Michael P. Dion

A Pair Production Telescope for Medium-Energy Gamma-Ray Polarimetry

abstract We describe the science motivation and development of a pair production telescope for medium-energy( 5–200 MeV) gamma-ray polarimetry. Our instrument concept, the Advanced Energetic Pair Telescope(AdEPT), takes advantage of the Three-Dimensional Track Imager, a low-density gaseous timeprojection chamber, to achieve angular resolution within a factor of two of the pair productionkinematics limit ( 0.6 at 70 MeV), continuum sensitivity comparable with the Fermi-LAT front detector( 10 GeV. However, neither instrument isoptimized for observations below 200 MeV or for polarizationsensitivity. Ground-based air Cherenkov telescopes have been usedto observe both galactic sources such as supernova remnants andextragalactic sources of very high energy (TeV) gamma-rays suchas active galactic nuclei (AGN) [3]. They have provided importantastrophysical information, but they also lack the capability todetect polarization. The Fermi and AGILE space-based telescopes,operating in the GeV energy range, are expected to continue tomake significant progress for the next several years. However,there remains a significant gap in our knowledge of astronomy inthe medium-energy ( 0.1–200 MeV) regime between the X-rayand high-energy gamma-ray energy ranges.The next major step in gamma-ray astrophysics, recognized asearly as the SAS-2 era [4], should be a medium-energy gamma-ray pair production telescope to fill this gap and provide answersto many important astrophysical questions. In the following, wedescribe the science motivation for this mission and the designof the Advanced Energetic Pair Telescope (AdEPT) a pair productiontelescope for medium-energy, 5to 200 MeV, gamma-raypolarimetry.2. Science motivationThe AdEPT pair production telescope for the detection of med-ium energy ( 5–200 MeV) gamma-rays with high angular resolu-tion and polarimetry capabilities will open a new window inobservational astronomy and astrophysics. Such an instrumentcan help provide answers to important questions in both astron-omy and physics. For example, it can shed light on the origin andacceleration of cosmic rays, the nature of the cosmic-ray accelera-tion of electrons in the Crab nebula to energies in excess of 10

High-Purity Germanium Spectroscopy at Rates in Excess of

In gamma spectroscopy, a compromise must be made between energy resolution and event-rate capability. Some foreseen nuclear material safeguards applications require a spectrometer with energy resolution typical of high purity germanium (HPGe) detectors, operated at event rates up to and exceeding 106 per second. We report the performance of an HPGe spectrometer system adapted to run under such conditions. Our system consists of a commercial semi-coaxial HPGe detector, a modified high-voltage-rail, resistive-feedback, charge-sensitive preamplifier and a continuous waveform digitizer. Digitized waveforms are analyzed offline with a novel time-variant trapezoidal filter algorithm. Several time-invariant trapezoidal filters are run in parallel and the slowest one not rejected by instantaneous pileup conditions is used to measure each pulse height. We have attained full-width-at-half-maximum energy resolution approximately 8 keV measured at 662 keV with 1.03 ×106 per second incoming event rate and 39% throughput. An additional constraint on the width of the fast trigger filter removes a significant amount of rising edge pileup that passes the first pileup cut, reducing throughput to 25%. While better resolution has been reported by other authors, our throughput is an order of magnitude higher than any other reported HPGe system operated at such an event rate.

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RadICalc was developed to address the need for a computer program that could calculate the composition, activity, and measurable radiation of arbitrary radionuclide mixtures over time without significant effort from end-users. It provides an interface to perform decay calculations and can search and display the resulting data in graphical or tabular form. RadICalc can also determine radiation expected at specific masses with user-defined molecules in addition to atomic species for use in mass-based isotope separations for radiometric counting applications, a novel method under development at Pacific Northwest National Laboratory.

Events/s

(241)Am has been deposited using a novel technique that employs a commercial inductively coupled plasma mass spectrometer. This work presents results of high-resolution alpha spectrometry on the (241)Am samples using a small area passivated implanted planar silicon detector. We have also investigated the mass-based separation capability by developing a (238)Pu sample, present as a minor constituent in a (244)Pu standard, and performed subsequent radiometric counting. With this new sample development method, the (241)Am samples achieved the intrinsic energy resolution of the detector used for these measurements. There was no detectable trace of any other isotopes contained in the (238)Pu implant demonstrating the mass-based separation (or enhancement) attainable with this technique.

