The present disclosure relates to radiation or radioisotope production. More particularly, the present disclosure relates to secondary radiation or radioisotope production controlled by adjusting alignment, proximity or exposure of a primary source to a target.
Emissions of natural radioactive isotopes occur with decay time and emitted radiation dictated by nuclear species. Many uses have been found for natural radioactive sources. Secondary radiation or radioisotopes can be produced when primary radiations by a natural source cause reactions or excited state populations in nuclei in a target. A great variety of radiation types characterized by emission type, time properties, and energy would be available if primary sources could be controllably paired with target materials.
Detecting fissionable and other dangerous chemical materials, inside well-constructed containers designed to subvert detection, has been a technical challenge for the national security apparatus and National Nuclear Security Administration (NNSA). Current methods used for detecting fissile material rely off passive and active interrogation. For chemical materials, passive detection is not an option and can only be identified using active interrogation or destructive testing. Passive detection relies on emitted radiation from the material in question, and the secondary radiation that material produces. Active interrogation includes many methods for determining material contents hidden inside of packages. For active neutron interrogation, fast neutrons are pulsed into the contents in question to measure the delayed neutron production and the associated delayed gamma spectrum. The active interrogation of pulsed neutrons lasts 100-900 seconds depending on geometry, distance, and amount of spent nuclear material (SNM) present assuming the neutron source strength is on the order of 108 n s−1. The number of neutrons needed for interrogation is dependent on the level of associated signal needed. By verifying that neutrons are present when the interrogating neutron beam is turned off, the only conclusion is they are produced through fission and sub-critical multiplication.
Prompt Gamma Neutron Activation Analysis (PGNAA) is another method of active interrogation that utilizes thermal neutrons being pulsed into the area of interest. Thermal neutrons are preferred as the cross sections for (n,p) reactions are orders of magnitude greater than those in the fast neutron spectrum. Neutrons are captured by the material, and these materials emit a prompt gamma specific to each isotope. PGNAA is mainly used for isotope identification for different material compositions, but it can be used for localization if multiple detectors are used. This concept can be extrapolated to detect fissionable material using the same methods. Except it utilizes fast neutrons to penetrate shield or moderating materials, which in turn slow the neutrons down for a larger gamma production rate from (n,p) reactions. (n,p) reactions that occur within neutron and gamma shielding materials will be identified as well, in most instances suggesting fissionable material is present.
Detecting special nuclear material that will potentially cross into the border of the United States, remains a high priority for the defense establishment. Ports of entry monitor and scan all arriving packages that will be dispersed throughout the country. The techniques that the government deploys rely on radiation portal monitors and radiation isotope devices. There are also vehicle scanning systems that can be used to determine the existence of contraband explosive materials.
When detecting nuclear material, there are two main strategies. One is passive interrogation, and this type of analysis relies upon a source emitting gamma radiation that directly impinges on a detector system. This is then used to trigger a response in the alarm systems at a port of entry if the associated signal is strong enough. Active interrogation, on the other hand, is the primary technique to determine if there is nuclear weapon material present in the container. This is usually done by interrogating a container with a 252Cf source to induce a secondary response from the material in question which impinges on a detector system. This can also be done with neutron generators such as deuterium-tritium interactions. 252Cf is a useful isotope for such use since it undergoes spontaneous fission and does not rely on electrical inputs. However, both techniques suffer when there is a well-constructed shielding apparatus around the material. For neutrons, the shield can be constructed from materials with high hydrogen contents, and for gammas, the shield can be constructed with materials of high effective proton numbers. One way to combat well-constructed shielding, that would reduce signal to undetectable levels, would be to increase the sensitivity of the detector system. However, this yields more false positives which can slow traffic at ports of entry. Thus, improvements are needed to determine not only if special nuclear material (SNM) is present, but where it is located within a container volume.
This summary is provided to introduce in a simplified form concepts that are further described in the following detailed descriptions. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it to be construed as limiting the scope of the claimed subject matter.
According to at least one embodiment, a system for detecting gamma radiation by neutron activation of a material includes a switchable radiation source and at least a first detector. The switchable radiation source includes a primary source assembly having an alpha particle emitter, and a target assembly in which, upon irradiation of the target assembly by alpha particles from the primary source assembly, secondary radiation comprising neutrons is produced. An alignment, proximity or exposure of the primary source assembly relative to the target assembly is adjustable to control irradiation of the target assembly by the primary source assembly and thereby selectively irradiate a material under interrogation with the secondary radiation. The first detector is configured to detect gamma radiation prompted by neutron activation of the material under interrogation.
In at least one example, the target assembly includes a stable isotope and irradiation of the target assembly by alpha particles from the primary source assembly creates compound nuclei and neutrons.
The primary source assembly may include at least one planar source tile, the target assembly may include at least one planar target tile, and alignment or proximity of the source tile and target tile may be adjustable by movement of the source tile or target tile.
In at least one example, a shielding shell is movable by rotation or translation between the primary source assembly and target assembly.
The target assembly may include a circular arrangement of multiple target panels, and the primary source assembly include a source panel relative to which the circular arrangement of multiple target panels is rotatable.
The first detector may include a high purity germanium detector.
The first detector may include a lanthanum-bromide detector.
The first detector may include a plastic scintillator.
A shielding material may partially surround the first detector.
The material under interrogation may be positioned in a container, and the switchable radiation source device may be positioned outside of the container.
The first detector may be configured to detect the gamma radiation through the container.
The system may include a second detector configured to detect gamma radiation prompted by neutron activation of the material under interrogation through the container.
The system may be configured to determine a location of the material under interrogation based on signals from the first detector and second detector.
The alpha particle emitter may include americium-241, and the target assembly may include beryllium.
In at least one embodiment, a method for detecting gamma radiation by neutron activation of a material includes: irradiating a target assembly with alpha particles from a primary source assembly, thereby producing secondary radiation comprising neutrons; irradiating a material under interrogation with the secondary radiation; and detecting gamma radiation prompted by neutron activation of the material under interrogation. Irradiating the target assembly with alpha particles from the primary source assembly includes controlling a switchable radiation source in which an alignment, proximity or exposure of the primary source assembly relative to the target assembly is adjustable to control irradiation of the target assembly by the alpha emitter and thereby selectively irradiate a material under interrogation with the secondary radiation.
