High-energy physics experiments sometimes require exceptionally large areas of photo detection. In many of these cases,
low noise and fast response make a photoelectric tube such as a photomultiplier the detector of choice. Hexagonal
photomultipliers have been made for a number of years to meet these requirements. This paper describes the design and
development of hexagonal image tubes to fulfill the need for improved spatial resolution combined with the ability to
closely pack detectors over a large working area.
The spectral transmission and electron emissivity responses, measured for a series of typical photocathodes, are
presented and analysed. Specifically, samples of S1, S20, S25, Bialkali and two types of solar-blind telluride
photocathodes were investigated in both transmission and reflection modes of operation. The transmission mode is more
convenient for imaging, night vision and for scintillation counting applications such as CT scanners and is more
commonly used than the reflection mode. However, more recent work has focussed on the reflection photocathode as a
source of electrons with low energy spread used for electron guns for microscopy and lithographic free electron lasers
. Our analysis provides a determination of the reflectivity of the substrate/cathode and cathode/vacuum interface,
enabling the refractive index to be deduced. The high apparent quantum efficiency (QE) of some conventional
photocathodes is shown to be due to the conversion of each photon to two or more electrons.
Currently, photo-cathodes hold the highest promise in the near term (next few years) of being able to detect low light level UV signals at high QE while being nearly blind to visible wavelengths. We briefly discuss the requirements for UV detection for astronomical applications, and then we describe our work on producing GaN based photo-cathodes. The p-type GaN films were grown on sapphire at Northwestern University. The films were then converted into opaque photo-cathodes inside photo-tubes at Hamamatsu. Hamamatsu tested detective quantum efficiencies (DQE) of these detectors to be as high as 30% at 200 nm. The ratio of peak DQE at 200 nm to the minimum DQE at 500 nm was measured to be about 6 X 103. We found a dramatic increase in the DQE at 200 nm versus the conductivity, with the break point being near 0.13 1/(Ohm-cm). Based on this dramatic increase, we believe that further improvement in photo-cathode quantum efficiencies can be achieved by increasing the conductivity. We have recently achieved more than an order of magnitude increase in conductivity by co-doping techniques. Improvements in the solar blindness of the devices depend both on characteristics of the film and its surface properties. A detailed discussion of decreasing the visible response and producing a sharper wave-length cutoff is beyond the scope of this work, but we briefly discuss the attributes that most likely affect the wavelength dependence of the photo-cathode response.
We have been testing the vacuum ultraviolet (UV) response of several types of detectors, supporting investigators developing photodiodes made of GaN, near UV photocathodes, and Electron-Bombarded CCDs (EBCCDs). We are currently supporting 4 independent research groups developing GaN most of which have produced devices with significant sensitivity down to 1200 Angstroms. Over the past year, we have also tested bare CCDs with coatings to enhance ultraviolet response. Detectors based on microchannel plate (MCP) have been used extensively for a wide variety of NASA mission and continue to be the standard to beat. Two particularly promising detector technologies are (1) EBCCDs which offer an immediate factor of 3 - 4 improvement in sensitivity and (2) devices made of GaN or GaAlN which may eventually offer factors of 6 - 8 increased sensitivity, both compared to MCPs for wavelengths between 1200 to 3000 Angstroms. We present latest results and plans to expand our vacuum UV testing.
The Space Telescope Imaging Spectrograph (STIS) is a versatile HST instrument covering the 115 - 1000 nm wavelength range in a variety of spectroscopic and imaging modes. Coverage of the ultraviolet range (115 - 310 nm) is provided by two Multi- Anode Microchannel Array (MAMA) detectors built by Ball Aerospace. The FUV MAMA covers the 115 - 170 nm range using an opaque CsI photocathode on the microchannel plate; the NUV MAMA covers the 165 - 310 nm range using a semi-transparent Cs2Te photocathode on the detector window. Both MAMAS utilize a 1024 X 1024 anode format, but detected photon events are positioned to half the spacing of the anode lines, leading to a 2048 X 2048 format for the final readout. The active area of each detector is 25.6 X 25.6 mm. Since the installation of STIS onto the Hubble Space Telescope (HST) in February 1997, the MAMAs have carried out a varied program of astronomical observing and in-flight calibration. The detectors have performed extremely well. In this report, we briefly describe the design of the STIS MAMA detectors, provide illustrative examples of their scientific use on HST, and summarize their technical performance in orbit, in such areas as sensitivity, resolution, flat-field uniformity and stability, signal-to-noise capability, dynamic range, and background.
Two multi-anode microchannel array (MAMA) detectors were fabricated at Ball Aerospace Technology Corporation for the Space Telescope Imaging Spectrograph (STIS) which was installed into the Hubble Space Telescope in February 1997. The photometric stability of the opaque CsI and semi- transparent Cs2Te sealed MAMA tubes has been characterized as a function of operating voltage and illumination conditions. A total exposure of 5 by 107 counts per pixel results in < 1 percent change in the detection quantum efficiency (DQE), which is attributed to conditioning of the microchannel plate (MCP) during tube processing. Employing good engineering practices to power supply design mitigates the effects of these components in long term detector stability. Other factors contributing to the photometric stability include the use of a curved channel MCP, photocathode processing, hydrocarbon free ultra high vacuum processing and sealed tube processing techniques. Contributions to the low resolution mode DQE stability are discussed along with empirical results on high resolution mode stability.
