Collected by Alfred B. Garrett Ohio State University
Columbus
The Flash o f Genius, 77
The Neutron Identified: Sir James Chadwick
In 1920, Sir Ernest Rutherford in the Bdcrian Lecture'.' proposed s. theory of a neutral particle to oxplnin the discrepancy between the charge and the mass of the neutron. He wrote
The effectappeared to be due to the ejectionof protons, and from further experiments they showed that the protons had ranges in air up to about 26 cm corresponding to a velocity of nearly 3 X 109 cm per "This assumption of a neutral particle involves the second,. . . . idea of the possible existence of an atom of mass which "I made further experiments to examine the properhas zero nucleus charge. Such an atomic structure ties of the radiation excited in beryllium. It was seems by no means impossible. On present views, the found that the radiation ejects particles not only from neutral hydrogen atom is regarded as a nucleus of hydrogen but from all other light elements which were unit charge with an electron attached a t a distance and examined. The experimental results were very diffithe spectrum of hydrogen is ascribed to the movements cult to explain on the hypothesis that the beryllium of this distant electron. Under some conditions, howradiation was a quantum radiation, but followed imever, it may be possible for an electron to combine mediately if it were supposed that the radiation conmuch more closely with the H nucleus, forming a kind sisted of particles of mass nearly equal to that of a proof neutral doublet. Such an atom would have very ton and with no net charge, or neutron^.^ novel properties. Its external field would be practically "It appears then that the beryllium radiation can zero, except very close to the nucleus, and in conseimpart energy to the atoms of matter through which it quence it should be able to move freely through matter. passes and the chance of an energy transfer does not Its presence would probably be difficult to detect by the vary widely from one element to another. spectroscope and i t may be impossible to contain it in "The Neutron Hypothesis-It is evident that we must a sealed vessel. On the other hand, it should enter either relinquish the application of the conservation of readily the structure of atoms, and may . . . unite with energy and momentum in these collisions or adopt the nucleus . . . ." another hypothesis about the nature of the radiation. I f we suppose that the radiation is not a quantum radiaIn spite of the known identifying chamcteristics of the neutron tion, but consists of particles of mass very nea~lyequal it remained unidentified until Chadwick in 1932 eathered tonellwr t l w divrrse ol.arrvst~ona that hut1 hvm unt.y~lairwd i l , to that of proton, all the difficultiesconnected with the w v ~ ~rest~iir~h+!s i~l . ~ n dfinally provrd I y 8 simplr. m~ttl~t.~t~:sri(.:!I collisions disappear, both with regard to their frequency prorc.hre thsr r~diatiousemitttd in n uuml.er of n u r l w r Irnrrand to the energy transfer to different masses. In esses must be neutrons. order to explain the great penetrating power of the Chadwick describes his results as followsa: radiation we must further assume that the partirle has no net charge. We may suppose i t to consist of a pro". . . It was shown . . . that, some light element,^ when ton and an electron in close combination, the neutron bombarded by a-particles of polonium emit radiations discussed bv Rutherford in his Bakerian lecture' of which appear to be of the X-ray type. The element 1920. beryllium gave a particularly marked effect of this kind, "When such neutrons pass through matte1 they suffer and later observations . . . showed that the radiation occasionally close collisions with the atomic nuclei and excited in beryllium possessed a penetrating power so give rise to the recoil atoms which are observed. distinctly greater than that of any X-radiation yet Since the mass of the neutron is equal to that of the found from the radioactive elements. . . Quite recently, proton, the recoil atoms produced when the neutrons Mme. Curie-Joliot and M. Joliot made the very pass through matter containing hydrogen mill have all striking observation that these radiations from berylvelocities up to a maximum which is the same as the lium and from boron were able to eject protons with maximum velocity of the neutrons. The experiments considerable velocities from matter containing hydroshowed that the maximum velocity of the protons gen. In their experiments the radiation from berylejected from paraffin wax was about 3.3 X lo9cm per lium was passed through a thin window into an ionsecond. This is therefore the maximum velocity of the ization vessel containing air a t room pressure. When neutrons emitted from beryllium bombarded by paraffin wax, or other matter containing hydrogen, a-particles of polonium. From this we can now calwas placed in front of the window, the ionization in the culate the maximum energy which can be given by a vessel was increased, in some cases as much as doubled. colliding neutron to other atoms, and we find that the results are in fair agreement with the energies observed ' R U ~ R P O R DE.,, "Bakerian Lecture" (1920). Proc. Roy. in the experiments. For example, a nitrogen atom will Sac. (London). acquire in a head-on collision with the neutron of mass P. AND MORRISON, E., Seientifi American, 185, MORRISON, No. 4, 44-53 (1951).
J., PTOC. Rq.Soe. (London), 136,692-708 (1932). CHADWICK,
638 / Journal o f Chemical Educafion
' CHADWICK, J., Nature, 129,312 (1932)
1 and velocity 3.3 X log cm per second, or a velocity of 4.4 X 108 ern per second." The identification of this mysterious radiation emitted when alpha particles were used to bombard beryllium was established by ealruhting the approximate mass of the particle of this radiation. I h t a to make this calculation were obtained by bombarding other stoms ar the result of the collisions. Chadnick writes,
"It is possible to prove that the mass of the neutron is roughly equal to that of the proton by combining the evidence from the hydrogen collision (with neutrons) with that from the nitrogen collisions (with neutrons)." If Af and V he the mass and velocity of the neutron, then the maximum vclooity given s hydrogen atom is vayapogem =
2df?'l(.U
+ ~nn)
or
1'
= UE~~.....
X (34
+ me)/lJl
and the maximum velocity given a nitrogen atom is vrrit.,.
=
2MV/(Af + mw)
V = u ~ i ~ , .X ~ ~ihf .
or V
H r ~ E ~~=n
=
+ ni~)/2Af
3 . 3 X 10) cn~:ecc,rnn = 1
4 . 7 X 10B e m / ~ e qn h = 14
and sinre V is the same in each case
Chadwick continues, "The total error in the estimation of the velocity of the nitrogen recoil atom may easily be about lo%, and it is legitimate to conclude that the mass of thr neutron is very nearly the same as the mass of the proton. ". . . The Nature of the Neutron-It has been shown that the origin of the radiation from beryllium hombarded by a-partirles and the behavior of the radiation, so far as its interaction with atomic nuclei is concerned, receive a simple explanation on the assumption that the radiation consists of particles of mass nearly equal to that of the proton which have no charge. The simplest hypothesis one can make about the nature of the particle is to suppose that it consists of a proton and an electron in close combination, giving a net charge 0 and a mass which should be slightly less than the mass of the hydrogen atom. This hypothesis is supported by an examination of the evidence which can be obtained about the mass of the neutron. ' I . . . Allowing for the errors in the mass measurements it appears that the mass of the neutron cannot be less than 1.003, and that it probably lies between 1.005 and 1.008."
Volume 39, Number 12, December 1962
/
639
