Douglas P. Wells

Extremely Radiation-Resistant Mutants of a Halophilic Archaeon with Increased Single-Stranded DNA-Binding Protein (RPA) Gene Expression

Abstract DeVeaux, L. C., Müller, J. A., Smith, J. R., Petrisko, J., Wells, D. P. and DasSarma, S. Extremely Radiation-Resistant Mutants of a Halophilic Archaeon with Increased Single-Stranded DNA-Binding Protein (RPA) Gene Expression. Radiat. Res. 168, 507–514 (2007). Extremely halophilic archaea are highly resistant to multiple stressors, including radiation, desiccation and salinity. To study the basis of stress resistance and determine the maximum tolerance to ionizing radiation, we exposed cultures of the model halophile Halobacterium sp. NRC-1 to four cycles of irradiation with high doses of 18–20 MeV electrons. Two independently obtained mutants displayed an LD50 > 11 kGy, which is higher than the LD50 of the extremely radiation-resistant bacterium Deinococcus radiodurans. Whole-genome transcriptome analysis comparing the mutants to the parental wild-type strain revealed up-regulation of an operon containing two single-stranded DNA-binding protein (RPA) genes, VNG2160 (rfa3) and VNG2162, and a third gene of unknown function, VNG2163. The putative transcription start site for the rfa3 operon was mapped ∼40 bp upstream of the ATG start codon, and a classical TATA-box motif was found centered about 25 bp further upstream. We propose that RPA facilitates DNA repair machinery and/or protects repair intermediates to maximize the ionizing radiation resistance of this archaeon.

Production of medical radioisotopes with linear accelerators

In this study, we discuss producing radioisotopes using linear electron accelerators and address production and separation issues of photoneutron (γ,n) and photoproton (γ,p) reactions. While (γ,n) reactions typically result in greater yields, separating product nuclides from the target is challenging since the chemical properties of both are the same. Yields of (γ,p) reactions are typically lower than (γ,n) ones, however they have the advantage that target and product nuclides belong to different chemical species so their separation is often not such an intricate problem. In this paper we consider two examples, (100)Mo(γ,n)(99)Mo and (68)Zn(γ,p)(67)Cu, of photonuclear reactions. Monte-Carlo simulations of the yields are benchmarked with experimental data obtained at the Idaho Accelerator Center using a 44MeV linear electron accelerator. We propose using a kinematic recoil method for photoneutron production. This technique requires (100)Mo target material to be in the form of nanoparticles coated with a catcher material. During irradiation, (99)Mo atoms recoil and get trapped in the coating layer. After irradiation, the coating is dissolved and (99)Mo is collected. At the same time, (100)Mo nanoparticles can be reused. For the photoproduction method, (67)Cu can be separated from the target nuclides, (68)Zn, using standard exchange chromatography methods. Monte-Carlo simulations were performed and the (99)Mo activity was predicted to be about 7MBq/(g(⁎)kW(⁎)h) while (67)Cu activity was predicted to be about 1MBq/(g(⁎)kW(⁎)h). Experimental data confirm the predicted activity for both cases which proves that photonuclear reactions can be used to produce radioisotopes. Lists of medical isotopes which might be obtained using photonuclear reactions have been compiled and are included as well.

Laser-Compton scattering as a tool for electron beam diagnostics

Laser-Compton scattering (LCS) experiments were carried out at the Idaho Accelerator Center (ICA) using the 5 ns (FWHM) and 22 MeV electron beam. The electron beam was brought to an approximate head-on collision with a 7 ns (FWHM), 10 Hz, 29 MW peak power Nd:YAG laser. We observed clear and narrow X-ray peaks resulting from the interaction of relativistic electrons with the 532 nm Nd:YAG laser second harmonic line on top of a very low bremsstrahlung background. We have developed a method of using LCS as a non-intercepting electron beam monitor. Unlike the method used by Leemans et al. ( 1996 ), our method focused on the variation of the shape of the LCS spectrum rather than the LCS intensity as a function of the observation angle in order to extract the electron beam parameters at the interaction region. The electron beam parameters were determined by making simultaneous fits to spectra taken across the LCS X-ray cone. We also used the variation of LCS X-ray peak energy and spectral width as a function of the detector angles to determine the electron beam angular spread, and direction and compared the results to the previous method. Experimental data show that in addition to being viewed as potential bright, tunable and monochromatic X-ray source, LCS can provide important information on electron beam pulse length, direction, energy, angular, and energy spread. Since the quality of LCS X-ray peaks, such as degree of monochromaticity, peak energy and flux, depends strongly on the electron beam parameters, LCS can therefore be viewed as an important non-destructive means for electron beam diagnostics.

Long-term variations in the surface air 7Be concentration and climatic changes.

We have used EML Surface Air Sampling Program (SASP) data to analyze the long-term trend in (7)Be surface concentration and address possible correlation between this long-term trend and climatic changes, namely changes in precipitation patterns and temperature. In this paper we present (7)Be concentration data from 23 sites, spanning over 25 years, all over the world, and extract long-term trend parameter using two independent techniques. The (7)Be concentrations in most stations show a pronounced decreasing trend, potentially corresponding to statistically significant changes in transporting (7)Be from upper atmosphere source to these sites. Weak negative correlation between (7)Be concentration and amount of precipitation was also observed. However, more data from more representative sites around the world are needed the statistical robustness of this trend.

