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The SYNTH Title Screen
Because all of the pertinent parameters were saved in a SYNTH
"project file" the first time the problem was solved, I can simply
recall them by using the "File Open" option on the menu bar of
the main form [primary user interface] to select a "project file"
to read in.
In this example, the actual experimental details of a real laboratory measurement
were used as input parameters to the SYNTH code in order to model
the experimental results. The generated spectrum is then visually compared
to the experimentally acquired spectrum.
The sample consisted of about 100 grams of 20 year old
low-burnup plutonium in the chemical form of PuO2. The sample was physically
contained in two concentric steel pipes. The isotopic data used for generating
the source term was taken from the original laboratory records generated when
the source was prepared. The contemporary gamma-ray spectrum was acquired using
a 30% HPGe detector.
The following steps are shown with screen shots and detailed explanations below:
Opening the Project File
All
of the pertinent parameters for each of the sections [Sample, Source term,
Absorbers, Detector, Electronics] are recalled, and loaded into the respective
forms. The information may then be edited, or just processed before going on to
the next form. The forms may be filled out in any order. In this example, the
information is worked from left to right.
Physical Properties of the Sample
Matrix
The Physical properties [dimensions, mass, bulk composition],
and the source-to-detector distance are entered on this form. For point
sources [dimensions and mass = 0], only the source-to-detector distance
is pertinent. For finite sources, the plot of self absorption as a function
of energy may be quite instructive.
Defining the Source Term(s)
- Element
First, select an element, by using the pulldown menu box located
inside the "Element" frame or pressing the "Periodic Table"
button, also located inside the "Element" frame. If the "Periodic
Table" button is pressed, a picture of the periodic table will appear
on the screen. You can select an element by pressing its corresponding
button (i.e., to select iron, press the button labeled "Fe").
Isotope
Second, if the RADIOACTIVE SOURCE option is selected, specify the
atomic number (A) of the radioisotope, if the correct value is not already
displayed. To change the atomic number, either type the correct value into
the text box inside the "Isotope" frame or click the spin button
(the two arrows). Clicking on the left arrow decreases the "A"
value by 1, and clicking on the right arrow increases the "A"
value by 1.
Note: DO NOT specify a value outside the min/max
range (displayed underneath the "Quantity" frame). No isotopes
for the selected element exist outside of the specified range (each element
has a different range). The scroll bar does not leave this range.
If the element is metastable, click the pulldown menu box and
select the "m." If it is not metastable, click the pull-down
menu box and select the blank line.
Quantity
Next, specify the quantity of the radioactive source, (or the composition
of a sample to be irradiated in a neutron flux if the NEUTRON ACTIVATION
option is selected). First, using the keyboard, type the numeric part of
the quantity (such as "5" for the quantity "5 grams")
into the text box inside the "Quantity" frame. Then, using the
pull-down menu box next to the value you just typed in, select the unit.
If the RADIOACTIVE SOURCE option is selected, the three choices are Bq
(disintegrations per second), Curies, and grams. For the NEUTRON ACTIVATION
option, the composition of the sample may be entered as percent (%), parts
per million (ppm), parts per billion (ppb), parts per trillion (ppt), or
grams. All fractional compositions are weight ratios, rather than atom
ratios!
Daughters
If you want to specify that daughters in equilibrium with the primary
source term be included, click the check box marked "Daughters"
in the "Decay Time" frame (if not, skip this step). If the NEUTRON
ACTIVATION option is selected, this is the place to enter the decay time
(Tdecay) between the End of Irradiation (EOI, or Tzero), and the beginning
of the counting period (Tcount). If this is done, the appropriate Bateman
equations for complex decay will be evaluated using the decay time described
in the next paragraph.
Now specify the numeric part of the decay time (such as "1000"
for the decay time of "1000 years") by typing it into the text
box inside the "Decay Time" frame. Then, using the pull-down
menu box to the right of the text box, specify the units. The choices are
(s)econds, (m)inutes, (h)ours, (d)ays, and (y)ears.
You can also specify the decay time by using the Decay Calculator.
