FROM CURRENT LITERATURE

EXPERIMENTS ON THE DECAY OF NEGATIVE MESONS

Recently the attention of physicists has been drawn to the experiments of a group of Italian investigators (Conversi, Pancini, and Piccioni¹), who carried out simple and elegant experiments on the decay of negative mesons. This attention is explained by the sharp disagreement between experiment and the usual theory. The leading theorists (Fermi, Teller, and Weisskopf², Euler³, Iwanenko⁴, Weisskopf⁵) have expressed their views on these questions (see the following abstract on this topic). Two months after the Italian work, analogous experiments were carried out at Princeton University by Sigurgeirsson and Yamakawa⁶.

I. THE WORK OF CONVERSI, PANCINI, AND PICCIONI¹

To study the decay of mesons the authors used recording apparatus that had been successfully employed in previous work⁷–⁸. Figure 1 schematically shows the arrangement of the counters, lead

Fig. 1. Arrangement of the counters, absorber, and magnetized iron plates in the experiments of Conversi et al. All the counters \(D\) are connected in parallel. Delayed pulses between \(A\) and \(III\) (and between \(B\) and \(III\)) are produced by a special multivibrator.

screens and the absorber. Two magnets (length 20 cm, magnetic induction 15,000 gauss), placed between the first (A) and second (B) groups of counters, first, absorb practically the entire soft component and, second, concentrate mesons of the same sign in the direction of the counters. Mesons of the opposite sign are almost completely led away beyond the measuring apparatus.

The third group of counters (C) must count the electrons produced in the decay of mesons stopped in the moderator, since “coincidences” are counted only if the pulse from the counters of the third group arrives in the interval from 1 μsec to 4.5 μsec after the pulse in the first two (A and B). The fourth group of counters (D) covers the solid angle formed by the other groups, and is connected with them for coincidence. The third and fourth groups of counters are separated by a lead plate 1 cm thick, so that decay electrons do not enter the counters D. Other lead blocks are placed in order to protect the counters from showers.

Making a measurement of triple coincidences (III), we obtain the number of decayed mesons (M) plus the number of accidental coincidences. Accidental coincidences may be of two kinds:

a) one particle accidentally gives three pulses shifted relative to one another;

b) one particle crosses the first two groups of counters, while another particle at a suitable time crosses the third group of counters. Or the same with three particles.

Thus:

\[ (III)=M+(a)+(b). \tag{1} \]

To eliminate (a) and (b), a measurement of quadruple coincidences (IV) is made. It is easy to derive the following equality:

\[ (IV)=(1-P_{123})(a)+(1-P_{12}P_3)(b), \tag{2} \]

where \(P_{123}\) is the probability that a particle which has passed through the first three groups of counters will not pass through the fourth group, \(P_{12}\) is the probability that a particle which has passed through the first two groups of counters will not pass through the fourth group, etc. Special measurements showed (by taking comparisons of the corresponding coincidences without delay) that \(P_{123}\simeq P_{12}P_3=0.05\).

Thus, the number of decayed mesons (M) can be obtained by excluding \((a)+(b)\) from equation (1) with the aid of equation (2):

\[ M=III-IV-0.055\,IV. \tag{3} \]

The authors carried out the following experiments:

A) Negative mesons stopped in \(4\) cm C \(+\) \(5\) cm Fe.
B) Negative mesons stopped in \(6.2\) cm Fe (\(6.2\) cm Fe is approximately equivalent, in energy losses, to \(4\) cm C \(+\) \(5\) cm Fe).
C) Positive mesons stopped in \(4\) cm C.
D) Positive and negative mesons stopped in \(5\) cm Fe.

The result of the measurements is summarized in Table I.

