Quasi-monochromatic Photon Beam Research Articles

Isotope-selective radiography and material assay using high-brilliance, quasi-monochromatic, high-energy photons.

We demonstrate the possibility to directly detect microgram amounts of the isotope 7Li using a quasi-monochromatic high-energy photon beam. The isotope selective detection is based on a witness scatterer absorbing and re-emitting photons via nuclear resonance fluorescence. This enables the detection of isotopes with microgram accuracy at long distances from the actual sample. Further, we demonstrate that the technique can deliver quantitative information without specific knowledge of the photon flux and no spectral capabilities or knowledge of the resonance fluorescence cross section. Detection of low-atomic-weight isotopes screened by heavy shielding is also shown. The techniques described are applicable to all next-generation, ultrahigh brilliance, laser-Compton light sources currently under construction.

Photoneutron reaction cross section measurements on Mo94 and Zr90 relevant to the p -process nucleosynthesis

The photodisintegration cross sections for the 94Mo({\gamma},n) and 90Zr({\gamma},n) reactions have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity {\gamma}-ray Source (HI{\gamma}S) facility of the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the photoneutron reaction cross sections was measured with high precision from the respective neutron emission thresholds up to 13.5 MeV. These measurements contribute to a broader investigation of nuclear reactions relevant to the understanding of the p-process nucleosynthesis. The results are compared with the predictions of Hauser-Feshbach statistical model calculations using two different models for the dipole {\gamma}-ray strength function. The resulting 94Mo({\gamma},n) and 90Zr({\gamma},n) photoneutron stellar reaction rates as a function of temperature in the typical range of interest for the p-process nucleosynthesis show how sensitive the photoneutron stellar reaction rate can be to the experimental data in the vicinity of the neutron threshold.

The concept of nuclear photon strength functions: A model-independent approach via ([formula omitted]) reactions

Most theoretical approaches used in nuclear astrophysics to model the nucleosynthesis of heavy elements incorporate the so-called statistical model in order to describe the excitation and decay properties of atomic nuclei. One of the basic assumptions of this model is the validity of the Brink–Axel hypothesis and the related concept of so-called photon strength functions to describe γ-ray transition probabilities. We present a novel experimental approach that allows for the first time to experimentally determine the photon strength function simultaneously in two independent ways by a unique combination of quasi-monochromatic photon beams and a newly implemented γ–γ coincidence setup. This technique does not assume a priori the validity of the Brink–Axel hypothesis and sets a benchmark in terms of the detection sensitivity for measuring decay properties of photo-excited states below the neutron separation energy. The data for the spherical off-shell nucleus 128Te were obtained for γ-ray beam-energy settings between 3 MeV and 9 MeV in steps of 130 keV for the lower beam energies and in steps of up to 280 keV for the highest beam settings. We present a quantitative analysis on the consistency of the derived photon strength function with the Brink–Axel hypothesis. The data clearly demonstrate a discrepancy of up to a factor of two between the photon strength functions extracted from the photoabsorption and photon emission process, respectively. In addition, we observe that the photon strength functions are not independent of the excitation energy, as usually assumed. Thus, we conclude, that the Brink–Axel hypothesis is not strictly fulfilled in the excitation-energy region below the neutron separation threshold (Sn=8.78MeV) for the studied case of 128Te.

Self-absorption with quasi-monochromatic photon beams

We present a new experimental technique to perform self-absorption experiments in combination with nuclear resonance fluorescence using a quasi-monochromatic photon beam for the determination of absolute level widths of excited low-spin states. By measuring simultaneously at two target positions the amount of absorbed photon intensity is directly measured in a single experiment, which strongly reduces systematic uncertainties and reduces the amount of needed beam time by a factor of two. In addition the method does not rely on a given shape of the energy profile of the photon beam and, thus, can also be applied with quasi-monochromatic beams produced via Laser-Compton-Backscattering (LCB). Due to the strongly reduced background with these kind of photon beams this new method in combination with LCB promises a highly improved sensitivity for self-absorption experiments compared to experiments using bremsstrahlung.

