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. Applets have been written to simulate four different detectors: a Geiger Counter, NaI gamma detector, a high resolution Ge gamma detector, and a liquid scintillation detector. The Geiger counter is designed to simulate a real one, and the gamma detectors use real data from our samples in the laboratory. The Liquid Scintillation detector is designed to simulate the detector in our 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 and UC Santa Cruz student Jonathan Siegel.

Below is a picture of a Geiger Counter in our lab. 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 Virtual Geiger Counter The Geiger counter has two sample holders. In each sample holder you can pick either an empty holder, Ba^137m or Mn⁵⁴ (5 mCi). 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 Ba^137m 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:

1. Dead time measurement: Measurement of the detector's dead time.
2. Statistics of Nuclear Decay: Examine if the detector's counts follow a Poisson distribution.
3. Efficiency measurement of the detector
4. Half-life of Ba^137m: Take data on Ba^137m and determine its half-life. Remember to account for background and dead time.

Below 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.

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. The total number of channels is 1024, and the "pan" slider allows you to pan left or right. There are two cursers. 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 left cursor with the "left cursor" slide bar. Then set the right cursor using the "window" slide bar. Then click on "Gaussian Curve fitting". Click on "autofit" to improve the fit. Each time "autofit" is clicked a grid search is performed to minimize the total &chi². Keep clicking on "autofit" until the total &chi² stops decreasing. The best-fit Gaussian parameters are displayed on the screen. Normal mode returns you to the full spectrum.

Energy Calibration Experiment

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 which gives off 2 gamma's when it decays. 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 listed below:

to determine the parameters of the linear 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. Note that three of the spectra in the applet contain two isotopes at once. This is to help you correct for any amplifier drift, since the Cs137 photopeak is present in all three spectra.

Natural Abundance of K40 Experiment

To run the applet, click on K40 Natural Abundance Experiment You will see the MCA screen with 1024 channels. The sample and activity are listed in the table below:

All the calibration samples were recorded for two minutes counting time. In addition to these calibration samples, 35.9 grams of KCl were counted for two hours. Also, a background recording for two hours was performed. All samples have approximately the same source-detector geometry. Your goal is to determine the natural abundance of K40 from this data. One approach you can take is to first find the efficiency of the detector at the energies 662KeV, 511KeV, 1275KeV, 570KeV and 1063 KeV 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 natural abundance of K40. Remember to subtract the background K40, to include the yield factors, and the half-life of K40 is 1.277 x 10^9 years.

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:

Collection time was two minutes in each case. Tables for the literature values of the attenuation coefficients can be found in tables.

Attenuation of X-ray radiation in Aluminum Experiment

The applet x-ray attenuation in aluminum experiment 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:

Tables for the literature values of the attenuation coefficients can be found in tables. Note: the amplification has been doubled from the previous 662 KeV data, so the x-ray peak can be more clearly measured.

Below 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. There are 8192 channels.

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 on Environmental Sample Experiment 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 gamma decay series for the natural occurring isotopes can be found in tables. Information about the characteristic x-rays given off by the various daughter isotopes can also be found in tables.

Data for Ge detector:

1. The file "Calibration" is a spectra obtained by counting for a few minute with calibration samples. The Calibration samples are: Cs137, Mn54, Na22, Co60, and Bi207.
2. The file "kcl1hr" is a spectra obtained by counting for one hour (3600 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 samples, and Brazil Nut sample.
3. The file bkrd20hr is a spectra obtained by counting for 20 hours with no sample present. It is a background recording.
4. The file "PasSoil Sample" is a spectra obtained by counting for 20 hours with a soil sample from Pasadena. The mass of the sample is 3960 grams, and the detector geometry is the same as the kcl1hr sample.
5. The file "Brazilnuts" is a spectra obtained by counting for 20 hours with a sample of Brazil Nuts. The mass of the nuts is 2000 grams, and the detector geometry is the same as the kcl1hr sample.
6. The file "pblend" is a spectra obtained by counting a sample of pitchblend, which contains a significant amount of uranium.
7. The file "BBearsoil" is a spectra obtained by counting for 20 hours with a soil sample from Big Bear. The mass of the sample is 4661 grams, and the detector geometry is the same as the kcl1hr sample.
8. The file "Monrosoil" is a spectra obtained by counting for 20 hours with a soil sample from the Monrovia foothills. The mass of the sample is 4928 grams, and the detector geometry is the same as the kcl1hr sample.

Click on: Liquid Scintillation Detector for our applet that simulates our liquid scintillation detector.

The applet RIA Calibration is used in our radioimmunioassay experiment for calibration. After you have finished calibrating the instrument, then click on: RIA Experiment to run the RIA experiment.

The applet Transport Assay Experiment is used in our Gaba transport assay experiment.