From: Mark Goans [Mark.D.Goans@nasa.gov] Sent: Thursday, April 07, 2005 11:40 PM To: Mark E. Melton Cc: 'Pat St Aubin'; Henning.W.Leidecker@nasa.gov; Mark Goans Subject: RE: Fwd: GLAST-LAT-CDR-0035 (Accelerated Performance Life Testing of PIN Diode Elastomer) Mark, Based on the input from Henning, I too consider this RFA closed. Henning, thank you very much for your input and continued support. Regards, Mark Goans -----Original Message----- From: Mark E. Melton [mailto:mamelton@pop400.gsfc.nasa.gov] Sent: Thursday, April 07, 2005 4:45 PM To: Mark Goans Cc: 'Pat St Aubin' Subject: FW: Fwd: GLAST-LAT-CDR-0035 (Accelerated Performance Life Testing of PIN Diode Elastomer) Mark, Henning had several questions that he sent to me and I forwarded to Neil Johnson, the LAT Calorimeter Manager. Neil replied to Henning below and you can see Henning's response below as well. Can I assume this RFA (LAT CDR 35) is now closed based on Henning's response? Thanks, Mark ____________________________________________ Mark E. Melton GLAST Instruments Systems Engineer Swales Aerospace Bldg 12, Rm N210A GSFC (301) 286-7936 (V) (240) 988-1057 (M) (301) 286-5717 (F) -----Original Message----- From: Henning W Leidecker [mailto:Henning.W.Leidecker@nasa.gov] Sent: Thursday, April 07, 2005 2:44 PM To: Johnson, Neil Cc: 'Mark E. Melton'; Pat Hascall; 'Henning.W.Leidecker@nasa.gov'; Bernie Graf; Nick Virmani (nvirmani@swales.com); Grove, Eric Subject: Re: Fwd: GLAST-LAT-CDR-0035 (Accelerated Performance Life Testing of PIN Diode Elastomer) Dear Neil Johnson, Your responses answer all my questions, and more. I have no objections to the use of this material in this application. I wish you a successful mission. Sincerely, Henning Leidecker On Thursday 07 April 2005 02:32 pm, Johnson, Neil wrote: > Mark, > > My responses to Dr Leidecker's questions are addressed in blue below > each of his questions. Let me know if you have any more questions. > > Regards, > Neil Johnson > > > W. Neil Johnson > Naval Research Lab Code 7651 > Washington DC 20375 > phn: 202-767-6817 > fax: 202-767-6473 > email: neil.johnson@nrl.navy.mil > > > -----Original Message----- > From: Henning Leidecker [ > mailto:henning@toadmail.com] > Sent: Monday, April 04, 2005 8:36 AM > To: Mark Goans; MMELTON@swales.com > Cc: Henning.W.Leidecker@nasa.gov; Michael J Sampson > Subject: Re: Fwd: GLAST-LAT-CDR-0035 (Accelerated Performance Life > Testing of PIN Diode Elastomer) > > > Dear Mark, > > I do not really know what properties this elastomer must have, in > order for it to be regarded as qualified. Does anyone have a list of > required properties? And allowable changes over the mission duration? > > >>>> The requirements for this elastomer - which is used as the > >>>> optical > > window for a PIN photodiode - are captured in the overall requirements > for the PIN diode. They are included in our diode part specification, > LAT-DS-00209-14. It is attached. > > <> > > I can guess the intent is to fly the stuff as an adhesive to bond a > window to a PIN used as a light detector. But I do not know the > spectral range over which this resin must remain transparent, or the > radiation environment (both > for UV fluence or for ionizing radiation, both of which darken > resins), or > temperature range, or duration of mission. > > >>>>The environment expected for the PIN diode is also specified in > >>>>the > > spec, LAT-DS-00209-14 (attached). It is not exposed to UV. > > >>>>The adhesive is used to seal the ceramic carrier of the diode and > > provide an optical window. The diode window is then bonded to CsI > crystals using DC 93-500 - also optical transparency requirements. > > Generally, any use of a silicone-based resin on a spacecraft calls for > measurement of the extent of outgassing, and an estimate of the > dangers that the outgassed species will blacken under exposure to UV > light or ionizing radiation or both. This has happened many times in > the past, including during preparations for the LITE mission when a > silicone-based adhesive outgassed a silicone-product that blackened > important optics when exposed to > UV. I do not see this addressed in the attached report --- has this > concern > been completely addressed elsewhere? > > >>>>The silicone resin is completely sealed from light since the > > photodiode is trying to detect scintillations in the CsI crystal. The > resin has been reviewed and tested by Fred Gross as part of the > materials approval process. It was approved vis-a-vis outgassing by > Fred with a 24 hour bakeout, which was done by the vendor. > > > Exposing this resin to 1000 hr at 85% R.H. at 85C seems apt evidence > that this material can be used for at least a few months in the > tropics. But what > > about exposure to an environment more apt for space? Specifically, > how much does this resin --- WHEN