Cal Poly Physics Department's Virtual Radiation Laboratory
The virtual radiation laboratory is a set of applets that allow the user to simulate detecting nuclear radiation. The purpose is to give the students an introduction to radiation detection and data analysis without being radiated. Three different detectors have been simulated: a Geiger Counter, NaI gamma detector, and a high resolution Ge gamma detector. The Geiger counter is designed to simulate a real one, and the gamma detectors use real data from our samples in the laboratory. For each instrument there is a picture of the real detector, instructions on how to use the applet, and suggested experiments that can be performed with the virtual detector. All the applets were written by Cal Poly Physics major Andres Cardenas as part of his senior project.
Geiger Counter
To the right is a picture of our Geiger Counter. The tube and counter are shown. On the counter, the user can set the counting time. The counter options include start, stop and reset. The display shows the number of radiation particles recorded during the counting time. /The virtual Geiger counter operates similar to the real one. Click here for the Geiger Counter. The Geiger counter has two sample holders. In each sample holder you can pick either an empty holder, Ba137m or Mn54 (5 Ci). The detector has a dead time, and there is a background. The buttons are similar to a real Geiger counter. To operate: set the counting time and click start. Counting stops after the counting time. Then clear the counter. To record counts from the Ba137m samples, you need to select the sample and click on “squeeze out Ba”. Squeezing out the sample refreshes the Ba source, which has a short half life. The button refreshes both sources when clicked. The sources are only counted when they are in the sample holder.
Some experiments that can be done:
- Dead time measurement: Measurement of the detector’s dead time.
- Statistics of Nuclear Decay: Examine if the detector’s counts follow a Poisson distribution.
- Efficiency measurement of the detector
- Half-life of Ba137: Take data on Ba137 and determine its half-life. Remember to account for background and dead time.
NaI Gamma Detector
To the right is a picture of our NaI detector / Multi-Channel-Analyzer (MCA) setup. The complete setup includes: the NaI detector with photomultiplier tube, Power supply and amplifier box, and computer with MCA card. // To the left, we show the computer display of the data. The source is Na22. The horizontal axis is channel number, which is proportional (approximately) to the energy of the gamma. The vertical axis is the number of counts for the corresponding channel number. There are 1024 channels. For the Na22 spectrum shown, the larger peak (lower channel number) has an energy of 511 KeV, and the smaller peak (higher channel number) has an energy of 1275 KeV.
For each applet described below the MCA screen is displayed with different sample options. Pick a sample from the list and click on collect, which displays the spectrum. Left (Right) curser moves each of the two cursers left or right by 20 or 1 channel number(s). The channel number and counts for each curser are displayed underneath the screen. The applet allows the user to perform Gaussian peak fitting as follows: first set the two cursers to the left and right of the desired peak to fit. Then click on a Gaussian Curve fitting. Click on “autofit” to improve the fit. Each time "autofit" is clicked a grid search is performed to minimize the total chi-square. Keep clicking on "autofit" until the total chi-square (chisq) stops decreasing. The best-fit Gaussian parameters are displayed on the screen. Normal mode returns you to the full spectrum.
1. Energy Calibration Experiment and Unknown
To run the applet, click on gamma detector (Calibration) . You will see the MCA screen with 1024 channels. The samples include three standards and an unknown. The unknown is a single isotope. Your goal is to determine the photopeak energies and the identity of the unknown. The energy of the detected gamma is (approximately) proportional to the channel number. Use the standards Cs137(661.64 KeV), Na22(511.0034 and 1274.5 KeV), and Mn54(834.827 KeV) to determine the parameters of the linear (or quadratic) relationship between channel number and energy. Then find the channel numbers of the photopeaks of the unknown, determine their energies from your calibration line, and interpolate to find the gamma energies of the unknown. To assist you, a table of gamma energies is supplied.
2. Half-Life of K40 Experiment
To run the applet, click on k40 half-life experiment. You will see the MCA screen with 1024 channels. The sample data are: One minute counting time for 0.84 micro-Curies (0.84uCi) of Cs137; One minute counting time for 0.35 micro-Curies (0.35uCi) of Mn54; One minute counting time for 0.75 micro-Curies (0.75uCi) of Na22; One hour counting time for 30.2 grams of KCl; and a four hour background count. All samples have approximately the same source-detector geometry. Your goal is to determine the half-life of K40 from this data. One approach you can take is to first find the efficiency of the detector at the energies 662KeV, 835KeV, 511KeV, and 1275KeV of the three standards. Make a graph of your results, and extrapolate to estimate the efficiency of the detector at 1460 KeV, which is the energy of the gamma emitted by K40. Using the counts from the KCl sample, you can determine the half-life of K40. Remember to subtract the background K40, to include the yield factors, and the natural abundance factor of K40 (0.0117%).
3. Attenuation of Gamma radiation in Lead Experiment
The applet Gamma attenuation in lead experiment contains gamma spectrum data with different absorbers between the source and detector. The source used was Cs137, which gives off a gamma with energy 662 KeV and an x-ray with energy of 32 KeV. For the data in the applet, we have used lead absorbers of various thickness to attenuate the gamma particles. The lead absorbers block the x-ray completely, but the 662 KeV gamma particles do pass through. All data were taken with the same source-detector geometry.
