Speaker
Description
Using a portable thermal neutron detector array, we have performed cartographic measurements of the neutron background in the reactor and experimental halls of the Institut Laue-Langevin. Four panels of twelve tube detectors have been mounted on a movable trolley, together with the required electronics and computing hardware, forming a mobile neutron detection instrument. The detectors are shielded at the rear and arranged in a square geometry, allowing directional information, as well as magnitude and temporal variations of neutron backgrounds, to be obtained. These measurements can be correlated with operational parameters such as reactor power and the open/closed status of neutron shutters, guides and instrumentation.
Depending on the instrument class, experimental practices and, in some cases, oversights in design and construction, some neutron instrumentation can release non-negligible neutron fluxes into the surrounding environment. These neutrons form a diffuse ‘neutron gas’ or ambient neutron field which, while generally of little radiological significance, can increase and modify the background observed by neighbouring instruments.
Recent infrastructure modifications implemented to comply with earthquake resistance requirements have significantly altered the shielding of neutron guides passing from the reactor building to the guide halls. These changes have resulted in a degraded environmental neutron background, negatively affecting the performance of some instruments. Cartographic measurements using thermal neutron detectors allow the spatial distribution and potential sources of environmental neutrons to be identified, providing a route towards reducing neutron background and improving instrument performance.
Key Words
Neutron backgrounds, Shielding, Instrument Performance
| Other | Neutron backgrounds |
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