RadICalc: a program for estimating radiation intensity of radionuclide mixtures

The effectiveness of conventional measurement techniques for environmental monitoring is limited by background and other interferences. We are exploring a new measurement approach involving the detection of α particles in coincidence with conversion electrons as a means to simultaneously assay environmental samples for multiple actinides without chemical separation. The initial target isotopes studied in this work are 238Pu, 239Pu, 240Pu and 241Am. We explore various aspects of the design, such as impact of the mounting of the source material, energy resolution requirements and impact of a background on isotopic uncertainties. We conclude that a dual gas-proportional counter and a dual-sided, large-area silicon detector could provide similar performance for the measurement scenario examined.

Alpha spectrometry applications with mass separated samples.

A novel, multi-point contact high-purity germanium detector has been developed for applications in high-rate gamma environments. The planar detector was fabricated with seven point contacts, a high-voltage steering grid and bias electrode using amorphous germanium technology. We have characterized this detector and report herein on the depletion profile, leakage current, energy resolution, and charge-sharing behavior.

Concepts for alpha coincidence detection

The high-energy side of peaks in alpha spectra, e.g. 241Am, as measured with a silicon detector has structure caused mainly by alpha-conversion electron and to some extent alpha-gamma coincidences. We compare GEANT4 simulation results to 241Am alpha spectroscopy measurements with a passivated implanted planar silicon detector. A discrepancy between the measurements and simulations suggest that the GEANT4 photon evaporation database for 237Np (daughter of 241Am decay) does not accurately describe the conversion electron spectrum and therefore was found to have discrepancies with experimental measurements. We describe how to improve the agreement between GEANT4 and alpha spectroscopy for actinides of interest by including experimental measurements of conversion electron spectroscopy into the photon evaporation database.

A Multi-Point Contact HPGe Detector

In recent research to compare detection sensitivities of gamma spectrometers applied to in situ and field laboratory scenarios, the authors lacked background data for comparable detection sensitivity calculations. To overcome this, experimental measurements and Monte Carlo modeling of terrestrial gamma radiation were undertaken. Inspired by Vojtyla’s research to reduce the computing burden of modeling bremsstrahlung from lead surfaces, this related approach defines a gamma-ray surface source representing emission of background gamma-rays from the earth. This work presents a surface source based on the 40K, 137Cs, uranium, and thorium content of Hanford soil, and compares modeled backgrounds to experimental in situ gamma measurements.

Alpha coincidence spectroscopy studied with GEANT4

While I appreciate the comments presented by Dr. Garcı́a-Toraño it is inaccurate to state that gross differences in alpha sample preparation techniques are only observable in high resolution systems as this work (and others) has shown that differences in sample development can be discriminated with large area, low-resolution detectors. Furthermore, it was directly stated that future research would be conducted with high-resolution silicon based alpha spectroscopy. Pertaining to the research under question, we compared two ‘‘standard’’ sample preparation techniques to a new and exciting method using available large area detectors maintained in our count lab. This allowed publication in a timely fashion while, in parallel, a high resolution system was developed. We do not have the capability of vacuum evaporation or other recent sample development methods. And in fact, the author/s never concluded that the ICP-MS samples would be superior to the aforementioned techniques, only that there was a repeatable observed improvement in energy resolution when compared to two other standard techniques.

Streamlined Monte Carlo simulation of environmental gamma-ray backgrounds for radiation detector sensitivity comparisons

Interferences in both decay counting and mass counting techniques limit their application for some environmental monitoring applications. For example, 238U interferes with 238Pu in mass spectrometry measurements, while in conventional alpha spectroscopy measurements it is nearly impossible to separate 238Pu from 241Am and 239Pu from 240Pu. These interferences are typically resolved by using chemical separation and/or different measurement techniques for different isotopes. We are investigating radiation detector concepts to simultaneously assay these four isotopes with minimal sample preparation by exploiting radiation signatures measured in coincidence with the predominate alpha decays of these isotopes. Particles in coincidence with the alpha decay include conversion electrons, gamma rays, x-rays, and Auger electrons. Each decay has a unique energy distribution enabling the separation of the isotopes. We are exploring two basic detector concepts to achieve these goals: a silicon-based design and a gas-detector design. The silicon system provides the potential for higher energy resolution at the cost of lower efficiency compared to a gas detector. In this paper, we will describe our evaluation of the different detector concepts, which will include estimations of potential detection efficiency, ability to resolve the isotopes, sample preparation and equipment requirements.