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Dosimetry up to an altitude of 30 km

29.12.2011

Since the beginning of the 1990s, PTB has carried out dosimetric investigations of the secondary cosmic radiation in the atmosphere [1, 2, 3]. In the course of time, the measuring instruments became more and more compact but also considerably smaller. The measurements were, however, always performed at cruising altitudes between 8 km and 12 km. In July 2011, a measuring instrument developed in cooperation with the Meteorological Observatory Lindenberg of the German meteorological service (Deutscher Wetterdienst, DWD) could measure the altitude dependence of the dose rate in two balloon probings up to an altitude of 30 km - i.e. into the stratosphere.

The measuring instrument used - "Liulin-6SG" - manufactured by the Solar-Terrestrial Influences Laboratory of the Bulgarian Academy of Sciences in Sofia, uses a silicon detector (size: 2 cm2, thickness: 0.3 mm). The signals of the detector are digitalized by means of a 12 bit analogue-to-digital converter and written on an SD memory card. The measuring instrument, its power supply and others sensors for determining the temperature, the air pressure, the air humidity, GPS data and the VHF transmitter were accommodated in a heat-insulated housing (size: approx. 24 cm × 24 cm × 28 cm; total weight: approx. 1.3 kg, see Figure 1) to ensure that the temperature in the housing will not fall below the freezing point at outdoor temperatures down to -60°C.

 Figure 1

The silicon detector measures the energy deposited in silicon or the dose absorbed in silicon, respectively. A field calibration at flight altitudes by means of the software code FDOScalc [4] developed by PTB furnishes a constant calibration factor with the aid of which the measured absorbed dose can be converted into ambient dose equivalent. The results from the two probing processes are shown in Figure 2. For comparison, the data of measurements on board a Concorde aircraft from the years 1997 - 1999 [5] (corrected for solar activity and geographic latitude) are shown. The PTB measurement with a Reuter Stokes chamber (RSS+n) [6] (corrected with the expected neutron contribution) and calculations with the EXPACScode of the Japan Atomic Energy Agency (JAEA) [7] show good agreement with the measured data. The agreement with the PTB code FDOScalc [4] is based on the field calibration method.

  Figure 2

Literature

  1. H. Schuhmacher, U. J. Schrewe:
    Dose Equivalent Measurements on Board Civil Aircraft.
    PTB-Bericht PTB-N13 (1992)
  2. U. Schrewe:
    Global Measurements of the Radiation Exposure of Civil Air Crew from 1997 to 1999.
    Radiation Protection Dosimetry 91, pp 347-364 (2000)
  3. F. Wissmann:
    Long-term Measurements of H*(10) at Aviation Altitudes in the Northern Hemisphere.
    Radiation Protection Dosimetry 121, 347-357 (2006)
  4. F. Wissmann, M. Reginatto, T. Möller:
    Ambient Dose Equivalent at Flight Altitudes: A Fit to a Large Set of Data using a Bayesian Approach.
    Journal Radiological Protection 30, 513-524 (2010)
  5. D. T. Bartlett:
    Radiation Protection Aspects of the Cosmic Radiation Exposure of Aircraft Crew.
    Radiation Protection Dosimetry 109, 349-355 (2004)
  6. F. Wissmann, A. Rupp, U. Stöhlker:
    Characterization of dose rate instruments for environmental radiation monitoring.
    Kerntechnik 72, 192-198 (2007)
  7. T.Sato, K.Niita:
    Analytical Functions to Predict Cosmic-Ray Neutron Spectra in the Atmosphere.
    Radiation Research 166, 544-555 (2006)