On the Absorption Law for Slow Neutrons

Abstract

on the basis of a simple model for the neutron-nucleus interaction (deep potential hole of radius of the order of magnitude of the nuclear radius) shows that, whatever the absolute value of the capture cross section may be, it must be inversely proportional to the velocity of the neutron. This statement can be expressed in other words by saying that the probability of absorption of a slow neutron depends only upon the time which the neutron spends in the material.However, phenomena of selective absorption observed by several experimenters 4 using different absorbers and detectors suggest that this law does not hold, at least for some of the elements investigated.In order to test the validity of the 1/v law in a more direct way, and also to find out in which direction the possible deviations lie, we performed the following experiment. It is known already from experiments with a velocity selector 5 that the slow neutrons are approximately in thermal equilibrium at room temperature; it must then be possible to produce appreciable changes in the absorption by altering the relative velocity of the neutrons and the absorber, i.e., by moving the latter. We rotated a 50-cm diameter steel disk coated with a thin film of cadmium (about 0.02 mm) at 6000 r.p.m., so that the linear velocity near the edge was about 140 meters/sec. A beam of slow neutrons was sent through the disk near the edge at an angle of about 25° with the face of the rotating wheel so that by rotating the disk in one direction or the other a large component of the velocity of the cadmium was added to or subtracted from the velocity of the neutrons. It is clear that under these conditions if the 1/v law holds, no change in absorption must be found when the direction of rotation is reversed, because the time spent by the neutron in the cadmium sheet is not changed; in other words, the longer or shorter effective path is compensated by the change in cross section. However, if the cross section does not vary inversely as the velocity but follows a different law, then a change in absorption should be observed. For instance, if the cross section is a constant we must find more absorption when the absorber moves against than when it moves with the neutrons.We found that 8.2 ±0.8 percent fewer slow neutrons came through when the cadmium was moved against than when it was moved with the neutrons (about 150,000 particles were counted by means of a lithium lined ionization chamber). This, under the experimental conditions, corresponds to a change in absorption coefficient of 6.3 percent. If we assume the Maxwellian distribution at room temperature for the slow neutrons, and calculate the apparent change in absorption coefficient resulting only from the changed length of path in cadmium (i.e., we assume the cross section to be constant), this turns out to be about 9 percent, which is not much larger than the observed effect.In the case of cadmium we conclude therefore that the anticipated 1/v law does not hold, but the cross section varies with the velocity less rapi...

Capture of Slow Neutrons

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