AIRBORNE GAMMA-SPECTROMETRY SYSTEMS R&D

The airborne gamma-spectrometry systems (AGSS) and methods R&D are being carried out in Moscow Engineering Physics Institute since 1980. The systems and methods designed are constantly used in our experimental researches. We have two base sets of AGSS at our disposal now:

• airborne gamma-spectrometry system AGSS-99S with scintillation detector;
• airborne gamma-spectrometry system AGSS-99 with combined scintillation and HPGe detector unit.

These base AGSS sets may be transformed depending on air-craft type and task to be solved.

Some AGSS R&D results are presented here.

• AGSS description
• experimental results

Airborne gamma-spectrometry system AGSC-99S with scintillator detector.

AGSC-99S is designed for radioactive sources search and for surveys of the relatively simple contaminated territories. The latter are:

• search for radioactive sources (nuclear and radiological terrorism problems, sources lost in transport operation, accidents and so on);
• scanning of "aged" radioactive pollution (peaceful nuclear explosions, global radioactive pollution and so on.)
• geophysical R&D.

The AGSS-99S prototype was tested in 1989. The main technical approaches introduced in this model were the following:

• Application of complete spectrometry tract for each of the multiple detectors and complete spectrum from each detector accumulation and storage.
• Application of digitally controlled spectrometry amplifiers for spectra stabilizing and scaling.
• The detectors and all spectrometer's electronics and supporting units are designed as a complete package (AGSS spectrometry module).
• Application of PC for flexible spectrometer control, data acquisition and storage, real-time data processing and visualization.

The Mi-8, Ka-26 helicopters and An-24, An-26 airplane AGSS operation was realized. The summed operation time is about 500 hours.

AGSC-99S COMPOSITION. Main parts and units.

• Spectrometry module.
  Spectrometry module is a container with four scintillation spectrometers inside. This container takes care of the spectrometers (electronics and detectors) in flight conditions. On the outer surface of the container the connectors for power supply and PC are placed. One to four containers may be used depending on the task to be solved.
• Airborne computer.
  Industrial computer. It serves for spectrometer control, spectrometry and navigation data storage, preliminary data processing and real-time data visualization.
• GPS navigational receiver with a pilot steering indicator.
• Uninterruptible Power Source.
• Laboratory data processing computer.
• System software.
  This is the set of software and methods for spectrometer control, data acquisition, processing and visualization.
• Documentation.

Main features.

• Spectrometry unit.
  Detectors type and size: NaI(Tl) scintillator, Æ200õ100mm, 3.1 l volume or 200õ200õ100 mm, 4 l volume.
  Energy resolution for 661.6 keV: 9% - 11%.
  Detectors by unit (total volume): 4 pcs; total volume 12.5 or 16 l.
  Data acquisition control by airborne computer.
  Spectrometers by unit: 4pcs.
  Number of channels of spectral data: 256-1024.
  Full spectrum storage for each detector.
  ADC conversion time: less than 6 ms, fixed.
  Automatic (both hardware and software controlled) spectrum stabilization.
  Spectrometry module dimensions: 550õ550õ450 mm.
  Spectrometry module weight: less than 85 kg.
  
• Software and methodic.
  The air-borne scintillation data processing methods and software has been developed in Moscow Engineering Physics Institute and differ radically from the universally accepted spectra processing methods. The methods devised make it possible to locate and identify the rather complex gamma-emitting nuclide composition point and extended sources.
  Software and methodic kit includes:
  
  • Spectrometer control software.
  • Gamma-survey realization method.
  • Point and extended gamma-emitting sources location method and software.
  • Gamma-emitting sources identification method and software.
  • Natural radionuclides (Ra, Th, K) concentration radon corrected calculation method and software.
  • ArcGis software by ESRI based methods and software for data mapping and processing.
  • Software for GPS receiver and altimeter. Special Moscow Engineering Physics Institute developed software is used for the point gamma-emitting sources precise localization.
  • Spectrometer calibration method and software.
• Navigational and pilot steering unit.
  The deviation from the route of interest is displayed by this GPS receiver based unit to simplify the route of interest piloting. It's an optional unit for AGSS-99S because any tacking type data may be processed by the AGSS-99S software.

Scientific and technical accompaniment.

• Optionally we can provide the following theoretical and practical training for our clients:
  • «The basis of environmental gamma-spectrometry»
  • «The theoretical and practical fundamentals of airborne gamma-spectrometry»
  • «Spectrometry results processing techniques»
  • «AGSC-99S design»
  • «AGSC-99S practical training».
• Our professionals can provide the full set of setup works on the client's ground.

Airborne gamma-spectrometry system AGSC-99 with combined scintillation and HPGe detector unit.

