—
290
12.
ALPHA, BETA AND GAMMA RAYS
.
EXPERIMENT 12—5
.
ABSORPTÏON OF GAMMA RAYS
Read the discussion of Counter Tubes in Chapter 18.
Danger.—Continued exposure to gamma rays can cause
'*serious burns.
Consult the instructor as to the precautions neces—
.
sary with the source which you are to use.
Âppamz‘us.—A small Geiger-Müller tube with its amplier
and high speed counter, sheets of lead or aluminum and the gamma -
ray source.
The strength Of the source and the maximum counting
rate will determine whether lead or aluminum is to be used and
the proper thickness.
For example, with a particular source of
radio-thorium located 35 centimeters from a counter tube, whose
residual count was 10 and maximum was 150 per minute, 8 sheets
of lead, each 1 centimeter. thick were found to be satisfactory.
'
Experiment—When the apparatus is functioning properly, (1)—
measure the residual count, using as long a time interval as possible,
say 10 minutes.
(2) In the absence of the source, place each of the
blocks near the counter and again measure the residual count in
.
order to be sure that none of the absorbers are radioactive.
ï
(3) Then, with the source at a xed distance, insert
sheets of absorbing material and rec0rd the _counting rate over as
-
long time intervals as possible.
,_
:,
(4) Plot the above data, With the net counting rates as
ordinates and centimeters thickness of the absorber as absc1ssae
'
Compute the absorption coefcient. Plot, also, the logar1thmsof
.
the net counting rates against thickness of the absorber. If de51red,
the absorption coefcient may be obtained from the slope of the
f'reSulting lirîe rather than from the equation for this quant1ty
1
The same equipment may be used forfother
.
as a test of the inverse square law. ”For this, measure
_
rate
With
source
variOus measured d13tances
290
12.
ALPHA, BETA AND GAMMA RAYS
.
EXPERIMENT 12—5
.
ABSORPTÏON OF GAMMA RAYS
Read the discussion of Counter Tubes in Chapter 18.
Danger.—Continued exposure to gamma rays can cause
'*serious burns.
Consult the instructor as to the precautions neces—
.
sary with the source which you are to use.
Âppamz‘us.—A small Geiger-Müller tube with its amplier
and high speed counter, sheets of lead or aluminum and the gamma -
ray source.
The strength Of the source and the maximum counting
rate will determine whether lead or aluminum is to be used and
the proper thickness.
For example, with a particular source of
radio-thorium located 35 centimeters from a counter tube, whose
residual count was 10 and maximum was 150 per minute, 8 sheets
of lead, each 1 centimeter. thick were found to be satisfactory.
'
Experiment—When the apparatus is functioning properly, (1)—
measure the residual count, using as long a time interval as possible,
say 10 minutes.
(2) In the absence of the source, place each of the
blocks near the counter and again measure the residual count in
.
order to be sure that none of the absorbers are radioactive.
ï
(3) Then, with the source at a xed distance, insert
sheets of absorbing material and rec0rd the _counting rate over as
-
long time intervals as possible.
,_
:,
(4) Plot the above data, With the net counting rates as
ordinates and centimeters thickness of the absorber as absc1ssae
'
Compute the absorption coefcient. Plot, also, the logar1thmsof
.
the net counting rates against thickness of the absorber. If de51red,
the absorption coefcient may be obtained from the slope of the
f'reSulting lirîe rather than from the equation for this quant1ty
1
The same equipment may be used forfother
.
as a test of the inverse square law. ”For this, measure
_
rate
With
source
variOus measured d13tances