Your goal in the experiment is to see if the attenuation is exponential with absorber thickness, and if so, determine the mass attenuation coefficient for the 662 KeV gamma for the lead absorber. Measure the number of gamma particles that pass through the lead for the absorbers given. Use Gaussian curve fitting to determine the area under the photopeak. The data are as follows:
Sample PB0 is with no absorber. Collection time was two minutes. Sample PBC is with a lead absorber of thickness 2.651 g/cm2. Collection time was two minutes. Sample PBD is with a lead absorber of thickness 4.451 g/cm2. Collection time was two minutes. Sample PBE is with a lead absorber of thickness 7.194 g/cm2. Collection time was two minutes. Sample PBCE is with a lead absorber of thickness 9.845 g/cm2. Collection time was two minutes. For more information about the samples see lead absorbtion data.
4. Attenuation of X-ray radiation in Aluminum Experiment
The applet x-ray attenuation experimen contains spectrum data with different absorbers between the source and detector. The source used was Cs137, which gives off a gamma with energy 662 KeV and an x-ray with energy of 32 KeV. For the data in the applet, we have used lead aluminum absorbers of various thickness to attenuate the radiation. The aluminum absorbers do attenuate the 32 KeV x-ray. All data were taken with the same source-detector geometry.
Your goal in the experiment is to see if the attenuation is exponential with absorber thickness, and if so, determine the mass attenuation coefficient for the 32 KeV x-ray for the aluminum absorber. As with the previous experiment, measure the number of x-rays that pass through the aluminum for the absorbers given. Use Gaussian curve fitting to determine the area under the photopeak. The data, for a two minute counting time, are as follows:
The sample AL0 contains spectra data for the Cs137 source with no absorber. Note: the amplification has been doubled from the previous 662 KeV data, so the x-ray peak can be more clearly measured. The x-ray photopeak is at around channel 90.
The sample AL7 is with an aluminum absorber thickness of 0.082 g/cm2. The sample AL13 is with an aluminum absorber thickness of 0.342 g/cm2. The sample AL17 is with an aluminum absorber thickness of 0.620 g/cm2. The sample AL20 is with an aluminum absorber thickness of 0.961 g/cm2. The sample AL22 is with an aluminum absorber thickness of 1.225 g/cm2. The file AL25 is with an aluminum absorber thickness of 1.602 g/cm2. For more information about the absorber data see Aluminum absorber data.
High Resolution Ge Detector
To the right we show our high resolution Germanium detector from Canberra. The complete setup includes the detector with liquid nitrogen cooling, power supply, amplifier and a computer with a multi-channel analyzer card. // To the left, we show the computer display of a sample spectrum, Na22. The horizontal axis corresponds to the channel number, which is proportional to the gamma’s energy. On the vertical axis are the counts for the corresponding channel numbers. The resolution is around 10 times better than with the NaI detectors shown above. There are 8192 channel numbers.
The germanium detector applet is similar to the NaI detector applets. However, since the data consist of 8192 channels, we do not display the whole spectrum at once. The screen displays 1024 channels at a time. To move left or right through the complete spectrum, hit the pan buttons: left moves the spectrum 512 channels left and right 512 channels right. There are also two cursers as with the NaI detector. For Gaussian peak fitting, move the left and right curser around the peak you want to analyze. Be sure to include enough of the flat background left and right of the peak. Hit "Gaussian Curvefitting", and then hit "autofit" repeatedly until the total chi-square doesn't decrease any more. Gaussian fitted photopeak data is displayed on the screen. Hit "return to normal mode" to pan the rest of the spectrum for peak analysis.
Environmental Sample Experiment
Click here for the germanium detector for soil and rock analysis. Your goal is to determine the radioactive isotope content of the soil and rock sample. The information for the data files are listed below. You can also obtain ascii listings of the data for your own spreadsheet by clicking on the files below. Information about the decay series for the natural occurring isotopes can be obtained from: Uranium 238, Thorium232, and Uranium235.
Data for Ge detector:
The file bk4hr.dat is a spectra obtained by counting for 4 hours with no sample present. It is the background spectra to be subracted from the other data files.
The file kcl1000.datis a spectra obtained by counting for 1000 seconds with a pure potassium chloride sample. The mass of the KCl sample is 3117 grams, and the sample geometry is the same as for the soil sample (monro.dat).
The file pblend.dat is a spectra obtained by counting for 2 minutes with a sample of pitch-blend rock. The main isotope in the pitch-blend is U238. This spectra is to be used to identify the peaks of the U238 decay series, and to obtain an approximate relative efficiency calibration curve. It does not have the same geometry as the soil or KCl samples.
The file lantern.dat is a spectra obtained by counting for 2 minutes with a lantern mantle as a sample. The main isotope in the lantern mantle is Th232. This spectra is to be used to identify the peaks of the Th232 decay series, and to obtain an approximate relative efficiency calibration curve. It does not have the same geometry as the soil or KCl samples.
The file monrovia.dat is a spectra obtained by counting for 4 hours with a soil sample from the Monrova foothills. The mass of the sample is 4735 grams, and the detection geometry is the same as the KCl sample.