Browsing by Author "Howell, Calvin R"

Over the past 200 years, the Earth's atmospheric carbon dioxide (CO₂) concentration has increased by more than 35%, and climate experts predict that CO₂ levels may double by the end of this century. Understanding the mechanisms of resource management in plants is fundamental for predicting how plants will respond to the increase in atmospheric CO₂. Plant productivity sustains life on Earth and is a principal component of the planet's system that regulates atmospheric CO₂ concentration. As such, one of the central goals of plant science is to understand the regulatory mechanisms of plant growth in a changing environment. Short-lived positron-emitting radiotracer techniques provide time-dependent data that are critical for developing models of metabolite transport and resource distribution in plants and their microenvironments. To better understand the effects of environmental changes on resource transport and allocation in plants, we have developed a system for real-time measurements of metabolite transport in plants using short-lived positron-emitting radiotracers. This thesis project includes the design, construction, and demonstration of the capabilities of this system for performing real-time measurements of metabolite transport in plants.

The short-lived radiotracer system described in this dissertation takes advantage of the combined capabilities and close proximity of two research facilities at Duke University: the Triangle Universities Nuclear Laboratory (TUNL) and the Duke University Phytotron, which are separated by approximately 100 meters. The short-lived positron-emitting radioisotopes are generated using the 10-MV tandem Van de Graaff accelerator located in the main TUNL building, which provides the capability of producing short-lived positron-emitting isotopes such as carbon-11 (¹¹C; 20 minute half-life), nitrogen-13 (¹³N; 10 minute half-life), fluorine-18 (¹⁸F; 110 minute half-life), and oxygen-15 (¹⁵O; 2 minute half-life). The radioisotopes may be introduced to plants as biologically active molecules such as ¹¹CO₂, ¹³NO₃^-, ¹⁸F^--[H₂O], and H₂^{15<\sup>O. Plants for these studies are grown in controlled-environment chambers at the Phytotron. The chambers offer an array of control for temperature, humidity, atmospheric CO₂ concentration, and light intensity. Additionally, the Phytotron houses one large reach-in growth chamber that is dedicated to this project for radioisotope labeling measurements.}

There are several important properties of short-lived positron-emitting radiotracers that make them well suited for use in investigating metabolite transport in plants. First, because the molecular mass of a radioisotope-tagged compound is only minutely different from the corresponding stable compound, radiotracer substances should be metabolized and transported in plants the same as their non-radioactive counterparts. Second, because the relatively high energy gamma rays emitted from electron-positron annihilation are attenuated very little by plant tissue, the real-time distribution of a radiotracer can be measured in vivo in plants. Finally, the short radioactive half-lives of these isotopes allow for repeat measurements on the same plant in a short period of time. For example, in studies of short-term environmental changes on plant metabolite dynamics, a single plant can be labeled multiple times to measure its responses to different environmental conditions. Also, different short-lived radiotracers can be applied to the same plant over a short period of time to investigate the transport and allocation of various metabolites.

This newly developed system provides the capabilities for production of ¹¹CO₂ at TUNL, transfer of the ¹¹CO₂ gas from the target area at TUNL to a radiation-shielded cryogenic trap at the Phytotron, labeling of photoassimilates with ¹¹C, and in vivo gamma-ray detection for real-time measurements of the radiotracer distribution in small plants. The experimental techniques and instrumentation that enabled the quantitative biological studies reported in this thesis were developed through a series of experiments made at TUNL and the Phytotron. Collimated single detectors and coincidence counting techniques were used to monitor the radiotracer distribution on a coarse spatial scale. Additionally, a prototype Versatile Imager for Positron Emitting Radiotracers (VIPER) was built to provide the capability of measuring radiotracer distributions in plants with high spatial resolution (~2.5 mm). This device enables detailed quantification of real-time metabolite dynamics on fine spatial scales.

The full capabilities of this radiotracer system were utilized in an investigation of the effects of elevated atmospheric CO₂ concentration and root nutrient availability on the transport and allocation of recently fixed carbon, including that released from the roots via exudation or respiration, in two grass species. The ¹¹CO₂ gas was introduced to a leaf on the plants grown at either ambient or elevated atmospheric CO₂. Two sequential measurements were performed per day on each plant: a control nutrient solution labeling immediately followed by labeling with a 10-fold increase or decrease in nutrient concentration. The real-time distribution of ¹¹C-labeled photoassimilate was measured in vivo throughout the plant and root environment. This measurement resulted in the first observation of a rapid plant response to short-term changes in nutrient availability via correlated changes in the photoassimilate allocation to root exudates. Our data indicated that root exudation was consistently enhanced at lower nutrient concentrations. Also, we found that elevated atmospheric CO₂ increased the velocity of photoassimilate transport throughout the plant, enhanced root exudation in an annual crop grass, and reduced root exudation in a perennial native grass.

