XLIII. Long-range Alpha Particles from Thorium. By 
Sir Ernest Rutherford, F.R.S., and A. B. Wood, 
M.Sc, Lecturer in Physics, University of Liverpool [Communicated by the Authors.]. 

IN the course of an examination of a strong source of the 
active deposit of thorium by the scintillation method, 
one of us observed the presence of a small number of bright 
scintillations which were able to penetrate through a thickness of matter corresponding to 11*3 cm. of air at 760 mm. 
and 15° C. These scintillations were undoubtedly due to 
alpha particles and of greater velocity than any previously 
observed; for the swiftest alpha particles hitherto known, 
viz. those from thorium C, have a range in air of 8*6 cm. 
The number of these long-range alpha particles is only a 
small fraction of the total number emitted by the source. 
The actual number of long-range particles decreased exponentially with time, falling to half value in 106 hours — 
the normal period of decay of the active deposit of thorium. 
Owing to the pressure of other work, the experiments were 
kindly repeated and extended by Mr. A. B. Wood, who 
examined in detail the variation of the number of scintillations with thickness of matter traversed. There are still a 
number of points that require further examination, but as 
neither of the authors is likely to have time to continue the 
experiments in the near future, it has been thought desirable 
to give a brief account of the preliminary results. 

Experimental arrangements. 

The end of a brass rod, 1 mm. in diameter, was exposed 
as negative electrode in a small vessel containing a strongly 
emanating preparation, either of radio-thorium or mesothorium. By suitable adjustment of the electrodes, the active 
deposit was concentrated almost entirely on the end of the 
rod — a condition essential to the accurate determination of 
the ranges. After two days' exposure to the electric field, 
the wire was removed and placed end-on at 4 mm. distance 
from a small screen of zinc sulphide viewed with a low-power microscope. Care was taken that the axis of the 
microscope passed through the centre of the rod. The screen 
was permanently covered with a mica plate whose thickness 
corresponded to 8*6 cm. of air — the maximum range of 
the alpha particle from thorium C. All scintillations then 
observed were due to alpha particles which had passed 
through the mica plate and 4 mm. of air, i. e. a distance 




380 [header]



corresponding to 9 cm. of air. With the most intense source 
available, about 20 scintillations per minute were counted on 
the microscope. The number fell off rapidly with increase 
of distance of the source, but an occasional [alpha] particle was 
still observed at a distance of 2 cm. from the source. In 
order to determine the variation of number of these particles 
with distance of matter traversed, thin screens of aluminium, 
each corresponding in thickness to 1*25 mm. of air, were 
successively interposed between the source and screen. It 
was found that the number of long-range particles remained 
constant between S'6 and 9*3 cm. of air, but decreased in 
number from 9*3 cm., vanishing at 11*3 cm. The grouped 
average of all the observations in a large number of 
experiments is shown in fig. 1. 

[figure redacted]


It is seen that the curve shows evidence of two fairly 
definite slopes AB, BC, as if there were two sets of alpha 
particles present of different ranges. This important point 



[header] 381 



was very carefully examined, and the results o£ a special 
series of observations are shown in fig. 2. It will be seen 
that there appears to be a fairly definite change in the 
slope when the number of scintillations is reduced to about 
two-thirds of the total. 


[figure redacted]

It will be seen from the curves that the alpha particles 
start decreasing in number from about 2 cm. of the maximum 
range. The variation of number with distance is much 
slower than that to be expected for a single group of alpha 
particles of corresponding range. This is brought out in 
fig. 3, which shows the results obtained when the scintillation-distance curve in air was obtained for the two well-known 
groups of alpha particles emitted from thorium C of ranges 
5'0 and 8*6 cm. respectively. In these cases, the scintillations 
fall off rapidly, beginning at about 1 cm. from the end of the 
corresponding range. The difference between the slopes of 
the scintillation- curve for the long-range alpha particles and 
those from thorium cannot be explained by the oblique 



382 [header]


path taken by some of the rays through the mica on account 
of the nearness of the source and screen. Calculations 
showed that the influence of obliquity could only account for 
a small fraction of the difference actually observed. 


[figure redacted]


The results we have so far obtained certainly seem to 
indicate either that (1) the long-range alpha particles are 
expelled with variable velocities over a comparatively narrow 
range, and in this respect differ markedly from alpha particles 
from ordinary radioactive products which are known to be 
expelled with identical velocity, or (2) that two homogeneous, 
groups of alpha rays of characteristic ranges are present. In 
order to distinguish definitely between these two hypotheses, 
it would be necessary to count many thousands of alpha 
particles, but other evidence suggests that (2) is the more 
probable explanation. 

The slope AB ends at about a range 10'2 cm. and when 
the number of alpha particles is reduced to about two-thirds 



[header] 383 

of the total. This suggests that two groups of homogeneous 
rays are present, one-third of maximum range 10*2 cm. and 
two-thirds of range 11*3 cm. The slope of the scintillation 
curve to be expected on this hypothesis agrees within the 
experimental error with the observed curve. This division of 
the alpha particles into two homogeneous groups may be 
compared with the two well-known groups of alpha particles 
emitted from thorium C, for it is known that one-third have 
a range 5*0 cm. and two-thirds a range 8' 6 cm. This 
suggests that the new groups of alpha particles have their 
origin in thorium C, and that one-third of range 10*2 cm. 
accompany the alpha particles of range 5*0 cm., and the 
remainder of range 11*3 cm. accompany the alpha particles 
of range 8'6 cm. While it is very difficult to prove the 
correctness of such a deduction, the numerical agreement in 
the divisions of alpha particles of different ranges is certainly 
striking. 