The primary source assembly may include at least one planar source tile; the target assembly may include at least one planar target tile, and controlling the switchable radiation source may include adjusting alignment or proximity of the source tile and target tile by movement of the source tile or target tile.
Controlling the switchable radiation source may include moving a shielding shell by rotation or translation between the primary source assembly and target assembly.
In at least one example, the target assembly includes a circular arrangement of multiple target panels, the primary source assembly includes a source panel relative to which the circular arrangement of multiple target panels is rotatable, and controlling the switchable radiation source includes selecting an angular position of the target assembly relative to the primary source assembly.
The previous summary and the following detailed descriptions are to be read in view of the drawings, which illustrate particular exemplary embodiments and features as briefly described below. The summary and detailed descriptions, however, are not limited to only those embodiments and features explicitly illustrated.
These descriptions are presented with sufficient details to provide an understanding of one or more particular embodiments of broader inventive subject matters. These descriptions expound upon and exemplify particular features of those particular embodiments without limiting the inventive subject matters to the explicitly described embodiments and features. Considerations in view of these descriptions will likely give rise to additional and similar embodiments and features without departing from the scope of the inventive subject matters. Although the term “step” may be expressly used or implied relating to features of processes or methods, no implication is made of any particular order or sequence among such expressed or implied steps unless an order or sequence is explicitly stated.
Any dimensions expressed or implied in the drawings and these descriptions are provided for exemplary purposes. Thus, not all embodiments within the scope of the drawings and these descriptions are made according to such exemplary dimensions. The drawings are not made necessarily to scale. Thus, not all embodiments within the scope of the drawings and these descriptions are made according to the apparent scale of the drawings with regard to relative dimensions in the drawings. However, for each drawing, at least one embodiment is made according to the apparent relative scale of the drawing.
For the sake of brevity, the following abbreviations may be used in these descriptions:
The following section describes, with reference to
The device may be scaled to allow for different intensities of radiation. Increasing the activity of the alpha source will result in more radiation produced by the target material through alpha-capture reactions.
These descriptions detail an approach to experimental validation of previous alpha-capture cross section values and comparison to the analytical values approximated by using Hauser-Feshbach calculations. These values are then compared to analytical validated cross sections found in the NON-SMOKER database, which contains statistical model results for a range of nuclei. These cross sections are dependent on the energy of the alpha particle. This energy will vary due to different alpha sources being used. The activity of the alpha source will depend on the geometry used and the alpha flux needed for irradiation. Due to the alpha-capture cross section of the target materials being energy dependent on the alpha particle, reaction rates will be calculated for each independent source. The energy of the gammas, neutrons, and protons are dependent on the energy of the alpha particle, binding energy of the stable target, and electron structure of the stable target.
Alpha-capture cross sections for X(α,γ)Y are evaluated experimentally using multiple alpha sources and foil targets. The different alpha sources and materials are listed in Table 1. An alpha source geometry example is provided in
Within Equation 1, compound nucleus values are used to analytically obtain the capture cross section. The summation occurs over the angular momentum values of the alpha particle, ranging from zero to n, which is the maximum angular momentum quantum number. The change in angular momentum, denoted as Δl, of the alpha particle from the ground state. The intrinsic spin of the alpha particle, denoted by sl, interacting with the target nucleus as the angular momentum changes through the summation, which is constant. The wavelength of the alpha particle is λ. The wave number just after the alpha particle enters the target nucleus is K. The compound nucleus radius of the alpha particle and the target is R. R and K remain constant throughout the summation. Equation 1 is generally true for any target atom, but has been used specifically for finding alpha-capture cross sections.
The alpha-capture cross section obtained from NON-SMOKER database was used as the microscopic capture cross section in Equation 2 to determine the expected activation of the foil targets. The flux used in Equation 2 varies depending on the activity of the alpha sources. The decay constant in Equation 2 is the decay constant of the newly formed compound nucleus after alpha capture, and t is the activation time. Equation 2 was integrated over time to determine the expected counts from alpha-activation. Using an activation time of 24 hours, the integral of Equation 2 predicts the number of counts emitted from the reaction.
Experimental Cross Sections and Theoretical Yields of Target Materials:
Analytical validation has been done and cross sections have been obtained from the NON-SMOKER database. Values for cross sections for multiple proposed foil target materials were found (see Appendix 1 of U.S. Provisional Patent Application 62/529,583).
Alpha Sources and Target Materials:
Multiple isotopes for the alpha source may be implemented based upon the desired emission rates of different particles. The nuclear properties of the alpha sources are listed in Table 1. Alpha particle energies and their half-lives are included in Table 1.
148Gd
210Po
226Ra
228Th
229Th
231Pa
232U
236Pu
239Pu
240Pu
241Am
243Am
242Cm
243Cm
244Cm
245Cm
246Cm
247Cm
248Cm
249Cf
250Cf
254Es
Target materials were chosen specifically for their transmutation products after absorbing an alpha particle. Equation 3 demonstrates an X(α,γ)W reaction, Equation 4 demonstrates an X(α,n)W reaction, and Equation 5 demonstrates an X(α,p)W reaction. Table 2 lists the Initial material and the products produced. Table 3 and Table 4 contain nuclear properties of the target materials and the corresponding products.
α+yzX→y+2z+4W+γ (Equation 3)
α+yzX→y+2z+3W+n (Equation 4)
α+yzX→y+1z+3W+p (Equation 5)
In Equation 3, α is the alpha particle, γ is the atomic number of the target material, z is the atomic mass of the target material, and γ is the photon emission from the reaction. X is the target material and W is the product of X after transmutation.
