Absorption first-pass effect

An alternative technique is the so-called Photothermal Beam Deflection Spectroscopy [PBDS], based on the so-called mirage effect first reported by Boccarra and coworkers [39, 40]. In this case, the periodic temperature rise caused by the absorption of the modulated IR radiation (i.e. the photothermal effect) is detected optically because it causes periodic deflections of a laser beam passing close to the surface of the solid sample. The PBDS technique has some advantages over the PAS technique, because of its lower Hmits of sample dimensions, but it has disadvantages because of the critical geometric setup. Like PAS, PBDS can have advantages with respect to traditional IR technique for the detection of surface... 

The absorption by the Pa is a more serious effect. This element is, according to Miss Way, probably not fissionable and will therefore cause a loss in two ways. First of all, by the neutrons which it absorbs and second by forming an element of a relatively short half-life which decays into 24 instead of the fissionable 23. For this reason it will be necessary to keep the absorption by the Pa at a minimum. This can be done by extracting the Pa at sufficiently frequent intervals so as to reduce the absorption of neutrons by this element to about 1% of the absorption by the thorium. This will be relatively easy if the absorption cross section of Pa is of the same order as the absorption of the thorium and if all the thorium can be used in the form of a slurry, since for a power output of 100,000 kW only about 120 gms of Pa are formed per day and since the mean life of Pa is 40 days, there will be, in steady state, 4.8 kg Pa in the reflector. Since the total amount of Th in the reflector is more than a ton, it will be sufficient to extract the Pa only n times a month if its cross section is n times that of the thorium. However, if it is necessary to use part of the thorium in form of a backing of the tank, a considerable fraction (about 25%) of all the neutrons will be absorbed by the first few cm of this backing. As a result, the Pa accmnulates in this part of the thorium much more rapidly than in the bulk... 

Non-productive neutron absorption in the absorption zone, however, has a more serious effect. For example, protactinium is an intermediate stage between the thorium and the uranium isotope desired, and as far as is presently known is not fissionable. This element can cause a neutron loss in two ways. First, a neutron loss by 0Q the neutrons which Pa absorbs, and second, by formation of an element decaying into instead of into a known fissionable isotope. This effect however can be kept to a minimum by extracting the Pa from the slurry at sufficiently frequent intervals to reduce the ab-55 sorption by the Pa to about. 5 percent of the absorption by the thorium. 

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