Lab Radiation

Radioactivity is the emission of energy from the nucleus of certain nuclides or elements due to the decay of the nucleus. Some naturally occurring radioactive elements include uranium and thorium and radon. A small amount of naturally occurring potassium is even radioactive.

There are three types of radioactive emissions:

  • Alpha - the least penetrating form of radiation can be stopped with a piece of paper or a few inches of air. Alpha rays are the nucleus of a helium atom, and are produced by certain radioactive materials such as thorium and uranium.
  • Beta rays are more penetrating than alpha rays, and can be stopped by a few millimeters of aluminum or other metals. They are very fast moving electrons.
  • Gamma rays are the most penetrating form of radiation. Depending on their energy, they can travel through up to several inches of steel, and hundreds of feet of air. They are usually produced in conjunction with either alpha or beta rays.

A Geiger counter lets you check the environment and items for radioactivity. You can use to check for the presence of radon on your house or basement, or even use it to go prospecting for uranium or other radioactive minerals.

A Geiger counter works by detecting the ionization produced by a radioactive particle. Each time a particle of radiation is detected, the counter records this event. The number of events recorded over a period of time indicates the amount of radiation present. Often this is done over one minute intervals, resulting in the familiar "counts per minute" or CPM. The higher the CPM, the higher the radiation levels.

Geiger counters are devices to detect and measure ionizing (nuclear) radiation. They are one of the oldest devices used to for this purpose, but are still one of the most sensitive, especially for the low radiation levels typically found in most situations.

The typical Geiger counter consists of a metal body, called the tube, which is filled with gas at a low pressure. The gas usually contains Argon and Neon, along with small quantities of other gases (the quenching agent described below).

There is a second metal conductor, which is usually in the form of a thin wire, which runs inside the tube to a connector on the tube body. A high voltage, typically 600 to 1100volts, is applied between this conductor and the Geiger counter tube body.

When a charged particle, such as an alpha or beta ray, or a gamma ray or x-ray, enters the Geiger counter tube, it can hit one or more of the gas atoms, knocking off electrons. This process is called ionization.

The ionized gas is able to conduct electricity. In the case of the Geiger counter, the applied voltage is high enough so that as the electric current flows, the electrons cause additional atoms to be ionized, resulting in even more current flow. In a very short amount of time, the feeble current caused by the radiation particle has grown to a larger current, which is easier to measure.

The quenching agent gas in the Geiger counter stops the flow of electrical current after a few microseconds. Older Geiger tubes used gases such as methane, which broke down each time there was a detection, resulting in a finite lifetime for the tube. Modern Geiger counter tubes use gases that don't break down, resulting in essentially an unlimited lifetime for the tube, providing it is operated within the specifications and not subjected to physical abuse.

Each gas discharge event is measured and counted. Often, the number of events or counts per minute is recorded, resulting in the typical Counts Per Minute or CPM reading so often seen.Some detectors produce a click each time an event is detected. The higher the clicking rate, the higher the radiation level.

Part 1: Determine the optimum voltage range at which the Geiger counter detects emissions from Co60.

Connect the Geiger tube to the connection wire and connect the other end of the connection wire to the Gm port on the back of the st360. Remove red rubber shield from Geiger tube and place tube in gray holding tower. Use USB wire to connect St360 to your computer. You should have already installed ST360 software on your computer. Plug in adapter cord and turn on your St360. Place the Co60 on level 2 from the top of the gray tower. Start the ST360 software by clicking on the desktop on your desktop.

In Setup:In Preset:Reading will be from 0v to 1100V

Description: Co60 rangeTime: 10 sec

HV setting: 0 Runs55

Step voltage:20 (click on)

Save your data: save as - Physics file/Radiation - Co60range

Transfer to excel:

Open excel spreadsheet - return to St program - click edit then copy - return to excel and paste

Type in Co60 range in description then save as Co60range.

