NeXT Mission Research Articles

Stardust–NExT NAVCAM calibration and performance

NASA’s Stardust–NExT mission used the Stardust spacecraft to deliver a scientific payload, including a panchromatic visible camera designated NAVCAM, to a close flyby of Comet 9P/Tempel 1 in February 2011. Proper interpretation of the NAVCAM images depends on accurate calibration of the camera performance. While the NAVCAM had been calibrated during the primary Stardust mission to Comet 81P/Wild 2 in 2004, that calibration was incomplete and somewhat lacking in fidelity. Substantial improvements in the NAVCAM calibration were achieved during Stardust–NExT in the areas of geometric correction, spatial resolution, and radiometric calibration (in particular, zero-exposure signal levels, shutter time offsets, absolute radiometric response, noise, and scattered light characterization). These improvements will allow upgrades to the calibration of images returned from the Stardust primary mission as well as high-quality calibration of the Stardust–NExT images. The upgraded calibration results have been incorporated into the Stardust–NExT image processing pipeline via new routines and updated constants and files in preparation for archiving calibrated images in the NASA Planetary Data System.

Performance study of Si/CdTe semiconductor Compton telescopes with Monte Carlo simulation

A Compton telescope with high angular resolution and high energy resolution is a promising detector for the next generation of astrophysics space missions aiming at hard X-rays and sub-MeV/MeV gamma-rays. We have been working on a semiconductor Compton camera based on silicon and cadmium telluride (Si/CdTe Compton telescope). The soft gamma-ray detector (SGD) employs a Si/CdTe Compton camera combined with a well-type active shield. It will be mounted on the NeXT mission, proposed to be launched around 2012. One Compton camera module in the SGD will consist of 24 layers of double-sided silicon strip detectors and four layers of CdTe pixel detectors. We carried out Monte Carlo simulations to investigate the basic performance of the detector. Design parameters of devices required in the simulation, such as energy resolution and position resolution of the detector, are based on the results from our prototype detector. From the simulation using current design parameters, the detection efficiency is found to be higher than 10% at ∼ 100 keV and the angular resolution to be 9° and 4.4° at 120 keV and 330 keV, respectively. The effects of changing the design parameters are also discussed.

Design and performance of the soft gamma-ray detector for the NeXT mission

The soft gamma-ray detector (SGD) onboard the Japanese future high energy astrophysics mission (NeXT) is a Compton telescope with narrow field of view, which utilizes Compton kinematics to enhance its background rejection capabilities. It is realized as a hybrid semiconductor gamma-ray detector which consists of silicon and cadmium telluride (CdTe) detectors. It can detect photons in a wide energy band (0.05-1 MeV) at a background level of 5 times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> counts/s/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /keV; the silicon layers are required to improve the performance at a lower energy band (<0.3 MeV). Excellent energy resolution is the key feature of the SGD, allowing it to achieve both high angular resolution and good background rejection capability. An additional capability of the SGD, its ability to measure gamma-ray polarization, opens up a new window to study properties of astronomical objects. We will present the development of key technologies to realize the SGD: high quality CdTe, low noise front-end application-specific integrated circuit, and bump bonding technology. Energy resolutions of 1.7 keV (full-width at half-maximum) for CdTe pixel detectors and 1.1 keV for Si strip detectors have been measured. We also present the validation of Monte Carlo simulation used to evaluate the performance of the SGD

Application of CdTe for the NeXT mission

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and γ -ray detection. The high-atomic number of the materials ( Z Cd = 48 , Z Te = 52 ) gives a high quantum efficiency in comparison with Si. The large band-gap energy ( E g = 1.5 eV ) allows to operate the detector at room temperature. Based on recent achievements in high-resolution CdTe detectors, in the technology of ASICs and in bump-bonding, we have proposed the novel hard X-ray and γ -ray detectors for the NeXT mission in Japan. The high-energy response of the super mirror onboard NeXT will enable us to perform the first sensitive imaging observations up to 80 keV. The focal plane detector, which combines a fully depleted X-ray CCD and a pixellated CdTe detector, will provide spectra and images in the wide energy range from 0.5 to 80 keV. In the soft γ -ray band up to ∼ 1 MeV , a narrow field-of-view Compton γ -ray telescope utilizing several tens of layers of thin Si or CdTe detector will provide precise spectra with much higher sensitivity than present instruments. The continuum sensitivity will reach several × 10 - 8 photons - 1 keV - 1 cm - 2 in the hard X-ray region and a few × 10 - 7 photons - 1 keV - 1 cm - 2 in the soft γ -ray region.