The Birth-Time of the World and Other Scientific Essays

by John Joly

the effect of the bending of some of the rays near the end of
their range must be to cause a decrease in the aggregate
ionisation near the very end of the ultimate range. For, in fact,
some of the rays complete their work of ionising at points in the
gas before the end is reached. This is the cause, or at least an
important contributory cause, of the decline in the ionisation
near the end of the range, when the effects of a bundle of rays
are being observed. The explanation does not suggest that the
ionising power of any one ray is actually diminished before it
finally ceases to be an alpha ray.

The full line in Fig. 13 gives the ionisation curve which it may
be expected would be struck out by a single alpha ray. In it the
ionisation goes on increasing till it abruptly ceases altogether,
with the entire loss of the initial kinetic energy of the
particle.

A highly remarkable fact was found out by Bragg. The effect of
the atom traversed by the ray in checking the velocity of the ray
is independent of the physical and chemical condition of the
atom. He measured the "stopping power" of a medium by the
distance the ray can penetrate into it compared with the distance
to which it can penetrate in air. The less the ratio the greater
is the stopping power. The stopping power of a substance is
proportional to the square root of its atomic weight. The
stopping power of an atom is not altered if it is in chemical
union with another atom. The atomic weight is the one quality of
importance. The physical

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state, whether the element is in the solid, liquid or gaseous
state, is unimportant. And when we deal with molecules the
stopping power is simply proportional to the sum of the square
roots of the atomic weights of the atoms entering into the
molecule. This is the "additive law," and it obviously enables us
to calculate what the range in any substance of known chemical
composition and density will be, compared with the range in air.

This is of special importance in connection with phenomena we
have presently to consider. It means that, knowing the chemical
composition and density of any medium whatsoever, solid, liquid
or gaseous, we can calculate accurately the distance to which any
particular alpha ray will penetrate. Nor have the temperature and
pressure to which the medium is subjected any influence save in
so far as they may affect the proximity of one atom to another.
The retardation of the alpha ray in the atom is not affected.

This valuable additive law, however, cannot be applied in
strictness to the amount of ionisation attending the ray. The
form of the molecule, or more generally its volume, may have an
influence upon this. Bragg draws the conclusion, from this fact
as well as from the notable increase of ionisation with loss of
speed, that the ionisation is dependent upon the time the ray
spends in the molecule. The energy of the ray is, indeed, found
to be less efficient in producing ionisation in the smaller
atomm.

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Before leaving our review of the general laws governing the
passage of alpha rays through matter, another point of interest
must be referred to. We have hitherto spoken in general terms of
the fact that ionisation attends the passage of the ray. We have
said nothing as to the nature of the ionisation so produced. But
in point of fact the ionisation due to an alpha ray is sui
generis. A glance at one of Wilson's photographs (Fig. 14.)
illustrates this. The white streak of water particles marks the
path of the ray. The ions produced are evidently closely crowded
along the track of the ray. They have been called into existence
in a very minute instant of time. Now we know that ions of
opposite sign if left to themselves recombine. The rate of
recombination depends upon the product of the number of each sign
present in unit volume. Here the numbers are very great and the
volume very small. The ionic density is therefore high, and
recombination very rapidly removes the ions after they are
formed. We see here a peculiarity of the ionisation effected by
alpha rays. It is linear in distribution and very local. Much of
the ionisation in gases is again undone by recombination before
diffusion leads to the separation of the ions. This "initial
recombination" is greatest towards the end of the path of the ray
where the ionisation is a maximum. Here it may be so effective
that the form of the curve is completely lost unless a very large
electromotive force is used to separate the ions when the
ionisation is being investigated.

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