Paul L. Hink

In-Flight Performance of the High Energy X-Ray Timing Experiment on the Rossi X-Ray Timing Explorer

The High Energy X-Ray Timing Experiment (HEXTE) is one of three scientific instruments aboard the Rossi X-Ray Timing Explorer (RXTE), which was launched on 1995 December 30. RXTE performs timing and spectral studies of bright X-ray sources to determine the physical parameters of these systems. The HEXTE consists of two independent clusters of detectors, each cluster containing four NaI(Tl)/CsI(Na) phoswich scintillation counters sharing a common 1° FWHM field of view. The field of view of each cluster is switched on and off source to provide near real-time background measurements. The net open area of the eight detectors is 1600 cm2, and each detector covers the energy range 15-250 keV with an average energy resolution of 15.4% at 60 keV. The in-flight performance of the HEXTE is described, the light curve and spectrum of the Crab Nebula/pulsar is given, and the 15-240 keV spectrum of the weak source, active galaxy MCG +8-11-11 is presented to demonstrate the weak source spectral capabilities of HEXTE.

MEASUREMENT OF THE SECONDARY RADIONUCLIDES 10Be, 26Al, 36Cl, 54Mn, AND 14C AND IMPLICATIONS FOR THE GALACTIC COSMIC-RAY AGE

We report on abundance measurements of ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn in the Galactic cosmic rays (GCRs) using the Cosmic-Ray Isotope Spectrometer (CRIS) instrument aboard the Advanced Composition Explorer spacecraft at energies from ~70 to ~400 MeV nucleon^(-1). We also report an upper limit on the abundance of GCR ^(14)C. The high statistical significance of these measurements allows the energy dependence of their relative abundances to be studied. A steady-state, leaky-box propagation model, incorporating observations of the local interstellar medium (ISM) composition and density and recent partial fragmentation cross section measurements, is used to interpret these abundances. Using this model, the individual galactic confinement times derived using data for each species are consistent with a unique confinement time value of τ_(esc) = 15.0 ± 1.6 Myr. The CRIS abundance measurements are consistent with propagation through an average ISM hydrogen number density n_H = 0.34 ± 0.04 H atoms cm^(-3). The surviving fractions, f, for each radioactive species have been calculated. From predictions of the diffusion models of Ptuskin & Soutoul, the values of f indicate an interstellar cosmic-ray diffusion coefficient of D = (3.5 ± 2.0) × 10^(28) cm^2 s^(-1).

CONSTRAINTS ON THE TIME DELAY BETWEEN NUCLEOSYNTHESIS AND COSMIC-RAY ACCELERATION FROM OBSERVATIONS OF 59 Ni AND 59 Co

Measurements of the abundances of cosmic-ray ^(59)Ni and ^(59)Co are reported from the Cosmic-Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer. These nuclides form a parent-daughter pair in a radioactive decay which can occur only by electron capture. This decay cannot occur once the nuclei are accelerated to high energies and stripped of their electrons. The CRIS data indicate that the decay of ^(59)Ni to ^(59)Co has occurred, leading to the conclusion that a time longer than the 7.6 × 10^4 yr half-life of ^(59)Ni elapsed before the particles were accelerated. Such long delays indicate the acceleration of old, stellar or interstellar material rather than fresh supernova ejecta. For cosmic-ray source material to have the composition of supernova ejecta would require that these ejecta not undergo significant mixing with normal interstellar gas before ~10^5 yr has elapsed.

On the Low Energy Decrease in Galactic Cosmic Ray Secondary/Primary Ratios

Galactic cosmic ray (GCR) secondary/primary ratios such as B/C and (Sc+Ti+V)/Fe are commonly used to determine the mean amount of interstellar material through which cosmic rays travel before escaping from the Galaxy (Λ_(esc)). These ratios are observed to be energy-dependent, with a relative maximum at ~1 GeV/nucleon, implying a corresponding peak in Λ_(esc). The decrease in Λ_(esc) at energies above 1 GeV/nucleon is commonly taken to indicate that higher energy cosmic rays escape more easily from the Galaxy. The decrease in Λ_(esc) at energies <1 GeV/nuc is more controversial; suggested possibilities include the effects of a galactic wind or the effects of distributed acceleration of cosmic rays as they pass through the interstellar medium. We consider two possible explanations for the low energy decrease in Λ_(esc) and attempt to fit the combined, high-resolution measurements of secondary/primary ratios from ~0.1 to 35 GeV/nuc made with the CRIS instrument on ACE and the C2 experiment on HEAO-3. The first possibility, which hypothesizes an additional, local component of low-energy cosmic rays that has passed through very little material, is found to have difficulty simultaneously accounting for the abundance of both B and the Fe-secondaries. The second possibility, suggested by Soutoul and Ptuskin, involves a new form for Λ_(esc) motivated by their diffusion-convection model of cosmic rays in the Galaxy. Their suggested form for Λ_(esc)(E) is found to provide an excellent fit to the combined ACE and HEAO data sets.

Observations of the Li, Be, and B isotopes and constraints on cosmic-ray propagation

The abundance of Li, Be, and B isotopes in galactic cosmic rays (GCRs) between E = 50 and 200 MeV/nucleon has been observed by the Cosmic Ray Isotope Spectrometer (CRIS) on NASAs ACE mission since 1997 with high statistical accuracy. Precise observations of Li, Be, and B can be used to constrain GCR propagation models. We find that a diffusive reacceleration model with parameters that best match CRIS results (e.g., B/C, Li/C, etc.) are also consistent with other GCR observations. A ˜15-20% overproduction of Li and Be in the model predictions is attributed to uncertainties in the production cross-section data. The latter becomes a significant limitation to the study of rare GCR species that are generated predominantly via spallation.

