We present on the detector characterization activities and flight results for fine-pitch (60um) cadmium telluride (CdTe) strip detectors designed for the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment. FOXSI, optimized for observations in the range 4-20 keV, is the first solar-dedicated instrument to utilize the technology of focusing optics for observing the Sun in hard X-rays. Each of the seven FOXSI optics modules is paired with a semiconductor strip detector from which the energy and position of each incoming photon can be derived. While the first FOXSI experiment (FOXSI-1) flew only silicon (Si) detectors, both of the next two FOXSI sounding rocket experiments (FOXSI-2 and FOXSI-3) upgraded some of the detectors to CdTe (60um pitch) for enhanced efficiency at energies >10 keV. Here we present the measurements and analysis performed to characterize components of the CdTe detector response for FOXSI-2 and FOXSI-3, including the gain, energy resolution, and efficiency. Additionally, we explore the effects of charge sharing for these fine-pitch detectors and describe how these effects are accounted for in our calibration and data analysis. Results from spectral analysis of a solar microflare using CdTe data from the FOXSI-2 flight will be shown.
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment demonstrates the technique of focusing hard X-ray (HXR) optics for the study of fundamental questions about the high-energy Sun. Solar HXRs provide one of the most direct diagnostics of accelerated electrons and the impulsive heating of the solar corona. Previous solar missions have been limited in sensitivity and dynamic range by the use of indirect imaging, but technological advances now make direct focusing accessible in the HXR regime, and the FOXSI rocket experiment optimizes HXR focusing telescopes for the unique scientific requirements of the Sun. FOXSI has completed three successful flights between 2012 and 2018. This paper gives a brief overview of the experiment, focusing on the third flight of the instrument on 2018 Sept. 7. We present the telescope upgrades highlighting our work to understand and reduce the effects of singly reflected X-rays and show early science results obtained during FOXSI's third flight.
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment aims to investigate fundamental questions about the high-energy Sun through direct imaging and spectroscopy of hard X-rays. The experiment utilizes Wolter-I type nested hard X-ray mirrors and fine-pitch semiconductor detectors, which are separated by a 2m focal length. Tol date, FOXSI has had two successful flights, on 2012 November 02 and 2014 December 11, demonstrating that the technology can measure small-scale energy releases (microflares and aggregated nanoflares) from the solar corona. The third flight for FOXSI is scheduled for August 2018. Significant improvements have been made on the FOXSI instrumentation, including upgraded optic modules with more nested mirror shells; specially designed collimators to mitigate the number of single bounce photons (ie., ghost rays) reaching the focal plane detector; and fine-pitch double-sided CdTe strip detectors to replace some of the Si-based hard X-ray detectors for better efficiency for hard X-rays. Furthermore, a CMOS based soft X-ray (SXR) instrument, “Phoenix”, will be added to FOXSI-3 by replacing one hard X-ray detector with a photon-counting SXR sensor. This will enable evaluation of the Sun via imaging spectroscopy simultaneously over a large X-ray energy range covering soft to hard X-rays. This paper will describe the overall instrument design of the FOXSI-3 experiment, which will be sensitive to solar soft and hard X-rays in the 1 – 20 keV range, as well as give a summary of insightful results and lessons from the first two flights. Possible observations for FOXSI-3 will also be discussed.