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We do not see any signal in the high energy diode in tower 4, layer X1, column 4, + side. From Eric Siskind: "Whether \[\] Note that currently there is no failure mode in CalRecon so events from this channel is not treated in any special way. NRL is working on this.

What this means is that (until xtal recon is fixed):

1. Any photon that MISSES this one crystal is not affected. It is correctly and properly reconstructed.
2. Any photon that deposits LESS THAN about 1 GeV in this one crystal is not affected. It is correctly and properly reconstructed.
3. Any photon that deposits MORE THAN about 1 GeV in this one crystal has an incorrect energy and position measurement in this one crystal, and therefore has an incorrect reconstructed incident energy and direction. The level of error in reconstructed incident energy and direction is surely energy-dependent, and I don't have an estimate of the magnitude yet.

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The combination of cases (1) and (2) covers the overwhelming majority of photons in the LAT dataset, so most events are perfectly fine, but clearly we need to implement a fix for this particular failure in the code that reconstructs crystal energy and position.

Eric

Once CAL xtal recon has been modified with the changes outlined below, the performance of this xtal will be essentially nominal, and the performance of the LAT will be unaffected.

Changes necessary to recon

As of July 2010, CAL recon code contained no mitigation against failures, despite our having discussed adding such code for the last 10+ years.  This failure requires that we do something.  The intent here is to fix the reconstruction of events that hit this xtal at the level of xtal recon so that the failure and fix are transparent to downstream energy reconstruction and clustering.  

We're adding status bits to xtal recon to indicate that the energy and position information returned by a xtal with compromised readout have been modified.  Any reconstruction code that follows xtal recon could make use of those bits to accept/reject that xtal or modify higher level recon algorithms, if the authors and architects of that code wish.

On 4 Aug 2010 (Day 216), at 4:59 PM EDT, J. Eric Grove wrote:

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case == bad plus-face HEX8 && bad plus-face HEX1 && ! plus-face HE autoranging disabled
/* this is the case for the current failure and current configuration */
/* if one of the HEX ranges is best range, fix it */
/* if plus-face first range == (HEX8 || HEX1),
	calculate E using the opposite face and the externally provided longitudinal position
	clear the bad energy bit
	set the externally provided longitudinal position bit
	(maybe set the failure-mitigation energy bit)
*/


case == bad plus-face HEX8 && bad plus-face HEX1 && plus-face HE autoranging disabled
/* this is the case for the current failure, but it requires a configuration we have not used */
/* if LEX1 is best range and saturated, fix it */
/* if plus-face first range == LEX1 && plus-face > 4050 (i.e. saturated),
	calculate E using the opposite face and the externally provided longitudinal position
	clear the bad energy bit
	set the externally provided longitudinal position bit
	(maybe set the failure-mitigation energy bit)
*/


case == bad plus-face LEX8 && bad plus-face LEX1 && bad plus-face HEX8 && bad plus-face HEX1
/* this is the case we may get to soon if this GCFE continues to degrade and the LE ranges fail */
/* in all cases,
	calculate E using the opposite face and the externally provided longitudinal position
	clear the bad energy bit
	set the externally provided longitudinal position bit
	(maybe set the failure-mitigation energy bit)
*/