Daniel Maier

Modeling of proton-induced radioactivation background in hard X-ray telescopes: Geant4-based simulation and its demonstration by Hitomi ’s measurement in a low Earth orbit

Abstract Hard X-ray astronomical observatories in orbit suffer from a significant amount of background due to radioactivation induced by cosmic-ray protons and/or geomagnetically trapped protons. Within the framework of a full Monte Carlo simulation, we present modeling of in-orbit instrumental background which is dominated by radioactivation. To reduce the computation time required by straightforward simulations of delayed emissions from activated isotopes, we insert a semi-analytical calculation that converts production probabilities of radioactive isotopes by interaction of the primary protons into decay rates at measurement time of all secondary isotopes. Therefore, our simulation method is separated into three steps: (1) simulation of isotope production, (2) semi-analytical conversion to decay rates, and (3) simulation of decays of the isotopes at measurement time. This method is verified by a simple setup that has a CdTe semiconductor detector, and shows a 100-fold improvement in efficiency over the straightforward simulation. To demonstrate its experimental performance, the simulation framework was tested against data measured with a CdTe sensor in the Hard X-ray Imager onboard the Hitomi X-ray Astronomy Satellite, which was put into a low Earth orbit with an altitude of 570 km and an inclination of 31 ° , and thus experienced a large amount of irradiation from geomagnetically trapped protons during its passages through the South Atlantic Anomaly. The simulation is able to treat full histories of the proton irradiation and multiple measurement windows. The simulation results agree very well with the measured data, showing that the measured background is well described by the combination of proton-induced radioactivation of the CdTe detector itself and thick Bi 4 Ge 3 O 12 scintillator shields, leakage of cosmic X-ray background and albedo gamma-ray radiation, and emissions from naturally contaminated isotopes in the detector system.

Hard x-ray imager onboard Hitomi (ASTRO-H)

Abstract. The hard x-ray imaging spectroscopy system of “Hitomi” x-ray observatory is composed of two sets of hard x-ray imagers (HXI) coupled with hard x-ray telescopes (HXT). With a 12-m focal length, the system provides fine (1  ′    .  7 half-power diameter) imaging spectroscopy covering about 5 to 80 keV. The HXI sensor consists of a camera, which is composed of four layers of Si and one layer of CdTe semiconductor imagers, and an active shield composed of nine Bi4Ge3O12 scintillators to provide low background. The two HXIs started observation on March 8 and 14, 2016 and were operational until 26 March. Using a Crab observation, 5 to 80 keV energy coverage and good detection efficiency were confirmed. The detector background level of 1 to 3  ×  10  −  4  counts s  −  1 keV  −  1 cm  −  2 (in detector geometrical area) at 5 to 80 keV was achieved, by cutting the high-background time-intervals, adopting sophisticated energy-dependent imager layer selection, and baffling of the cosmic x-ray background and active-shielding. This level is among the lowest of detectors working in this energy band. By comparing the effective area and the background, it was shown that the HXI had a sensitivity that is same to that of NuSTAR for point sources and 3 to 4 times better for largely extended diffuse sources.

A laboratory test setup to study the stability of operation of the CdTe detectors within Astro-H HXI

Astro-H1 is a JAXA/NASA X-ray satellite launched in 17th Feb. 2016. The hard X-ray imager (HXI)2 is a Si/CdTe stacked detector system which is placed in the focus of a hard x-ray telescope. HXI constitute one of the four different instruments onboard Astro-H. We are presenting the current status of a stacked detector setup which consists of two mini-HXI double sided CdTe strip detectors (CdTe DSDs)|similar to those used in HXI|that are read out with the low-noise readout ASIC IDeF-X BD. We describe the configuration of the setup, its spectroscopic performance, and a long-term operation of the setup. The long-term test simulates the orbital operation of HXI using identical detector temperatures, bias voltages, and switch-on/switch-off cycles with the goal to study the detector stability and the evolution of its performance during operation.

Development of a stacked detector system for the X-ray range and its possible applications

We have constructed a stacked detector system operating in the X-ray range from 0.5 keV to 250 keV that consists of a Si-based 64×64 DePFET-Matrix in front of a CdTe hybrid detector called Caliste-64. The setup is operated under laboratory conditions that approximate the expected environment of a space-borne observatory. The DePFET detector is an active pixel matrix that provides high count-rate capabilities with a near Fanolimited spectral resolution at energies up to 15 keV. The Caliste-64 hard X-ray camera consists of a 1mm thick CdTe crystal combined with very compact integrated readout electronics, constituting a high performance spectro-imager with event-triggered time-tagging capability in the energy range between 2 keV and 200 keV. In this combined geometry the DePFET detector works as the Low Energy Detector (LED) while the Caliste-64 - as the High Energy Detector (HED) - detects predominantly the high energetic photons that have passed the LED. In addition to the individual optimization of both detectors, we use the setup to test and optimize the performance of the combined detector system. Side-effects like X-ray fluorescence photons, electrical crosstalk, and mutual heating have negative impacts on the data quality and will be investigated. Besides the primary application as a combined imaging detector system with high sensitivity across a broad energy range, additional applications become feasible. Via the analysis of coincident events in both detectors we can estimate the capabilities of the setup to be used as a Compton camera and as an X-ray polarimeter - both desirable functionalities for use in the lab as well as for future X-ray missions.

Towards Radiotherapy Enhancement and Real Time Tumor Radiation Dosimetry Through 3D Imaging of Gold Nanoparticles Using XFCT

To enhance the efficiency of radiotherapy, a promising strategy consists in tumor exposure simultaneously to ionizing radiation (IR) and gold nanoparticles (GNPs). Indeed, when exposed to the radiation beam, these GNPs exhibit a photoelectric effect that generates reactive oxygen species (ROS) within the tumor and enhances the direct IR related deleterious effects. The measurement of this photoelectric effect thanks to an additional detector could give new insight for in vivo quantification and distribution of the GNPs in the tumor and more importantly for measuring the precise dose deposition. As a first step towards such a challenge, we present here materials and methods designed for detecting and measuring very low concentrations of GNPs in solution and for performing 3D reconstruction of small gold objects whose size is representative with respect to the considered application. A matrix image detector, whose sensitivity is first validated through the detection of few hundreds of micrograms of GNPs, is combined with a pinhole element and moved along a limited circular trajectory to acquire 2D fluorescence images of a motionless object. We implement a direct back-projection algorithm that provides a 3D image of these objects from this sparse set of data.

In-orbit performance and calibration of the Hard X-ray Imager onboard Hitomi (ASTRO-H)

Abstract. The Hard X-ray Imager (HXI) onboard Hitomi (ASTRO-H) is an imaging spectrometer covering hard x-ray energies of 5 to 80 keV. Combined with the Hard X-ray Telescope, it enables imaging spectroscopy with an angular resolution of 1′.7 half-power diameter, in a field of view of 9′  ×  9′. The main imager is composed of four layers of Si detectors and one layer of CdTe detector, stacked to cover a wide energy band up to 80 keV, surrounded by an active shield made of Bi4Ge3O12 scintillator to reduce the background. The HXI started observations 12 days before the Hitomi loss and successfully obtained data from G21.5–0.9, Crab, and blank sky. Utilizing these data, we calibrate the detector response and study properties of in-orbit background. The observed Crab spectra agree well with a powerlaw model convolved with the detector response, within 5% accuracy. We find that albedo electrons in specified orbit strongly affect the background of the Si top layer and establish a screening method to reduce it. The background level over the full field of view after all the processing and screening is as low as the preflight requirement of 1  −  3  ×  10−4  counts s−1 cm−2 keV−1.