HETn: Exploring the Solar Orbiter Neutron Window with Innovative Energetic Particle and Neutron Instrumentation, 15-9469

Principal Investigator
Arik Posner

Inclusive Dates:  04/01/04 – 04/01/05

Background - Solar Orbiter is planned as an ambitious space science mission that delineates links between the sun, the solar corona, and the surrounding heliosphere. It is a single three-axis-stabilized spacecraft mission of the European Space Agency (ESA). Solar Orbiter will make unprecedented near-sun observations in distances between 0.22 and 0.88 Astronomical Units (AU) and out of the ecliptic plane to heliographic latitudes of 30 to 38 degrees. The orbit becomes near-heliosynchronous for periods of time near closest approach to the sun, when the relative angular motion of the sun and the Solar Orbiter is small. The spacecraft will use a combination of in situ particle (including neutron) and field measurements, radio sounding, and visible, UV, and EUV imaging. With this instrument complement and a perspective near synchronous with the sun's rotation, Solar Orbiter will investigate the inner workings of the solar dynamo, track the birth and evolution of solar activity, and make the first out-of-the-ecliptic images of coronal mass ejections (CMEs) during their creation and early evolution stages. Solar Orbiter will also make the first images of the Sun's poles. A high-energy telescope, suggested here with neutron detection capabilities (HETn), will play a key role within the Solar Orbiter payload. HETn will investigate the acceleration of particles and three-dimensional heliospheric magnetic structure with the observation of energetic particles. Solar Orbiter provides the ideal vantage points for HETn, where for the first time, particle propagation effects can be separated from temporal and spectral variability of energetic particle sources. The unique orbit of Solar Orbiter will allow us to study the importance of different energetic particle populations at different locations in the inner heliosphere. HETn, is a charged- and neutral particle analyzer system combining the capabilities of the flight-instruments COSTEP on the Solar and Heliospheric Observatory (SOHO) and Matroshka on the International Space Station (ISS). The active sensor components utilized for HETn are solid-state detectors, as used for COSTEP, and photo-diodes, as used for Matroshka. The goal of this SwRI project is to demonstrate the feasibility of the HETn concept and to prove key technologies for upcoming NASA and ESA proposal opportunities.

Approach - With this project, the Institute for the first time seriously enters into the technology for high-energy charged particle detection in a space environment. Our collaborators at University of Kiel, Germany (i.e., Prof. Robert F. Wimmer-Schweingruber and Dr. Rudolf Beaujean) leveraged this technology development process with valuable hardware contributions. The applied technologies include the use of solid-state detectors and photo-diodes. The core of the HETn instrument consists of two scintillators, a CsI(Tl) crystal and a plastic scintillator. Charged particles stopping in these materials generate scintillation light at optical wave lengths, with a maximum emission in the red part of the spectrum. Reflective surfaces are used on all edges of the scintillators to keep generated light within. The only absorber is the read-out photo-diode. We use red-sensitive silicon positive-intrinsic-negative (PIN) photo-diodes for the read-out of scintillators. PIN photo-diodes provide output signals that are proportional to the intensity of the measured light. The detectors were tested using both an artificial source - a signal generator providing pulses comparable to a radiation source - and several SwRI-owned radiation sources. A radiation source placed close to the scintillator generates signals to be analyzed with digital oscilloscopes and multi-channel analyzers (MCAs). MCAs detect the pulses and bin them according to amplitude. The result is a histogram of pulse-height distributions.

Accomplishments - This project strengthened HETn's role as the baseline for the high-energy particle spectrometer of the upcoming Solar Orbiter mission. The ongoing collaboration with our partners from the University of Kiel will provide an opportunity for SwRI to propose a competitive energetic particle package. Several important calibration and simulation tasks were accomplished within this project. The gamma ray calibration of the instrument provided us with the spectra of 137Cs and 60Co. Simulations and measurements show that linearity and energy resolution of the HETn system are sufficient for distinguishing several main ion species, e.g., p, He, C, O, Fe, of galactic cosmic ray origin. Furthermore, this calibration task demonstrated that the low-energy threshold for the CsI scintillator package is well below 500 keV, the critical value that allows HETn to observe minimally ionizing particles (MIPs) successfully. Singly charged MIP particles at relativistic energies are of critical importance. They are the main particle components (~90 percent) of galactic cosmic rays (protons and electrons), which are clearly covered by the response of HETn at gamma-ray energies. Also, energetic electrons of solar energetic particle events are in the scope of HETn, allowing use of the detector as a warning device for hazardous solar energetic ion events.

2005 Program Home