducted 5691 measurements of gamma background. The gamma background in territory is defined by background from environing soils 0.18±0.02 μZv/h and background from low-act ive wastes 0.11±0.02 μZv/h. During conducting gamma shooting the ownerless gamma source Cesium-137, with equivalent dose rate 40 μZv/h, has been detected. For localization and source extraction specialists of Committee on emergency situations under government RT and Agency on Radiation and Nuclear Safety at Academy of Sciences RT have been invited. Fluency of neutrons on surface does not exceed background values and that testifies about absence of neutron sources.


Introduction
After collapse of the USSR the centralized monitoring system over storage and disposal of radioactive sources in Tajikistan has been destroyed. And only after 11 years, in 2003 the Agency on Control for Rad iation and Nuclear Safety has been created. During the civil war years and after it there could be events of losses or illegal disposal of radioactive nuclear sources at the Republic factories. For the purpose of search of ownerless radioactive sources IAEA financed the project of the Agency on Radiation and Nuclear Safety.
Recently the decision on rearrangement of Municipal landfill (M L) of Dushanbe city was made. W ithin the limits of rearrangement it is supposed to carry out a soil covering of a surface of a landfill du mp. Befo re realizat ion of these works there was a necessity of conducting gamma and neutron shooting of a surface of a landfill, for the purpose of detection of illegally buried of ownerless sources.
ML of Dushanbe city is located in its southeast part, in 12 km fro m the centre and 2 km fro m a route of Dushanbe -Vahdat, fig. 1. The relief of territory of a du mp represents complex undulating system with comparat ive deep ravine, somet imes with steep slop es, with a g rad e fro m 5-10° to 25-30°. The territory is located at altitudes 870 -930 m above sea level. Landfill exp loitation has begun in 1977. The landfill total area makes more than 20 hectares, the maximu m depth of a body of landfill exceeds 30 m. Territory of landfill has no accurately certain boundary lines and fences. The landfill surface was not covered with a ground for many years, therefore all wo rking zone for today is accessible. The present work is dedicated to discussion of results of gamma and neutron shooting made on landfill territory. As a result of work the ownerless gamma quanta source has been detected. For the source extract ion specialists of Co mmittee on Emergency Situations under Govern ment of Republic of Tajikistan and Agency on Nuclear and Radiation safety at Academy of Sciences have been invited. The taken source has been directed to laboratory of Agency for identification and the subsequent disposal.

Method of Conducting of the Gamma Shooting
Preliminary reconnaissance investigations of the ML were carried out before beginning of works. With shuttle route landfill and its natural co mplex were familiarized, correctness and conformity of a modern situation with topographic maps were checked. Landfill contours were plotted, wind rose beam direct ions defined.
Two basic monitoring points (BMP), for the monitoring over correctness of work of dosimeters have been chosen. The first place of BM P was chosen in territory of Physical Technical institute in 4 km fro m landfill, the second place was chosen near to a landfill office build ing. Chosen BMPs were noted on a district map. The topographic grid was put on the map-scheme of landfill with step of 5 meters, fig. 2. The grid has been tied up to outstanding points on locality (electric poles) and fixed in the working notebook. A knot of this topographic grid has been numbered. In an aspect of relief co mplexity the itineraries for access the measurement points were chosen. The beginning and the end of a grid, a position of monitoring knots on it and coordinate of BMPs were measured by means of the GPS-navigator and were recorded in the notebook. Dosimeters of type "MKS-А 03" and "LUDLUM 2241-2" were used in-process which was adjusted on an optimu m regime accord ing to service manuals of devices.
Before beginning of the work the measurements of an equivalent dose rate (EDR) of gamma background at altitude of 0.1 and 1 meter and fluency of neutrons on two points of BMP were performed, were thus spent not less than 5 measurements, the obtained data was averaged. Co mparison of measurements were made in the beginning and in the end of the working day, they were correlated among themselves on BMP. Indications of dosimeters did not differ fro m each other more than on +/-10%.
Along the chosen routing lines a gamma shooting was performed. Distance between measuring points was choose equal to 5 m. The fixed measurements of gamma background and neutrons intensity (on a point) have been performed in the grid knots. Speed of foot shooting did not exceed 2,0 km/hour. By conducting gamma shooting continuous audition of noise level in earphones and observation on a scale of dosimeters was performs. Audition of intensity of noise (caused by intensity gamma-background) was made at as much as possible pushed detector. In the course of conducting of the gamma shooting test-check of sensitivity of dosimeters by test source was performed periodically each 2 hours and battery power levels of devices were mon itored.
At measurements in the nodal points the detector of device was placed on distance of 1 meter and performed fivefo ld measurement of gamma background and neutron's intensity, the data was averaged and registered in the notebook, then the detector was hauled down on distance of 0,1 meter and procedure repeated. At detection of excess gamma background -on 30% fro m a natural background careful measurement of rad iation intensity in radius of 5 meters was performed. Fo r this purpose the grid broke into sections on 1 meter, and additional measurements and search of an abnormal zone were performed.

