Volume 68, Issue 4

Exposure studies with voxel phantoms were carried out to understand the new personal dose measurand Hp(10) measured by personal dosimeters under realistic wearing conditions. Two simulated realistic voxel phantoms and the dosimeter assembly are described in a first step. Conditions such as photon fluence, radiation field geometry, and photon energy for simulating the external photon fields at the realistic model are specified. Person-related effective doses are calculated to evaluate the personal dose. The relation between measured deep personal dose and effective dose is demonstrated and discussed by means of the simulated response function of the dosimeters at realistic phantoms.

For neutron radiation and mixed neutron/photon radiation fields, only a few types of direct reading dosemeters are commercially available or under development. Some important features of these dosemeters are briefly outlined and their energy dependent dose equivalent response and suitability for operational monitoring tasks in nuclear facilities, in particular, are discussed.

Mathematical efficiency calibration with the LabSOCS software was introduced for two detectors in the measurement laboratory of the Central Safety Department of Forschungszentrum Karlsruhe. In the present contribution, conventional efficiency calibration of gamma spectroscopy systems and mathematical efficiency calibration with LabSOCS are compared with respect to their performance, uncertainties, expenses, and results. It is reported about the experience gained, and the advantages and disadvantages of both methods of efficiency calibration are listed. The results allow the conclusion to be drawn that mathematical efficiency calibration is a real alternative to conventional efficiency calibration of gamma spectroscopy systems as obtained by measurements of mixed gamma ray standard sources.

The decommissioning of nuclear facilities demands an optimisation of analytical procedures to determine a variety of radionuclides in a variety of different materials on a wide activity scale ranging from very low to very high activities. Severe analytical problems arise from disadvantageous decay properties of a number of radionuclides. In the following the opportunities and limitations of recently developed analytical and nuclear detection methods are presented which has been applied successfully determining radionuclides like 55 Fe, 59 Ni, 63 Ni, 90 Sr, Th, U, Pu, Am, Cm. The applied analytical methods have to fulfil high quality standards including very low lower limits of detection, very high selectivity, sufficient precision and accuracy even in samples containing enhanced amounts of interfering elements. A new powerful tool is the application of collimated in-situ-γ-spectrometry. A recently developed calibration phantom named K-RISK is presented.

To draw the balance of anthropogenic 129 I input into European soils, the concentrations of 129 I and 127 I in seven soils from Lower Saxony were analyzed down to a depth of 250 cm. In comparison with pre-nuclear soils and Ukrainian soils, we detected a significant anthropogenic input of 129 I. Considering the different time periods for the input of 129 I and 127 I it was possible to assess the dynamics of sorption processes which dominate migration and accumulation. Anthropogenic 129 I was successfully used as a tracer for a clarification of the migration processes of trace elements in soils.

On September 1st, 2001, the Austrian standard OENORM S 5255-1 with the title “Uncertainties of measurements and limiting values in radiation protection – Part 1: Basics” was published. A part 2 of the standard with the subtitle “Assessment of measurements with regard to limiting values” was published on April 1st, 2002. Part 1 of the standard is a consequent application of the “Guide to the expression of uncertainty in measurement” (GUM) for measurements in radiation protection. It contains a summary of the most important definitions and mathematical methods, which are relevant for measurements in radiation protection. An important prerequisite is the standardisation of the coverage factor k = 1 for measurements in radiation protection. Finally, the use of the standard is demonstrated by a detailed example there (dose rate meter). First, the uncertainty of a dose rate meter is stated, using only information on tolerances available e. g. from instrument specifications. Additionally, the strategies to reduce the uncertainty and the necessary detailed knowledge of the dose rate meter are demonstrated. Part 2 of the standard gives a simple concept for interpretation of limits using the uncertainties calculated based on part 1 of the standard.

