Preliminary Assessment of Wavelength Shifter Data

  The measurements made during the December 96 user shift were performed with the PTB Wavelength Shifter (WLS) beamline. With the incident radiation going well beyond 15 keV, it was hoped the measurements would extend the absolute calibration of the reference standards up to the high energy limit of the AXAF observatory. Another goal of this measurement was to provide an independent determination of the depletion depths. Unlike the White Light beamline which can only constrain the low energy quantum efficiency dominated by the gate thicknesses, the harder spectra of the WLS beamline produces photons that easily transverse the length of depleted silicon-the requisite condition to accurately model the high energy quantum efficiency and the depletion depth.

Initial analysis of the WLS data reveals two potential problems with the quality of the data. Reduced data products indicate that the electronics were performing sub-optimally during half of the measurements. The problems can be simply characterized as a spatial and temporal variation in the bias levels [Pivovaroff1997b]. Techniques do exist to correct for such effects, but it is unclear how well they will work. It was known that the WLS would produce many more photons than the White Light beam for a given ring current. Anticipating problems with pileup, the storage ring currents were reduced from typical White Light operating conditions by at least a factor of two. Unfortunately, the extent that pileup influences high energy spectra has only recently been fully appreciated. Given the way the BESSY data was collected, it is quite possible that the data may prove difficult to accurately correct for pileup . Initial XSPEC fitting indicate that pileup is much more severe than for the White Light data.

Footnotes

...pileup 

The beating of the chopper wheel with the readout time of the CCDs leads to different pileup conditions for some fraction of the data  [Pivovaroff1997a].