Detection of Interstellar Pickup Ions within our Solar System: New Instrument Concept

We discuss the importance of measuring the composition of the local interstellar medium. Composition measurements of the local interstellar medium pickup ions are confined to mass less than or equal to ²⁰Ne⁺. It is desireable to extend the mass range up to ⁵⁶Fe⁺ if not up to ⁸⁴Kr⁺ which will require 2-3 stop coincidence designs. It's not clear the IMAP mission can detect pickup ions with mass > ²⁰Ne⁺. At present most techniques are similar to the SWICS design, which uses a combination of energy/charge (E/Q) analyzer, post-acceleration V_PA > |-15| kV for positive ion measurements, followed by time-of-flight (TOF) section to measure the ion's velocity to give the ion's mass/charge (M/Q). Then by combing the ion's M/Q and E/Q, and then using a solid ststae detector (SSD) to measure the ion's total energy E = (E/Q + V_PA)*Q, allows one to separate ion's M and Q for high charge state solar wind ions with 2-stop detection. But for singly charged heavy ions with mass M ≥ 16 it's difficult to detect pulses from the SSD so only a single stop design is achieved. We're developing a Tandem Ion Mass Spectrometer (TIMS) (1 V ≤ E/Q ≤ 50 kV) composed of frontend which measures the ion's approximate E/Q and M/Q, followed by a post-acceleration V_PA ~ -15 kV (i.e., ion E/M (keV/nucleon) is estimated) after which the ion enters TIMS 3-stop TOF unit (does not use SSD) where the ions mass resolution is increased (10 ≤ M/ΔM ≤ 60 at FWHM). With the ion approximate E/Q, M/Q and E/M provided by the frontend to the TOF unit, an algorithm that corrects for carbon foil induced ion scattering (path length changes) and energy straggling corrections can be made to make the TOF peaks narrower and boost the instruments mass resolution. In addition dE/dx effects can separate ions of similar M/Q and by making the TOF peaks narrower they can be separated. Finally, the 3-stop design allows for long integration times > years, so the only limiting factor for rare species is signal (i.e., instrument's maximum E/Q and its geometric factor).