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Preparing Ultraviolet Instrumentation for Extreme Radiation Environments, 15-9515

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Principal Investigators
Randy Gladstone
John Stone
John Scherrer
David Slater
Greg Dirks

Inclusive Dates:  10/06/04 – 02/06/05

Background - A modified Alice ultraviolet spectrograph (UVS) was proposed as part of the Juno science payload, when Juno was one of two remaining missions in competition for NASA's New Frontiers 2 mission. NASA made the decision between the two candidate missions on the basis of a concept study report (CSR) submitted in March 2005 and a site visit to each team's spacecraft provider in May 2005. This project supported several radiation studies that were used in preparing for the CSR and site visit presentations.

Approach - Six tasks were attached to this project, all revolving around the basic theme of preparing an ultraviolet (UV) instrument for operations in an extreme radiation environment. These tasks included the following: 1) review the Juno requirements and the Jovian radiation environment; 2) ensure the instrument electronics are suitable for and protected by the spacecraft vault and that appropriate interfaces to the spacecraft are maintained; 3) determine the thickness and placement of spot shielding to protect the detector and its pre-amps, which cannot be placed in the vault; 4) determine any additional radiation hardening for the detector and pre-amps and mitigate any instrumental sources of secondary radiation, e.g., fluorescence from window materials; 5) evaluate the effect of the proposed changes on the instrument thermal design, e.g., determine whether additional heaters are needed, since the heat from the electronics is no longer available for the optics and detector; and 6) report the results and provide the revised design to the Juno project.

Accomplishments - The current Juno UVS electronics design is qualified to meet with the required mission radiation tolerance of 50 kRad (RDM = 2) of the vault. As for shielding, it was found that 150 mil Ta walls would reduce the incident electron flux rate by approximately three orders of magnitude (in the most important portions of the Juno orbit), making the incident flux comparable to the signal. To further reduce the effects of the remaining radiation flux, we are adding a scintillator that covers a large fraction of the solid angle in back of the detector. This scintillator will be connected to the detector electronics such that within a given time period (approximately tens of nanoseconds), if both the scintillator and the detector see an event, the event is discarded. Thermal modeling indicated that the detector electronics should be co-located with the UVS sensor to avoid adding replacement heaters to maintain operational temperature limits. The revised design of the Juno UVS was made part of the Jet Propulsion Laboratory's CSR and forms the basis of the current design now that Juno has been selected.

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