Cross-section measurements of the 94Mo(γ,n) and 90Zr(γ,n) reactions using real photons at the HIγS facility

The photodisintegration reaction cross-sections for 94Mo(γ,n) and 90Zr(γ,n) have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity γ-Ray Source (HIγS) facility, Triangle University Nuclear Laboratory (TUNL). The measurements were focused primarily on studying the energy dependence of the photoneutron cross sections, which is the most direct way of testing statistical models, and were performed close to the respective neutron thresholds and above up to ~ 20 MeV. Neutrons from the (γ,n) reactions were detected using a 4π assembly of 3He proportional counters developed at Los Alamos National Laboratory and presently available at TUNL. While the 94Mo(γ,n) cross section measurement aims to contribute to a broader investigation for understanding the γ-process (the mechanism responsible for the nucleosynthesis of the so-called p-nuclei), the information from the 90Zr(γ,n) data is relevant to constrain QRPA calculations of γ-ray strength functions in this mass region. In this contribution, we will present our preliminary results of the total (γ,n) excitation functions for the two photoneutron reactions on 94Mo and 90Zr.

A portable x-ray source with a nanostructured Pt-coated silicon field emission cathode for absorption imaging of low-Z materials

We report the design, fabrication, and characterization of a portable x-ray generator for imaging of low-atomic number materials such as biological soft tissue. The system uses a self-aligned, gated, Pt-coated silicon field emitter cathode with two arrays of 62 500 nano-sharp tips arranged in a square grid with 10 μm emitter pitch, and a natural convection-cooled reflection anode composed of a Cu bar coated with a thin Mo film. Characterization of the field emitter array demonstrated continuous emission of 1 mA electron current (16 mA cm − 2) with >95% current transmission at a 150 V gate-emitter bias voltage for over 20 h with no degradation. The emission of the x-ray source was characterized across a range of anode bias voltages to maximize the fraction of photons from the characteristic K-shell peaks of the Mo film to produce a quasi-monochromatic photon beam, which enables capturing high-contrast images of low-atomic number materials. The x-ray source operating at the optimum anode bias voltage, i.e. 35 kV, was used to image ex vivo and nonorganic samples in x-ray fluoroscopic mode while varying the tube current; the images resolve feature sizes as small as ~160 µm.

Decay pattern of the Pygmy Dipole Resonance in140Ce

The decay behavior of low-lying dipole states in 140Ce was investigated exploiting the γ3-setup at the HIγS facility using quasi-monochromatic photon beams. Branching ratios of individual excited states as well as average branching ratios to low-lying states have been extracted using γ – γ coincidence measurements. The comparison of the average branching ratios to QPM calculations shows a remarkable agreement between experiment and theory in the energy range from 5.0 to 8.5 MeV.

Characterization of a spatial light modulator as a polarization quantum channel

Spatial light modulators are versatile devices employed in a vast range of applications to modify the transverse phase or amplitude profile of an incident light beam. Most experiments are designed to use a specific polarization which renders optimal sensitivity for phase or amplitude modulation. Here we take a different approach and apply the formalism of quantum information to characterize how a phase modulator affects a general polarization state. In this context, the spatial modulators can be exploited as a resource to couple the polarization and the transverse spatial degrees of freedom. Using a quasi-monochromatic single photon beam obtained from a pair of twin photons generated by spontaneous parametric down conversion, we performed quantum process tomography in order to obtain a general analytic model for a quantum channel that describes the action of the device on the polarization qubits. We illustrate the application of these concepts by demonstrating the implementation of a controllable phase flip channel. This scheme can be applied in a straightforward manner to characterize the resulting polarization states of different types of phase or amplitude modulators and motivates the combined use of polarization and spatial degrees of freedom in innovative applications.