We have not so far examined experimentally whether the 
new alpha particles are expelled from the alpha ray product 
thorium C, but this seems very probable. To settle this 
point, it will be necessary to prepare a strong preparation 
of thorium G and to determine whether the period of 
transformation, measured by the new alpha particles, is in 
agreement with the accepted value for thorium C, viz. half 
value in 60 minutes. 

Number of Long-range Alpha Particles. 

Since the number of long-range particles decreases at the 
same rate as the alpha-ray activity of the active deposit of 
thorium, it is convenient to express their number as a 
fraction of the total number of alpha particles emitted per 
second from thorium C. For this purpose, the number of 
long-range particles per minute was measured with the 
source fixed at a known distance from the screen. The 
active deposit was then allowed to decay in situ for 32 hour, 1 -. 
The absorbing screen was then removed and the number of 
alpha particles from thorium C measured at distances from 
6 to 7 cm., so as to include only the longer range alpha 
particles (8*6 cm.) from thorium C. In this way it was 
found that the fraction of long-range alpha particles was 
1/6700. Taking into consideration that the alpha particles 
of range 8'6 cm. from thorium C are two-thirds of the total, 
the fraction becomes 1/10000. 

Preliminary observations by different methods gave a 
somewhat lower value, but the above number cannot be much 
in error. We thus see that the long-range alpha particles 



384 [header]


are expelled in a very small proportion (1/10000) compared 
with the ordinary alpha particles. Unless a very intense 
source be employed, it will not be easy to detect the presence 
of the long-range alpha particles when the ordinary alpha 
particles are first absorbed by a layer of 8' 6 cm. of air. 

In the Bragg ionization curves from thorium C given by 
Marsden and Perkins [citation redacted], a small residual activity is to be 
noticed beyond the distance 8'6 cm. which is relatively more 
marked than for the corresponding curve for radium G. 
This no doubt is to be ascribed to the effect of the very long-range alpha particles. 

Discussion of Results. 

It is now well established that thorium C is anomalous in 
breaking up in two distinct ways. One-third of the atoms 
are transformed with the emission of alpha particles of range 
5*0 cm., and the remainder gives alpha particles of range 
8' 6 cm. These modes of transformation of thorium C have 
been examined in detail by Marsden and Darwin [citation, and an 
ingenious scheme of changes has been suggested to account 
for the facts observed. It is known that the products corresponding to thorium G in the radium and actinium series, 
viz. radium C and actinium C, also have two distinct modes 
of transformation. Fajans [citation redacted] showed that 1/6000 of the 
atoms of radium C give rise to a new product of half period 
1*38 minutes, which emits beta rays in its transformation. 
In a similar way actinium C has been found to emit two sets 
of alpha particles of range 5*4 and 6*4 cm. [citation redacted] This is ascribed 
to a double mode of transformation, 1*5/1000 of the atoms 
breaking up with the emission of alpha .particles of range 
6*4 cm. 

Assuming that the new alpha particles of thorium can be 
divided into two homogeneous groups of range 10*2 and 
11*3 cm., it is seen that thorium C must break up in four 
distinct ways with the expulsion of alpha particles of ranges 
5-0, 8'6, 10-2, and 11*3 cm. at 15° C. 

The possible modes of transformation of thorium C are 
thus more complicated than was at first supposed, and it is 
obvious that the suggestions given by Marsden and Darwin 
as to the modes of transformation of this substance can be 



[header] 385 



only a partial explanation. From the close analogy of the 
" 0" products of radium, thorium, and actinium, it is probable 
that further examination will show an analogous complexity 
in the modes of breaking up of radium and actinium C. 
The loss of energy in the form of expelled alpha particles is 
very different in the four modes of transformation of thorium 
C, and in consequence it does not seem likely that the 
resulting products can be the same in all cases. The differences in the energies emitted by the two branch products of 
thorium C formed a serious difficulty in the original explanation given by Marsden and Darwin of the two main modes 
of transformation of thorium C, and this difficulty is now 
further increased. A more detailed discussion on these 
interesting points will be reserved until further experimental 
information is available. The relation found by Geiger 
between the range of the expelled alpha particles and the life 
of the radioactive product, suggests that the average life of 
the atoms which expel the long-range alpha particles must be 
exceedingly short, and of the order of 10 ~ 13 and 10 ~ 16 sec. 
for the products emitting alpha particles of range 10*2 and 
]1*3 cm. respectively. 

The following table gives the velocity of the four groups 
of alpha particles from thorium C, taking as the basis of 
calculation the measurements of Rutherford and Robinson 
that the velocity V of the alpha particles from radium C of 
range 6*94 cm. at 15° C. is [formula redacted] cm. per second, and 
assuming Geiger's relation [formula redacted] where II is the range. 





Range at 15° C. 


Ratio of velocities. 


Calculated velocity. 

[table redacted] 



Recently one of us [citation redacted] showed by direct measurement that 
the velocities in two main groups of alpha particles from 
thorium C were in good agreement with the calculated 
values if the range of the alpha particles from thorium Ci is 
4'95 cm. instead of 4*80 cm. — the value usually taken. From 
our measurements, there appears to be no doubt that the 
higher value is more correct. 

[footer]


386 [header]

Summary. 

Evidence is given that the active deposit o£ thorium emits 
a small number of alpha particles of greater velocity than 
any previously observed. These alpha particles are believed 
to have their origin in the transformation of thorium C, 
and appear to be divided into two homogeneous groups of 
maximum range 10*2 and 11*3 cm. The number of these 
alpha particles is about 1/10000 of the total number emitted 
from thorium C, two-thirds of the number having a range 
11.3 cm. 

The results indicate that the atoms of thorium can break 
up in three and probably four distinct ways with the emission 
of four characteristic groups of alpha particles of ranges 5'0, 
8'6, 10-2, and 11*3 cm. 

University of Manchester, 
February 1916.