Measuring Beam Intensity and Fluxes

Approximate values of the target thicknesses can also be obtained by noting the energy loss of monoenergetic ions as they pass through a foil. The thicknesses are calculated from the observed dE/dx and empirical stopping power relationships. 

Measurement of charged particle beam intensities is largely done using physical methods, although some older radiochemical methods are used occasionally. The most common techniques to measure the intensity of a charged particle beam is through the use of a Faraday cup. The beam is stopped in an electrically isolated section of beam pipe referred to as a Faraday cup (Fig. 19.3). 

When the energy of the charged particle beam is too large to easily stop the beam in a Faraday cup, the beam intensity is frequently monitored by a secondary ionization chamber. These ion chambers have thin entrance and exit windows and measure the differential energy loss when the beam traverses them. They must be calibrated to give absolute beam intensities. If the charged particle beam intensity is very low ( 106 particles/s), then individual particles can be counted in a plastic scintillator detector mounted on a photomultiplier tube. 

When performing irradiations with neutrons or high-energy protons, it is common to measure the beam intensity using a monitor reaction. A thin foil of a 

Assuming that the monitor and target foils are exposed to the same irradiating flux, we have, for the activity of the monitor and target foils,, 4rnon and Atgt, respectively, at the end of the irradiation  

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