Results of Measurements
During the performed works 5691 measurements of gamma background in the points has been registered. Landfill has been divided into 128 lines of measurementsrouting lines least of wh ich was 75 meters and the greatest -355 meters. Gamma background on a surface of soils equals 0.18±0.02 µZv/h, and this background is typical for a Dushanbe city and its neighbourhoods. Background measurements on BMP points have shown similar values, 0.18 µZv/h near Physical Technical institute and 0.19 µZv/h near office building of M L, measuring points were placed on distance more than 20 meters fro m structures.
The wastes on landfill in the basic consist of pieces of polyethylene, glass, paper and rag scraps, and also food wastes. At unloading of wastes from trucks all visib le metals (black and colour), po lyethylene tare and large plastic products was being collected, for recycling. The remained wastes are slightly rad ioactive and their gamma background does not exceed 0.11±0.02 µZv/h, fig. 3.
Considering the big square of an urban du mp -mo re than 20 hectares, 20 days have been spent for conducting of a field work. Results of measurements are introduced in drawing 4. At measurement the ground roads with background of 0.16-0.20 µZv/h were clearly v isible. Ground roads edged a dump boundary line, also there are the inner roads with the raised background presented.  The landfill is located on a slope of a hill and the thickness of wastes on it varies. On northern slope a thickness of wastes less and does not exceed 3 meters, and the soil shows up in some p laces. The gamma background on this section equals 0.15-0.18 µZv/h. On a southern slope (located lower) the thickness of wastes in places makes 30 meters, moreover there are no ground roads on this slope. Gamma background on this section was low compared to other places of the landfill and it's equal 0.09-0.13 µZv/h.
On one of the point on the northern section the abnormal zone with EDR more than 1 µZv/h was detected, the neutron intensity remained constant and did not exceed background values. Detected place has been investigated follow-up, with more fine steps and was revealed the place with EDR mo re than 10 µZv/h. Discovered place has been recorded by external outstanding points and GPS, fig. 5.
By rules of IA EA detection of a radioactive source of unknown type and activity, is considered as emergency situation. Specialists of Co mmittee on Emergency Situations under the Govern ment of Republic Tajikistan and Agency on Nuclear and Radiat ion Safety of the Academy of Sciences have been invited to the place. The discovered section has been surrounded. A repetitive measurement of EDR in a finding point by means of devices of the above-stated structures has been performed. Attempt to determine the type of source was made with using the scintillation gamma-ray spectrometer "InSpektor 1000", but it was failed to detect it on-site. Decision to dig out the source was made. The source was placed on depth less than 10 cm that simplified a problem on its ext raction. The taken source represented the metal container of 4 cm in length and 1 cm in diameter. EDR in a source surface has equal 40 µZv/h. The source has been disposed in the container and carried out to laboratory of Agency on Nuclear and Radiation Safety. Du ring researches in laboratory it has been found out that the source is gamma emitter of Cs-137.

Conclusions
In connection with forthcoming rearrangement of the Municipal landfill of Dushanbe city the gamma and neutron shooting of a surface of landfill has been made. During the performed works 5691 measurements of gamma background in the points has been registered. Landfill has been divided into 128 lines of measurements -routing lines least of which was 75 meters and the greatest -355 meters. Gamma background of adjoining soils equal 0.18±0.02 µZv/h and wastes equal 0.11±0.02 µZv/h were measured. Gamma background on a dump varied dependent on these two values and varies with changes of waste thickness too. The neutron background in all territory of a du mp did not exceed background values, which testify the absence of disposal of neutron sources.
During shooting the ownerless illegally buried gamma source of Cs-137, with EDR on a surface 40 µZv/h was detected. For liquidation of the emergency situation specialists of Co mmittee on Emergency Situations under Govern ment of Republic Tajikistan and Agency on Nuclear and Radiation Safety at Academy of Sciences have been invited. The source has been taken, placed in to the container and carried out to laboratory of Agency for identification and the subsequent disposal.
In the conclusion authors express gratitude to specialists of Co mmittee on Emergency Situations under Govern ment of Republic Tajikistan and Agency on Nuclear and Radiation Safety of Academy of Sciences of Republic of Tajikistan for the help and participation in process.