Four duraluminum disks were provided with sectors of sheet cadmium, the sectors subtending 3.7° with spacing of 3.5° between. Two of these disks were mounted 54 cm apart to rotate on the same shaft, while the other two disks were fixed within 5 mm of the rotating disks, respectively, as shown in Fig. 1. Since the cadmium is practically opaque to slow neutrons and the duraluminum transparent, the two pairs of disks form a mechanical velocity selector for slow neutrons. Speeds up to 5000 r.p.m. were used.The Rn-Be source, about 600 millicuries, was placed in a Cd shielded paraffin cylinder 16 cm diameter and 22 cm long, and the neutrons were detected by an 8-cm diameter Li-lined ion chamber specially constructed to be insensitive to noise and connected to a linear amplifier-thyratron recording system.Better resolution is desirable, but decreasing the ratio of width of spacing to width of Cd sectors not only reduces the number of slow neutrons as the cube of this ratio, but unfortunately leaves unchanged the large background count due to neutrons of intermediate and high velocities which are not appreciably absorbed by the Cd sectors. These background counts due to the disintegration of Li and the projection of various nuclei 1 by these higher speed neutrons were in these experiments slightly larger than the counts due to slow neutrons. For example, about 29.25 counts per minute were recorded with the selector operating slowly, around 250 r.p.m., about 24.7 counts per minute were obtained at 2500 r.p.m., and a background of about 16 counts per minute was obtained with a piece of Cd interposed.The results prove by direct measurement that many of the slow neutrons are in the thermal velocity range. The curve in Fig. 2 shows the decrease in the number of slow neutrons detected at various speeds of the selector disks. When the selector is run at a speed of about 2500 to 3000 r.p.m., such that it intercepts best particles with speeds of 225,000 to 270,000 cm/sec, there is the greatest decrease in the number of slow neutrons coming through. Hence the peak of the velocity distribution of these slow neutrons is in this region, which corresponds to the maximum of the velocity distribution to be expected if the neutrons were in thermal equilibrium in the paraffin. The precision in these experiments is not very high, and the exact energy distribution curve will require further data and further analysis. ION CHAMBER E 54 CM / III f^~\ I II I I Pn 11 1 I 1 Ml] 1000 90,000 2000 RPM 3000 180,000 £M 270,000 4000 360,000 5000 450,000 FIG. 2. Curve showing change (decrease) in number of slow neutrons detected after passing through the two shutter systems, as the speed of the sectors was changed. The speed of the sectors is indicated in revolutions per minute, and the neutron velocity for which the selector is most effective is also indicated. The vertical lines indicate the probable precision calculated on the basis of the square root of the number of counts.The curve appears to indicate that on the high velocity side the number of n...
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...
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