Investigation of Nuclear Reactions

This equation is similar to eq. (4.47). The solution for the initial conditions N = N,  

The activation method requires the use of high-puiity targets, in order to exclude the influence of other nuclear reactions. Chemical procedures may be applied to separate the reaction products and to identify their atomic number Z. If short-lived radionuclides are to be measured, fast separation methods are required, for instance on-line separation in a gas stream that passes a temperature gradient (thermochromatography). In the case of half-lives of the order of milliseconds or less, however, only physical methods are applicable, in particular separation by a sequence of electric and magnetic fields. Stable or long-lived products may be determined by use of mass spectrometry, provided sufficient masses are available. 

Flux densities are determined by means of monitor foils for which the cross sections are well known. Sample and monitor have to be irradiated rmder exactly the same conditions. Sandwich arrangements or stacks of sample and monitor foils are preferred. 

In all nuclear reactions the product nucleus suffers a recoil, which may be marked if nucleons or a particles are emitted (chapter 9). Due to this recoil, a certain amount of the product nuclei are thrown out from the target and may be collected in catcher foils. Stacks of samples and thin monitor and catcher foils are used to obtain as many data as possible. 

Measuring the change of the flux density within the target provides only information about the total cross section of all interactions in the target. This method is mainly used for determination of neutron absorption. In the case of charged particles it has to be taken into account that these are also losing energy by ionization processes. 

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