Kinematically complete measurements of three-body photodisintegration of $^3$He were performed at the High Intensity $\gamma$-ray Source (HI$\gamma$S) with nearly monoenergetic 15 MeV photons. The experiment relied on two-nucleon coincidence measurements in which the nucleons are emitted on opposite sides of the incident $\gamma$-ray beam axis. The setup consisted of seven 10 cm long cylindrical gas targets pressurized near 4 atm with thin windows to allow low-energy charged particles to exit with acceptable energy loss. Charged particles were detected in silicon strip detectors with angular acceptance constrained by a collimator system. Neutrons were detected in arrays of liquid organic scintillator cells. Data for neutron-proton (np) coincidences were acquired in configurations which selectively include or exclude the np final state interaction. Measurements of proton-proton (pp) coincidences along the same kinematic locus containing the np final state interaction (FSI) were also taken in-situ. Products from the two-body reaction were used as a luminosity monitor. Theory predictions were propagated through a GEANT4 simulation of the experimental setup. There was good agreement between predictions and measurements in the vicinity of the collinear point in which a proton remains at rest as measured by np coincidences. The measured np FSI peak included additional low-energy neutrons not anticipated by the simulation, which are likely associated with intermediate neutron scattering. The np FSI peak was found to be underpredicted by about 20$\%$. The pp coincidence data were consistently about 39$\%$ above predictions.

The lifetime of $^{19}$Ne is an important parameter in precision tests of the Standard Model. Improvement in the uncertainty of experimental observables of this and other $T=\frac{1}{2}$ mirror isotopes would allow for an extraction of

V$_{ud}$ at a similar precision to that obtained by superallowed $0^+\rightarrow0^+$ Fermi decays. We report on a new high precision measurement of the lifetime of $^{19}$Ne, performed at the Kernfysich Versneller Instituut (KVI) in Groningen, the Netherlands. A 10.5 $\frac{MeV}{A}$ $^{19}$F beam was used to produce $^{19}$Ne using inverse reaction kinematics in a H$_2$ gas target. Contaminant productions were eliminated using the TRI$\mu$P magnetic isotope separator. The $^{19}$Ne beam was implanted into a thick aluminum tape, which was translated to a shielded detection region by a custom tape drive system. Collinear annihilation radiation from the emitted decay positrons were detected by two high purity germanium (HPGe) detectors. Event pulse waveforms were digitized and stored using a CAEN V1724 Digitizer. Systematic studies were performed to characterize rate-dependent data acquisition effects, diffusion, backgrounds, and contamination from the separator. We have obtained the result for the lifetime of $\tau = 24.9344 \pm 0.0073(stat) \pm 0.0083(sys)$ seconds.

Neutron-neutron (nn) quasifree scattering (QFS) in neutron-deuteron (nd) breakup is the kinematic configuration in which the proton remains at rest in the laboratory frame. The cross section for this process is sensitive to the 1S0 component of the nn interaction. Recent measurements of the cross section for nn QFS at 26 and 25 MeV indicate that rigorous ab-initio theory calculations underpredict the data by more than 15%. This discrepancy can be resolved by increasing the strength of the 1S0 component of the nn interaction; however, this solution creates a level of charge-symmetry breaking in the effective range parameter thatis much larger than currently accepted.

The goal of this work is to investigate the discrepancy between ab-initio three-nucleon calculations and data for nn QFS cross section in nd breakup. Two measurements of the nn QFS cross section were performed at the Triangle Universities Nuclear Laboratory tandem accelerator facility. Standard time-of-flight techniques were used to determine the energies of the two breakup neutrons detected in coincidence. The integrated beam-target luminosity was determined from the nd elastic scattering yields measured simultaneously with the coincidence yields from nd breakup.

