288
12.
ALPHA, BETA AND GAMMA RAYS
circle, is turned. A vernier is mounted on  and moves along the
scale indicated.
Across the ends of the rods, a brass bar supports a
low-power eyepiece which, by rotation, may be focused on theuorescent screen (C).
The preparation of a suitable screen was
'
described in experiment 12—1. The brass bar also supports the thin
foil (B) whose stopping power is to be measured. A sheet of thin
spring brass is doubled over and pierced with a central hole about
1.5 centimeters in diameter.
Various foils may be slipped between
the sheets and the whole slid into guides at a distance of, say, 2
millimeters from the screen.
The apparatus is to be used in a
dark room.
.
=
Pracedure.—(1) Mount the foil in its holder. Two sheets of
aluminum such as used for electroscope leaves will prove satis—
factory. Hold them up
to the light and see that there are no
obvious holes or areas of unequal thickness.
(2) Record the barometric pressure and the temperature.
(3) Without the foil (B), place the source (Â) two centimeters
from the screen (C) and wait (about 15 minutes) in the dark room
until scintillations can be seen clearly. Move the source back and
’
forth and determine the distance (d) between the surface of the »
active substance and that of the uorescent material where
scintillations just cannot be seen.
There will always be a few
spurious ashes of light which are to be disregarded. The
of scintillations will not be sharp and a large number of rapid
settings will give better results than a single reading taken with "
-
great care; The lack of sharpness in the cut-off is due not only to
_
the straggling of the particles but also to their lack of parallelism
and to slight absorbing layers, perhaps grease, on the source and
'
screen.
‘
,
(4) Insert the foil in its holderin the path of the rays and
.
repeat the range measurement as in (3). This distance is a”
.
,
(S) Compute the air equivalent, la, equation 12—7.
'
(6) Determine the thickness (t) of the foil by weighing a known
.
area on an accurate balance.
The density of aluminum is 2.70
=
gram_s per cm3. The quantity pt in equation 12—11 is equal to the
'
°
.
_
mass in gra‘ms of one square centimeter of the
(pl = m/a), '
,
'
‘
'
(7) Compute the equivalent and the relative atomic stoppîg
powers from .equations 12—8 and 12—11, and compare
With, 'ÇhC
accepted values.
_
_
,
12.
ALPHA, BETA AND GAMMA RAYS
circle, is turned. A vernier is mounted on  and moves along the
scale indicated.
Across the ends of the rods, a brass bar supports a
low-power eyepiece which, by rotation, may be focused on theuorescent screen (C).
The preparation of a suitable screen was
'
described in experiment 12—1. The brass bar also supports the thin
foil (B) whose stopping power is to be measured. A sheet of thin
spring brass is doubled over and pierced with a central hole about
1.5 centimeters in diameter.
Various foils may be slipped between
the sheets and the whole slid into guides at a distance of, say, 2
millimeters from the screen.
The apparatus is to be used in a
dark room.
.
=
Pracedure.—(1) Mount the foil in its holder. Two sheets of
aluminum such as used for electroscope leaves will prove satis—
factory. Hold them up
to the light and see that there are no
obvious holes or areas of unequal thickness.
(2) Record the barometric pressure and the temperature.
(3) Without the foil (B), place the source (Â) two centimeters
from the screen (C) and wait (about 15 minutes) in the dark room
until scintillations can be seen clearly. Move the source back and
’
forth and determine the distance (d) between the surface of the »
active substance and that of the uorescent material where
scintillations just cannot be seen.
There will always be a few
spurious ashes of light which are to be disregarded. The
of scintillations will not be sharp and a large number of rapid
settings will give better results than a single reading taken with "
-
great care; The lack of sharpness in the cut-off is due not only to
_
the straggling of the particles but also to their lack of parallelism
and to slight absorbing layers, perhaps grease, on the source and
'
screen.
‘
,
(4) Insert the foil in its holderin the path of the rays and
.
repeat the range measurement as in (3). This distance is a”
.
,
(S) Compute the air equivalent, la, equation 12—7.
'
(6) Determine the thickness (t) of the foil by weighing a known
.
area on an accurate balance.
The density of aluminum is 2.70
=
gram_s per cm3. The quantity pt in equation 12—11 is equal to the
'
°
.
_
mass in gra‘ms of one square centimeter of the
(pl = m/a), '
,
'
‘
'
(7) Compute the equivalent and the relative atomic stoppîg
powers from .equations 12—8 and 12—11, and compare
With, 'ÇhC
accepted values.
_
_
,