Roy R. Johnson

Large Format Si:As IBC Array Performance for NGST and Future IR Space Telescope Applications

A mid-infrared(5-30 micron) instrument aboard a cryogenic space telescope can have an enormous impact in resolving key questions in astronomy and cosmology. A space platforms greatly reduced thermal backgrounds (compared to airborne or ground-based platforms), allow for more sensitive observations of dusty young galaxies at high redshifts, star formation of solar-type stars in the local universe, and formation and evolution of planetary disks and systems. The previous generations largest, most sensitive infrared detectors at these wavelengths are 256 x 256 pixel Si:As impurity band conduction devices built by Raytheon Infrared Operations for the SIRTF/IRAC instrument. Raytheon has successfully enhanced these devices, increasing the pixel count by a factor of 16 while matching or exceeding SIRTF/IRAC device performance. NASA-Ames Research Center in collaboration with Raytheon has tested the first high performance large format (1024 x 1024) Si:As IBC arrays for low background applications, such as for the mid-IR instrument on NGST and future IR Explorer missions. These hybrid devices consist of radiation-hard SIRTF/IRAC-type Si:As IBC material mated to a readout multiplexer that has been specially processed for operation at low cryogenic temperatures (below 10 K), yielding high device sensitivity over a wavelength range of 5-28 microns. In this paper, we present laboratory test results from these benchmark devices. Continued development in this technology is essential for conducting large-area surveys of the local and early universe through observation and for complementing future missions such as NGST, TPF, and FIRST.

Radiation environment performance of JWST prototype FPAs

As the logical extension of the 20-year mission of the Hubble Space Telescope, NASA plans to launch the James Webb Space Telescope (JWST, formerly NGST) near the end of this decade. As Hubbles scientific and technological successor, equipped with a 6-meter-class deployable mirror, JWST will allow observations of the very early universe and initial formation of galaxies at levels not achievable today. JWSTs unprecedented sensitivity cannot be utilized without a new class of IR focal plane arrays whose performance matches that of the telescope. In particular, JWST focal planes must be able to withstand the ionizing-particle radiation environment expected for its Lagrange-point (L2) orbit and ten-year mission lifetime goal. To help determine their suitability for JWST, NASA is evaluating prototype megapixel-class readouts and hybrid detector arrays under proton bombardment to simulate the anticipated JWST lifetime radiation dose. This report describes the results of early tests on devices from two manufacturers using photovoltaic (HgCdTe or InSb) candidate near-infrared detector structures. Results to date have shown encouraging performance, along with some areas of continuing concern.

Radiation-induced transient effects in HgCdTe IR focal plane arrays

The operability requirements of NASAs James Webb Space Telescope (JWST) impose specific challenges on radiation effects mitigation and analysis. For example, the NIRSpec Instrument has the following requirements: •The percentage of pixels defined as operable for target acquisition shall not be less than 97% (TBR) (goal 99%) of the total number of pixels... An inoperable pixel is: ο A dead pixel: a pixel with no radiometric response o A noisy pixel: a pixel with a total noise greater than 21 e-, per Fowler 8 exposure •The percentage of pixels defined as operable for science observations shall not be less than 92% (TBR) (goal 98%) of the total number of pixels... An inoperable pixel is: ο A dead/low-DQE pixel: a pixel deviating by >30% from the DQE mean value ο A noisy pixel: a pixel with a total noise greater than 12 e- (goal 9e-). With these performance requirements and operation in space, the radiation environment from galactic cosmic rays (GCR), energetic solar particles, and activation of spacecraft materials can contribute significantly to the number of inoperable pixels. The two most important issues to date are radiation-induced transient effects and hot pixels. This paper focuses on the methods used to assess the impact of ionizing radiation induced transients on the HgCdTe SCA selected by JWST. Hot pixel effects in these detectors has been previously presented. Both effects are currently under investigation.