The Space Telescope Imaging Spectrograph (STIS) is a second- generation instrument for the Hubble Space Telescope (HST), designed to cover the 115-1000 nm wavelength range in a versatile array of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. STIS was successfully installed into HST in 1997 February and has since completed a year of orbital checkout, capabilities that it brings to HST, illustrate those capabilities with examples drawn from the first year of STIS observing, and describe at a top level the on-orbit performance of the STIS hardware. We also point the reader to related papers that describe particular aspects of the STIS design, performance, or scientific usage in more detail.
The space telescope imaging spectrograph (STIS) was designed as a versatile spectrograph capable of maintaining or exceeding the spectroscopic capabilities of both the Goddard High Resolution Spectrograph and the Faint Object Spectrograph (FOS) over the broad bandpass extending from the UV through the visible. STIS achieves performance gains over the aforementioned first generation Hubble Space Telescope instruments primarily through the use of large a real detectors in both the UV and visible regions of the spectrum. Simultaneous spatial and spectral coverage is provided through long slit or slitless spectroscopy. This paper will review the detector design and in-flight performance. Attention will be focussed on the key issue of S/N performance. Spectra obtained during the first few months of operation, illustrate that high signal-to-noise spectra can be obtained while exploiting STIS's multiplexing advantage. From analysis of a single spectrum of GD153, with counting statistics of approximately 165, a S/N of approximately 130 is achieved per spectral resolution element in the FUV. In the NUV a single spectrum of GRW + 70D5824, with counting statistics of approximately 200, yields a S/N of approximately 150 per spectral resolution element. An even higher S/N capability is illustrated through the use of the fixed pattern split slits in the medium resolution echelle modes where observations of BD28D42 yield a signal-to-noise of approximately 250 and approximately 350 per spectral resolution element in the FUV and NUV respectively.
AlGaN based interdigital metal-semiconductor-metal (MSM) photodetectors with 14 percent Al have been successfully grown and fabricate don sapphire substrates. The devices exhibit large gains up to 106 at high bias voltages, but with very high dark currents, > 1 mA and very long detector responses, > 60 seconds. A negative temperature coefficient for the breakdown voltage was observed indicating that tunneling is occurring. However, at high bias voltages, avalanche breakdown also appears to be present since a constant breakdown field of 105 V/cm was obtained independent of MSM geometry. Avalanche breakdown is nucleated at the non-uniform field distribution at the edge of the MSM finger.
The STIS instrument was installed into HST in February 1997 during the Servicing Mission 2. It has almost completed checkout and is beginning its science program, and is working well. Several scientific demonstration observations were taken to illustrate some of the range of scientific uses and modes of observation of STIS.
Large-format ultraviolet image sensors have been and are actively being developed for a variety of space-borne astronomy missions. The detector is historically one of the most problematic parts of any astronomical spacecraft and it plays a critical role in the overall capability of the instrument. There are numerous detector systems with one being ideal for all applications. This paper focuses on the physical processes responsible for the inherent strengths and weaknesses of a few important UV image sensors as well as the recent technological progress that has been mae to improve performance of these devices.
The Space Telescope Imaging Spectrograph (STIS), a next-generation instrument for the Hubble Space Telescope, has begun the fabrication of the flight units (plus spares) of the multi-anode microchannel array (MAMA) detectors. STIS will fly two MAMA detectors, one with a CsI photocathode covering the short-wavelength (1150-1750 angstrom) band and a second with Cs2Te covering the 1650-3100 angstrom band. Good tube yields continue to be realized with many of the MAMAs exceeding flight specifications by substantial amounts in key parameters. Evnironmental testing on the earlier engineering model units (EMUs) demonstrate the ruggedness of the tube design. We present performance results of the STIS flight MAMAs which have been fabricated to date. Life and other engineering tests on EMU detectors will be presented as well.
The space telescope imaging spectrograph (STIS), a next-generation instrument for the Hubble Space Telescope, has fabricated several engineering model units (EMUs) of the multi-anode microchannel array (MAMA) detectors. Good tube yields have been realized in producing these EMUs and some have performances suitable for flight. One of these EMU MAMAs has been operated for substantial periods of time after having undergone both shake and thermal environmental testing. A second will undergo similar environmental tests later this year. An earlier demonstration tube has been used extensively for over a year to evaluate STIS gratings in the Goddard Diffraction Grating Evaluation Facility. The STIS MAMA detectors have now matured to the point where half of the total test and evaluation effort is concerned with the characterization of subtle processes, a level of characterization needed to achieve data with a signal-to-noise ratio in excess of 100. We present test results from these EMUs including detailed analysis of data collected with vacuum chambers specifically designed for the evaluation of these detectors.
Curved-channel microchannel plate (C-plate) improvements resulting from an ongoing NASA STIS microchannel plate (MCP) development program are described. Performance limitations of previous C-plates led to a development program in support of the STIS MAMA UV photon counter, a second generation instrument on the Hubble Space Telescope. C-plate gain, quantum detection efficiency, dark noise, and imaging distortion, which are influenced by channel curvature non-uniformities, have all been improved through use of a new centrifuge fabrication technique. This technique will be described, along with efforts to improve older, more conventional shearing methods. Process optimization methods used to attain targeted C-plate performance goals will be briefly characterized. Newly developed diagnostic measurement techniques to study image distortion, gain uniformity, input bias angle, channel curvature, and ion feedback, will be described. Performance characteristics and initial test results of the improved C-plates will be reported. Future work and applications will also be discussed.