The applicability of several neutron detectors for dose measurements at a neutron field typical for high energy particle accelerators is investigated. The response of four commercially available active neutron dose meters and two passive detectors to neutrons from a 241 Am-Be(α,n) source and to neutrons at the CERN EU high energy reference field was determined experimentally and simulated using the Monte-Carlo code FLUKA. Fluence response functions and dose responses for the different detectors were calculated in the energy range between 1 keV and 10 GeV. The results show that the dose response to the high energy neutron field at CERN of the conventional rem-counters is lower by a factor of 2 to 2.5 if compared to the dose response to a 241 Am-Be(α,n) neutron source. The rem-counters exhibiting an additional layer of lead inside the moderating structure showed dose readings which differ only up to 25 %. A thermoluminescent based neutron detector was tested for comparison. These passive detectors revealed a neutron response similar to a rem-counter and may be preferable for situations with highly pulsed beams.

A method for the determination of Sr-89/-90 in different types of environmental samples (German IMIS program) was to be adopted to the 2 nd Saxon state laboratory for environmental radioactivity. Resulting from literature studies a combination of variable extraction steps for Y and Sr was chosen to be able for determining Sr-90 or Sr-89/-90 in a common as well as a rapid way. Measurements with a liquid-scintillation counter (LSC) and a low-background proportional counter (LBC) were performed after the same chemical separation steps in each case to find out the detector which is the best suitable one for our requirements. Different steps of the analytical procedure are discussed in detail and its characteristic parameters are given.

Radioactive isotopes of strontium, mainly 90 Sr, released into the environment due to nuclear accidents may contribute significantly to the internal radiation exposure of members of the public after ingestion of strontium with contaminated foodstuffs. The committed radiation dose is significantly dependent on the fraction of the ingested activity that crossed the gut wall (f 1 value). The directive 96/29/EURATOM has adopted f 1 values of 0.3 for adults. This study was aimed to investigate how far the calcium content of foodstuffs influences the uptake of ingested strontium. For aqueous solutions without calcium containing 1 mg strontium, f 1 values of 0.62 ± 0.13 (mean ± SD) were obtained. However, the uptake of 1 mg Sr from milk, and respectively from solutions with calcium was lower than from aqueous solution (f 1 : = 0.32 ± 0.06, respectively f 1 : = 0.31 ± 0.06). The data obtained show a significant reduction of the uptake of ingested strontium from milk in comparison to aqueous solution. The calcium content of milk could explain this reduction.

For more than 25 years, thermoluminescence dosemeters (TLDs) have been used at Research Centre Jülich for the dose determination of photon radiation in environmental monitoring. Dose measurements are performed in six-month exposure intervals at 78 measuring locations in total at the outer fence and some additional locations inside and outside the research centre. Long-term variations in natural background radiation are analysed in comparison with climate data and the local conditions near the measuring locations and discussed with a view to determining the area-specific parameters. Taking the special conditions of Research Centre Jülich into account, the problems relating to the detection limit of 0.1 mSv/a, stipulated in the guideline for the emission and immission monitoring of nuclear facilities, are considered in more detail compared to natural background radiation.

Light was thrown on the nuclear aspects of Herndon’s georeactor. His thesis is that a power producing droplet of liquid uranium, possessing a radius of about 5 km, operated during the last 4.5 billions of years in the center of the Earth and producing 3–6 TW of thermal power. It could be shown that indeed initial criticality was possible due to the high U-235/U-238 enrichment of approximately 30% at that time. Furthermore, the U-238/Pu-239/U-235 conversion cycle guarantees also a stabilized U-235/U-238 enrichment of about 10% during the whole history of Earth. For this, a constant critical neutron flux in the realm of 10 8 n/(cm 2 s) has to be present to convert U-238 via the Pu-239 route and its eddecay into U-235 with a conversion ratio of exactly unity. Herndon’s georeactor is compared with other exotic fission reactors in Nature, too, but the decisive answer whether such a georeactor really exists needs further – particularly chemical, thermodynamical and geological – research work.