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:
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 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 microCuries (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.