Experiment:- Half-Life of Barium 137

Physics – Unit 1

Experiment:- Half-Life of Barium 137

Name: ______

Barium has a number of isotopes, one of which is Ba-137 (Z= 56). Caesium Cs-137 (Z = 55) decays to B –137 via β- decay with a 30.1y half-life. The result is an excited and unstable form of Barium: Ba-137m, this then decays to the stable form of Barium Ba-137 via γ-decay with a short half-life. This is the half-life we will measure.

The decay mode is as follows:

Ø  The Cs-137 is always slowly producing the Ba-137m isotope.

Ø  The apparatus uses an eluting solution to separate the Ba-137m from the Cs-137.

Ø  The GM tube detects the gamma emissions from the Ba-137m decaying quickly to Ba-137 (stable).

Aim: To determine the half-life of Ba-137 by measuring a sample’s activity over about 15 mins and graphing the results.

Apparatus: Ba-137 source, Geiger Counter, Timer.

Method:

1/ Record the Geiger- Counter display (cumulative measurement) for 15 minutes for the sample.

2/ Calculate the Count-per-minute (count rate for each minute) and the Activity (counts per sec).

3/ Graph the results.

4/ Determine the time it takes for the activity to fall by 50% = T1/2 (the Half-Life).

Results:

1/ Complete the following table for the Ba-137 sample:

Time (minutes) / Total Count (Cumulative) / Counts/Min / Activity (Bq) / % Activity
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

2/ Graph the %-Activity against the time in minutes on graph, using an appropriate scale that uses most of the graph paper’s area.

-Clearly show each data point.

-Sketch a curve-of-best-fit through the points.

-Use this curve to estimate the half-life of Ba-137 in minutes and seconds. Show this method clearly on your graph.

3/ Half-Life of Ba-137 = ______

Example of how to complete the table on the previous page:

*The following cumulative count numbers are simply made up as a guide:

Time (minutes) / Total Count (Cumulative) / Counts/Min / Activity (Bq) / % Activity
1 / 15000
2 / 20,500
3 / 24,000
4 / 28,400

To complete the remaining columns:

Time (minutes) / Total Count (Cumulative) / Counts/Min / Activity (Bq)
= counts/sec / % Activity
1 / 15,000 / 15,000 / 12,000 /60
= 200 / 100
2 / 25,500 / 25.500 – 15,000
= 10,500 / 10,500 / 60
= 175 / 175 / 200 x 100
= 87.5
3 / 33,000 / 33,000 – 25,500
= 7,500 / 7,500 / 60
= 125 / 125 / 200 x 100
= 62.5
4 / 38,400 / 38,400 – 33,000
= 5.400 / 5,400 / 60
= 90 / 90 / 200 x 100
= 45

Result : (then graph % Activity versus time

Time (minutes) / Total Count (Cumulative) / Counts/Min / Activity (Bq)
= counts/sec / % Activity
1 / 15,000 / 15,000 / 200 / 100
2 / 25,500 / 10,500 / 175 / 87.5
3 / 33,000 / 7,500 / 125 / 62.5
4 / 38,400 / 5.400 / 90 / 45

Now graph Activity (%) versus time (minutes)

P.J.Cuthbert