T. Gregory Guzik

Large-area submillimeter resolution CdZnTe strip detector for astronomy

We report the first performance measurements of a sub-millimeter CdZnTe strip detector developed as a prototype for space-borne astronomical instruments. Strip detector arrays can be used to provide two-dimensional position resolution with fewer electronic channels than pixellated arrays. Arrays of this type and other candidate technologies are under investigation for the position-sensitive backplane detector for a coded-aperture telescope operating in the range of 30 - 300 keV. The prototype is a 1.4 mm thick, 64 multiplied by 64 stripe CdZnTe array of 0.375 mm pitch in both dimensions, approximately one square inch of sensitive area. Pulse height spectra in both single and orthogonal stripe coincidence mode were recorded at several energies. The results are compared to slab- and pixel-geometry detector spectra. The room-temperature energy resolution is less than 10 keV (FWHM) for 122 keV photons with a peak-to-valley ratio greater than 5:1. The response to photons with energies up to 662 keV appears to be considerably improved relative to that of previously reported slab and pixel detectors. We also show that strip detectors can yield spatial and energy resolutions similar to those of pixellated arrays with the same dimensions. Electrostatic effects on the pulse heights, read-out circuit complexity, and issues related to design of space borne instruments are also discussed.

Energetic helium isotopes trapped in the magnetosphere

Helium nuclei have been measured within the magnetosphere (L ≤ 6) during the 1990/1991 CRRES mission over an energy range of ∼40–100 MeV nucleonl−1. The Office of Naval Research 604 instrument resolves helium isotopes with a mass resolution of ∼0.1 amu. Each helium isotope can be represented by a power law energy spectrum, and the energy spectrum of the 3He is different from that of 4He; that is, the 3He/4He ratio is energy dependent. In the energy range 51–86 MeV/nucleon the 3He/4He ratio is 7.4 ± 2.6 for L = 1.15–1.3 and 2.2±0.6 for L = 1.8–2.15. All observed helium events at L < 2.65 show pitch angle distributions consistent with equatorially trapped ions, whereas the distributions at larger L values are more uniform, within a factor of 2. For 2.15 < L < 6, as L decreases, the 3He/4He ratio increases and the helium energy spectrum softens. A possible origin of the geomagnetically trapped helium isotopes at L <2.15 is proton interactions in the residual atmosphere. The CRRES satellite was operational during the large geomagnetic storm on March 24, 1991, and we have divided the analysis into prestorm and poststorm time periods. Following the storm, the magnetosphere became more dynamic, and at L ∼ 2.3 an enhanced helium flux was observed. Over the energy range of 51–86 MeV nucleonl−1 both the average 3He and 4He fluxes at L = 2.15–2.65 increased by a factor of 6 after the storm relative to the prestorm period, while at L = 1.8–2.15 the average 3He and 4He fluxes decreased by factors of 3 and 11, respectively, following the storm. The implications of these observed results are discussed.

Energetic helium particles trapped in the magnetosphere

High energy (approximately 40-100 MeV/nucleon) geomagnetically trapped helium nuclei have been measured, for the first time, by the ONR-604 instrument during the 1990/1991 Combined Release and Radiation Effects Satellite (CRRES) mission. The helium events observed at L less than 2.3 have a pitch angle distribution peaking perpendicular to the local magnetic field and are contained in peaks located at L = 1.2 and 1.9. The events in each peak can be characterized by power law energy spectra with indices of 10.0 +/- 0.7 for L = 1.9-2.3 and 6.8 +/- 1.0 for L = 1.15-1.3, before the large storm of 24 March 1991. CRRES was active during solar maximum when the anomalous component is excluded from the inner heliosphere, making it unlikely that the observed events derived from the anomalous component. The trapped helium counting rates decrease gradually with time indicating that these high energy ions were not injected by flares during the 1990/91 mission. Flare injection prior to mid-1990 may account for the highest energy particles, while solar wind injection during magnetic storms and subsequent acceleration could account for the helium at lower energies.

Advanced Thin Ionization Calorimeter (ATIC) balloon experiment: expected performance

An advanced thin ionization calorimeter (ATIC) will be used to investigate the charge composition and energy spectra of ultrahigh energy primary cosmic rays in a series of long- duration balloon flights. While obtaining new high priority scientific results, this balloon payload can also serve as a proof of concept for a BGO calorimeter-based instrument on the International Space Station. The ATIC technical details are presented in a companion paper at this conference. Here we discuss the expected performance of the instrument based on a GEANT code developed for simulating nuclear- electromagnetic cascades initiated by protons. For simulations of helium and heavy nuclei, a nucleus-nucleus interaction event generator LUCIAE was linked to the GEANT based program. Using these models, the design of the ATIC detector system has been optimized by simulating the instrument response to particles of different charges over the energy range to be covered. Results of these simulations are presented and discussed.

The He-3/He-4 ratios for solar energetic particle events during the Combined Release and Radiation Effects Satellite Mission

Helium data measured by the University of Chicago instrument, ONR-604, are employed to determine the ratio of He-3 to He-4 for solar energetic particle (SEP) events over an energy range 50-110 MeV/nucleon during the 1990/1991 Combined Release and Radiation Effects Satellite mission. The Sun in this period is extremely active. A total of 29 separate SEP events have been identified; among them 16 major events have been analyzed to obtain He-3/He-4 ratios, with a mass resolution of 0.10 amu. Thirteen events have a He-3/He-4 ratio larger than 0.005, one order of magnitude greater than the solar coronal value. The He-3/He-4 ratio at energies of 50-110 MeV/nucleon is independent of the size of the SEP event, for the moderately large flares analyzed here. The helium energy spectra are represented by power laws. During the 1991 June flare period, different large-particle injections associated with different solar flares, but occurring from the same active region, have a similar average spectral index and a similar He-3/He-4 ratio. The spectral index of He-4 varies from event to event, i.e., from as small as 1.5 to as large as 7.5. A correlation is found between the inferred spectral index from gamma-ray measurements and our measured spectral indices for the 1991 June 11 and June 15 events, suggesting that the high-energy SEPs may come from the same acceleration event as the particles that interact at the Sun and produce the gamma rays. The implications of these results for particle acceleration and propagation at the flare site and in the solar corona are discussed.