39K
43Sc
42Sc
42Ca
40K
44Sc
43Sc
43Ca
40Ca
44Ti
43Ti
43Sc
47Ti
51Cr
50Cr
50V
58Ni
62Zn
61Zn
61Cu
63Cu
67Ga
66Ga
66Zn
79Br
83Rb
82Rb
82Kr
48Ca
52Ti
51Ti
51Sc
39K
40K
40Ca
47Ti
58Ni
63Cu
79Br
48Ca
42Sc
43Sc
44Sc
51Sc
42Ca
43Ca
44Ti
51Ti
52Ti
53Ti
50V
50Cr
51Cr
61Cu
61Zn
62Zn
66Zn
66Ga
67Ga
82Kr
82Rb
83Rb
Device Geometry:
A switchable radioisotope source device can utilize alpha particles to irradiate stable target materials to emit photons, neutrons, and or protons. Depending on the need of the consumer and the application, the size and strength of the switchable isotope source device may be changed. The probability of producing each particle and product may be found in the preceding under Experimental Cross Sections and Theoretical Yields of Target Materials and in Appendix 1 of U.S. Provisional Patent Application 62/529,583, and are dependent on the alpha particle energy. Specific activities of the produced products may be between 1000 and 10000 Bq/g.
As shown in
In at least one example according to
Switchable Source Device:
In
While a square tile geometry is illustrated, other shapes that overlap into alignment and stagger out of alignment are within the scope of these descriptions. For example, other rectangular shapes other than squares may be used, and triangular, hexagonal, and other shapes may be used.
In
Spherical Source:
In the particularly illustrated design of
Plate with Shielding:
Expected Activities and Products:
The activities of activated foils were calculated using Mathcad 15 by utilizing coupled differential equations. The alpha-capture cross sections for each of the foil materials were interpolated for an energy of 5.48 MeV. Activities of activated foils were calculated for 1 Ci, 0.1 Ci, and 10 mCi sources of 241Am. Based on the results, the activated material for spectroscopy is obtained using a 1 Ci 241Am on nickel and copper foil targets. Potassium was disregarded due to it being a very reactive metal. These calculations are to show proof of concept for the switchable radioisotope and are to be used as a template to calculate product activities.
Similar calculations as those for which the results are shown in
A reaction in the above or other embodiments as described below is produced by having an alpha emitting isotope bombard a stable isotope, creating a compound nucleus and the ejection of a neutron. Radiations such as gammas and or neutrons are then emitted depending on the target material. Target materials are specifically chosen that will emit the desired type of neutron energy after alpha-capture; the scattering angle of the radiation may also vary by changing the target material. The amount of each type of radiation produced depends on the energy of the alpha particle and the target material. A rotary device may be used to place targets in front of or away from alpha particles, thus acting as a switch for the neutron source generator. The rotary device may be moved through an electric signal or mechanically.
Alpha-capture cross sections for 27Al(α,n)30P are evaluated experimentally using a 90 μCi 241Am alpha source and foil targets. The 241Am alpha source information is provided in
Cross sections found within ENDF were integrated over energy, then averaged to find the average interaction rate for each material depending on the starting energy of the alpha particle. This may be seen in Equation 6. The averaged cross section obtained from Equation 6 was used as the microscopic capture cross section in Equation 7 to determine the expected activation of the foil targets. The flux used in Equation 7 varies depending on the activity of the 241Am source. The decay constant in Equation 7 is the decay constant of the newly formed compound nucleus after alpha capture, and t is the activation time. Equation 7 was integrated over time to determine the expected counts from alpha-activation. Using an activation time of 24 hours, the integral of Equation 7 predicts the number of counts incident on the NaI detector. Equation 8 is used to determine the neutron production rate from the foil and the expected count rate on the 3He neutron detector.
Stopping power charts for each target material was used in combination with Srim & Trim to find the range of alpha particles in various target materials. This allowed the optimal target thickness to be determined. An example of a stopping power chart is provided in
Expected Neutron Production:
Calculating the neutron production rate for a foil target included the change in alpha energy as it moves through the foil target, secondary energy of the neutrons, angle of scattering, and energy dependent cross sections. Stopping power was used to determine the target thickness for the highest production rate. Simulations in SRIM were run to show the path alpha particles travel as they enter a 27Al target, as seen in
For the simulation, one hundred thousand individual particles were run. Values for 27Al were integrated and averaged to account for the change in energy of the alpha particle as it moves through the 27Al target. Stopping power figures shown in
Experimental Cross Sections and Theoretical Yields:
Analytical validation has been done and cross sections have been obtained from the ENDF database. Values for cross sections for the proposed foil targets were determined. The cross sections were integrated then averaged to find expected interaction rates used in calculations.
Alpha Sources and Target Materials:
Multiple isotopes for the alpha source may be implemented based upon the desired emission rates of neutrons and their respective energy. The nuclear properties of the alpha sources are listed in Table 1 in the preceding. Alpha particle energies and their half-lives are included in the table.
Target materials were chosen specifically for their transmutation products after absorbing an alpha particle. Equation 9 demonstrates an X(α,n)W reaction. Table 5 lists the initial material, the products produced, and the kinetic energy and scattering angle of the produced products. Table 6 and Table 7 contain nuclear properties of the target materials and the corresponding products.
α+yzX→y+2z+3W+n (Equation 9)
In Equation 9, α is the alpha particle, γ is the atomic number of the target material, z is the atomic mass of the target material, and n is the neutron emission from the reaction. X is the target material and W is the product of X after transmutation.
9Be
12C
10Be
13C
19F
22Na
22Ne
25Mg
23Na
26Al
25Mg
28Si
27Al
30P
29Si
32S
41K
44Sc
45Sc
48V
48Ti
51Cr
51V
54Mn
9Be
10Be
19F
22Ne
23Na
25Mg
27Al
29Si
41K
45Sc
48Ti
51V
12C
13C
22Na
25Mg
26Al
28Si
30P
32S
44Sc
48V
51Cr
54Mn
Device Geometry:
Above descriptions of particular embodiments of switchable source devices and the corresponding drawings are to be taken as cumulative with further embodiments. For example, a rotary switchable source device 500 according to at least one other embodiment is shown in
In
The rotary device 500 will allow for multiple neutron energies and fluxes depending on the target materials. The neutron flux generated by the target materials may be changed by changing the size and thickness of the targets, increasing the distance between the source and the target materials, and the strength of the source. The geometry of the rotary device and the materials may vary in size depending on the application.
