HalfLife of Ba137

Purpose: To determine the halflife of the radioactive isotope Ba137by measuring the activity of a sample as it decays using a Geiger counter.

Introduction:

In this experiment, 137mBa will be obtained from 137Cs by the decay

t1/2 = 30 years

The 137mBa then decays by from the metastable excited state to the ground state with emission of a gamma () particle:

t1/2 = ??

The halflife for this decay is relatively short. The gamma energy is released because the Ba137 daughter nucleus is left with some of the energy from the Cs137 nuclear decay. This metastable excited state is analogous to a ball balanced on the peak of a roofany slight "push" will cause it to fall to the ground state.

The decay constantof a radioactive sample cannot be measured instantaneously. The number of radioactive decays must be measured over a series of short time intervals with a Geiger counter. The decay rate of the sample decreases exponentially with time. The number of particles detected with a GM tube is a fraction of the total number of decays since the radiation is emitted from the source in all directions and the detector only intercepts a small portion of these directions

Background radiation is a common source of error when measuring radioactive decay. This error can lead to inaccuracies in the determination of the half-life. The background radiation comes from many naturally occurring sources and from cosmic rays entering our upper atmosphere from space. Background radiation is a constant addition to the activity from the sample and must be subtracted out of data.

The source of the Ba-137m is an isotope generator consisting of an exempt quantity of Cs-137.An exempt quantity is a quantity small enough so that no license is required to purchase the material. It is deemed to be a minimal health hazard. The Cesium atoms are in molecules of a Cesium salt, CsCl, that have been adsorbed by small beads of a resin material. The Ba-137m nuclei are a daughter product that results from the -decay of Cs-137.

As the Cs-137 nuclei decay to the Ba-137m nuclei, the Ba-137m atoms remain adsorbed on the surface of the resin. But because Barium has a different chemistry than Cesium, it is more loosely bound to the resin and can be de-adsorbed with a weakly acidic salt solution containing HCl and NaCl. Small amounts of the short lived Ba-137m isotope can be extracted from the resin by this eluting solution.The Cesium nuclei have a half-life of 30 years and Cesium isalways decaying, building up Ba-137m. Ba-137m has a short half-lifeand quickly decays down to its stable ground state by the emission of gamma energy.

Prelab Questions:

  1. What does Cesium-137 decay into? What is Cesium-137’s half-life?
  2. What is emitted when metastable Ba-137 decays? How can we measure this?
  3. What is background radiation? Why is it important that it is subtracted out of our data?
  4. How is eluting solution used to isolate the Ba-137m isotope?
  5. How does half life determination in this lab relate to radioactive dating?

Materials:

Geiger-Mueller (GM) counter, 137Cs source (Isogenerator), Logger Pro

Procedure:

  1. Carefully set up G.M. tube as instructed.
  2. Turn dial to high voltage and push up button until you reach the HV listed on the Geiger counter.
  3. Turn dial to time and set time to 30.00 (use up button)
  4. Determine the Background radiation every 30 sec over a five minute interval in a data table.
  5. Press count button. Turn dial back to time. At the end of 30 sec turn dial back to count and record count. Repeat step 5 for a five minute time period. Determine the average background radiation.
  6. Make sure everything is ready and Mrs. Scheff will provide you with the Ba-137m sample.
  7. Place the planchet on the sample tray to be used
  8. Mrs. Scheff will provide you with 7 drops in the planchet.
  9. Note the time the sample was created______and IMMEDIATELY begin counting.

7. Record count every 30 sec for a 15 minute period in a second data table.

Data:

Create two data tables- one for background radiation counts per unit time and a second for Ba-137m decay counts per unit time

Graph: Appropriately graph the Ba-137m decay vs. unit time.

Analysis: From your initial graph, graphically determine the half-life of the Ba-137*. Show your work on your graph. You can do an interpolation (go to analyze-interpolate) and find the exact half-life for the decay. This is the time when half of the original amount is decayed.

Conclusions:

  1. What is your half-life of Ba-137*, determined graphically?
  1. Calculate your percentage error for the half-life of Ba-137*, determined from the graph, using the standard table value for the correct value, Mrs. Scheff will provide you with the actual half-life of Ba-137m. Show your work.

Experimental – Actual

Actual x 100 = ______% error

  1. Using your graph and the original time the sample was created, estimate the original radioactive count of the Ba-137 sample that was created.

Lab FormatTitle

Purpose

Background- answers to Pre-Lab Questions

Materials

Procedure

Data Tables- handwritten or from LoggerPro for background radiation and Ba-137 decay

Graph- Ba-137 decay data

Analysis- shown on graph

Conclusion questions