APPLIED AT THE THICKNESS IT WILL BE > USED IN ITS > > INTENDED APPLICATIONS --- darken under UV and under ionizing > radiation? > > Years ago, when a new material was proposed for use as a cover-glass > adhesive for solar cells, the Materials Branch exposed a 1 cm by 1 cm > by 1 cm cube to Cobalt-60 radiation, and it turned utterly opaque at a > typical mission dose: > this turned out not to matter at all, since in use, the thickness is > about 3 > mils, and this thickness of stuff remained completely transparent. > That is, > the "internal transmission" goes as T_int = exp[ - kappa * > thickness] > where kappa depends on wavelength of light, and is proportional to > the ionizing dose (up to very roughly 100 krads). Thus, we can have > a slab be > transparent at 3 mils and opaque at 1 inch. > > >>>>The HASP testing was specifically to examine any issues with the > > mechanical properties of the resin over life. It is an optical window > but it sees strains caused by CTE mismatch of the PIN diode to the CsI > crystal transferred to the resin thru the DC93-500 bonding material. > > >>>>Optical properties after radiation exposure were addressed and > > presented elsewhere and not part of the RFA. I attach for your > information the report on optical properties after irradiation. No > changes in optical properties of the material as measured by diode > optical sensitivity were detected. > > <> > I note that the plot attached to this report shows (effectively) the > change in transmission (as well as the specrtral response of the > silicon PIN diode), and not the more useful "kappa": it is worthwhile > reprocessing the experimental transmission, which depends on the > surface properties as well as > the thickness, to exhibit "kappa" which is a more-nearly material > dependent > property. But I agree that this curve can be satisfactory for a > purely engineering "pass/fail". I note that the pre- and post-aging > curves are > > nearly indistinguishable for wavelengths longer than about 800 nm, > which is typical. But there are easily observable decreases in > post-exposure transmission for shorter than 500 nm --- do we need to > attend to that region? The requirements should say, somewhere. > > >>>>The peak in scintillation light yield for CsI(Tl) is around 590 > >>>>nm. > > The issue that needs to be addressed here is signal to noise in the > collection of light from the CsI and transferred to the electronics. > The transmission of the optical window of the diode is but one > component of this. The other bigger component is the loss of light > from the CsI caused by radiation damage in the CsI itself. This is > expected to be on the order of 40% of the light lost over the life of > the mission. The system has been designed with such losses in mind. > Consequently, the relatively minor change in the photosensitivity seen > in the figure are acceptable. > > Call Fred Gross in the Materials Branch, mention "silicone adhesive", > and stand back from the phone for a few minutes until Fred stops > foaming at the mouth, and then discuss with Fred what is needed from > the Materials point of > view. > > >>>>Fred Gross was a great help in developing the specification for > >>>>the > > PIN diode and identifying the optical window material. The original > design included hard epoxy optical window that could not take the CTE > mismatch loads. The silicone was the best solution. > > Best, > Henning Leidecker > Please return any emails to my work address: > Henning.W.Leidecker@nasa.gov > > > Henning, > > > > In discussions with Mike Sampson, the recommendation was made that I > > forward this RFA and response to you for your assessment. The RFA > > was > > > > generated by an individual at the CDR that was not on the > > Independent > > > > Review Board and I'm looking for input from someone with expertise > > in > > > > this area prior to closure. I'd appreciate any assistance you can > > provide. The projects response is attached. > > > > Regards, > > > > Mark Goans > > 6-9763 > > > > --------------- > > > > GLAST-LAT-CDR-0035 (Accelerated Performance Life Testing of PIN > > Diode Elastomer) > > > > Title: Accelerated Performance Life Testing of PIN Diode Elastomer > > Originator: Goans, Mark > > Date Assigned: 5/16/2003 > > Due Date: 10/31/2003 > > Initial Due Date: 10/31/2003 > > Review Date: 5/16/2003 > > Initial Response Date: 2/28/2005 > > > > Action: [Action originated by Ron Ray and assigned to Mark Goans of > > the IIRT] Perform accelerated performance life testing of PIN diode > > elastomer and set aside a few devices for long term monitoring. > > > > Rationale: The elastomer being used for the PIN diodes has no flight > > history. > > > > Response: See attached. > > > > ------------------------------------------------------- << File: LAT > > CDR RFA 35 Response.doc >>