Effects of capillary reflection in the performance of the collimator of the Large Area Detector on board LOFT

Abstract

The Large Observatory For X-ray Timing (LOFT) is one of the candidate missions selected by the European Space Agency for an initial assessment phase in the Cosmic Vision programme. It is proposed for the M3 launch slot and has broad scientific goals related to fast timing of astrophysical X-ray sources. LOFT will carry the Large Area Detector (LAD), as one of the two core science instruments, necessary to achieve the challenging objectives of the project. LAD is a collimated detector working in the energy range 2-50 keV with an effective area of approximately 10 m 2 at 8 keV. The instrument comprises an array of modules located on deployable panels. Lead-glass microchannel plate (MCP) collimators are located in front of the large-area Silicon Drift Detectors (SDD) to reduce the background contamination from off-axis resolved point sources and from the diffuse X-ray background. The inner walls of the microchannel plate pores reflect grazing incidence X-ray photons with a probability that depends on energy. In this paper, we present a study performed with an ad-hoc simulator of the effects of this capillary reflectivity on the overall instrument performance. The reflectiv-On behalf of LOFT collaboration T. 2 Teresa Mineo et al.ity is derived from a limited set of laboratory measurements, used to constrain the model. The measurements were taken using a prototype collimator whose thickness is similar to that adopted in the current baseline design proposed for the LAD. We find that the experimentally measured level of reflectivity of the pore inner walls enhances the off-axis transmission at low energies, producing an almost flat-top response. The resulting background increase due to the diffuse cosmic X-ray emission and sources within the field of view does not degrade the instrument sensitivity.

Characterization of the NNVT capillary plate collimators

“…This is visible especially towards the peak of the response (figure 16). This phenomenon is likely to be caused by photon reflection on the inner walls of the MCP channels, as it is expected according to the calculations described below, whose results are shown in figure 17, as well as the simulations in [6]. If this is the case, it also means that the walls are very well polished, as qualitatively suggested by the very large angle at which the measured curve departs from the quasi-triangular analytic response.…”

Future Perspectives for Gamma-ray Burst Detection from Space

“…The LAD is exploiting the technology of innovative large-area Silicon Drift Detectors [22] (SDDs), to provide an unprecedented large effective area, reaching 3.4 m 2 at 8 keV. The instrument is not designed for imaging purposes, and the FoV is limited to 1 deg using micro-channel collimator plates [23] to simultaneously reduce source confusion and background. The LAD achieves an energy resolution of 260 eV at 6 keV and a timing resolution of 10 µs in the whole operating energy range.…”

Lobster Eye X-ray Optics

“…but where the optic to source distance is larger than the optic to detector distance. Collimation can be achieved by chemically roughening the channel walls for example, in this configuration a small FoV is defined, and rays from outside this filed of view are efficiently blocked, reducing the sky background [52]. Beam splitters are created by rotating a slumped optic so that the concave surface of the optic is towards the X-ray source.…”