New Evidence for the Neutron

Irène Curie and F. Joliot

Editor’s Note

The existence of the neutron here receives support from Irène Curie and Frédéric Joliot. Experimenting with particles emitted from lithium bombarded by alpha particles, they found a penetrating power less than that of gamma rays from radioactive decay. Moreover, these particles were absorbed more readily by paraffin than by lead, ejecting protons in the process, indicating that they were not electrons or light quanta (which would not do so because of their small or zero mass). The evidence points to the particles in question being different from previously known radiation. Curie and Joliot completed some of their experiments before Chadwick’s, and might have discovered the neutron first had their interpreted them properly. Their early results stimulated Chadwick’s own decisive experiments.ft　　中文

SEVERAL important communications dealing with the properties of rays emitted by atomic nuclei when bombarded with α-particles have recently appeared,1 on which we should like to make a few comments.ft　　中文

It has been shown by F. Joliot2 that the rays emitted by boron under the action of α-particles from polonium are much more penetrating than had originally been indicated. Their penetrating power, while superior to that of the most powerful γ-rays obtained from radio-active sources, is inferior to that of the rays obtained from beryllium bombarded by α-particles from polonium. This result does not agree with Webster’s findings, but agrees with the fact that the protons ejected from boron are slower than those ejected from beryllium. Secondly, we have shown that the ejection of protons is a general phenomenon. By means of the Wilson chamber, we have photographed the paths of the helium nuclei ejected by beryllium rays, and from absorption measurements were able to conclude that other atoms are also ejected. Further, our experiments showed for the first time the important part played by the nuclei in the absorption of the rays emitted by beryllium under the influence of α-particles, a phenomenon which clearly marked them off from all previously known radiation.ft　　中文

J. Chadwick was led simultaneously to the same generalisation concerning the ejection of nuclei, and he put forward the view that the penetrating rays produced by the bombardment of beryllium by α-particles from polonium are neutrons. This interpretation is necessary if energy and momentum are conserved in the collision.ft　　中文

Recent experiments which we have carried out with M. Savel clearly show that the rays emitted by lithium have a penetrating power, in lead, less than that of the γ-rays of polonium (they are completely absorbed by 5 mm. of lead), and that they are much more readily absorbed, at equal surface mass, by paraffin than by lead. This proves that they cannot be of an electronic or electromagnetic nature. Since for various reasons it is extremely improbable that we are dealing with hydrogen nuclei or α-particles (the energy of which would be enormous), these results prove—independently of the ejection of nuclei and the laws of elastic collisions—that the rays emitted by lithium under bombardment by α-particles from polonium are different from previously known radiation and are probably neutrons. The above reasoning does not apply to the rays ejected from beryllium, boron, or to those emitted by lithium when bombarded with the α-rays from the active residue of radium3, because in such cases we do not have γ-rays of equivalent penetrating power, for comparison.ft　　中文

Our latest experiments, in collaboration with M. Savel, indicate that the protons ejected from beryllium form two groups. This suggests that there are also two groups of neutrons (each group not necessarily homogeneous); one group has a range of 28 cm. in air, and an energy of 4.5×106 electron volts; the other has a range of about 70 cm. and an energy of approximately 7.8×106 electron volts. We find it difficult to reconcile Chadwick’s result of a maximum range of 40 cm. with the curves which we have obtained for the absorption of protons.ft　　中文

The mass of the neutron calculated by Chadwick4 (based upon the experimentally estimated energy of the neutrons from boron), according to the reaction B11 + α = N14 + n,5 is about 1.006 (He=4), and the atomic mass of Be9, based on the energy of the fast neutrons (7.8×106 ev.), is 9.006. This suggests that the binding energy between the two α-particles and the neutron in the Be9 nucleus is relatively weak. Further, we know that the rays emitted by beryllium are composed of neutrons and photons, and we may therefore suppose that they are emitted simultaneously according to the equation

Be9 + α = C12 + n + hv.

ft　　中文

The photons of 2 to 4.5×106 ev. energy, which we have detected, would correspond to the group of neutrons of maximum energy 4.5×106 ev. (protons having a range of 28 cm.).ft　　中文

(130, 57; 1932)

Irène Curie and F. Joliot: Institut du Radium, Laboratoire Curie, 1, Rue Pierre-Curie, Paris (5e), June 25.

References:

  1. Webster, H. C., Chadwick, J., Feather, N., and Dee, P. I., Proc. Roy. Soc., A, 136, 428, 692, 708 and 727 (1932).

  2. Joliot, F., C.R. Ac. Sci., 193, 1415 (1931).

  3. Broglie, M. de, and Leprince-Ringuet, L., C.R. Ac. Sci., 194, 1616 (1932).

  4. Chadwick, J., Proc. Roy. Soc., A, 136, 702 (1932).

  5. Curie, I., and Joliot, F., C.R. Ac. Sci., 194, 1229 (1932).