This is the first lab of a three-lab sequence on radioactivity. In this lab, you will get some practice handling radioactive sources and measuring intensities, as well as getting a feeling for how different materials shield against radioactive particles. Your work in this lab will prepare you for the next modules which explore the lifetimes of radioactive materials how we can use that information to perform radiodating. 

By the end of this lab, students will have…

• … observed alpha, beta, and gamma decay;
• … learned to use a Geiger-Mueller particle detector and counter;
• … studied how different materials shield against radiation.

Working with radiation requires some additional safety precautions.

We will be working with radioactive sources which emit alpha, beta, and gamma particles. The sources we use are all relatively low activity and are sealed so you cannot access the radioactive material directly. Your potential exposure is low and well below safe limits.

• If you discover that a radioactive source is broken open or damaged, alert the TA immediately.
• Do not remove sources from the room.
• Do not place source in your pockets or bags.
• Use only one radioactive source at a time, and return sources to their storage location after use.

One of the materials we will be using as shielding is lead.

• Use nitrile gloves whenever handling lead blocks.
• Wash your hands after finishing lab (whether you touched lead or not).
• There is absolutely no food or drink allowed in the lab. Food which comes in contact with lead (or lead contaminated surfaces) can lead to lead ingestion.
• If you need to eat or drink during lab, wash your hands and step outside the room.

The TA will introduce you to the Geiger-Mueller detector and counter and will show you where the radioactive sources are kept. You will begin by making some observations about count rate using different emitters and different shielding materials and conditions. Based on what you see, you will make a hypothesis that leads to a prediction, and you will design an experiment to test that hypothesis.

At the end of the period, you will need to share your experiment and its results with the class. It may be helpful to use the whiteboards to make diagrams, notes, tables, or plots of information you want to communicate to others as you go. Also, remember to keep track of your work in your group's lab notebook.

In this experiment, we want a device which can detect when a radioactive particle passes through it and which will then produce an electronic pulse which can be counted. There are many different types of detectors – some of which can provide additional information such as particle energy or particle type – but for this experiment we will use what is called a Geiger-Mueller (GM) tube.

A GM tube is a sealed cylinder filled with an inert gas (like xenon or argon) which has a wire running down the center which is held at a constant voltage. When a high-energy radioactive particle passes through the gas, it ionizes the gas (i.e., it strips off electrons from the atoms) and the liberated electrons are attracted to (and collected on) the central wire. This accumulation of electrons causes the voltage on the wire to momentarily drop, which can be read out as a detection. If a GM tube is connected to an electronic counter, the pulses can be tallied. 

For counting, we will use the STX x64 software (which should be bookmarked on the desktop).

Open the software, and set the high voltage for the detector tube to 900 V. If you don't turn on the voltage, you won't see any counts!

• To start collection, press the green diamond in the upper left corner.
• To stop collection, press the red stop sign in the upper left corner.
• To erase the number of counts, press the “X” button in the upper left corner.

Every time you hit “stop”, the software will automatically record a “run” and display the number of counts and the time.

• If the “preset time” is set to zero, the software will run until you select “stop”.
• If the “preset time” is set to any finite number of seconds, it will run for that amount of time and then automatically stop.
• If you increase “runs remaining” to any finite value, it will execute consecutive runs, each for a length of time set by “preset time”, erasing the data between runs.

The STX software

Spontaneous decays of radioactive particles occur randomly. If we look at a large collection of particles of the same type, we can say something about the average (mean) lifetime of particles of that type, but we can’t say anything for sure about any one particular member of the group. It may decay right now, 1 second from now, 1 year from now, etc.

Because of the random nature of nuclear decay, there is an inherent uncertainty when we measure count rates. If we count the number of decays that happen in a radioactive source for a time interval $t$, the number we observe, $N$, will fluctuate from trial to trial. The number of decays obeys what is known as a Poisson distribution. We don’t need to go into the details of this type of distribution, here, but it means that if you observe a number of decays $N$, your best estimate for the true number of decays is $N \pm \sqrt{N}$. The fractional uncertainty, $\sqrt{N}/N = 1/\sqrt{N}$, decreases as $N$ increases. Think about this when you consider how to reduce the uncertainty in your measurements.

Consider: Can you detect the radiation coming from all the different sources? How do you carefully measure count rate? What variables have an influence on count rate? How does shielding affect count rate? How can you estimate uncertainty in the count rate?

Consider: What is your independent variable? Your dependent variable? Your control variables? What is your hypothesis for this experiment and what specific prediction does it make?

Consider: How will measurements be taken? By what methods are control variables being considered? How can you estimate (and ultimately reduce) the experimental uncertainty?

Consider: Do you trust your measurements? What uncertainties are present in the data and how can you quantify them? If measurements are repeated, do you get similar results?

Consider: How can you represent your results? Does any relationship exist between the variables being looked at? Are your data consistent with your prediction? Inconsistent? Inconclusive?

Consider: What do you want others to know about your experiment? How can you present your methods and results succinctly and clearly? Can you anticipate questions others might have? Can you think of improvements you’d make if you had more time?

At the end of the lab, you will need to record your final conclusions (about 1 or 2 paragraphs) in your lab report summing up the important results and take-away points from your experiment. Remember that you should only draw conclusions which are supported by the data, so be ready to back up any statements you make!

When you're finished, save your file as a PDF and upload it to Canvas. (Only one student needs to submit the report, but make sure everyone's name is on it!) If you make a mistake, you can re-submit, but work done after the end of the lab period will not be accepted.