At the beginning of each day, the kinetic energy of the NSRL Beam is measured using the technique known as the Bragg Peak. For some low-LET beams, the amount of material required to stop the beam particles is too great and this measurement is not possible. Of the several ways used to determine the kinetic energy of the beam and the associated LET, the most routine measurement is performed by measuring the Bragg peak. The relative LET is measured using the secondary ion chambers as greater and greater thicknesses of high density polyethylene are inserted into the path of the beam. When a critical thickness is reached, the beam particles will slow down enough in the polyethylene to stop in the ion chamber, giving a peak in the observed LET. A plot of the LET measured in the ion chamber as a function of the thickness of intervening plastic typically looks something like Figure 1. From the location of the stopping peak, we derive the kinetic energy of the beam, and the LET that a beam of that kinetic energy would deposit in water.
A typical Bragg peak for 1000 MeV/n Fe is shown below, as well as Bragg peaks for 600 MeV/n Fe and Silicon at various intervals between 100 and 1000 MeV/n.
Last Modified: February 1, 2008