Expected Activities and Products:
The activities of activated foils were calculated using Mathcad 15 by utilizing coupled differential equations. The alpha-capture cross sections for each of the foil materials were interpolated for an energy of 5.48 MeV. Activities of activated foils and neutron production rates were calculated for 1 Ci, 0.1 Ci, and 10 mCi sources of 241Am. Each foil had a mass of 1 g.
Similar calculations as those for which the results are shown in
In the following descriptions, alpha-neutron reactions are used for active interrogation techniques to identify materials and their concentrations by using pulsed neutrons and Prompt Gamma Neutron Activation Analysis (PGNAA). In non-limiting implementation of the switchable radioisotope generators described above, alpha-induced reactions are used to create the neutrons. An alpha emitting isotope will react and bombard a stable isotope, creating a compound nucleus between an alpha particle and the target nucleus. The compound nucleus will be in an excited state, and emit gammas, neutrons, and or protons depending on the material of the target. Target materials are specifically chosen to maximize secondary neutron production. The angular-energy distribution of the secondary neutrons is dependent on the energy of the incoming alpha particles and the target material. A thin sheet of non-metal material may shield the alpha particles from interacting with the target nucleus, allowing to turn the target material source on and off. This is utilized to create a pulsed effect to identify fissile materials by measuring delayed neutrons from subcritical multiplication. The systems and devices may be scaled to allow for different neutron fluxes, which is proportional to the count rate when using PGNAA. Increasing the activity of the alpha source will result in more neutrons produced by the target material through alpha-neutron reactions. Non-limiting examples of use include National Nuclear Security Agency (NNSA) and Homeland Security uses. Determining material composition is dependent on the amount of fluence created by the switchable neutron generator.
The following section describes, with reference to
Special Nuclear Material (SNM) including 95% 235U and 239Pu can be identified by utilizing a quasi-forward directional AmBe source using Prompt Gamma Neutron Activation Analysis (PGNAA). Non-limiting examples of such sources are described in U.S. non-provisional utility patent application Ser. No. 16/027,632, published as US2019/0013109 A1. Simulations using Monte-Carlo N-Particle transport 6.2 (MCNP) and modeled HPGe and LaBr3 detector arrays were used to identify and quantify the peak-to-background and peak-to-total ratios of the associated photon spectra from SNM encased in a polyethylene shield. The conducted simulations varied the volume of the SNM and neutron source strength in the MCNP data card to conduct an uncertainty analysis. Photopeaks identified include K-shell X-rays from 235U and 239Pu, 61 keV from fission, 2.223 MeV prompt gamma from hydrogen, 511 keV annihilation, and a single and double escape peak from the prompt gamma interaction from hydrogen. Relationships between peak-to-total and peak-to-background as a function of SNM and particle history were investigated to aid in the analysis. The capabilities of both detector systems to acquire well resolved photopeak with a 5% relative error or less, with a 1 Ci source activity, and a peak-to-background ratio of 1.15. This was determined to take 326 seconds for LaBr3 and 163 seconds for HPGe which is comparable to current methods for material detection which take between 100-900 seconds to acquire.
In new methods and systems described herein, PGNAA is used to identify SNM, consisting of High Enriched Uranium (HEU) and Weapons Grade Plutonium (WGPu) using High Purity Germanium (HPGe) and Lanthanum-Bromide (LaBr3) detectors. Semiconductors such as HPGe have significantly higher energy resolution (0.3-1% at 662 keV) whereas scintillators such as LaBr3 have an energy resolution of 2.8-4.0% at 662 keV. This energy resolution improvement has benefits in terms of identifying materials that are similar in photopeak energy, which should be obvious to the reader. The better the energy resolution the more potential photopeaks the detector can resolve within a given energy range. However, these two detectors have downsides associated with operation. HPGe detectors require cryogenic cooling to maintain the internal band structure. LaBr3 scintillators are relatively stable and do not require cooling. The only potential problem with LaBr3 is the fact that the crystal is hygroscopic which can be mitigated by water resistant detector housing. For this analysis both detectors can resolve the photopeaks of interest, but the low energy k-shell peaks (61-105 keV) is better resolved by HPGe. Ultimately the choice for detector should come down to user needs and operational environment.
By using PGNAA, as opposed to measuring delayed neutron production, the SNM and shielding material may still be identified though photopeak analysis along with the location and quantity based on the counts under each full energy photopeak for each detector. Sensitivity analysis was done by comparing peak-to-total and peak-to-background ratios of each photopeak as the number of source particles and amount of SNM changes. These simulations were done within MCNP 6.2 with ENDF VIII.0 cross section libraries. The neutron source used in these simulations is a switchable radioisotope generator, for example as described in patent application publication no. US20190013109A1.
Neutrons from the source are emitted with spectrum peaks at 3 and 4.5 MeV, which are more readily lower energy than neutrons produced from D-T (deuterium-tritium) generators. The switchable source has certain benefits over current methodologies utilized at ports of entry. One, the system is relatively self-contained, which means that there is not a need for a large power draw in potentially remote areas. Two, the radioisotope generator concept is far more portable than large scale D-T generators or X-ray imaging systems. The methods and systems described herein, however, are mean to supplement, not necessarily replace the Vehicle and Cargo Inspection System (VACIS). The methods and devices described herein may be implemented, for example, without acquiring reconstructed images of shipping containers. However, this switchable radioisotope generator can provide data regarding material content without the need for tracks or complex reconstruction algorithms. It would also be possible to utilize source localization to acquire an approximate location of the material without the need for imaging equipment.
METHODS: To simulate the PGNAA photon spectra created from varying amounts of different SNM, input cards within MCNP 6.2 were constructed. The modeled geometry is visualized using MCNPX visual editor as displayed in
To obtain the net photon spectrum from each variation, F8 photon tallies were applied to each detector cell in the array. Each F8 tally was equally divided into 4096 equal bins from zero to 10 MeV for both LaBr3 and HPGe detectors. Simulations with and without SNM were done, and each set of corresponding tallies were subtracted from one another to generate the net spectra. Each F8 tally was processed in Python 3.6 to use gaussian broadening parameters found in a FT GEB tally in the MCNP manual. This was done with post-processing as the gaussian broadening parameters are not final. Gaussian broadening parameters used correspond to previous work, and may be adjusted in simulations according to these descriptions to match experimentations. This was accomplished by parsing MCNP output files with Python. Areas under each of the photopeaks from PGNAA were compared to the area of the total spectrum for various particle history. The number of particles per simulation ranged from 107 to 1011 to replicate the statistics of counting in real time. This resulted in finding the minimum particle history and acquisition time required for determining if an isotope is present and to what degree of certainty. The signal from the PGNAA spectra is dependent on the amount of SNM present and is proportional to the amount of SNM. Varying amounts of SNM were used to quantify the mass of SNM to the signal of the PGNAA spectra.