Si:As IBC IR focal plane arrays for ground-based and space-based astronomy

Raytheon/SBRC has demonstrated high quality Si:As IBC IR FPAs for both ground-based and space-based Mid-IR astronomy applications. These arrays offer in-band quantum efficiencies of approximately 50 percent over a wavelength range from 6 micrometers to 26 micrometers and usable responses from 2 micrometers to 28 micrometers . For high background, ground-based applications the readout input circuit is a direct injection (DI) FET, while for low background, space-based applications a source follower per detector (SFD) is used. The SFD offers extremely low noise and power dissipation, and is implemented in a very small unit cell. The DI input circuit offers much larger bucket capacity and better linearity compared with the SFD, and is implemented in a 50 micrometers unit cell. SBRCs Si:As IBC detector process results in very low dark current sand our Raytheon/MED readout process is optimized for very low redout noise at low temperature operation. SBRC is committed to achieving still better performance to serve the future needs of the IR astronomy community.

Proton-induced secondary particle environment for infrared sensor applications

We present measurements of the proton-induced secondary particle environment in the vicinity of an infrared focal plane array. Measurements were made of the energy depositions from secondary electrons and scattered protons from the interior of a cryogenic test dewar using an infrared detector array. The results are compared with model predictions and analyzed for implications to space-based infrared sensors.

Si:As IBC array performance for SIRTF/IRAC

The goal of achieving background-limited performance in SIRTFs cryogenic telescope environment places stringent demands on focal plane sensitivity. SIRTFs prime imaging instrument, the InfraRed Array Camera (IRAC), employs 256 X 256 Si:As Impurity-Band Conduction (IBC) arrays for its two longest wavelength channels at 5.8 micrometers and 8.0 micrometers . Background-limited performance is achieved at very low levels of zodiacal background radiation with cryo-optimized readout and detector technology from Raytheon. Presented here are performance measurements of IRAC flight candidate IBC arrays. Operating at a temperature of 6 K, these devices meet all IRAC sensitivity requirements, with dark currents well below the 10 e-/s specification, Fowler-sampled noise levels of 16 e-, and excellent photometric stability.

High-performance HgCdTe infrared detectors for the GOES long-wave sounder

GOES long wave sounder (LWS) detector requirements have always pushed the state-of-the-art for longwave detectors operating in the vicinity of 102 K. Performance and yield of acceptable detectors have always been problems and continue to be important issues affecting the performance of instruments of both present and future design. GSFC has been examining new device and operational concepts aimed at producing significant improvements in performance and yield. Our approach has been directed towards mitigating the deleterious effects of operating small geometry HgCdTe PC devices under heavy bias, that is, under minority carrier sweepout, as is typical in conventional LWS detector operation. Specifically, theory indicates that detectors of the new design operating under optimal bias conditions have significantly higher responsivity, lower power dissipation,and lower 1/f noise knees than conventional LWS detectors. In this paper we will describe the new LWS detectors fabricated at GSFC, present detector data, and review the theory of operation of these devices.

A highly stable spectrophotometric capability for the Origins Space Telescope (OST) mid-infrared imager, spectrometer, coronagraph (MISC)

This paper describes the transit spectrograph designed for the Origins Space Telescope mid-infrared imager, spectrometer, coronagraph (MISC) and its performance derived through analytical formulation and numerical simulation. The transit spectrograph is designed based on a densified pupil spectroscopy design that forms multiple independent spectra on the detector plane and minimizes the systematic noise in the optical system. This design can also block any thermal light incoming into pixels around the transit spectra. The gain fluctuations occurring in the detector and readout electronics are accurately corrected by use of a number of blanked-off pixels. We found that the transit spectrograph for the OST concept 1 with a diameter of 9.3m potentially achieves the photon-noise-limited performance and allows detection of biosignature gases through transmission spectroscopy of transiting planets orbiting late- and middle-M type stars at 10 pc with 60 transit observations.