To use the calculator, press the button with the picture of the calculator
on it. First, enter the starting date and time into the top row of text
boxes. Second, enter the ending date and time into the second row of text
boxes. Third, press the button labeled "Calculate" to find the
time difference between the two dates.
Finally, press "Return" to accept this value or "Cancel"
to exit the Calculator. (Note that you cannot press "Return"
until "Calculate" is pressed.) After you press "Return,"
the difference between the two dates will be placed in the "Decay
Time" frame on the Source Terms form.
Note: The valid range for starting and ending
years is from 1753 to 2078, inclusive. You will not be able to specify
a decay time unless the check box labeled "Daughters" is checked.
Results of the Library Search
After the library search is completed, the detailed results
are available for viewing. The code makes an attempt to identify possible
"redundant peaks" that may exist in the result set. The user
has the opportunity to review, and accept or reject the items on the list.
Defining the Absorbers
A graph of the composite transmission function as a function of energy is displayed during the absorber selection process, and is quite useful in itself. The display has an extra feature that allows the user to interrogate the graph with the mouse to extract numerical values, if needed. The latest version of SYNTH retains all original selections and allows up to nine additional "User Defined" regions in which any element may be specified as an absorber. Each element selected is initially offered at a default density, but the density may be adjusted to any desired value. This capability allows almost any absorber material to be simulated.
How to Define the Absorbers
First, select the absorber you want to include by clicking
on its corresponding check box (i.e., to select Water, click on the "Water"
check box). This will place a check mark in the box next to the absorber
type.
Note: The absorber "Air" is calculated
automatically by subtracting all of the other absorber thickness' from
the total distance from the source to detector.
To deselect an absorber, just click on it again and the check
mark will be removed.
Next, type in the absorber thickness, in centimeters, in the text box to
the right of the absorber name.
Note: Only positive, real numbers are allowed
in these text boxes.
If you want to see how this absorber affects the attenuation
curve as displayed on the graph, push the button labeled "Update."
Repeat the above steps for any other absorbers you wish to include.
Note: If you wish to specify an absorber that
is not on the list, there are two options. 1) select the User Defined menu
option, and define your own, or 2) select one that is closest to it in
composition and density (i.e., If you wanted aluminum as one of the absorbers,
you might choose "Concrete," and then adjust the thickness to
compensate for the difference in densities).
It is also worth noting that almost all organic
materials (i.e., most woods, rubber, cardboard, linoleum, paraffin, etc.) can be
described by the same surrogate material; H20, or CH2. As an aid to this
process, the following Table is provided.
Densities of Some Common Materials*
| Asphalt 1.10 -
1.50
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Porcelain
2.30 - 2.50 |
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| Brick 1.40 -
2.20
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Rubber,
hard 1.19 |
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| Cardboard
0.69
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Rubber,
soft 0.91 -
1.10 |
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| Cork 0.22 -
0.26
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Tar
1.02 |
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| Glass, common
2.40 -
2.80 |
Wood (seasoned): |
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| Linoleum
1.18
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Oak
0.60 - 0.90 |
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| Mica 2.60 -
3.20
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Cedar
0.49 - 0.57 |
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| Paper 0.70 -
1.15
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White Pine 0.35 -
0.50 |
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| Paraffin
0.87 - 0.91 |
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* g/cm3, from the CRC Handbook of Chemistry and Physics (1966)
Defining the Detector Parameters
How to Set the Detector Characteristics
The first step is to select the desired detector material.
The choices include planar and coaxial germanium, NaI(Tl) [1" x 1",
2" x 2", 3" x 3", and 5" x 2"], and BGO [1.5"
x 1.5"].