Decay pattern of the pygmy dipole resonance in60Ni

Spin-1 states in ${}^{60}$Ni were excited with the $(\stackrel{P\vec}{\ensuremath{\gamma}},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reaction, exploiting the High Intensity $\stackrel{P\vec}{\ensuremath{\gamma}}$-ray Source at Triangle University Nuclear Laboratory. This facility is capable of providing fully linearly polarized, quasimonochromatic, Compton-backscattered photons in the entrance channel of the reaction. The depopulation of low-lying levels in an energy region far below the incident quasimonochromatic photons allows us to obtain average branching ratios of the excited spin-1 states. Levels within the energy region associated with the PDR showed regular behavior and $\ensuremath{\approx}$75$%$ of their decays are direct ground-state decays. The levels in the energy region above the PDR exhibit a statistical decay behavior to a large number of low-lying excited states and have only $\ensuremath{\approx}$50--60$%$ branches to the ground state. Within the framework of the quasiparticle phonon model this feature can be explained with the number of quasiparticles contributing to the wave functions of the excited spin-1 states. Quasimonochromatic photon beams provide a new method to test the microscopic nature of ${1}^{\ensuremath{-}}$ levels.

Nondestructive identification of isotopes using nuclear resonance fluorescence

Nondestructive identification of heavy isotopes concealed in a thick iron box has been demonstrated by using nuclear resonance fluorescence. A quasi-monochromatic photon beam produced by the collision of laser quanta with high energy electrons was used for resonant excitation of nuclear levels in (206)Pb and (208)Pb. By measuring the resonant γ rays emitted from (206)Pb and (208)Pb, each of these isotopes were clearly identified. The ratio of the effective thickness, i.e., concentration distribution, of these isotopes was deduced from the relative intensities of the measured nuclear resonance fluorescence strengths.

Photodisintegration of Pb isotopes

Yields of photonucleon reactions of different multiplicity of natural isotopic mixture of Pb were measured on a bremsstrahlung photon beam with a maximum energy of 67.7 MeV using gamma-activation methods. Cross-sections of photonucleon reactions of different multiplicity were calculated on stable Pb isotopes within the bounds of the combined model. The calculated cross sections are compared to those measured in experiments on quasimonochromatic photon beams. They are also used to split the measured yields into yields of particular photonucleon reactions.

Transmission-based detection of nuclides with nuclear resonance fluorescence using a quasimonoenergetic photon source

We provide a detailed experimental validation of the concept of transmission-based isotope detection. The dominant background processes in this class of systems were measured by studying the detection of U238 with a quasimonochromatic (ΔE∕E∼3%) photon beam. A notch develops in the spectrum transmitted through our test objects due to the preferential attenuation of photons with an energy that resonantly excites a bound nuclear state in U238 near 2 MeV. The notch was measured downstream of our test objects by means of resonant photon scattering from a secondary U238 target. The dominant backgrounds measured in the notch detector due to radioactive decay and elastic scattering of the transmitted beam are presented. Processes that refill the notch with off-resonance photons will obscure the signal and result in a higher probability of false negatives. A measurement of the refill process produced a null result, and we report an upper limit on the magnitude of the notch fill factor.

レーザと電子ビームによる軟X線，ガンマ線光源の研究

Soft x-ray and gamma-ray beam sources are developed by using laser and high energy electron beams. Laser plasma soft x-ray source is a cryogenic Xe target irradiated by a high average power (>100 W) and high peak power (30MW, 0.3 J/10 ns) laser. A conversion efficiency from the laser energy to the soft x-ray (70-200eV) energy of 22% was achieved. Laser Compton gamma-ray source on a synchrotron radiation facility, NewSUBARU, is new quasi-monochromatic high energy photon beam. World highest class gamma-ray flux of more than 106 photons/s is generated at photon energy range of 6 to 17 MeV.

