Geel, Belgium, 16 July 2002
Neutron induced charged-particle production cross-sections on light elements like 6Li and 10B are widely used standards in neutron reaction measurements and evaluations. Such measurements are very important for various applications, including radiotherapy (e.g. for the treatment of certain types of cancers concentrating carbon, nitrogen and oxygen) or the development of shielding components, made of lithium, boron or carbon, in future thermonuclear fusion reactors.
Usually neutron cross-section measurements are carried out using gridded ionisation chambers. However, these detectors are not capable of resolving multiple particles hitting the chamber simultaneously. If a neutron-induced reaction of light nuclei, like 10B(n, alpha)7Li, emits the two produced particles (alpha and 7Li) in forward angels, they appear as a virtual "alpha+7Li" quasi-particle rather than as individual alpha or 7Li particles.
Previous experiment evaluations have discarded these quasi-particles as background radiation - consequently some particles were "leaking" from the total emission events count. Recent experiments at the Van de Graaff accelerator of IRMM have correctly taken into account "particle leaking" for the first time worldwide in this context.
IRMM scientists have developed new methods for obtaining accurate neutron cross-sections - this is a major breakthrough in the field of nuclear spectrometry. Comparison of the recently obtained results with previous measurements of the 10B(n,alpha)7Li reaction revealed the sizeable contribution of this effect.
While now total cross-sections can be correctly determined with the existing experimental equipment of IRMM, the correct consideration of "particle leaking" for other measureables, like the angular or the ground/exited state distribution of the emitted alpha-particles, requires an upgraded version of the gridded ionisation chamber, the three-dimensional
IRMM and the Russian Institute of Physics and Power Engineering are developing versions of the latter instrument to be used at the Van de Graaf and the Linear Electron Accelerator of IRMM.