J. F. Harmon

Development of accelerator-based γ-ray-induced positron annihilation spectroscopy technique

Accelerator-based γ-ray-induced positron annihilation spectroscopy performs positron annihilation spectroscopy by utilizing MeV bremsstrahlung radiation generated from an accelerator (We have named the technique “accelerator-based γ-ray-induced PAS,” even though “bremsstrahlung” is more correct here than “γ rays”. The reason for that is to make the name of the technique more general, since PAS may be performed by utilizing MeV γ rays emitted from nuclei through the use of accelerators as described later in this article and as in the case of positron lifetime spectroscopy [F.A. Selim, D.P. Wells, and J.F. Harmon, Rev. Sci. Instrum. 76, 033905 (2005)].) instead of using positrons from radioactive sources or positron beams. MeV γ rays create positrons inside the materials by pair production. The induced positrons annihilate with the material electrons emitting a 511-keV annihilation radiation. Doppler broadening spectroscopy of the 511-keV radiation provides information about open-volume defects and plastic ...

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.

High depth nondestructive stress measurements on thick steel alloys

Stress measurements were performed using accelerator-based γ-ray induced positron annihilation spectroscopy technique, which allows probing of defects at high depths in thick materials up to several centimeters. Induced stresses due to tensile, fatigue, cold work, and bending tests were investigated in steel alloys of about 1-cm thickness. The measurements showed the dependence of the line-shape parameter of the annihilation peak S on the induced deformation in the four tests. They also revealed an interesting behavior for the change of S parameter with tensile deformation, related to the engineering stress-strain curve of the material. Transmission electron microscopy measurements of dislocation density in cold work deformation suggested that the saturation of positron annihilation parameters often observed in cold work data is not due to compete positron trapping at defects. It was also shown that the S parameter has a weak sensitivity and quickly saturates in fatigue test when compared with the other m...

Excitation of the 180Tam isomer in (γ, n) reactions

The yield has been determined for the excitation of 180Tag(Jπ=1+) from the 181Ta(γ,n) reaction by measurements of γ rays emitted following the electron capture and β decays of 8.15 h ground state. The probability σm/(σm+σg) for the production of the 180Tam isomer (Jπ=9−) after γ absorption was deduced. The role of initial and final spin on reaction yield, along with the relevance for nucleosynthesis of 180Tam, is discussed.

Sensitivity Upgrades to the Idaho Accelerator Center Neutron Time of Flight Spectrometer

Past experiments have shown that discrimination between between fissionable and non‐fissionable materials is possible using an interrogation technique that monitors for high energy prompt fission neutrons. Several recent upgrades have been made to the neutron time of flight spectrometer at the Idaho Accelerator Center with the intent of increasing neutron detection sensitivity, allowing for system use in nonproliferation and security applications.

Measurements of Dynamic Structural Changes in Laser‐Shocked Materials by Positron Annihilation

Standard shock wave measurements provide no information about the structural changes in the lattice during the generation and propagation of shock waves. It is important to acquire such information to understand the mechanisms of defect formation in laser processing of materials. Positron annihilation spectroscopy has been shown to be a valuable tool to study electron states and open volume defects in solids for more than 40 years. We report the first dynamic measurements of positron annihilation during short‐pulsed laser irradiation of materials. Nanosecond high‐power laser pulses were synchronized with nanosecond bremsstrahlung pulses from a 30 MeV electron linac to irradiate the materials. Bremsstrahlung photons generate positrons inside the material. These positrons, in turn, annihilate with the shocked material’s electrons emitting 511 keV radiation, which is influenced by the momentum distributions of the electrons. Due to positron trapping at defects, structural changes of the lattice are reflected...

Non‐Exponential Decay of Nuclei — A Search for a Fundamental Quantum Effect in Nuclei

Modern tests of grand unification theories have spent considerable experimental effort in pursuit of rare decays. A common feature of these experiments is that they involve extremely rare decay processes and probe regions of the systems’ decay curves which are very short in time compared to their expected mean lifetimes. A potential complication to interpretation of such experiments is the approximate nature of the exponential decay law for quasi‐stationary states. Using the decay of the isomeric nuclear state 207mPb in the short time limit, we search for predicted deviations from the exponential decay law. These experiments address the short‐time electromagnetic decays of nuclei with half‐lives on the order of a few seconds, and explore the as‐yet untapped electromagnetic sector for short‐time (tmin/t12 ≈ 10−5) violations of the exponential decay law. Isomeric states are photo‐populated via (γ, n) reactions with bremsstrahlung beams.