7.3.1 Related Topics

Reference: Weidner and Sells

Created October 19, 1995 Neutron Detection

Obviously neutrons do not interact as charged particles, but their effect once they have interacted is that of a heavy charged particle such as an accelerated proton. Boron-10 Reaction

Boron-10 has a high cross section for neutron capture with the subsequent emission of an alpha particle:

When the walls of an ionization chamber are lined with a solid boron compound, this reaction can take place in the presence of a neutron flux and the alphas will deposit their energy in the gas and the ions created will be collected as a signal.

It is of interest to note that this reaction has been of experimental interest for the treatment of tumors by loading the tumor with boron and putting the patient in a neutron flux. The short range alphas have been shown to kill tumor cells. Elastic Collisions

An elastic collision is one in which the total kinetic energy of the interacting particles is the same before as it is after the collision. In a head-on collision with a proton, a neutron will come to rest because their masses are nearly equal and the proton will be ejected with the kinetic energy of the neutron. The proton, being a charged particle, will be detected by the ionization it causes. Therefore neutron detectors tend to be plastic scintillators or other materials which have high proton densities.

The specifics of neutron detectors themselves are left for the next course. Activation

A neutron capture can take place and result in a radioactive reaction product. The detection of the decay of this product is another way of detecting neutrons. As example is the reaction

where Ag-108 is a beta minus emitter. From measurement of the activity of the Ag-108 and knowledge of the capture cross section, and the target geometry, one can estimate neutron flux.

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Douglas J. Wagenaar, Ph.D., wagenaar@nucmed.bih.harvard.edu