25 research outputs found
Nuclear Disarmament Verification via Resonant Phenomena
Nuclear disarmament treaties are not sufficient in and of themselves to
neutralize the existential threat of the nuclear weapons. Technologies are
necessary for verifying the authenticity of the nuclear warheads undergoing
dismantlement before counting them towards a treaty partner's obligation. This
work presents a novel concept that leverages isotope-specific nuclear resonance
phenomena to authenticate a warhead's fissile components by comparing them to a
previously authenticated template. All information is encrypted in the physical
domain in a manner that amounts to a physical zero-knowledge proof system.
Using Monte Carlo simulations, the system is shown to reveal no isotopic or
geometric information about the weapon, while readily detecting hoaxing
attempts. This nuclear technique can dramatically increase the reach and
trustworthiness of future nuclear disarmament treaties
Capabilities and Limitations of Dual Energy X-ray Scanners for Cargo Content Atomic Number Discrimination
To combat the risk of nuclear smuggling, radiography systems are deployed at
ports to scan cargo containers for concealed illicit materials. Dual energy
radiography systems enable a rough elemental analysis of cargo containers due
to the -dependence of photon attenuation, allowing for improved material
detection. This work studies the capabilities for atomic number discrimination
using dual energy MeV systems by considering dual energy MeV, MeV, and MeV bremsstrahlung beams. Results of this analysis
show that two different materials can sometimes produce identical transparency
measurements, leading to a fundamental ambiguity when differentiating between
materials of different atomic numbers. Previous literature has observed this
property, but the extent of the limitation is poorly understood and the cause
of the degeneracy is generally inadequately explained. This non-uniqueness
property stems from competition between photoelectric absorption and pair
production and is present even in systems with perfect resolution and zero
statistical noise. These findings are validated through Monte Carlo
transparency simulations. Results of this study show that currently deployed
commercial radiographic systems are fundamentally incapable of distinguishing
between high- nuclear materials and miscellaneous mid- cargo contents.Comment: 15 pages and 7 figure
A Semiempirical Transparency Model for Dual Energy Cargo Radiography Applications
Cargo containers passing through ports are scanned by non-intrusive
inspection systems to search for concealed illicit materials. By using two
photon beams with different energy spectra, dual energy inspection systems are
sensitive to both the area density and the atomic number of cargo contents.
Most literature on the subject assumes a simple exponential attenuation model
for photon intensity in which only free streaming photons are detected.
However, this approximation neglects second order effects such as scattering,
leading to a biased model and thus incorrect material predictions. This work
studies the accuracy of the free streaming model by comparing it to simulation
outputs, finding that the model shows poor atomic number reconstruction
accuracy at high- and suffers significantly if the source energy spectra and
detector response function are not known exactly. To address these challenges,
this work introduces a semiempirical transparency model which modifies the free
streaming model by rescaling different components of the mass attenuation
coefficient, allowing the model to capture secondary effects ignored by the
free streaming model. The semiempirical model displays improvement agreement
with simulated results at high- and shows excellent extrapolation to
materials and thicknesses which were not included during the calibration step.
Furthermore, this work demonstrates that the semiempirical model yields
accurate atomic number predictions even when the source spectra and detector
response are not known exactly. Using the semiempirical model, manufacturers
can perform a simple calibration to enable more precise reconstruction
capabilities, which has the potential to significantly improve the performance
of existing radiographic systems.Comment: 15 pages and 4 figure
Measurements of Compton Scattering on the Proton at 2 - 6 GeV
Similar to elastic electron scattering, Compton Scattering on the proton at high momentum transfers(and high p⊥) can be an effective method to study its short-distance structure. An experiment has been carried out to measure the cross sections for Real Compton Scattering (RCS) on the proton for 2.3-5.7 GeV electron beam energies and a wide distribution of large scattering angles. The 25 kinematic settings sampled a domain of s = 5−11(GeV/c)2,−t = −7(GeV/c)2 and −u = 0.5−6.5(GeV/c)2. In addition, a measurement of longitudinal and transverse polarization transfer asymmetries was made at a 3.48 GeV beam energy and a scattering angle of θcm = 120o. These measurements were performed to test the existing theoretical mechanisms for this process as well as to determine RCS form factors. At the heart of the scientific motivation is the desire to understand the manner in which a nucleon interacts with external excitations at the above listed energies, by comparing and contrasting the two existing models – Leading Twist Mechanism and Soft Overlap “Handbag” Mechanism – and identify the dominant mechanism. Furthermore, the Handbag Mechanism allows one to calculate reaction observables in the framework of Generalized Parton Distributions (GPD), which have the function of bridging the wide gap between the exclusive(form factors) and inclusive(parton distribution functions) description of the proton. The experiment was conducted in Hall A of Thomas Jefferson National Accelerator Facility(Jefferson Lab). It used a polarized and unpolarized electron beam, a 6% copper radiator with the thickness of 6.1% radiation lengths (to produce a bremsstrahlung photon beam), the Hall A liquid hydrogen target, a high resolution spectrometer with a focal plane polarimeter, and a photon hodoscope calorimeter. Results of the differential cross sections are presented, and discussed in the general context of the scientific motivation