In the first measurement, the breakup reaction was induced with an uncollimated beam of 10.0-MeV neutrons and the emitted neutrons were detected with two heavily shielded detectors placed at equal angles on opposite sides of the beam. This measurement was optimized to be insensitive to changes in the strength of the nn interaction in order to validate the technique for determining the integrated

beam-target luminosity. In the second measurement, a collimated beam of 15.6-MeV neutrons was used with eight unshielded detectors. Two nn QFS configurations were measured, one with detectors placed at equal angles (40°) and one with detectors placed at asymmetric angles (26° and 56°) on opposite sides of the beam. A configuration near the nn final-state interaction was also measured. Several other nd breakup configurations were measured but are not discussed in this dissertation.

Theory calculations of the nd breakup cross section were averagedover the energy spread and finite geometry of the experiment using a Monte-Carlo simulation. The simulation was also used to compute the average detector efficiencies, the average neutron transmission factors, and corrections to the raw yields for multiple scattering.

In the first measurement conducted at 10.0 MeV, our results agree well with theory, validating our experimental technique, in particular for measuring the integrated beam-target luminosity used to normalize the nn coincidence yields to determine the absolute breakup cross section. The results of the second measurement conducted at 15.6 MeV agree with the standard theory calculations, in contrast to the previously observed discrepancies. However, the systematic uncertainties are too large to rule out the previously observed discrepancy between theory and data with high confidence level.

Experiment to measure the differential cross-section for photodisintegration of triton is being developed at Triangle Universities Nuclear Laboratory (TUNL). The goal of this experiment is to provide data for assessing the theoretical treatment of meson-exchange currents in photodisintegration of nuclei and for investigating long-range features of three-nucleon interactions. Measurements will be performed using a linearly polarized gamma-ray beam at the High Intensity Gamma-ray Source (HIγS) at TUNL. This thesis describes the Monte-Carlo simulation developed as a tool for aiding in the optimization of the experimental design and the data analysis software. The Monte-Carlo simulation is important for guiding the data acquisition strategy and interpretation of data. The simulation is based on the GEANT4 toolkit and theoretical cross-section predictions for current experimental setup.

In 2009 the researchers at Triangle Universities Nuclear Laboratory (TUNL) participated in a series of experiments with the Radiation Countermeasures Center of Research Excellence (RadCCORE). This thesis project is a component of the research done at TUNL that was partially supported by the RadCCORE collaboration. The primary goals of this work are: (1) to measure the neutron fluence (and hence the dose) from the standard neutron beam source at TUNL delivered to a small animal target to an accuracy of better than ± 10% and (2) to develop techniques for real time monitoring of the absolute dose delivered to small animal targets from neutron beam irradiation. These two projects are interconnected as the development of the real-time monitoring techniques depends on the results of the absolute fluence measurements.

Measuring the absolute neutron beam fluence necessitates the use of a reaction in which the neutron cross section is accurately known over the relevant energy range and a detection technique which is insensitive to gamma-rays or is capable of distinguishing gamma-rays from neutrons. In this work, neutron activation of aluminum and gold foils was used to make absolute measurements of the fast neutron (E_n ~ 10 MeV) fluence. Neutron activation of gold foils was also used to make a relative measurement of the thermal neutron fluence. The neutrons produced nuclear reactions in the foils, converting a small quantity of the stable atoms in the foils into radioactive ones which subsequently generate gamma-rays in their decay process. The activated foils are then removed from the beam and placed in front of a high-purity germanium (HPGe) detector that measures the energy spectrum of the gamma-rays emitted by the foil. By counting the number of gamma-rays detected over a set time, the incident neutron fluence at the foil location was determined using the known reaction cross sections. The measured neutron fluence was used to calculate the imparted dose to live mouse targets via the muscle tissue neutron kerma factors. Liquid and plastic scintillation detectors were also used to monitor the relative neutron flux in real time during the experiments. These relative detectors were subsequently calibrated using flux results obtained from the foil activation measurements and were used for real time dose monitoring.

The neutron beam produced at TUNL also has an intrinsic gamma component that adds to the dose received by a small animal target. The gamma contribution to imparted dose is generally taken to be around 10% or less for neutron beams created by linear accelerators utilizing the 2H(d,n)3He reaction, but no confirming measurements of this type have been performed at TUNL prior to this work. To verify this claim, an experiment was conducted to quantify the gamma-ray contribution to the target dose at several incident neutron energies and gas cell pressures.

