Time-resolved neutron/gamma-ray data acquisition for in situ subsurface planetary geochemistry

Abstract

The current gamma-ray/neutron instrumentation development effort at NASA Goddard Space Flight Center aims to extend the use of active pulsed neutron interrogation techniques to probe the subsurface elemental composition of planetary bodies in situ. Previous NASA planetary science missions, that used neutron and/or gamma-ray spectroscopy instruments, have relied on neutrons produced from galactic cosmic rays. One of the distinguishing features of this effort is the inclusion of a high intensity 14.1 MeV pulsed neutron generator synchronized with a custom data acquisition system to time each event relative to the pulse. With usually only one opportunity to collect data, it is difficult to set a priori time-gating windows to obtain the best possible results. Acquiring time-tagged, event-byevent data from nuclear induced reactions provides raw data sets containing channel/energy, and event time for each gamma ray or neutron detected. The resulting data set can be plotted as a function of time or energy using optimized analysis windows after the data are acquired. Time windows can now be chosen to produce energy spectra that yield the most statistically significant and accurate elemental composition results that can be derived from the complete data set. The advantages of post-processing gamma-ray time-tagged event-by-event data in experimental tests using our prototype instrument will be demonstrated.

Modeled Martian subsurface elemental composition measurements with the Probing In situ with Neutron and Gamma ray instrument

“…Experiments using optimized time windows as described above were performed at NASA GGAO using a 14.1 MeV PNG, an high‐purity Ge (HPGe) gamma ray detector, and a He‐3 neutron detector on a large basalt monument [ Bodnarik et al ., ]. The PNG pulsing scheme with 100 µs pulse width and 1000 µs pulse period was chosen to be applicable to a variety of material compositions that PING might encounter on Mars.…”

“…At the end of the neutron capture window, the capture gamma ray rate is sufficiently low to allow the measurement of gamma rays from delayed activation and natural radioactivity. The results from the analysis of the gamma ray spectra acquired by the HPGe detector showed that the optimization of the time windows reduces the background and the separation of gamma ray spectra by nuclear processes improves the overall gamma ray line measurement precision [ Bodnarik et al ., ].…”

Subsurface In situ elemental composition measurements with PING

“…The diagram in Figure 2 and spectra in Figure 3 show how restricting gamma-ray data acquisition to specially timed coincidence windows between neutron pulses results in the separation of gamma-ray spectra by nuclear process. Bodnarik et al, (2012) [6], explain in detail how the coincidence time window boundaries and the PNG pulse width and period can be optimized to separate gamma ray lines from these different processes. Our calculation in section 4 makes use of this ability to correct for spectral contamination from different processes as discussed when computing the rate of the 28 Si inelastic line at 1779 keV.…”

Material Classification by Analysis of Prompt Photon Spectra Induced by 14-Mev Neutrons

“…Neutron systems are utilized in industry for examination of coal (Sowerby, 2009), cement (Womble et al, 2005), metal alloys (James and Fuerst, 2000), soil (Wielopolski et al, 2008) and in oil well logging applications (Frankle and Dale, 2013). Neutron methods are also used for the detection of toxic chemicals and explosives in cargo and vehicles (Koltick et al, 2007;Gozani, 2004), for humanitarian demining, for in situ astrochemical analysis of planetary samples (Bodnarik et al, 2013) and for in vivo cancer detection (Bender et al, 2007).…”