A quasi-isothermal method of operating an absorbed-dose graphite calorimeter is described in theory and practice. In contrast with the well-known quasi-adiabatic operation, which entails temperature increases during measurements, in the quasi-isothermal mode the temperatures of the different graphite bodies remain constant except for small temperature drifts throughout the measurement. This implies that the temperature dependence of the specific heat of the absorber and of the sensitivity of the temperature sensor influence the absorbed-dose determination significantly less. The method is characterized by a power-compensating measuring principle which is illustrated with a 3-body graphite calorimeter. Comparisons of the quasi-isothermal with the quasi-adiabatic method of operation showed good agreement.

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Cross-border movement of radioactive materials and contaminated items, in particular metallurgical scrap, has become a problem of increasing importance. Radioactive sources out of regulatory control, now often called 'orphan sources', have frequently caused serious, even deadly, radiation exposures and widespread contamination. The United States Nuclear Regulatory Commission reported over 2,300 incidents of radioactive materials found in recycled metal scrap and more than 50 accidental smeltings of radioactive sources. A further potentially serious problem is illicit trafficking in nuclear and other radioactive materials. In 1995 the International Atomic Energy Agency (IAEA) started a programme to combat illicit trafficking in nuclear and other radioactive materials, which includes an international database on incidents of illicit trafficking, receiving reports from some 80 member states. For the period 1993-2000 the IAEA database includes 345 confirmed incidents. While from 1994-1996 the frequency declined significantly, this trend has been reversed since 1997, largely due to radioactive sources rather than nuclear material. This paper compares monitoring techniques for radioactive materials in scrap applied at steel plants and scrap yards with monitoring at borders, a completely different situation. It discusses the results of the 'Illicit Trafficking Radiation Detection Assessment Program', a large international pilot study, conducted in cooperation between the IAEA, the Austrian Government and the Austrian Research Centre Seibersdorf. The aim of this exercise was to derive realistic and internationally agreed requirements for border monitoring instrumentation. Finally the present extent of border monitoring installations is discussed.

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