The dosage from the mixed beam was measured using two ionization chambers that have different sensitivities to neutron and gamma radiation. The chambers were placed in the neutron beam, and the total charge induced in the ionization chamber by the mixed radiation field was monitored. The percent gamma-ray contribution to total target dose was calculated utilizing the procedures outlined in AAPM Report No. 7 and Attix.

Using the foil activation technique, the neutron fluence incident on target and dose delivered were measured to within ± 10%. The target dose estimated using the scintillation detectors was found to be accurate to within ± 20%. The results of the ion chamber measurements imply the gamma-ray component of the neutron beam at TUNL contributes less than 5% to the total target dose. Given the large difference in quality factors between gamma-rays (=1) and fast neutrons (~10), the contribution by gamma radiation to the total equivalent dose was determined to be negligible.

Nuclear resonance fluorescence (NRF) is a highly sensitive technique for measuring the energies and strengths of dipole excitations in nuclei. For incident photon energies below the particle separation threshold, excited nuclear states decay solely through gamma-ray emission, providing a unique de-excitation energy spectrum corresponding to the differences in energy between excited and final states. When excitation cross sections and transition energies are known, the presence (or absence) of a particular isotope within a sample can be determined and quantified. This dissertation describes the use of the NRF technique in two separate areas of active research: nuclear security and medical diagnostics.

In recent years, the nuclear security community has been seeking to develop new photon-beam based technologies for screening cargo for contraband and special nuclear materials (SNM), especially weapons grade material. However, because the current knowledge of dipole excitations in the actinides is highly limited, much greater study of the energy and strengths of dipole excitations in these isotopes is required, with particular focus on isotopes of uranium and plutonium. Nuclear resonance fluorescence studies using mono-energetic gamma-ray beams have previously been conducted at the High Intensity Gamma-Ray Source (HIS) facility at Triangle Universities Nuclear Laboratory (TUNL) on 235U [Kwa11], 238U [Ham12], and 232Th [Ade11]. Additionally, bremsstrahlung surveys below 3 MeV have been conducted at the Massachusetts Institute of Technology’s High Voltage Research Laboratory, examining dipole excited states in 239Pu [Joh07, Ber08] and 240Pu [Qui12].

The first portion of this dissertation provides details of our high-sensitivity measurements of the distribution of dipole excited states observed in 240Pu between 2–3 MeV using the linearly-polarized, quasi-monoenergetic photon beam at the HIS. Measurements were taken at eleven mean beam energies ranging from 1.95 to 2.95 MeV (in 100-keV steps). The target material was powdered plutonium oxide containing 4.65(8) g of 240Pu. Gamma rays corresponding to transitions between excited states in 240Pu were observed using high-purity germanium (HPGe) detectors. The narrow energy resolution of the HIS beam (e.g., E/E ~4% FWHM achieved using a 0.75" lead collimator) greatly reduced background from Compton scattering off the high-Z target material compared to bremsstrahlung measurements, improving the signal-to-noise ratio (SNR) of observed transitions and allowing the observation of low-intensity transitions that would otherwise be below detection threshold. The linear polarization of the HIS beam also allowed the determination of the parity of excited states based on differences in the polar and azimuthal angular distribution of the gamma rays emitted by the target. For each observed transition, the transition energy, partial integrated cross section, partial level width, and reduced transition probability for de-excitation were determined. The excitation energy, spin, parity, total integrated cross section, total level width, reduced transition probability for excitation, and ratio of the reduced transition probabilities (Rexp) were determined for each excited state. A total of 27 discrete transitions were observed from 14 excited states, of which 10 transitions were observed for the first time in the present experiments. Cross sections measured for ground-state decay were higher on average than those previously measured by Quiter et al., while those for 1st excited state decays were largely consistent with the prior measurements. All ground-state transitions were determined to be M1 in character.