High-accuracy Geant4 simulation and semi-analytical modeling of nuclear resonance fluorescence
Nuclear resonance fluorescence (NRF) is a photonuclear interaction that
enables highly isotope-specific measurements in both pure and applied physics
scenarios. High-accuracy design and analysis of NRF measurements in complex
geometries is aided by Monte Carlo simulations of photon physics and transport,
motivating Jordan and Warren (2007) to develop the G4NRF codebase for NRF
simulation in Geant4. In this work, we enhance the physics accuracy of the
G4NRF code and perform improved benchmarking simulations. The NRF cross section
calculation in G4NRF, previously a Gaussian approximation, has been replaced
with a full numerical integration for improved accuracy in thick-target
scenarios. A high-accuracy semi-analytical model of expected NRF count rates in
a typical NRF measurement is then constructed and compared against G4NRF
simulations for both simple homogeneous and more complex heterogeneous
geometries. Agreement between rates predicted by the semi-analytical model and
G4NRF simulation is found at a level of in simple test cases and
in more realistic scenarios, improving upon the level
of the initial benchmarking study and establishing a highly-accurate NRF
framework for Geant4.Comment: 16 pages, 6 figures, revised for peer revie
Direct atomic number reconstruction of dual energy cargo radiographs using a semiempirical transparency model
Dual energy cargo inspection systems are sensitive to both the area density
and the atomic number of an imaged container due to the Z dependence of photon
attenuation. The ability to identify cargo contents by their atomic number
enables improved detection capabilities of illicit materials. Existing methods
typically classify materials into a few material classes using an empirical
calibration step. However, such a coarse label discretization limits atomic
number selectivity and can yield inaccurate results if a material is near the
midpoint of two bins. This work introduces a high resolution atomic number
prediction method by minimizing the chi-squared error between measured
transparency values and a semiempirical transparency model. Our previous work
showed that by incorporating calibration step, the semiempirical transparency
model can capture second order effects such as scattering. This method is
benchmarked using two simulated radiographic phantoms, demonstrating the
ability to obtain accurate material predictions on noisy input images by
incorporating an image segmentation step. Furthermore, we show that this
approach can be adapted to identify shielded objects after first determining
the properties of the shielding, taking advantage of the closed-form nature of
the transparency model.Comment: 19 pages and 6 figure
Experimental demonstration of an isotope-sensitive warhead verification technique using nuclear resonance fluorescence
Future nuclear arms reduction efforts will require technologies to verify
that warheads slated for dismantlement are authentic without revealing any
sensitive weapons design information to international inspectors. Despite
several decades of research, no technology has met these requirements
simultaneously. Recent work by Kemp et al. [Kemp RS, Danagoulian A, Macdonald
RR, Vavrek JR (2016) Proc Natl Acad Sci USA 113:8618--8623] has produced a
novel physical cryptographic verification protocol that approaches this treaty
verification problem by exploiting the isotope-specific nature of nuclear
resonance fluorescence (NRF) measurements to verify the authenticity of a
warhead. To protect sensitive information, the NRF signal from the warhead is
convolved with that of an encryption foil that contains key warhead isotopes in
amounts unknown to the inspector. The convolved spectrum from a candidate
warhead is statistically compared against that from an authenticated template
warhead to determine whether the candidate itself is authentic. Here we report
on recent proof-of-concept warhead verification experiments conducted at the
Massachusetts Institute of Technology. Using high-purity germanium (HPGe)
detectors, we measured NRF spectra from the interrogation of proxy 'genuine'
and 'hoax' objects by a 2.52 MeV endpoint bremsstrahlung beam. The observed
differences in NRF intensities near 2.2 MeV indicate that the physical
cryptographic protocol can distinguish between proxy genuine and hoax objects
with high confidence in realistic measurement times.Comment: 38 pages, 19 figures; revised for peer review and copy editing;
addition to SI for realistic scenario projections; minor length reduction for
journal requirement
Validation of Geant4's G4NRF module against nuclear resonance fluorescence data from U and Al
G4NRF is a simulation module for modeling nuclear resonance fluorescence
(NRF) interactions in the Geant4 framework. In this work, we validate G4NRF
against both absolute and relative measurements of three NRF interactions near
2.2 MeV in U and Al using the transmission NRF data from the
experiments described in arXiv:1712.02904. Agreement between the absolute NRF
count rates observed in the data and predicted by extensive Geant4+G4NRF
modeling validate the combined Geant4+G4NRF to within -- in the
U NRF transitions and in Al, for an average
discrepancy across the entire study. The difference between simulation and
experiment in relative NRF rates, as expressed as ratios of count rates in
various NRF lines, is found at the level of , and is
statistically identical to zero. Inverting the analysis, approximate values of
the absolute level widths and branching ratios for U and Al are
also obtained.Comment: 12 pages, 4 figures, 4 tables; revisions after peer review comments,
chiefly making the paper more concise and the reporting of results more clea