In previous work, the angular-energy distribution of a directional AmBe source was investigated. This was done using series of conical surfaces. Each cone was spaced by a 1° angle and bound by inner and outer spheres. Each of these angular cones was labeled as a different cell, and F4 neutron tallies were applied to 181 cells. Each tally had a bin width of 0.1 MeV from 0-12 MeV to find the energy dependence at each angle. The angular-energy dependence from each tally was used to write an equivalent source definition in MCNP.
The SDEF used for all MCNP input cards included a volumetric neutron source with energy being a function of direction and angle. There were 185 source distributions used for the volumetric source. Three of the source distributions bound the geometry of the volumetric neutron source inside the beryllium foil. One source distribution was used primarily for emitting neutrons along the x-axis. The remaining source distributions were used to define the neutron energy spectrum for each direction of emission. This was accomplished using a distribution card. To speed up computational time and increase sampling history for PGNAA, the neutron source was biased to emit neutrons within a forward 10° angle of emission along the x-axis. A convolution factor of 125.6 was used as a correction factor when calculating acquisition times with varying source activities.
The materials used in MCNP input card include beryllium, polyethylene, tungsten, air, and the material definition of the 90 microCi 241Am source. The 241Am source contains a silver backing and a gold plating, with americium between the two. Air was used to fill areas around the neutron source, detectors, and inside the hollow polyethylene cube. Tungsten was used as a shielding material, referenced as walls 612 in
The system 600, as represented in
Material cards used in the MCNP simulations are listed in the following MCNP 6.1.1 example material input deck:
RESULTS: F8 photon tallies were processed through Python to export the data. MATLAB was then used to produce the following figures. PGNAA was utilized to examine varying amounts of SNM and particle histories to see how the produced spectra changes over the two detector arrays and their constituents. The figures presented isolate each of these variables to see the change in the spectra.
The difference between HPGe and LaBr3 SNM detectors for PGNAA is shown in
Peak-to-total and peak-to-background values for each photopeak were compared to as particle history and volume of the fuel changed. The relationships between peak-to-total and peak-to-background as NPS changes are shown in
Uncertainty of the photopeaks within the spectra was found to have a power law relationship with the number of particles sampled. This led to diminishing returns as the NPS within the SDEF card increased. Increasing the amount of SNM left the uncertainty unchanged. With these metrics defined, the minimum particle history needed to properly identify SNM in this geometry was 2×108 neutrons for a volume of 16 cm3 of SNM for a LaBr3 detector system. The particle history value is less for HPGe detector systems with a particle history value of 108 neutrons. These values were obtained with all photopeaks of interest and surrounding channels having less than five percent error, or two standard deviations. This corresponds to 1.1×1013 alpha particles from the switchable AmBe source used in previous work. Assuming an activity of 1 Ci, the total acquisition time would take 326 seconds for a LaBr3 system and 163 seconds for an HPGe system. These time metrics assume no dead time within each detector array but do account for the efficiency. This is faster than previous methods used, but acquisition time may be scaled depending on the alpha source activity and the alpha emitter used. Ideally, 148Gd would be used as an alpha emitter, as there is no gamma emitted with each alpha particle. This would allow for no extra gamma dose to personnel using this PGNAA system, and acquisition times proportional to the 148Gd activity. The limiting factor is the current supply of this material, as quantities are only man-made. To improve acquisition times, a higher fluence of neutrons may be used to induce reactions within the SNM or other contents within the package which will scale linearly. It should be noted that longer acquisition times are inherently safer, as too many neutrons activating the SNM or contents of the package may lead to a criticality accident upon performing active interrogation.
The uncertainty of the PGNAA spectra for detecting SNM with a switchable AmBe source was quantified using both HPGe and LaBr3 detectors within MCNP. Photopeaks from fission, K-shell x-rays, annihilation, and prompt gamma emission were identified within the spectra. The number of particles sampled in the simulations affected the variance of the peak-to-background and peak-to-total which corresponded to ratios that were not converged in each spectrum. The amount of SNM used in active interrogation was found to have minimum impact on the photon spectra with the geometry used in the simulations. The acquisition time needed to determine the presence of SNM was found to be comparable to previous methods used. The added benefit to this system is that it may be used for detecting other hazardous materials, unlike delayed neutron counting. A forward biased neutron source can implemented for active interrogation using PGNAA, as the neutrons are forward emitting and contain high energies. These findings will be used to support further development of U.S. patent application publication US 2019/0013109 A1 and neutron sources used for fast neutron imaging for non-destructive testing.
The following section describes, with reference to
According to systems and methods described herein, source location of Special Nuclear Material (SNM) including 95% 235U and 239Pu is identified by utilizing a quasi-forward directional AmBe source, referenced as 701 in
In these descriptions, prompt gamma neutron activation analysis (PGNAA) is used in conjunction with a source localization regime to attempt to determine not only if special nuclear material (SNM) is present, but where it is located within the container volume. PGNAA is an active interrogating technique that utilizes neutrons, preferably thermal neutrons, to induce an (n,p) reaction. The resulting prompt gamma can be used for isotope identification. To induce this reaction, the chosen neutron source is a switchable radioisotope generator described in U.S. patent application publication US 2019/0013109 A1. The switchable radioisotope generator has some benefits over spontaneous fission sources and D-T generators. The spontaneous fission sources such as 252Cf are limited by the flux of spontaneous fission neutrons, and therefore can be problematic in determining the presence of SNM in well shielded environments. D-T generators require a power draw in order to work, unlike the switchable radioisotope generator which can be self-sufficient.