The second portion of this dissertation involves measurements made to determine the feasibility of applying the NRF technique to medical diagnostics. Nuclear resonance fluorescence is a potentially valuable new tool for aiding in the diagnosis of medical conditions which cause the natural abundances of elements in the body to be altered. Evidence has been found for statistically significant asymmetries in certain trace element concentrations between healthy (benign) and malignant (cancerous) breast tissue. One of the elements for which a strong (> factor 2) asymmetry has been found is calcium [Riz84, Ng97, Raj06, Sil12]. A likely source of this asymmetry is differences in the calcium concentrations between the two principle forms of calcium containing minerals that comprise breast calcifications, calcium oxalate and calcium phosphate [Hak02]. Calcium oxalate (Type 1) calcifications generally occur in the form of calcium oxalate dihydrate (CaC2O4•2(H2O)) (also called Weddellite) and have been found to always be associated with benign breast tissue. Calcium phosphate (Type 2) calcifications in breast tissue are primarily composed of hydroxylapatite (Ca5(PO4)3(OH)) and are highly associated with malignancy [Win93, Sco17]. While the density of hydroxyapatite containing lesions is typically greater than calcium oxalate, it is not always possible to distinguish them with traditional mammography. A secondary procedure that could determine the isotopic composition of a calcification would be of benefit in reducing the number of biopsies to which patients are subjected.

The experiment was designed to test the feasibility of using the NRF technique to distinguish between calcium oxalate and hydroxylapatite through examination of known strong dipole transitions in calcium and phosphorus. The first part of the experiment involved benchmark measurements of the cross sections of the strongest recorded NRF transitions from excited states in 40Ca (Ex = 10318.8(4) keV) and 31P (Ex = 7141.1(18) keV) using bulk samples of calcium oxide and red phosphorus, respectively. Cross sections for non-ground state transitions from these excited states as well as transitions from additional excited states populated during the experiment were also measured. The cross sections measured for ground-state transitions from the principally targeted states in calcium and phosphorus were consistent with previously published values, and the uncertainty in these cross sections was decreased. Other transition cross sections measured were mostly in agreement with the published values.

For the second part of the experiment, three cylindrical PRESAGE® radiochromic dosimeters, containing either calcium oxide or calcium hydroxylapatite targets were irradiated with the photon beam at the HIS targeting the same transitions in calcium and phosphorus as during the bulk measurements. The goal of these measurements was to determine the ability to distinguish between calcium oxalate and hydroxylapatite calcifications contained in a tissue-simulating medium as well as to determine the corresponding detection limits for calcium and phosphorus. PRESAGE® material was chosen due to its ability to provide 3D depth-dose information as well as effectively mimicking the photon scattering properties of human tissue. The dosimeters were irradiated for sufficient time so as obtain a dose of several grays (Gy) or higher in regions of interest in order to provide a strong signal for post-irradiation optical scanning. Experimentally measured depth-dose curves were then compared to simulated values. The two calcium compounds contained in the PRESAGE® material were able to be clearly distinguished via the strong counting asymmetry observed. The best detection limit for calcium achieved was 99.0(15)(49) mg/cm2 using a single HPGe detector and 73.5(8)(36) mg/cm2 for two HPGe detectors. Likewise, a minimum detection limit (MDL) for phosphorus of 45.6(5)(20) mg/cm2 was achieved using a single HPGe detector and 34.8(3)(15) mg/cm2 for two HPGe detectors. These MDLs were determined to be sufficient for detection of 1 mm hydroxylapatite calcifications in tissue by both calcium and phosphorus detection. The corresponding doses to tissue for photon beams that will likely be available in the near future (e.g., 0.1% FWHM energy resolution) are too high for consideration of NRF-based breast screening in the near term. Minimum detection limits will have to be lowered further and detector system efficiencies increased substantially before in vivo diagnostics becomes viable.

This experiment consisted of the planning of the photodisintegration of the triton at the High Intesity γ-ray Source and supporting experiments. The primary supporting experiment was photofission of 18O. The 18O photofission experiment verified the experimental techniques, analysis methods, and computational approaches that will be used for the photodisintegration of the triton. Neutrons were detected using an array of 30 liquid organic scintillators positioned at 3 angles in θ. Neutrons were detected both individually and in coincidence to map differential cross section as a function of angle. Measurements were made using circularly polarized γ-rays at 23.7 and 32.0 Mev. These measurements are the first anuglar differential cross section measurements performed for the 18O(γ,n) and 18O(γ,nn) reactions.

Also in support of photodisintegration of the triton, background neutron spectra were taken for the 27 Al(γ,n) reaction using the planned detector array. This provides in-situ feedback on expected signals and informed the proposed experimental setup for the photodisintegration of the triton. Theoretical predictions for tritium breakup were used to produce events processed by a GEANT4 simulation to assess the true coincidence versus interference coincidence rates. The results for these measurements and predictions are presented in this thesis.