One at a time, specify each characteristic of the detector by typing in
the value you want into the corresponding text box. Depending on the type
of detector selected, some of the fields may be unchangeable or unavailable
(i.e., when an NaI 3" x 3"; detector is selected, you cannot
change the diameter or length -- they are fixed at 3 inches. Also, the
deadlayer field is not applicable to this type of detector.) | End Cap Thickness: | Thickness of the end cap material. The value is generally less than 1 mm. |
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End Cap Material: | To select the end cap material, use the pull-down menu box to choose between aluminum, beryllium and stainless steel. | |
End Cap Spacing: | Using the manufacturers data sheet, enter the distance between the end cap and the surface of the detector. | |
Dead Layer Thickness: | For P-type germanium detectors, enter the thickness of the dead layer. The value should be on the order of 1 mm. This should be zero [or very small] for N-type germanium, and all scintillation crystals. | |
Diameter: | Outside diameter of the crystal. | |
Length: | Overall length of the crystal. | Efficiency: |
For germanium detectors, this is the standard stated efficiency value [relative to a 3x3 NaI(Tl) @25 cm]. Unused for Scintillators. | |
Resolution: | For germanium detectors, this is the standard stated resolution value [FWHM @ 1332.5 keV]. For Scintillators, it is usually given in % [FWHM @ 662 keV]. |
| Note: The models used here were designed to describe the
types and range of detectors most commonly used in the laboratory environment.
Although the models are fairly robust, they cannot be extended without bound,
and thus, can give non-physical results if extended too far. IF IT DOESN'T
MAKE SENSE, DON'T BELIEVE IT! |
Defining the Electronics Configuration
How to Specify the
Counting Parameters
Type in numeric values for any of
the categories (count time, zero, gain, linearity, or channels) if they are not
already suitable. As an aid in setting up the energy calibration, the Full Scale
Energy (keV) for the current settings is calculated, and displayed.
Graph of the Generated Gamma-Ray
Spectrum
The spectrum
generated from the parameters specified in the previous forms is plotted as a
function of channel. The spectrum may be saved in a number of different data
formats, compared to a reference spectrum, or viewed in detail. Some of the
available options are described below.
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| Statistical Noise: |
To add statistical noise to the generated spectrum,
click on the check box labeled "Statistical Noise." Press the "Update"
button to see the results. |
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| Background: |
To add an existing background spectrum to the
generated spectrum, click on the check box labeled
"Background."
Choose an ADCAM, IAEA, IEEE, S100, CAM, or ASCII
format spectra file for the background spectrum.
Specify the Normalization Factor. Choose from: "Use
As Is," "Ratio by Live Time," or "User Defined Value." If you select
"User Defined Value," a text box will appear where you can enter the
normalization factor.
Note: Because the
Normalization factor may be negative as well as positive, this option
may also be used for spectrum stripping as well as for building up a
complex spectrum from a number of simpler
components.
Press the "Accept & Normalize"
button.
Press the "Update" button to see the results.
Note: It is sometimes useful to generate
a "background" spectrum with one set of counting parameters, absorbers,
and nuclides, then add it to a specific source term to be
modeled. |
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| Reference: |
To compare the generated spectrum to an existing
Reference spectrum, click on the check box labeled
"Reference." Follow all of the steps listed above for
adding a "Background" spectrum.
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An
Expanded View of the Spectrum
The Pan / Zoom tool allows the generated spectrum
[and a reference spectrum, if loaded] to be examined in greater detail. To
zoom-in on any portion of the spectrum, just click the left mouse button on
the upper left corner of the region to be expanded, and drag the mouse to the
lower right corner of the region. When the mouse button is released, the
specified region will be expanded to fill the viewing area. By default, a
mini-graph showing the entire spectrum, with the selected region outlined in
blue, will appear in the upper right corner of the window. Multiple zooms are
allowed, and you can zoom-out one step at a time by clicking the right mouse
button anywhere on the expanded portion of the spectrum. By clicking the right
mouse button when the cursor is over the mini-graph, the view will revert to
the entire spectrum.
Some of the data items displayed:
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Energy: Displays the value of the energy, and Full Width at Half
Maximum (FWHM) in keV for the current cursor position.
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Channel: Displays the value of the channel number for the current
cursor position.
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Counts: Displays the value of the counts for the current cursor
position.
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Integral: Displays the integral of the currently displayed region for
the synthetic spectrum, and if present, the reference spectrum.
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ROI: If
the cursor is in a Region of Interest (as defined in the ROI Setup screen,
and shown in red on the plot) the Gross, and Net integral will be
displayed.
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