Look at your data and record the voltage that the Geiger counter started to record counts ______

Return to excel spreadsheet and make a XY scatter plot graph of the data. (Choose the 3rd one)

X will be the voltage and y will be the counts – label on graph

To do this: Highlight the voltage and count columns starting at the beginning of the counts and click graph on toolbar - choose XY scatter plot the 3rd choice – Next –Next – Chart title 1Co60 plateau – x axis voltage – y axis counts - next – label 1Co60plateau – finish – save as 1Co60plateau in physics/radiation

On your graph identify the straight horizontal section of the graph. This is called the plateau for Co 60 and is used to determine the optimum voltage to run the st360.

Record the starting voltage of the straight-line ______, record the ending voltage of straight

line ______. Add the LV and HV and divide by 2 to find the optimum voltage.

Record optimum voltage ______+ ______= total/2 = ______

This will be the voltage you operate your st360 for the remainder of your lab trials.

Part 2:

Determine which of the 5 radioactive samples gives off the greatest amount of radiation at 2cm from the detector.

In Setup:In Preset:

Description: GreatestTime: 10 sec

HV setting: (your setting) Runs5

Step voltage:0

Put in Co60 on the 2cm slide – hit start – when recording is complete remove Co60 and put in Cs137 – hit start – then do Tl204, PO210, and Sr90. save data as greatest

When all five samples have been recorded transfer the data you collected to excel. Save as 3 greatest.

To interpret this data you will make a bar graph of the average counts for each sample.

After data is transferred – cut and paste so that each of the five runs are lined up horizontally across the page – cut out all the repetitive data after the first listing – your paper will read run number – high voltage – then the counts of each sample – put the names of each sample above their corresponding column – highlight each column and use the auto sum to add each column. Below each sum use the program to determine the average for each column – under each sum hit = then/5 then enter – this will be the average - skip a line below and record the averages from greatest to lowest in a vertical column – type in name of each sample beside the average – highlight the 2 columns and click on graph – choose the first column to make a bar graph – label x as elements – y as counts – name as number of counts – label graph as 2 greatest - save data as 2 greatest.

Part 3:

Determine how the distance from the radioactive source affects the strength of the radiation or the number of counts given off by the source. You will use your HV setting and determine how changing the distance from the source affects all five of our samples. Start all five at 2 cm set the time at 10 seconds and do five runs. Transfer the data of each to excel and graph all five on separate graphs using the number of counts as y and the distance as x – label distance 2cm, 4cm, 6cm, 8cm, and 10cm. Use a scatter plot graph. Save each graph 4 Co60 distance, 5 Cs 137 distance, 6 Sr90 distance, 7 Po210 distance, and 8 Tl204 distance.

Part 4:

Determine how shielding effects each sample. Put all samples at level 6. Set Hv to the proper setting, time 10 sec, runs 5. Place the 1st Al foil shield on level 3 with the finished side facing start your test, when 5 runs are completed remove the shield and place the next shield in level 3 and do 5 runs. Continue until all the shields have been used. Transfer data to excel. Average the 5 runs for each shield then create a bar graph that shows the relationship between the shields and the counts. Label shields on x axis. You will have 5 graphs, one for each sample. Save each graph 91 Co60 shield, 92 Cs 137 shield, 93 Sr90 shield, 94Po210 shield, and 95Tl204 shield.

What was your optimum voltage ______.

List the samples from the greatest amount of radiation emitted to the lowest ______, ______, ______, ______, and ______.

Explain how the amount of radiation absorbed by an object is effect as the distance from the source increases. From your graphs in general what type of relationship is this?

What would be the best shield for all types of radiation ______?

From your data and your notes determine which type of radiation each sample emits. Most samples emit more than one type of radiation. Use your notes and draw a conclusion from your data as to which type your sample emits. Choose one type each according to your data.

Co60 _____, Cs 137 ______, Sr90 _____, Po210 ______, and Tl204 ______.