Source localization allows the user to determine, using an array of detector positions, an approximate location of where a source is located. This technique can be applied to stationary port of entry scanning systems to detect SNM or constituent gamma emitters that can be utilized for a dirty bomb. While this technique is not new in engineering, it can be a useful addition to the development of new and more robust scanning systems for national security applications. Source localization as a technique relies on utilizing detection positions in tandem to determine through statistical means where a source may reside. It should be noted that an increased count rate within any given detector is not necessarily indicative of the location of a source. Multiple detector positions are needed to localize. This localization can manifest as a statistical process such in which point source localization is defined as a function of Bayesian statistics.
Within a Bayesian framework the user has the ability to know the probability distribution for activity and position of an unshielded source. This does differ from the proposed purpose as that work focused on mobile source localization whereas the manuscript focuses on a stationary system. Other techniques such as difference of time of arrival (DTOA) have been used as well. DTOA relies on utilizing the distance differences of sensors to determine an approximate location of a source. This is a common technique in the aerospace field. DTOA does have a drawback, however. If fewer sensors are used it is possible for noise to be introduced either through measurement errors or a stochastic process such as radioactive decay. The main way to counteract this is to use more than three sensor systems. The main issue with utilizing a robust regime is the addition of attenuating material. Most localization regimes are valid in non-attenuating media where the gamma ray or neutron can travel further distances.
This method is a three-dimensional Cartesian real-time algorithm that can be used for accurate source positioning within a non-attenuating medium. This method utilizes a discretized domain which meshes the source domain into finite cell volumes, and it is possible to localize a source within 2-3 minutes of measurement. However, these descriptions apply it to an attenuating medium for neutrons and gammas.
For the source localization analysis using PGNAA, a LaBr3, HPGe, and BC-408 detector systems will be examined for isotopic identification and source localization. Semiconductor and Scintillators are in constant competition in the realm of gamma spectroscopy. The tradeoff between efficiency and energy resolution is a constant point of concern for detection parameters. Scintillators such as LaBr3(Ce) have much lower energy resolution (2.8-4.0% at 662 keV) compared to HPGe which has a significantly better resolution (0.3-1% at 662 keV). The improvement in energy resolution from scintillator to semiconductor allows users to identify full energy peaks in proximity together. However, scintillators have greater full peak efficiencies. LaBr3 is a decent tradeoff between efficiency and energy resolution. Compared to NaI(Tl), which has an energy resolution of 7% at 662 keV, LaBr3(Ce) allows the user to resolve more full energy photopeaks within a given energy width. This improvement in energy resolution still pales in comparison to the HPGe. However, scintillators offer stability at room temperature and require no cryogenic or electric cooling unlike HPGe semiconductor detectors. The main downside to outside use of scintillators is that many of them are hygroscopic, which means the properties change in the presence of water, therefore water-resistant housing is needed. This is true for neutron scintillators as well, including BC-408, which has polyvinyltoluene as a base material. The BC-408 scintillator emits a spectrum of light with a wavelength 425 nm and has a softening point at 70° C. BC-408 scintillators have small rise times of 0.9 ns and a decay time of 2.1 ns, which are important for neutron time-of-flight (nTOF) measurements.
METHODS: A source location method used for determining the location of SNM within the package utilizes four different detector positions to be known and the radiation intensity at each detector for a three-dimensional geometry. A system of four nonlinear equations is solved to determine the location of the SNM inside of the package. The source location method used relied on an inverse square law, meaning that the radiation intensity given off by either neutrons or gammas is inversely proportional to the square of the distance from the source. In the case for multiple detectors, this means the radiation intensity multiplied by the distance squared is equal for all detectors, as seen in Equation 10 [17]. For a three-dimensional geometry, Equation 11 is used for each detector to relate a radius to Cartesian coordinates.
I
1
r
1
2
=I
2
r
2
2
=I
i
r
i
2 (Equation 10)
(x−xi)2+(y−yi)2+(z−zi)2=ri2 (Equation 11)
r
i
2
=I
1
/I
i
r
12 (Equation 12)
(x−xi)+(y−yi)2+(z−zi)=I1/Iir12 (Equation 13)
By solving for one radius in terms of another and multiplying by the ratio of radiation intensities in Equation 12 and substituting back into Equation 11, this yields Equation 13 with four unknowns. The four unknowns include the x, y, z coordinate of the SNM and the distance of one detector from the SNM. This would take a minimum of four detector positions and responses to solve the system of equations. The standard deviation for this method is given in Equation 14. This method was developed to be used in a non-attenuating medium, which holds true for photons interacting within polyethylene due to low interaction probabilities. For neutrons, it is assumed that since the amount of polyethylene between the SNM source and each detector is relatively the same, the attenuation and scattering within the polyethylene towards each detector is equal and will still provide a valid source prediction. This is later proved to be true within the results section. For the simplicity of this work, nTOF analysis was not done in MCNP due to resolving the energy spectra between two BC-408 scintillator detectors. If two BC-408 scintillator plates were used in conjunction, the time taken for each neutron passing between then would be recorded and used to find the energy of each neutron. This has been well established in prior work, so F4 tallies were used instead to obtain the neutron energy spectra and total fluence for each BC-408 scintillator plate.