The NaI(Tl) scintillation detector relatively poor energy resolution is a restricting factor for the correct gamma-emitting source identification sometimes. So the high energy resolution Ge(Li) detector based airborne gamma-spectrometer was developed in MEPhI in 1983.
Such a spectrometer was used simultaneously with the NaI(Tl) scintillation airborne gamma-spectrometry to compensate the lack of Ge(Li) detector efficiency.
In joint data processing procedure the spectra from semiconductor detector are used for nuclide identification and scintillation spectra are used to locate the source and determine its spatial distribution. This approach has been called the high energy resolution airborne gamma-spectrometry. We used such an approach in 1986 in Chernobyl and at the beginning of August 1986 the significantly contaminated ETU regions were revealed and the suitable surface activity maps for ¹³¹I, ¹³²Te, ⁹⁵Zr+⁹⁵Nb, ¹⁰³Ru, ¹⁰⁶Ru, ¹⁴⁰Ba+¹⁴⁰La, ¹⁴¹Ce, ¹³⁴Cs, ¹³⁷Cs radionuclides were constructed.

Later the numbers of modifications of the first variant has been made. These AGSS were successfully used on Mi-8 and Ka-26 helicopters and An-24/26 airplanes. The total time of operation is about 500 hours.

AGSC-99 composition.

• AGSC-99S with one or several spectrometry modules.
• Airborne gamma-spectrometer with HPGe detector unit incorporates:
  • HPGe detector based unit with full energy absorption peak sensitivity spatial anisotropy.
  • Portable multi-channel analyzer.
  • Airborne computer.
  • Software and methodic.

Main parts and units. Features.

• Full energy absorption peak sensitivity spatial anisotropy HPGe detector based unit.
  This is the main high energy resolution gamma-spectrometer part. The full energy absorption peak sensitivity anisotropy is realized by passive collimation of the detector.
  Full energy absorption peak sensitivity (ANSI/IEEE 325-1986): 60%-150%.
  Absolute energy resolution (IEEE): 1.80-2.20 keV at 1332.5 keV.
• Portable multi-channel pulse-height analyzer.
  One of the commercially available portable multi-channel analyzer. The analyzer PC control software is to be somewhat modified to insure the specific airborne data acquisition.
• Airborne computer.
  Industrial computer. It serves for spectrometer control, spectral and navigational data storage, real time data pre-processing and visualization.
• Software and methodic.
  The methodic specifies the order of operations during survey. The software is intended to provide gamma-spectrometer functionality and results calculation.

Experimental results.

The airborne gamma-spectrometry methods and devices designed were successfully used for the investigation of the USSR European territory Chernobyl accident contamination in 28 April - 26 September 1986 period. The radioactive contamination of the territories of Semipalatinsk Test Site (1989-1990) and Novaja Zemlja Islands South part (1995) after nuclear tests stop were also investigated by the MEPhI specialists by means of the methods and devices designed.

In June-July 2003 the airborne survey of some territories of Moscow region was conducted by MEPhI and "Radon" specialists collaboration. The "Radon" Ka-26 helicopter was equipped with MEPhI AGSS-99S for this purpose. The flights over territories of interest were organized by regular parallel tacking (200-400 meters from each other) at 100 meters height and 30 m/sec velocity. The coordinate information was GPS based and by MEPhI methods corrected for better gamma-emitting sources localization and identification.

Survey plan.

GPS time-lag effect on positioning accuracy during parallel tacking
(here Dt – GPS data output interval,
V – average aircraft velocity).

Longitude (DB), latitude (DL) and altitude (DH) correction switching from WGS-84 to Pulkovo-42 datum.

The airborne data were processed by MEPhI designed software and programs. The spatial distributions and the corresponding maps for point source localization criterion NSI (Normalized Spectral Index) and ¹³⁷Cs, ⁴⁰K, ²²⁶Ra, ²³²Th were constructed.

NSI distribution along the flight trajectory.

NSI distribution on territory surveyed.

To calculate the artificial or natural nuclide activity (concentration) we have developed the "peak" method for airborne spectra processing. The essence of the method is the full energy absorption peak count rate and its total uncertainty correct calculation.

In case the spectra is obtained in AGSS measurements the full energy absorption peak count rate correct calculation appears to be a problem because of the scintillation detector poor energy resolution, short measuring time (that is poor statistics) and a great amount of continuum in spectra due to the air scatted radiation. To overcome this problem we have developed the "resolution improvement" method that enables to calculate accurately the full energy absorption peak count rate and its total uncertainty in the airborne spectra for the energy range 150-3000 keV. The essence of the method is a number of transformations of the measured spectrum resulting in peak width critically (2-3 times) decrease. The results of these methods application are displayed for three different gamma-emitting source types.

The measured and the transformed spectra are presented for A, B, C gamma-emitting source type. Place mouse cursor on picture to see the results of processing.

A - the flight over the ¹³⁷Cs significantly contaminated territory and the ⁴⁰K, ²²⁶Ra and ²³²Th normal concentration;

B - the flight over the ¹³⁷Cs slightly contaminated territory and the ⁴⁰K, ²²⁶Ra and ²³²Th normal concentration;

C - the flight over the operating nuclear reactor, producing the ¹⁶N high air concentration; the ¹⁶N decay is accompanied by high energy gamma-radiation emission producing a very strong annihilation peak in this spectra type.

The rather strong ¹³⁷Cs local contamination was discovered near town of Podolsk (Moscow visinity). The concentration maximums were localized and identified. One can see from the ¹³⁷Cs contamination map presented that two contaminated maximums register with two plant positions.

Found radioactive source spectrum.

Cs-137 count rate distribution.

Two Cs-137 sources localization.