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12.
REFERENCES
291
from the counter tube.
Plc'>t counting rates vertically and inverse
_
squares
of the distances horizontally. An approximately straight
line should result.
It will deviate from linearity because of the
absorption of the rays by the air. The deviation will be pronounced
when a weak source, such as three or four radium 'D tubes (emitting
mostly beta rays), is used.
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The absorption coefcient of air can be measured as follows:
due to the inverse square law, the net counting rate will be given by
Ï/ll = 1/a’2 and due to the exponential absorption, [1 = las—“d,
‘
where Io is the counting rate at the origin and d is the distance
_
from the source to the tube. Then, [d2 ,= Zoe—"d or loge (la?)
=
——
pd + loge Io. A_straight line will be obtained when loge (Id?)
is plotted against J. The absorption coefcient (p)“can be calculated from the slope, as in previous cases.
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.
—
1. S. Rosenblum, ]r. de Phys.,1, 438 (1930). Compt. ,Ren'd., 190, 1124
(1930); 194, 1919 (1932).
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.
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2. Rutherford, Wynn-Williams, Lewis and Bowden, Proc. Roy. Soc. A139, ’
_
617 (1933).
*
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”
Lewis and Bowden, Proc. Roy. SOc., A145, 235 (1934).
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3. See General Reference D, page 116, andGener_al Reference B, page 534.
4. See General Reference D, page62‘. ' '
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5. See General Reference A, page 78.
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6. See General Reference A,page'408_._
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GENERALREFÊRENCES
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