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Radioactivity: Discover the lowest amounts with new methods

September 17, 2008

PTB invitation to international conference on environmental radioactivity Sept. 22

Detecting ever lower amounts of ionising radiation with ever better methods - sci-entists have had this goal since the start of the nuclear age. In addition to natural radiation, mankind is exposed to a multitude of other sources of radiation which result from the military and technical use of radioactive substances. To protect health, but also for technical and medical applications, it is necessary to determine even the smallest amounts of radionuclides with high precision. On the invitation of the Physikalisch-Technische Bundesanstalt (PTB), scientists from 27 countries are meeting from 22nd to 26th September 2008 to exchange ideas about new measurement and analytical techniques and possible ways of applying them: at the "5th International Conference on Radionuclide Metrology Low-Level Radioactivity Measurement Techniques".

PTB scientists contribute to this international exchange with three dissertations:

1. The German thoron progeny chamber - Concept and Application

Abstract: For the measurement of the thoron progenies, a reference field has been estab-lished within the scope of a project funded by the Federal Ministry of Environment, Nature Protection Reactor Safety. This field consists of an air-conditioned walk-in testing chamber in which the environmental parameters temperature, air humidity and aerosol content can be adjusted and controlled. Analogue to the radon reference chamber, Carnauba wax aerosol is used as model aerosol. The activity concentrations of thoron and of the thoron progenies shall be adjusted by means of several open exhalation sources with 228Th which are distrib-uted in the chamber. Homogeneity of the environmental parameters and of the activity con-centrations of thoron and its progenies in the chamber is ensured by a special ventilation system. With this system, a compromise has been made between high circulation rates due to the low half-life of thoron, on the one hand, and low flow velocities due to the otherwise possible separation of the aerosol particle at surfaces of the chamber, on the other hand. The thoron progenies which mainly accumulate on the aerosol particles, shall be measured analogously by measuring the radon progenies after their separation by means of wire meshes (free fraction) or filters (fraction attached on aerosol partiles) in a follow-up product measuring set-up. By simultaneous spectrometry of the α- and γ-radiation emitted from the sample, quantifying of all thoron progenies as well as traceability of the measurement to the national activity standard is guaranteed.

2. Improvements of low-level gamma-ray Spectrometry Systems at UDO

Abstract: The Physikalisch-Technische Bundesanstalt (PTB), the German metrology institute, operates various low-background gamma-ray spectrometry systems at the underground laboratory (UDO) in the Asse salt mine, near Braunschweig. The experiences gained with these detector systems within ten years of operation lead to various changes and improve-ments of the experimental installations. In 2004, the whole UDO facility had to be moved within the Asse mine from its former location at a depth of 925 m to the present depth of 490 m. At its new location, the radon activity concentration in air (about 60 Bq/m3) is by a factor of three higher than at the 925 m level. Therefore, it became necessary to signifi-cantly improve the shielding against radon, at least for the most sensitive detector system. This radon shield consist of an almost air-tight box which surrounds the whole passive shielding (the latter is composed of 20 cm of lead and 10 cm of electrolytic copper). In addi-tion, an improved nitrogen flashing of the inner volume of the detector chamber was realized by changes of the design. At the same time, the end-cap of this detector was exchanged with the aim to reduce the inherent background of the system (especially from 40K). The background characteristics achieved with an electrolytic copper end-cap equipped with a thin carbon epoxy entrance window are compared with results obtained after the change to a low-activity aluminum end-cap operated at the same detector.

In this paper, the performance of the low-level gamma-ray spectrometry systems at UDO will be described. It focuses on technical aspects and the experiences gained after changes and improvements of the most sensitive detector had been performed.

3. On Monte Carlo Simulation of HPGE Gamma-Spectrometry Systems

Abstract: A complete calibration of gamma-spectrometry systems cannot be achieved exclu-sively on the basis of experimental measurements. Comprehensive experimental calibration is especially difficult in the case of low-level measurements, when volume sources, for which a proper standard is unavailable, are measured in close to detector high efficiency configura-tions, favoring important nuclide specific coincidence-summing effects. In such cases Monte Carlo simulation methods are among the best tools available for complementing experimen-tal calibration.

A recent intercomparison of Monte Carlo codes used in gamma-ray spectrometry revealed surprising differences between the results of different codes [1]. In this work our experience in using GESPECOR and GEANT 3.21 to solve problems in gamma-ray spectrometry (with emphasis on low-level measurements) is reviewed. The focus is on the problems resulting from differences between the two codes. Several topics related to the physics database, to the simulation parameters and algorithms and to problem definition are discussed.

Differences in interaction cross sections applied in the two programs are pointed out and the consequences on the simulation results in various measurement configurations are dis-cussed.

Physikalisch-Technische Bundesanstalt (PTB)