Neutron fluence measurements using underground HPGe-detectors

Full text: IRMM performs gamma-ray spectrometry in the underground laboratory HADES located at a depth of about 500 m water equivalent at the premises of the Belgian nuclear centre SCK CEN in Mol and operated by Euridice. By performing gamma-ray spectrometry with HPGe-detectors deep underground one can reduce the background counting rate with a factor of 1000 compared to a low background detector above ground. Underground HPGe-detectors are being employed in an increasing number of different fields. One field in which the use of underground HPGe-detectors has proven to be particularly useful is in the field of neutron fluence measurements. It is useful at this stage to point out that according to the ISO guides the entities particle flux, fluence and fluence rate are used together with the units s-1, m-2 and m-2s-1, respectively. The name flux density can be used as synonym to fluence rate. Measurement of neutrons (energy and fluence) is a delicate task in nuclear measurements. By making use of underground gamma-ray spectrometry it is possible to extend the present use of the so called foil activation technique (a common technique to measure neutron fluxes in nuclear reactors) to encompass very low neutron fluences (even environmental neutron fluences) as well. As we started working on this technique a couple of years ago, 4 interesting applications have evolved and will be briefly described below. Application no. 1, ALARA principle and benchmarking: Around nuclear installations like accelerators and reactors there is generally an enhanced neutron field. The neutron dose meters used for monitoring purposes are often not reliable at very low doses. By using metal discs of various materials it is possible to measure also only slightly enhanced neutron flux levels. This can be used to better implement the ALARA principle. Such work has been carried out at the IRMM Van de Graaff accelerator as well as at the BR-1 reactor of SCK CEN. The neutron field near medical accelerator is often overlooked. We aim at studying this in the future. Application no. 2, use of fast neutron induced reactions: An accurate neutron energy spectrum can be produced using unfolding techniques in combination with activation foils/discs. Although the activation technique is very sensitive to thermal neutrons it is not so sensitive to fast neutrons due to the generally low activation cross sections for high energy neutrons. The use of threshold reactions and underground gamma-ray spectrometry allows us to produce a neutron spectrum with relatively high resolution, which is useful for benchmarking of other (on-line) neutron monitors or machine (accelerator/reactor) properties. Application no. 3, retrospective neutron fluence measurements: In 1999 there was a criticality accident at the JCO fuel factor in Tokai-mura. The accident was characterised by: 1) High release of neutrons (for 20 hours) to the surroundings, 2) Only negligible amounts of fission products were released, 3) Poor neutron monitoring facilities outside the plant In order to understand the effects of the accident, activated samples (table salt, gold items, steel spoons etc.) were collected from houses in Tokai-mura by the Japanese investigation team headed by Prof. Komura. Some of these items had low activities due to either too large distance from epicentre and/or long time (in relation to the half-life in question) between activation and measurement. Man's knowledge of the effect of ionising radiation on the human body is to a large extent based on follow-ups of Hiroshima/Nagasaki victims. In a project similar to the 'JCO-measurements', IRMM measured steel activated by the A-bomb in 1945 in order to better quantify the dose received by the population. At present the dose is calculated using computer models and measurements are necessary to verify these models. Application no. 4, environmental neutron flux inside various material: With thin discs/foils it is possible to measure the neutron flux inside various materials with high depth resolution. This is important for examp le in geoscience, which relies on cosmogenically induced activation in order to make e.g. age determinations and erosion histories. (author)