Utilization of high-energy neutrons for the detection of fissionable materials

The detection of high-energy prompt fission neutrons was investigated as a method of fissionable material detection. Neutron energy spectra of U238 and several nonfissionable materials were measured using a neutron time of flight spectrometer. The photonuclear reactions were induced in the targets using a pulsed bremsstrahlung beam for several endpoint energies between 8 and 15MeV. While fission neutrons can have energies in excess of 10MeV, neutrons emitted from nonfissionable materials have distinct energy limits that depend upon binding and incident particle energies. Hence the presence of high-energy neutrons can be used to discriminate fissionable from most nonfissionable materials.

Residual stress characterization in structural materials by destructive and nondestructive techniques

Transmutation of nuclear waste is currently being considered to transform long-lived isotopes to species with relatively short half-lives and reduced radioactivity through capture and decay of minor actinides and fission products. This process is intended for geologic disposal of spent nuclear fuels for shorter durations in the proposed Yucca Mountain repository. The molten lead-bismuth-eutectic will be used as a target and coolant during transmutation, which will be contained in a subsystem vessel made from materials such as austenitic (304L) and martensitic (EP-823 and HT-9) stainless steels. The structural materials used in this vessel will be subjected to welding operations and plastic deformation during fabrication. The resultant residual stresses cannot be totally eliminated even by stress-relief operations. Destructive and nondestructive techniques have been used to evaluate residual stresses in the welded and cold-worked specimens. Results indicate that tensile residual stresses were generated at the fusion line of the welded specimens made from either austenitic or martensitic stainless steel, with reduced stresses away from this region. The magnitude of residual stress in the cold-worked specimens was enhanced at intermediate cold-reduction levels, showing tensile residual stresses in the austenitic material while exhibiting compressive stresses in the martensitic alloys. Comparative analyses of the resultant data obtained by different techniques revealed consistent stress patterns.

Positron lifetime measurements by proton capture

A positron lifetime spectroscopy (PLS) technique was developed using coincident γ rays induced by proton capture. Proton capture in some light elements induce coincident MeV γ rays, allowing positron lifetime to be measured. One γ quantum provides a start signal for the positron lifetime spectrometer, whereas the other γ quantum bombards the sample under investigation, generating a positron inside it through pair production. The stop signal is obtained from the detection of one of the two 511keV photons emitted from positron annihilation with the sample electrons. This new technique can extend PLS, which is a powerful tool to identify the size and concentration of defects, to thick materials and a broad range of applications. It also eliminates the source contribution from the measured spectra, which may lead to the identification of more defect types in a sample.

Doppler Broadening Analysis of Steel Specimens Using Accelerator Based In Situ Pair Production

Positron Annihilation Spectroscopy (PAS) techniques can be utilized as a sensitive probe of defects in materials. Studying these microscopic defects is very important for a number of industries in order to predict material failure or structural integrity. We have been developing gamma‐induced pair‐production techniques to produce positrons in thick samples (∼4–40 g/cm2, or ∼0.5–5 cm in steel). These techniques are called ‘Accelerator‐based Gamma‐induced Positron Annihilation Spectroscopy’ (AG‐PAS). We have begun testing the capabilities of this technique for imaging of defect densities in thick structural materials. As a first step, a linear accelerator (LINAC) was employed to produce photon beams by stopping 15 MeV electrons in a 1 mm thick tungsten converter. The accelerator is capable of operating with 30–60 ns pulse width, up to 200 mA peak current at 1 kHz repetition rate. The highly collimated bremsstrahlung beam impinged upon our steel tensile specimens, after traveling through a 1.2 m thick concre...

Photon Activation Analysis at the Idaho Accelerator Center

Activation methods require minimal sample preparation and provide sufficiently high sensitivity for detecting the vast majority of the elements throughout the periodic table. In this paper we shall discuss photon activation analysis (PAA) at the Idaho Accelerator Center. The process of PAA begins with exposing a sample with photons in the energy range of 10 to 30 MeV. Many nuclides in the sample will become activated and, in turn, these radionuclides will decay by emitting characteristic radiation. These characteristic radiation decays are the telltale signatures for identifying elements which can then be measured with spectrometers such as a high‐purity Germanium detector. PAA is not an “absolute” method, as the samples under investigation must be irradiated along with a reference or calibrating material having a well‐known elemental composition. The quantitative evaluation is performed through comparing the two resulting element spectra from the unknown sample and reference material. Besides the obvious...

Real‐Time Dosimetry for Radiobiology Experiments Using 25 MeV LINAC

The next generation of radiobiology research requires increasingly more complex radiation sources to address questions ranging from the effects of space‐based radiation to the influence of dose rate on biological systems. The Idaho Accelerator Center (IAC) has developed a radiobiology research facility to address some of these questions. The irradiation challenge is to deliver stable and reproducible conditions of high dose rate with well‐controlled beam uniformity, dose, and dose rate under controlled temperature. In this work, we used a 25 MeV modified medical grade linear accelerator (LINAC) to obtain a high and adjustable electron dose rate. To overcome electron beam drift we used a collimator that both assisted the LINAC operator to steer the beam and ensured that regardless of beam drift, only the fixed collimated beam would irradiate the specimens. In addition, we utilized a beam flattener to keep the beam variation as low as 3% at 2.5 cm from the beam’s center, and 1% variation between the simultaneously irradiated sample tubes. We also demonstrated that a segmented Faraday “cup” (FC) array provides a useful real‐time beam scanning and monitoring system, and is promising for implementing real‐time dosimetry and control.