George Thomas Baldwin

Dynamics of electron flow in extended planar-anode diode operating at 19 MV and 700 kA

The electron flow in a planar‐anode diode having an extended anode‐cathode gap operating on the HERMES III accelerator is characterized and compared with predictions of a computational model. The model combines a particle‐in‐cell code with Monte Carlo radiation transport. The comparisons confirm the model and show that the diode provides both a matched load and a versatile large‐area source of γ rays for the study of nuclear radiation effects. Electrical and spatial parameters of the beam at the diode and the downstream radiation fields from a graphite target are presented as a function of the anode‐cathode gap.

Radiation field from an extended planar-anode diode on HERMES III

The bremsstrahlung field from an extended planar-anode diode with an annular cathode tip on the the 16-TW HERMES III electron accelerator is measured and compared with predictions of the MAGIC-CYLTRAN model. Measurements confirm predictions and demonstrate that the diode provides a versatile large-area source of gamma radiation. Versatility is obtained by adjustment of the anode-cathode gap, which affects electron trajectories while simultaneously maintaining constant diode impedance. The adjustment permits the generation of average dose rates from about 1.2*10/sup 12/ rad/s over 3100 cm/sup 2/ to about 5.6*10/sup 12/ rad/s over 700 cm/sup 2/ without destruction of the bremsstrahlung target. >

Time Projection Compton Spectrometer (TPCS)

We propose a new technique for measuring the time-integrated energy spectrum of X rays emitted by high-intensity, medium-voltage flash x-ray sources. The method sorts Compton electrons by drift velocity time-of-flight in a non-uniform magnetic field. We explain a conceptual spectrometer design, show how we have modeled it using Monte Carlo methods, and discuss how one would unfold x-ray spectrum information from electron time-of-flight data. The technique appears promising, especially for short-pulse x-ray sources.

Flash X-ray measurements with a time projection Compton spectrometer

Abstract We have developed a “time projection Compton spectrometer” (TPCS) to measure the time-integrated photon energy spectrum between 0.1 and 2 MeV of short-pulse, intense bremsstrahlung sources. A target in the TPCS produces Compton electrons when illuminated by the flash X-ray source. Target electrons are trapped magnetically by current in a rod on the axis of an evacuated drift tube. The curvature and gradient of the magnetic field cause the electrons to drift parallel to the spectrometer axis. The time of electron arrival at the end of a 1 m drift encodes the energy spectrum of the X-ray burst. The detector is a plastic scintillator coupled to a fast photomultiplier tube. Despite heavy shielding, enough prompt X-rays penetrate to give a time fiducial. Background is measured on shots with the X-ray target removed. The response of the TPCS is calculated using the TIGERP computer code to model photon/electron transport in the target and a sophisticated algorithm to compute electron drift velocity. The response functions and signal are the known components of a matrix equation, which is solved to determine the unknown spectrum. We tested the TPCS with flash X-ray sources on the Saturn accelerator. Signals have been analyzed, spectra unfolded and results compared with other methods.

Operation of a bidirectional voltage‐dividing bremsstrahlung diode

Operation of a new type of high‐power electron diode, the bidirectional voltage‐dividing (BVD) diode, has been demonstrated on a 1‐MV, 2.5‐MA, 20‐ns accelerator. This diode consists of a forward‐ and an inverse‐triaxial diode in series. The diode impedance and electron flow pattern agreed reasonably well with particle‐in‐cell code calculations. The radiation spectrum was measured using two different diagnostics, a differential absorption spectrometer and the time‐projection Compton spectrometer. Operating at a voltage of 850–1000 kV, the BVD diode produced a bremsstrahlung spectrum similar to that produced by a standard diode operating at 350–500 kV.

Field Trial of the Enhanced Data Authentication System (EDAS)

The goal of the field trial of EDAS was to demonstrate the utility of secure branching of operator instrumentation for nuclear safeguards, identify any unforeseen implementation and application issues with EDAS, and confirm whether the approach is compatible with operator concerns and constraints.

Material control and accountability for laboratory biosecurity

Laboratory biosecurity refers to a suite of measures to protect legitimate stocks of dangerous pathogens and toxins from being acquired by individuals with malicious intent. Intentional misuse presents a different problem than accidental misuse, which is the subject of laboratory biosafety. In particular, the material control and accountability (MC&A) component of laboratory biosecurity addresses an insider threat. MC&A directly involves the personnel who work with pathogens and toxins that could be attractive to bioterrorists. Control of material comprises both procedural and physical measures for both material and MC&A-relevant information. Accountability requires a one-to-one correspondence between materials and people, together with a system of records, reporting and audit. Key elements of MC&A should be understood intellectually, rather than being applied simply as a prescriptive list of security measures. MC&A measures reflect an additional stewardship responsibility that technical people accept in working with material that could harm others if used improperly.

Extending the range of the time projection Compton spectrometer to lower energy

The Time Projection Compton Spectrometer (TPCS) has previously been used with a 12.7- mu m-thick polypropylene target to measure the time-integrated energy spectrum of intense flash X-ray sources. In the present work, the authors investigated the application of the TPCS to photoelectron spectrometry, substituting a 2- mu m thick gold target to improve device sensitivity to low-energy photons. Experiments with a gold target, a polypropylene target, and no target (background) were done at the Saturn pulsed X-ray source for various endpoint bremsstrahlung up to about 2 MV. Response functions were calculated using the TIGERP electron/photon Monte Carlo code. The signal from gold is larger and persists longer, as expected. The spectrum unfolded from the gold signals has problems, however. Alternative approaches to the unfold problem that combine data from both targets may prove more successful. If so, a future spectrometer could be designed to incorporate both low-Z and high-Z targets in a single instrument. >