bei48482_FM

Nuclear Structure 413

Hideki Yukawa (1907–1981) grew up

in Kyoto, Japan, and attended the uni-

versity there. After receiving his doctor-

ate at Osaka, he returned to Kyoto

where he spent the rest of his career. In

the early 1930s Yukawa tackled the

problem of what keeps an atomic

nucleus together despite the repulsive

forces its protons exert on one another.

The interaction must be extremely

strong but limited in range, and Yukawa

found it could be explained on the basis of the exchange between

nucleons of particles whose mass is in the neighborhood of

200 electron masses: “Could the neutrons and protons be play-

ing catch?” In 1936, the year after Yukawa published his proposal,

a particle of such intermediate mass was found in cosmic rays by

C. D. Anderson, who had earlier discovered the positron, and oth-

ers. But, this particle, today called the muon, did not interact

strongly with nuclei, as it should have. The mystery was not

cleared up until 1947 when British physicist C. F. Powell discov-

ered the pion, which has the properties Yukawa predicted but de-

cays rapidly into the longer-lived (and hence easier-to-detect)

muon. (The pion and muon were originally called the and 

mesons by Powell because, according to legend, these were the

only Greek letters on his typewriter.) Yukawa received the Nobel

Prize in 1949, the first Japanese to do so.

The Japanese physicist Hideki Yukawa was more successful with his 1935 proposal

that particles intermediate in mass between electrons and nucleons are responsible for

nuclear forces. Today these particles are called pions.Pions may be charged (, )

or neutral (^0 ), and are members of a class of elementary particles collectively called

mesons.The word pion is a contraction of the original name meson.

According to Yukawa’s theory, every nucleon continually emits and reabsorbs pions.

If another nucleon is nearby, an emitted pion may shift across to it instead of returning

to its parent nucleon. The associated transfer of momentum is equivalent to the action

of a force. Nuclear forces are repulsive at very short range as well as being attractive

at greater nucleon-nucleon distances; otherwise the nucleons in a nucleus would mesh

together. One of the strengths of the meson theory of such forces is that it can account

for both these properties. Although there is no simple way to explain how this comes

about, a rough analogy may make it less mysterious.

Let us imagine two boys exchanging basketballs (Fig. 11.18). If they throw the balls

at each other, the boys move backward, and when they catch the balls thrown at them,

Repulsive force due to particle exchange

Attractive force due to particle exchange

Figure 11.18Attractive and repulsive forces can both arise from particle exchange.

bei48482_ch11.qxd 1/23/02 3:14 AM Page 413 RKAUL-9 RKAUL-9:Desktop Folder: