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On the Temperature Effect of Underground Cosmic Radiation

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Letter to the Editor

On the Temperature Effect of Underground Cosmic Radiation T. Miyazima

Physics Department, Tokyo Bunrika Daigaku. Feb. 20, 1948

According to a long period of observa-tion by M. Forrb,(1)the intensity of the vertical, hard component of the cosmic radiation at 1000-m water equivalent depth has good correlation with the outer-air temperature, but not with the barometric pressure. The temperature coefficient is

about 0.74±0.13 percent per degree. On

the other hand, it is well known that the intensity at sea level decreases with in-creasing temperature and also with increas-ing pressure. and the temperature coef-ficient is -0.15 percent per degree. This striking contrast seems poteworthy, and I want to remark that this fact gives a strong evidence for validity of the two-meson hypothesis.

Barnothy and Forro(2) interpreted this phenomenon as follows: The most penetrat-ing component of cosmic radiation consists of neutrini created by the disintegration of the mesons in the atmosphere, and the

intensity of the neutrini increases with increasing temprature, since the meson-producing place is shifted upwards and a larger number of mesons disintegrate be-fore reahcing sea level. This interpretation seems correct qualitatively, but the life time of the meson,2.15 microsec., will be too long to produce sufficient number of neutrini to explain quantitatively intensity of underground cosmic radiation.

Now, Lattes et al.(3) have recendy obtained splendid photographs which show there are two kinds of mesons, one being heavier than the other. The heavy meson interacts strongly with the atomic nuclei; it is sometimes found in the"stars"or it is absorbed by the nuclei and causes nuclear destruction. Morever, it decays into the light meson, and this phenomenon

is called"μ-decay."Though the life time of the μ-decay is not yet known, it will

be shorter than that of the ordinary decay of the light meson, because heavy mesons are supposed to be created by the primary protons in the upper atmosphere and then decay into light mesons which constitute the hard component in the lower atmos-phere. Marshak and Bethe(4)called atten-tion to the possibility of explaining the fact that the rate of decrease of the un-derground intensity increases with increas-ing depth by assuming that the heavy

meson cannot penetrate into deep under-ground owing to their strong interaction with the atomic nuclei and the mean free path of heavy mesons with energy 2.1011 ev. is of comparative order of magnitude with the height L of the producting place of the heavy mesons. In fact. most heavy mesons of energy less than 2.101( ev. than decay in the atmosphere and the light mesons produced by their decay can pene-. trate only up to 500-m equivalent depth. Thus the light mesons at deeper place can be produced only by heavy mesons of ener gy greater than 2.1011 ev, whose mean free path l is longer than L and inversely proportional to the energy. The rate of decay of these mesons in the atmosphere and, consequently, the intensity at 100-m water equivalent depth are proportional to L. The temperature coefficient of the underground intensity is, therefore, equal to that of L, which is, according to Forr6, about +0.16 percent per degree, when L is assumed to be 15 km, for the standard atmosphere. Though this is somewhat smaller than the observed value, this dis-crepancy need not be taken too seriously, because the real atmospheric condition is not well known.

The conclusion arrived at is the same as that of Forr6, but the life time of the heavy meson becomes 5.10-3 sec when its mass is assumed 400 times of the electron mass. This value of the life time is much shorter than that of the light meson, but this is necessary in order that heavy me-sons of low energy produce in the upper atmosphere sufficient light mesons as observed in the lower atmosphere.

In connection with the two-meson hypo-thesis, it will be desirable that tempera-ture effect and other natempera-ture of the cosmic radiation at various depth are examined experimentally as well as theoretically from the new point of view.

Detail of this report will be published shotly.

(1) M. Forr6, Phys. Rev., 72 (1947) . 506. 2) J. Barnothy 104 and M. Forr6, ZS. f . Phys.,

, (1937), 744; Phys. Rev., 53 (1938), 848; 55 (1939), 368; 58 (1940) , 844. (3) C. M. Lattes et al ., Nature 160 (1947),

478.

(4) R. E. Marshak and H. Bethe , Phys, R

ev. 72 (1947), 506.

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