ABSORPTION EXPERIMENT (12—4)
289
EXPERIMENT 12—4
-
ABSORPTÏON OF BETA RAYS
'
The source of the beta rays may consist of a shallow cup whose
bottom is covered uniformly With pulverized radium D tubes held
in place with a very thin layer of binding material such as gelatin
or sodium silicate.
A sheet of aluminum 0.006 to 0.01 centimeter
-
_thick is placed over the cup
to cut off all alpha rays.
This source is placed in an electroScope of the type shown in
gure 17—1. The, student should read the discussion on electroscopes given in chapter 17 before performing this experiment.
The activity is measured as successive sheets of aluminum,
.
each about 0.007 centimeter thick, are placed over the source.
Twelve of these sheets will be sufcient for a good curve. Care -
should be taken that the sheets_are not and do not
become con—
taminated by direct contact with the active material.
'
Place a non-contaminated sheet of lead about 3 millimeters
thick over the source and make sure that there are no appreciable
gamma radiations; ie,
fall of the leaf is the same as the
natural leak.
If it is not, the gamma activity as well as the
natural leak must be subtracted to obtain the beta ray activities.
Plot a curve of the beta ray
ordinates (vertical)
and the thickness of the aluminum as ab8cissae. Also, using semi—
logarithm paper, plot logarithms of the activities
vs.
total th1ck—
_
nesses of the absorbing screens. Common logarithms may be used.
Then,- the slope of the line is 04343 times the coefc1ent of absorption of- these beta rays
in aluminum. Equation 12—21 may be used
instead of the graPhical method if desired. Take an average of the
coefcients computed for various thicknesses.
Compare your
value of ;u with the accepted value;
_
_
'
If the source emits gamma rays
of sufcient 1ntensrty,
measure
'
the absorption coefdent of lead for these rays, using sheets
approximately 2 millimeters thick.
_
289
EXPERIMENT 12—4
-
ABSORPTÏON OF BETA RAYS
'
The source of the beta rays may consist of a shallow cup whose
bottom is covered uniformly With pulverized radium D tubes held
in place with a very thin layer of binding material such as gelatin
or sodium silicate.
A sheet of aluminum 0.006 to 0.01 centimeter
-
_thick is placed over the cup
to cut off all alpha rays.
This source is placed in an electroScope of the type shown in
gure 17—1. The, student should read the discussion on electroscopes given in chapter 17 before performing this experiment.
The activity is measured as successive sheets of aluminum,
.
each about 0.007 centimeter thick, are placed over the source.
Twelve of these sheets will be sufcient for a good curve. Care -
should be taken that the sheets_are not and do not
become con—
taminated by direct contact with the active material.
'
Place a non-contaminated sheet of lead about 3 millimeters
thick over the source and make sure that there are no appreciable
gamma radiations; ie,
fall of the leaf is the same as the
natural leak.
If it is not, the gamma activity as well as the
natural leak must be subtracted to obtain the beta ray activities.
Plot a curve of the beta ray
ordinates (vertical)
and the thickness of the aluminum as ab8cissae. Also, using semi—
logarithm paper, plot logarithms of the activities
vs.
total th1ck—
_
nesses of the absorbing screens. Common logarithms may be used.
Then,- the slope of the line is 04343 times the coefc1ent of absorption of- these beta rays
in aluminum. Equation 12—21 may be used
instead of the graPhical method if desired. Take an average of the
coefcients computed for various thicknesses.
Compare your
value of ;u with the accepted value;
_
_
'
If the source emits gamma rays
of sufcient 1ntensrty,
measure
'
the absorption coefdent of lead for these rays, using sheets
approximately 2 millimeters thick.
_