σ=Std(I1r12,I2r22, . . . Iiri2) (Equation 14)
To simulate the PGNAA photon spectra and neutron spectra created from varying amounts of different SNM and to determine the location of the SNM inside the package, input cards within MCNP 6.2 were constructed. The MCNP input cards used included a 241Am source with a beryllium foil target. The 241Am source was modeled after a source obtained through Eckert and Ziegler. The beryllium foil target has a thickness of 25 micrometers, and a width and length of 2.54 cm. The SDEF was specified within the beryllium foil, as all neutrons produced from (α, n) reactions occur there. The neutrons were directed towards a hollow polyethylene cube with a thickness of 14 cm between the detectors and the SNM. The detectors used in the simulations included HPGe and LaBr3 for PGNAA. Each of the LaBr3 and HPGe detectors had a diameter of 2.54 cm and 2.54 cm in length. The HPGe and LaBr3 detectors were placed in four 3×3 arrays for increased sampling. For neutron detection, BC-408 was used as the neutron scintillator for neutron spectroscopy. A single 2.54×10.54×10.54 cm3 sheet was used to record the energy of the neutrons. These dimensions were chosen to match the array size of each HPGe and LaBr3 detector array. Four HPGe/LaBr3 arrays or BC-408 plates were placed around the package geometry at varying locations. The location of each detector array or BC-408 scintillator plate is displayed in Table 8. The center location of each detector array was used for determining the location of the SNM inside the hollowed cube. Inside the hollow cube, varying volumes of 239Pu and 235U were placed, none of which exceeded 0.95 Keff. The modeled systems 700 and 800, as visualized using MCNPX visual editor, are represented respectively in
Each of the systems 700 and 800 may further include the control system 620 as shown in
To obtain the net photon spectrum from each variation, F8 photon tallies were applied to each detector cell in the array. Each F8 photon tally was equally divided into 4096 equal bins from zero to 10 MeV for both LaBr3 and HPGe detectors. Simulations with and without SNM were done, and each set of corresponding tallies were subtracted from one another to generate the net spectra. Each F8 tally was processed in Python 3.6 to use Gaussian broadening parameters found in a FT GEB tally in the MCNP manual. This was done with post-processing as the Gaussian broadening parameters are not final. Gaussian broadening parameters used in this study correspond to previous work, and will be changed later to match experimentation. This was accomplished by parsing MCNP output files with Python. The net area under each of the photopeaks from PGNAA were used for locating the SNM source, along with the total counts found in the net tallies. F4 tallies were placed over the BC-408 plate cells to record the neutron fluence. Each F4 tally used was equally divided into 80 equal bins from zero to 10 MeV for BC-408 scintillator detectors. This course binning structure was chosen due to detector binning used in current neutron spectroscopy systems and there being no visible peaks coming from the HEU and WGPu. Only the total fluence of each F4 tally was used to determine the location of the SNM source. The number of particles per simulation was 109 for reduced uncertainty less than one percent. The source locations determined by HPGe and LaBr3 detector arrays and BC-408 scintillator plates were compared for accuracy. Signal from the PGNAA spectra is dependent on the amount of SNM present and is proportional to the amount of SNM. Varying amounts of SNM were used to quantify the source location and how the source location method broke down as the size of the volumetric source increased.
The angular-energy distribution of a directional AmBe source was determined using series of conical surfaces. Each cone was spaced by a 1° angle and bound by inner and outer spheres. Each of these angular cones was labeled as a different cell, and F4 neutron tallies were applied to 181 cells. Each tally had a bin width of 0.1 MeV from 0-12 MeV to find the energy dependence at each angle. The angular-energy dependence from each tally was used to write an equivalent source definition in MCNP
The SDEF used for all MCNP input cards included a volumetric neutron source with energy being a function of direction and angle. There were 185 source distributions used for the volumetric source. Three of the source distributions bound the geometry of the volumetric neutron source inside the beryllium foil. One source distribution was used primarily for emitting neutrons along the x-axis. The remaining source distributions were used to define the neutron energy spectrum for each direction of emission. This was accomplished using a distribution card. To speed up computational time and increase sampling history for PGNAA, the neutron source was biased to emit neutrons within a forward 10° angle of emission along the x-axis. A convolution factor of 125.6 was used as a correction factor when calculating acquisition times with varying source activities.
The materials used in MCNP input card include beryllium, polyethylene, tungsten, air, SNM, LaBr3, HPGe, BC-408, and the material definition of the 241Am source. The 241Am source contains a silver backing and a gold plating, with americium between the two. Air was used to fill areas around the neutron source, detectors, and inside the hollow polyethylene cube, referenced as container 804 in
For SNM, 239Pu and 95% enriched 235U were used in the simulations, with 238Pu and 238U making up the remainder. Alpha-phase 239Pu and 235U were used in the simulations with densities of 19.2 cm3 and 18.9 cm3 respectively. Material cards used in the MCNP simulations are shown below (See MCNP 6.1.1 EXAMPLE INPUT DECK).
RESULTS: F8 photon tallies and F4 neutron tallies were processed through Python to export the data. MATLAB was then used to produce the following figures. The figures presented compare the source location prediction using four non-linear equations and four individual detector responses. Source location predictions were done for each PGNAA photopeak for the HPGe and LaBr3 detector systems as volume of the SNM varied. The total photon counts from HPGe and LaBr3 detector systems were compared with the total neutron count from the BC-408 detector system as the amount of SNM varied. All simulations utilized a particle history of 109. The method used for determining source location is based off point detectors and point sources; this method was used under the assumption that volumetric sources and detectors are still valid for identifying source location.
It was found that this assumption does not hold true for PGNAA photopeaks if they are not well defined or identifiable on each detector array. This was more prevalent within the HPGe detector arrays, as Gaussian broadening parameters used within Python, mirroring the MCNP GEB card, had little effect on peak broadening. The broadening parameters used for the HPGe detectors had no noticeable effect on the PGNAA spectra due to the Full Width at Half Maximum (FWHM) and standard deviation being less than the binning structure of the F8 photon tallies. For the broadening parameters used in the HPGe F8 tallies to be more noticeable, the binning structure would have to be increased to obtain better resolution. This led to inefficient PGNAA peak identification within HPGe detector arrays. Gaussian broadening was applied to each PGNAA spectra to simulate real detector response and replicate spectra obtained in previous work. The broadening parameters for the HPGe detectors included a, b, and c values of 5.868·10−4, 3.951·10−4, and 7.468 respectively. The broadening parameters for the LaBr3 detectors used a, b, c values of 6.8·10−3, 5.8·10−3, and 14.95. These parameters were based on previous work since experimental validation of the simulations have not been completed. The values used in this study were vailed for the energy range up to 2.6 MeV. The a, b, c values were used to recreate the GEB card within MCNP for Gaussian broadening and were implemented using Python. The larger broadening parameters used for the LaBr3 detectors smoothed the spectra to eliminate the randomness of the F8 tallies from the simulations, leading to better source location prediction. This is seen in
SNM volumes ranged from 16-244 cm3. Increasing the amount of SNM present increased the view factor to each detector array and the amount of subcritical multiplication. This was found to not have a significant effect on predicting source location as much as detector binning combined with the gaussian broadening parameters used, specifically within HPGe detector arrays. This led to an increase in the variance and false predictions of the source location for all PGNAA photopeaks using HPGe detectors, but the source location prediction for the total counts using HPGe detectors was correct for most cases with variance considered. LaBr3 detector arrays did not have this issue occur as the PGNAA photopeaks were well defined at all detectors. For this reason, source location predictions off HPGe PGNAA photopeaks were discarded. Both the HPGe and LaBr3 detector arrays were not able to identify the correct source location of the SNM using the PGNAA photopeaks as the volume of SNM went above 185 cm3. This is shown in the Tables 8-12 and is due to the larger view factor between the SNM and the HPGe and LaBr3 detector arrays, which the methodology doesn't account for. Channel error within the PGNAA photopeaks were less than 1% relative error. Each source location prediction was compared to the center of the SNM, which was (20, 0, 0) for the primary location.
