320 
THE SOURCE OF STELLAR ENERGY 
gives a total flux of 7-7.10~ 13 x ЗЛО 10 = -023 ergs per sq. cm. per sec. 
Hence the flux of penetrating radiation derived from М г is 
M € 
•023 -дтг 1 - ergs/cm. 2 sec. 
7u 2 e 2 
The quantum for the formation of helium is 4-8.10~ 5 ergs, and in any 
case the penetrating power of the radiation shows that its quantum is of 
this order of magnitude. Hence the flux is 
500 M } gl quanta/cm. 2 sec. 
7u 2 e 2 
Since this flux is considerably reduced in passing through our atmosphere, 
we take the reciprocal of the absorption coefficient to correspond to 
5 km. of air. Then the amount absorbed in 1 cu. cm. per sec. is 
10-3 ^ l€l quanta. 
For such penetrating radiation the stoppage is not by true absorption 
but by scattering. Nearly all the energy is transferred to the scattering 
electron, which becomes a high speed /3 particle and creates a large number 
of ions before it is brought to rest. The total number of ions resulting from 
one such scattering will be well over 100,000. 
In the terrestrial experiments the ionisation observed is of the order 
3 or 4 ions per cu. cm. per sec. I do not know whether it is fair to take 
the number produced in a small vessel as equal to the number in free air; 
if so, this would indicate that M 1 e 1 is of the order т ^Ж 2 е 2 . Since M x is 
not likely to exceed M 2 this indicates that e x is at least . Accordingly, 
the rate of liberation of subatomic energy in the nebulae though probably 
less than in an average star is not of greatly different order of magnitude. 
We have here followed up (without necessarily endorsing) the theory 
that the ionisation chamber experiments reveal a penetrating radiation 
of frequency higher than any known у rays, coming to us from outer space. 
It leads to astronomical conclusions of great interest, which in themselves 
seem to us entirely reasonable. It seems to be established that (up to 
considerable altitudes) the radiation is coming downwards and therefore 
its source is either in the upper atmosphere or extra-terrestrial. If we felt 
certain that radiation of such extraordinary energy could only be produced 
by subatomic transformations we might, in discussing celestial versus 
terrestrial origin, prefer the hypothesis of celestial origin as the less 
sensational of the two. But there is the Wilson effect (explained in § 210) 
to be reckoned with; the high frequency radiation might be caused by 
“runaway ” electrons in the earth’s atmosphere without involving unknown 
subatomic changes. The decision as to origin depends on the more detailed 
experimental results—rate of change with altitude, and relation to the 
altitude of the Milky Way. It would be outside our province to judge 
critically the trustworthiness of these results.