Ilja Frank (1985) - Some peculiarities of neutron-nuclear interaction

Dear colleagues, students, Damen und Herren, I'm very glad to have an opportunity to take part in the meeting of Nobel Prize Winners for Physics. This year we celebrate the centenary of Niels Bohr. I'm glad to remind you of that since Bohr has a great relation to the subject I begin my lecture with. Nearly fifty years ago or to be more exact in 1936, Bohr explained the peculiarity in the neutron-nuclear interaction which seemed incomprehensible at that time. It was known that slow neutrons penetrate easily into the nucleus. It seemed that the same should be true for a backward process. The neutron must leave the nuclei as easily as they penetrate into it. However, it is not so. As a rule a slow neutron penetrate into the nucleus remains in it. Instead gamma rays are emitted in majority of cases. I was lucky to hear Bohr explaining it himself at his lecture in Moscow in 1937, it was unexpected. Without a complicated theoretical argumentation, without any formulas, there was a simple model, more like a toy and Bohr obviously enjoyed demonstrating it. A wooden plate was placed on the table, Bohr put there a few steel balls. The plate represented the nucleus and the balls protons and neutrons in it. Along the inclined wooden rails one more ball was rolled into the plate. A neutron penetrated into the nucleus. Unfortunately, I have not a photo of that model. On slide taken from paper by Bohr of 1937 it is done a little different. It quite illustrates what Bohr said, you see this. If there were no other balls in the plate the ball would come over to the opposite edge. The neutron penetrated into the nucleus would easily leave it. The situation is different if there are other balls in the plate They rolled in, ball strikes one ball after another, the balls striking make each other to move but none of them as a rule has enough kinetic energy to go over the edge. The neutron penetrate into the nucleus cannot leave it, since it has transmitted its energy to many particles. Of course it is an illustration only, but under it lies a beautiful theory so called ‘Model of Compound Nucleus’ developed by Bohr. The theory of compound nucleus is the basis of our understanding for majority of nuclear reaction. Bohr of course did not at all refuse probable quantum representation there and immediately pointed out to its very important consequences. Since a very complicated motion of many particles arise in the nucleus, there must be many quantum states, so called neutron resonances excited by neutrons. This is example of neutron spectrum for one of nucleus. X is neutron energy, Y is the probability of its absorption. Therefore each resonance has a shape of a beak resembling a line in the spectrum of an atom. It is seen that the resonances are narrow and this also follows from the Bohr model. So the neutron having the energy coinciding, if resonance energy is strongly absorbed and if its energy is little different reabsorption is much bigger. The probability of neutron absorption in the resonances differ from resonance to resonance and besides there may be bigger resonance not seen on this slide. Another consequence of quantum theory is that the resonance has quantum characteristic. The so-called spin and parity, positive or negative. Certain rules must be held for connecting spin and parity of the absorbent nucleus with the quantum characteristics of the resonance. The only thing essential in the theory is that as a rule a slow neutron excites in the nucleus state with parity equal to that of initial nucleus. It is a consequence of effect that the excitation of nucleus is connected with so-called s-wave neutrons. However, with a very low probability the slow neutron excites also resonance with parity different from that the nucleus has initially. These resonances hardly reveal themselves and the absorption of slow neutrons is small. Here the p-wave neutrons are important, their contribution being insignificant for slow neutrons. So there is an axis, where different parities are excited. Some are weaker and some are stronger. When the question arises which I would like to discuss, each parity of this nuclear states quite definite or there may be cases when to some extent or other, there are simultaneously present both negative and positive parities. If it were possible when it would mean that the so-called violation of parity takes place. What does this mean? A problem is rather complicated. Let us try to explain it on examples. Looking at the clock, this is to the right on this slide you know the direction of seconds hands rotates. You are sure that the minutes hand and hours hand going in the same direction, though their motion is so slow that an eye can hardly catch it at once. If you are looking in the mirror at the image of a clock face you see a surprising thing. The seconds hand rotates not in the usual but opposite direction. The same happens with other hand if you follow the rotation. At the same time, the clock with the hands rotating counter clockwise like one sees in the mirror would evidently be not worse when conventional clock, for not customary to us. If there appeared any difference in its operation when it would mean the violation of parity, but in the clinical processes the parity conserves. Of course there is yet no answer to the question, why with two direction of rotation are not equivalent for us and why the definite sign rotation was chosen for the clock. You shall not consider possible reasons. Maybe our arms are made so that the right one prefers the clockwise direction while the left one being its mirror image prefers the opposite side of rotation. In different phenomena in nature you meet these signs of rotation. For example a lot of molecules are structured with definite asymmetry. It reveals an optical phenomena among others. In transmission of polarised light through a substance containing with molecules the plane of polarisation rotates