TETRA observation of gamma-rays at ground level associated with nearby thunderstorms.

[1] Terrestrial gamma-ray flashes (TGFs)-very short, intense bursts of electrons, positrons, and energetic photons originating from terrestrial thunderstorms-have been detected with satellite instruments. TGF and Energetic Thunderstorm Rooftop Array (TETRA), an array of NaI(Tl) scintillators at Louisiana State University, has now been used to detect similar bursts of 50 keV to over 2 MeV gamma-rays at ground level. After 2.6 years of observation, 24 events with durations 0.02-4.2 ms have been detected associated with nearby lightning, three of them coincident events observed by detectors separated by ∼1000 m. Nine of the events occurred within 6 ms and 5 km of negative polarity cloud-to-ground lightning strokes with measured currents in excess of 20 kA. The events reported here constitute the first catalog of TGFs observed at ground level in close proximity to the acceleration site.

Balloon-borne coded aperture telescope for arc-minute angular resolution at hard x-ray energies

We are working on the development of a new balloon-borne telescope, MARGIE (minute-of-arc resolution gamma ray imaging experiment). It will be a coded aperture telescope designed to image hard x-rays (in various configurations) over the 20 - 600 keV range with an angular resolution approaching one arc minute. MARGIE will use one (or both) of two different detection plane technologies, each of which is capable of providing event locations with sub-mm accuracies. One such technology involves the use of cadmium zinc telluride (CZT) strip detectors. We have successfully completed a series of laboratory measurements using a prototype CZT detector with 375 micron pitch. Spatial location accuracies of better than 375 microns have been demonstrated. A second type of detection plane would be based on CsI microfiber arrays coupled to a large area silicon CCD readout array. This approach would provide spatial resolutions comparable to that of the CZT prototype. In one possible configuration, the coded mask would be 0.5 mm thick tungsten, with 0.5 mm pixels at a distance of 1.5 m from the central detector giving an angular resolution of 1 arc-minute and a fully coded field of view of 12 degrees. We review the capabilities of the MARGIE telescope and report on the status of our development efforts and our plans for a first balloon flight.

Geomagnetically trapped energetic helium nuclei

Geomagnetically trapped helium nuclei, at high energy (∼40–100 MeV/nucleon), have been measured by the ONR‐604 instrument during the 1990/1991 CRRES mission. The ONR‐604 instrument resolved the isotopes of helium with a mass resolution of 0.1 amu. The energetic helium observed at L<2.3 have a pitch angle distribution peaking perpendicular to the local magnetic field, which is characteristic of a trapped population. Both the trapped 3He and 4He show two peaks at L=1.2 and 1.9. Each isotope’s flux, in each peak, can be characterized by a power law energy spectrum. The energy spectrum of the 3He is different from that of 4He, indicating that the 3He/4He ratio is energy dependent. Over the energy range of 51–86 MeV/nucleon, the 3He/4He ratio is 8.7±3.1 at L=1.1–1.5 and is 2.4±0.6 at L=1.5–2.3. The trapped helium counting rates decrease gradually with time during the CRRES mission, when the anomalous component is excluded from the inner heliosphere, indicating that these high energy ions were not injected by f...

Coded-aperture x-ray/gamma-ray telescope for arc- minute localization of gamma-ray bursts

MARGIE will be a large-area, wide field-of-view, hard x- ray/gamma-ray imaging telescope capable of providing accurate positions for faint gamma-ray bursts in near-real- time and of performing a sensitive survey of both steady and transient cosmic sources. The instrument is designed to image faint bursts at the low-intensity end of the log N - log S distribution. MARGIE was recently selected by NASA for a mission-concept study for an Ultra Long Duration Balloon flight. We describe a program to develop an instrument based on the new detector technology of either cadmium zinc telluride room-temperature semiconductors or pixelated cesium iodide scintillators viewed by fast timing charge- coupled devices.

The High Altitude Student Platform (HASP) as a model multi-payload balloon platform

One of the primary advantages of balloon flights is that payloads can be exposed to a near-space environment for an extended period of time, recovered, revised and then flown again. Thus, instrument and satellite component designs can be tested and refined without the need for orbital launches. This not only reduces development costs but can also reduce the time necessary to increase the technology readiness level (TRL) of a particular component. Over the last several years, interest in developing miniaturized satellites, with low mass, power and telemetry requirements, has significantly increased. An inexpensive method for testing and validation of “nanosats” components, or full systems, would be to fly them on a balloon platform at 120,000 feet. These systems could be clustered onto a single balloon payload carrier that provides standardized power, telemetry and a physical interface for each experiment. Such an approach reduces the payload development overburden, so the investigator can focus exclusively on experiment, or subsystem, development and, consequently, can reduce experiment cost and improve turn-around time. Here we report on our experience with the High Altitude Student Platform (HASP), the first balloon carrier specifically designed with a standard interface to support up to 12 independent experiments per flight. Since 2006, HASP has flown nine times from Ft. Sumner, New Mexico and carried close to eighty (77) experiments to an altitude of ~120,000 feet for an average duration of 14 hours at float. We will discuss the HASP system design, development and capabilities, the kinds of experiments that have flown on HASP, and the lessons-learned that are applicable to future multiple payload balloon platforms.