THE ORIGIN OF PRIMARY COSMIC RAYS: CONSTRAINTS FROM ACE ELEMENTAL AND ISOTOPIC COMPOSITION OBSERVATIONS

Cosmic-ray isotope observations from NASA’s Advanced Composition Explorer (ACE) mission have been used to investigate the composition of cosmic-ray source material. Source abundances relative to 56Fe are reported for eleven isotopes of Ca, Fe, Co, and Ni, including the very rare isotopes 48Ca and 64Ni. Although the source abundances range over a factor ∼104, most of the ratios to 56Fe are consistent with solar-system values to within ∼20%. However, there are some notable differences, the most significant being an excess of ∼(70±30)% relative to the solar system for the cosmic-ray source ratio 58Fe/56Fe. The possible association of such an excess with a contribution to the cosmic-ray source from Wolf–Rayet star ejecta is discussed.

CdZnTe arrays for astrophysics applications

The scientific objectives, status, and future instrumental requirements of high energy X-ray astronomy (20 to 200 keV) are discussed. Two particularly compelling requirements are: (1) an improvement in sensitivity to a level of about 5 microCrab and (2) a survey of the sky at a sensitivity of about 0.1 milliCrab, which will discover and characterize about 10,000 new sources. The first requirement can be fulfilled by imaging telescopes that use large-area focusing X-ray mirrors, which are effective over 5-30 arcminute fields, and the second requirement can be met by arrays of large area coded mask imagers with wide fields, about 50 deg. Multilayer mirror and CdZnTe detector technology now in development offers the potential to meet these objectives. Position-sensitive CdZnTe detectors are well-suited to both of these imaging techniques, and instrument concepts that use these detectors are described. Detectors with pixel readout are better suited for focusing telescopes, and those with crossed-strip readout are better suited for coded mask imagers. Technical aspects of these detectors are discussed. Recent work at UCSD and WU on CdZnTe strip detectors is described in detail. Studies with small, 40 micron, X-ray beams have mapped a crossed-strip detectors spatial response with fine spatial resolution.

Scintillating optical fiber trajectory detectors

Abstract Measurements of attenuation in several types of plastic scintillating optical fibers give attenuation lengths varying from 0.8 to 1.5 m. By comparing attenuation as a function of wavelength in fibers of different thicknesses we infer the contributions to the attenuation from reflection losses and bulk scintillation losses. We find good agreement between these values and calculated estimates of attenuation in scintillator. We have also calculated the effective scintillation efficiency of small fibers relative to that of bulk scintillator (for scintillator with dimethyl POPOP as the waveshifting dye) for the two cases of optically coupled and decoupled fibers. Scintillating fiber ribbons made of 200 μm square cross section fibers were exposed to relativistic iron nuclei at the LBL Bevalac, and positional resolution of 70 μm was obtained. Relativistic neon and carbon were also detected in these ribbons. In a similar exposure of 100 μm fibers to 50 MeV/n nitrogen nuclei at the NSCL cyclotron, Michigan State University, a positional resolution of about 50 μm was obtained.

Solar minimum spectra of galactic cosmic rays and their implications for models of the near-earth radiation environment

The radiation dose from galactic cosmic rays during a manned mission to Mars is expected to be comparable to the allowable limit for space shuttle astronauts. Most of this dose would be due to galactic cosmic rays with energies < 1 GeV nucleon^(−1), with important contributions from heavy nuclei in spite of their low abundance relative to H and He. Using instruments on NASAs Advanced Composition Explorer (ACE) spacecraft, we have made the most statistically precise measurements to date of the solar minimum energy spectra of cosmic ray nuclei with charge Z = 4–28 in the energy range ∼ 40–500 MeV nucleon^(−1). We compare these measurements obtained during the 1997–1998 solar minimum period with measurements from previous solar minima and with models of the near-Earth radiation environment currently used to perform shielding and dose calculations. We find that the cosmic ray heavy-element spectra measured by ACE are as much as 20% higher than previously published solar minimum measurements. We also find significant differences between the ACE measurements and the predictions of available models of the near-Earth radiation environment, suggesting that these models need revision. We describe a cosmic ray interstellar propagation and solar modulation model that provides an improved fit to the ACE measurements compared to radiation environment models currently in use.

High-altitude balloon flight of CdZnTe detectors for high-energy x-ray astronomy: II

Cadmium Zinc Telluride (CZT) is a room temperature semiconductor detector well suited for high energy x-ray astronomy. We have developed a CZT detector with 500 micron crossed strip readout and an advanced electrode design that greatly improves energy resolution. We conducted two balloon flights from Fort Sumner, NM, to study the cross strip detector and a standard planar detector both sensitive in the energy range of 20-350 keV. The flights utilized a total of seven shielding schemes: 3 passive, 2 active and 2 hybrid passive-active. In the active shielding modes, the anti- coincidence shield pulse heights were telemetered for each CZT event, allowing us to study the effect of the shields energy threshold on the spectral shape and magnitude of the background. We are also developing an energy-dependent background rejection technique based on the charge collection properties of the CZT detector. This technique employs the depth of interaction, as inferred by the ratio of cathode to anode pulse height, to reject events inconsistent with incident source x-rays. The long duration of the May flight enabled us to study activation effects. We present result of the effectiveness of each of the shielding schemes on both detectors, the rejection power of depth of interaction technique on the crossed strip detector, inferred aperture background flux and the level of activation after 22 hours as float.