in a definite way. You say that there are right-hand and left-hand light rotating molecules. Both types of molecules of one and the same substance are a mirror image of each other - in the rest being equivalent. For example if it appears that one type of molecules absorbs the polarised light stronger than other type then it means that the parity does not conserve. This is not observed. There is again a question: Can one observe something of kind in the interaction of neutrons with nuclei? The question is not idle. We made this, that the neutron process its own momentum spin. Speaking with classical language it is in rotation. So if a nucleus is bombarded by polarised neutrons with spin in the same or in the opposite velocity direction, then the nucleus will see them rotating clockwise or counter-clockwise, you see these two cases on the slide. One is accepted to speak then about two sides of neutron helicity. Maybe neutron capture probability of nucleus depends on neutron helicity. On this slide there is shown above the transmission neutron through the nucleus and below neutron absorption by the nucleus. May one observe the similar difference in absorption probability. The theory gives a seemingly negative answer to this question. Really, the interaction of neutrons with nuclei belongs to the type of the so-called strong interaction where parity must be conserved. Therefore, if there is no preferable direction of rotation in the nucleus itself; that is the spin of nucleus is in different orientation that it makes no difference for nucleus in which direction the neutron rotates. However, there is still a seemingly unimportant peculiarity which from the first side can be neglected. There is nature besides strong interaction, the so-called weak interaction, where the parity does not conserve. In non-electromagnetic interaction of the electron with nucleus is an example of weak interaction. Weak interactions must take place and in the presence of strong interactions. But they are about a dozen million times less. Before the parity conservation law being true for the strong neutron-nucleus interaction must hold with very high precision. The parity violation effects if one expects them to be proportional to weak interaction contribution, must be less than a millionth part of measurable value. In the experiment it’s impractical and undetectable. But in physics one should be careful about such statement. Something that can be neglected under certain conditions may unexpectedly become essential under other conditions. This happens with the absorption of polarised neutron binuclear, especially of resonance neutrons. It was already noted that the main resonances excited in a given nucleus by slow neutrons have equal parities. To be definite let us suppose it is positive. This corresponds to the Lanthanum 139 nucleus which you shall consider. However, a careful analysis of neutron absorptions spectrum shows slightly revealing resonance with an opposite what is negative here parity. Both types of resonances separately must have definite parity. But in the neutron spectrum they are neighbours and this leads to additional peculiarities. It is a big interaction that serves a peculiar catalysator which causes a partly mixing of property of resonances with different parity. According to the theory this is revealed more significantly in p-wave neutron resonances with a parity opposite to that of main ones. A parity violation effect may as it appeared increase by a factor a million it becomes significant. In particular in the experiment illustrated on this slide there is really observed the difference in the absorption of neutron polarised in the direction of velocity and the opposite direction. The greatest effect is observed with Lanthanum 139 nucleus. It has together with positive parity resonances a very big negative parity resonance that in neutron energy of 0.75 MeV. Slide demonstrates a part of neutron absorption. Spectrum near this energy see on the top. Even if it absorbs from Lanthanum this resonance is seen as a small hollow in the spectrum transmitted through its neutrons. In slide, in the bottom there is shown a relative value of the difference in neutron absorption or the polarisation. In the resonance the effect reveals itself clearly and the difference in effective cross section it is 7%. Outside the resonance it is practically equal to zero. So the nucleus really distinguishes the same velocity but with parity violation effect it reveals very distinctly. These results were recently obtained in Dubna. Such a significant value of violation effect was observed for the first time. Similar for less than for Lanthanum effects were also discovered with some more nuclei. Study of parity violation in nuclear interaction has a long history. These effects were first discovered in the experiment carried out about 20 years ago in Moscow. Here an understanding is very important of the effect that their value may be much greater than the relative value of contribution of weak interaction. It appeared true and it helped to find them out, but still the observed effects were very small, about 10^-4 of measured value. Theoretical and experimental studies in this direction continued by various methods and in different places. In Moscow, Grenoble, Dubna, Gatchina near Leningrad, Novosibirsk. Finally we brought forward the understanding of the condition under which parity violation effect in neutron-nuclear interaction would reveal itself more distinctly as well as the experiments about which results I spoke about. I began my lecture with a description of a simple model illustrating the story of compound nucleus advanced by Niels Bohr. The parity violation effects, discussed here, are also as you may see connected with compound nucleus. Thank you.

Ilja Frank (1985)

Some peculiarities of neutron-nuclear interaction

Ilja Frank (1985)

Some peculiarities of neutron-nuclear interaction

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