Experiments D show that, in accordance with theory, negative mesons, unlike positive ones, are absorbed by the iron nucleus and do not have time to decay. A comparison of A) and B) gives the difference in the behavior of negative mesons in iron and graphite. The authors were unable to obtain a rectangular graphite plate, and therefore used a cylindrical block with mean thickness 4 cm. A 1 cm iron plate was placed, in case A), under the graphite in order to avoid scattering of low-energy mesons, which might have disturbed the focusing effect of the magnets.

Table I

The sharp difference in the behavior of negative mesons in iron and graphite cannot be explained by the present theory (see the following report for more detail), since for the explanation it is necessary to assume that the probability of decay and the probability of capture are quantities of one and the same order.

The comparison of A and C gives a difference in the behavior of positive and negative mesons in graphite. This comparison can be regarded only as qualitative, owing to the different focusing action of the magnetic field in the two cases. The authors believe that the difference between A and C may be explained by a twenty-percent excess of positive mesons in the soft component.

II. THE WORK OF SIGURGEIRSSON AND YAMAKAWA6

The authors studied the decay of mesons in several light materials. The arrangement of the counters is shown in Fig. 2. The moderator has dimensions \(2\times 4\times 14\) inches. The diameter and the sensitive length of the counters are, respectively, 1 and 12 inches. The number of mesons entering the moderator was counted by means of double coincidences of counters I and II. The decay electrons were counted by four counters of group III, which operated only in the interval from \(1\ \mu\text{sec}\) to \(6\ \mu\text{sec}\) after the I–II coincidence. The following substances were used as meson moderators: Be, C, NaOH, Al, SiC, and S. At present no sufficiently accurate decay curve for mesons has yet been obtained, but what has been obtained shows no definite disagreement with the lifetime \(2.2\ \mu\text{sec}\). (The decay curve changes when the delay interval of the counters of group III is changed.)

Fig. 2. Arrangement of the counters in the experiments of Sigurgeirsson and Yamakawa.

In order to compare the decay of mesons in different moderators, it must be borne in mind that different moderators will stop \(\pi\)-mesons and absorb decay electrons in different ways. The most comparable results were obtained in beryllium and sulfur moderators, having equal tor-

...stopping power. In this case one and the same number of mesons will be stopped in the moderators, and the same percentage of electrons will undergo absorption before reaching the counters. The authors made measurements with and without a moderator, changing the moderator every day. Table II gives the results obtained after 8 days of operation (the unit in parentheses means that the quantity has conventionally been taken as unity).

Table II

Analogous results were obtained for other moderators and are given in Table III. In comparing the results it must be remembered that the authors did not make a correction for the absorption of decay electrons; however, they consider that, for emitted electrons, the numbers in Table III should be reduced by approximately 20% for C and by 10% for NaOH and SiC.

Table III

The figures given in Tables II and III show: 1) that almost all negative mesons are captured by the nuclei of heavy elements and are not captured by the nuclei of light elements (the number 1.67 for the relative number of decayed mesons in Be and S, or the number 1.7 for C, is close to 1.8—the ratio of the total number of mesons to the number of positively charged ones);

2) that the probability of capture of negative mesons gradually increases with the atomic number of the capturing nucleus;

3) that the absolute probabilities of capture are in complete contradiction with the theory (see the following abstract for more detail).

M. Rabinovich

References

1. M. Conversi, et al. Phys. Rev. 71, 209 (1947); 71, 557 (1947).
2. E. Fermi, at. al. Phys. Rev. 71, 314 (1947).
3. J. A. Wheeler, Phys. Rev. 71, 340 (1947).
4. G. Gamov, Phys. Rev. 71, 560 (1947).
5. V. Weisskopf, Bull. of Amer. Phys. Soc., 22, No. 2 (May), p. 2 (1947).
6. T. Sigurgeirsson and A. Yamakawa, Phys. Rev. 71, 319 (1947).
7. M. Conversi and O. Piccioni, Phys. Rev. 70, 859 (1946).
8. M. Conversi and O. Piccioni, Phys. Rev. 70, 874 (1946).
9. D. J. Hughes, Phys. Rev. 57, 592 (1940).