The polyethylene being a scattering and attenuating medium for neutrons was found to not have a noticeable effect on source location predictions. This is seen in the comparison between the total counts from the HPGe, LaBr3, and BC-408 detector arrays in Tables 13-17. The source location predictions done with total neutron counts using BC-408 scintillator detectors yielded better results than the total photon counts from HPGe and LaBr3 detector arrays. This was contributed to more neutrons being produced from fission and subcritical multiplication than photons in different volumes of SNM, yielding better sampling. For the energy spectra of photons and neutrons produced from fission and subcritical multiplication, the average mean-free path for photons was smaller than the average mean-free path of neutrons within the polyethylene. The extra scattering and absorption of photons was expected to yield less accurate results than the neutrons produced. Photons scattering in the polyethylene at higher volumes of SNM hindered source prediction as well. The total number of counts in each spectrum is proportional to the amount of SNM present in the simulations and the type of SNM present. More counts were seen in the plutonium spectra compared to the uranium spectra due to subcritical multiplication. The error for each total count for photons and neutrons were less than 0.015% relative error.
The SNM source 710 was moved to another location of the hollow polyethylene cube container 706 as seen in
The alternative location of the SNM within the package was not able to be predicted correctly with the methodology used. This was due to the change in view factor between each of the four detectors and the SNM. The first source location centered at (20, 0, 0) was more centered in the package and had relatively the same amount of polyethylene around the SNM. The second source location centered at (10, −9, −9) was in the corner of the package. This change in location affected the source location results by having uneven amounts of polyethylene between each detector and the SNM, and the view factor from each detector to the SNM. This justifies the over prediction of the secondary location due to scattering and absorption of neutrons and photons as particles traveled to the top and right detector seen in
The methodology used in these descriptions was intended for gamma point source identification using point detectors to yield a standard deviation of zero; as the size of the source, detectors, or view factor between the two increases, so does the standard deviation of the source location. It was found that a weak scattering and absorbing medium may be used if it is uniform throughout the geometry, such as in the polyethylene package. It was observed that strong scattering and absorbing mediums using this method were found to over predict the location of the source. Difference in view factor between the source and each detector increases the standard deviation of the source location prediction. To increase the accuracy of the results for each SNM location, the detector size can be minimized and the distance between the package and each detector can be increased.
This would make the detectors used better resemble point detectors used in the source location methodology. Increasing the number of detectors would decrease the standard deviation of the source prediction and yield better results by increasing the maximum detector combinations factorially. Neutrons were found to work for the source localization as well with this methodology using nTOF to produce neutron energy spectra; since only the total neutron count was used in this work, 3He proportional counters may have been used in supplement. If peaks within the neutron spectra were to be identified, nTOF would have been advantageous. Peak identification using nTOF or PGNAA may allow for identification of multiple sources with unique energy signatures, such as a package containing 137Cs and 60Co, multiple neutron sources, or a combination of both neutron and gamma sources. Neutron source identification becomes improbable if the neutron source is hidden within a neutron scattering material with the material composition and thickness being unknown. With a 1 Ci 241Am source activity it was determined that 1630 seconds were needed to obtain the results for each detector system with the quasi-forward directional AmBe source. Coupling source and material identification together would increase acquisition time but would only require one system to determine.
The location of the SNM was able to be correctly identified using photopeaks from LaBr3 detector arrays and the total counts incident on LaBr3, HPGe, and BC-408 detectors for locations near the center of the polyethylene package. The volume and gaussian broadening parameters used on the photon spectra was found to play a significant role in determining the correct source location of the SNM. In this parametric approach, source predictions for PGNAA detectors started to degrade after 185 cm3 of SNM. The source predictions from the total neutron count incident on BC-408 scintillator detectors was found to be unaffected as the volume of SNM increased. As the location of the SNM was changed to the secondary location, this changed the view factor between the SNM and each detector. The amount of polyethylene scattering and absorbing photons and neutrons between the SNM and each detector changed significantly. The combination of these two factors lead to the methodology over predicting the location of the SNM at the secondary location, as it does not account for either of them. This led to higher variances and standard deviations of detector responses for the second location of the SNM. The methods described herein are valid for identifying relative source location. Results may be improved by reducing the size of each detector and increasing the distance between each detector and the package. The results demonstrate that a forward biased neutron source is an advantageous choice for source location using PGNAA and neutrons from fission, as the neutrons are forward emitting and contain high energies.
Particular embodiments and features have been described with reference to the drawings. It is to be understood that these descriptions are not limited to any single embodiment or any particular set of features, and that similar embodiments and features may arise or modifications and additions may be made without departing from the scope of these descriptions and the spirit of the appended claims.
This application is a continuation-in-part (CIP), and claims the benefit of priority, of U.S. non-provisional utility patent application Ser. No. 16/027,632, titled “SWITCHABLE RADIATION SOURCES,” filed Jul. 5, 2018. This application, by way of the co-pending Ser. No. 16/027,632 application, claims the benefit of priority of U.S. provisional patent application No. 62/529,583, titled “SWITCHABLE RADIATION SOURCE,” filed on Jul. 7, 2017. The above-referenced applications are incorporated herein in their entireties by this reference.
Number | Date | Country | |
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62529583 | Jul 2017 | US |
Number | Date | Country | |
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Parent | 16027632 | Jul 2018 | US |
Child | 16685499 | US |