216
.
9.
X+RAYS
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x-ray beam to pass through as docs de centimeters of graphite,
‘
pm is the density of the aluminum, iron or copper, and pc is that
of graphite. The u’s
mass absorption coefcients.
‘
The mass absorption coefcient pc for graphite (Which is the
same for any form of carbon) is to be taken as the standard. This
is chosen since its value does not change with wave-length as much
_
as other substances.
(See table 14 at the end of the book.)
Inasmuch as the wave—length emitted by the tube is unknown, the
correct value of p,, cannot be obtained at rst.
Let us assume that
the wave-length is around 0.5 angstrom. Then, from the table,
the value of p.c is about 0.32.
Substituting this and the other
quantities in equation 9—16 gives an approximate value of p….
Say ,u.m for copper was thus found to be 7.00. This corresponds to
a wave-length of 0.350 angstrom as seen from the table or, better,
from a graph plotted from this data. The process is now to be
,
repeated, i.e., ,uc at 0.350 angstrom is equal to 0.224. Using
0.224 instead of 0.32 in equation 9—16 gives the mass absorption
coefcient of copper_as 3.63 which, from the table, corresponds to
a wave—length of 0.269 angstrom. The process is then repeated
again. If, for the average gas—lled tube, the value of … is
as 0.20, the steps need be repeated but once.
;ÏÏ
_
'
.
This method of successive approximations is to be repeated
for the other absorbing materials. It is not to be expected that, ;Ï
the wave-lengths so determined Will all be exactly the same Since
the x-ray beam is not strictly monochromatic but the values
should be less for those substances having greater absorption
,
coeicie'nts.
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‘
'
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.
Ÿ
EXPERIMENT 9—2
—
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.
-
LAUE‘ PHOTOGRAPHS ‘*_
g
_
_
(Read_secti0n 9—5 about “ Protection from X-rays )
of this experiment is to take a photograph‘of the
Laue spots
salt crystal, using the general radiation
to identify
with the crystä1