Photodisintegration cross section measurements onW186,Re187, andOs188: Implications for the Re-Os cosmochronology

Cross sections of the $^{186}$W, $^{187}$Re, $^{188}$Os($\gamma,n$) reactions were measured using quasi-monochromatic photon beams from laser Compton scattering (LCS) with average energies from 7.3 to 10.9 MeV. The results are compared with the predictions of Hauser-Feshbach statistical calculations using four different sets of input parameters. In addition, the inverse neutron capture cross sections were evaluated by constraining the model parameters, especially the $E1$ strength function, on the basis of the experimental data. The present experiment helps to further constrain the correction factor $F_{\sigma}$ for the neutron capture on the 9.75 keV state in $^{187}$Os. Implications of $F_{\sigma}$ to the Re-Os cosmochronology are discussed with a focus on the uncertainty in the estimate of the age of the Galaxy.

Transmission computerized tomography with a high-energy and quasi-monochromatic photon beam

A high-energy and quasi-monochromatic photon transmission Computerized Tomography system has been developed. The system is used for a nondestructive inspection of industrial products with high-atomic number and high density, and for high-quality inspection of homogeneity/heterogeneity of sintered materials or metal diecasts. Overall system description and some experimental results measured with the prototype CT system are presented.

The 1.2 GeV photon tagging system at LNS-Tohoku

A tagged photon beam line has been constructed in the Stretcher–Booster ring (STB ring) at Tohoku University. The quasi-monochromatic photon beam is produced by the internal photon tagging system which consists of a solid fiber radiator and the compact tagging counter installed in a bending magnet of the STB ring. The energy range of the tagged photon is from 0.8 to 1.1 GeV with the electron energy of 1.2 GeV . The performance of the tagging system has been investigated. The system provides good quality of γ -rays for the nuclear and hadron physics experiments.

High-energy photon radiography system using laser-Compton scattering for inspection of bulk materials

Transmission radiography of bulk materials using continuously energy-tunable quasimonochromatic photon beams has been performed with the photons of a few tens MeV. The photon beam is generated with the laser-Compton scattering of the 300–800 MeV electrons in the electron storage ring “TERAS” with laser lights. The present radiography system is applicable to an inspection of bulk materials and to nondestructive testing of large industrial products. The effectiveness of the method has been examined using metals, ceramics, and concrete blocks. The spatial resolution of the radiography system was measured to be 650 μm using a 1 mm collimator system.

Gamma-ray imaging with the Compton-backscattered laser photons

Gamma-ray imaging using a quasi-monochromatic and energy-tunable photon beam was performed. The photon was produced by the laser-Compton backscattering of 300-800 MeV electrons with laser photons. Radiographs of several objects were measured using 3-20 MeV quasi-monochromatic photons.

Photofission of 27Al nucleus in the quasi-deuteron region of photonuclear absorption

Quasi-monochromatic photon beams of 108 MeV Compton-edge energy produced in the ROKK-1M facility at the storage ring VEPP-4M (BINP, Novosibirsk) and makrofol sheets as fission track detectors were used to measure the photofission yield of 27Al. The fission yield was found to be (160±55) µb. From this result and previously measured 27Al photofission yield at ~79 MeV end-point energy with the LADON apparatus, the trends of fission cross section and fissility have been obtained in the incident photon energy range ~40-110 MeV. The present results and data for other intermediate-mass nuclei obtained with the same methodology indicate an increase of fissility with decreasing Z2/A as predicted by the liquid drop model for fissioning systems lighter than ~Ag.

Photofission cross section and fissility of pre-actinide and intermediate-mass nuclei by 120 and 145 MeV Compton backscattered photons

Cross section measurements for photofission induced in , , , and by 120 and 145 MeV quasi-monochromatic photon beams have been performed at the ROKK-1M facility (BINP, Novosibirsk). The fission yields have been obtained using Makrofol sheets as solid-state fission track detectors. Nuclear fissility values have been deduced on the basis of Levinger's modified quasi-deuteron model of photonuclear interaction, and compared with available literature data. The trend of fissility in the 60-145 MeV energy range has been analysed for various target nuclei as a function of energy and of the parameter .