News from the Guelph PIXE Group
Our research continues to revolve around particle-induced X-ray emission (PIXE) both in the Guelph Pelletron Accerator Lab and on the Mars Science Lab’s Curiosity rover. GUPIX development continues, as does our derived GUAPX code for the Mars rover alpha particle X-ray spectrometers.
- The APXS Emulator system is now being used in several new projects:
- Comparison of PIXE results from solid rocks before and after powdering and pelletizing;
- Refining APXS analysis of rocks containing chlorine and sulfur salts;
- Detailed study of a suite of Hawaiian rocks that has undergone progressive weathering; thi suite is a model for analogous rocks on Mars observed by the Curiosity APXS
- The APXS Emulator system has been used to study a number of rock types recently discovered on Mars (eg basatlic trachy-andesites) for which the APXS calibration suite has no standrads. Portions of these samples had undergone highly accurate XRF analysis after fusion to glass, enabling use of the pressed powder as equivalent to GRMs. The PIXE analysis showed that these materials followed the various trends already established for the igneous members of the APXS calibration suite.
- We have built and calibrated a new broad-beam PIXE system using 2.5 MeV protons and two X-ray detectors [SDD and Si(Li)]. Its function is to emulate the APXS. It provides superior energy resolution and precisely defined geometry, and can analyze ten rock samples in 2.5 hours. This compares favourably with APXS, where a single sample takes several hours and often overnight. The new system has been used to verify the mineral phase effects in geochemical reference materials (GRMs) which we observed earlier in APXS spectra.
- We have improved the accuracy of PIXE analysis of major and minor elements in mono-mineralic rocks to the level of 1-2%. This involved various projects which are described in the PhD thesis of Chris Heirwegh (now at Cal Tech):
- comparison of PIXE yields from light elements with GUYLS predictions in which both XCOM and FFAST mass attenuation coefficients were used; the outcome was a new blended MAC database for GUPIX;
- development of a correction within GUPIX for non-linear channel-energy response in SDDs.
- Working with Brianna Ganly (visitor from CSIRO, Australia), we have improved the quality of GUAPX fits to spectra from the Curiosity rover’s alpha particle X-ray spectrometer; this necessitated solving the problem of apparent energy shifts of light element X-rays. These shifts arise from two sources: non-linearity and alpha-induced multiple ionization satellites. A database for MIS has been constructed and incorporated into GUAPX; it will also be incorporated in GUPIX for terrestrial alpha-PIXE.
- We are collaborating with Matjasz Kavčič (Institute Josef Stefan, Slovenia) and Stjepko Fazinič (Ruder Boskovič Institute, Croatia) and their local colleagues on further development of the MIS database mentioned above. This involves high-resolution wavelength-dispersive X-ray at the IJS and RBI ion beam facilities.
- In collaboration with G.M. Perrett at Cornell University we have succeed in quantitative prediction from first principles (geochemical, mineralogical and physical) of the mineral phase effects upon APXS analytical results for light elements. These effects were reported by us following our 2012 fundamental parameters calibration of the Curiosity APXS.
- The MSc thesis work of Erin Flannigan involved the development and calibration of an accelerator-based PIXE system which would emulate the Mars APXS. It was used by her to measure the mineral phase effects first noted in geo-standards during calibration of the Curiosity APXS. Excellent agreement was found between PIXE and APXS, and the APXS calibration was “expanded” by adding new rock types (e.g. basaltic trachy-andesites) to the geostandard suite.
- It is planned to use this APXS-emulator to study pseudo-Martian rocks synthesized by Mars Science Lab colleagues.
- Again with Dr Perrett, we have devised a computational method for analyzing the element content of thin layers deposited (presumably) by fluids on Martian rocks. The method uses the “as is” APXS spectrum of the layered rock plus the substrate of the brushed substrate. It has been applied to dust covers on Gale Crater mudstones and to a layer on the Mazatzal sample examined in the early days of the Spirit Rover at Gusev Crater.
- We have developed a new code GUMAP which will assist workers doing micro-PIXE imaging. The user can construct 2D element maps from Oxford Microbeams list mode files and then select small regions of interest on her monitor. For each of these a PIXE spectrum is created which can be sent to GUPIXWIN for determination of element concentrations.
J. L. Campbell
Department of Physics
University of Guelph
Guelph, Ontario, Canada
email: firstname.lastname@example.org / email@example.com
Fax: +1 (519) 836-9967
Phone: +1 (519) 824-4120 x52325