In the past, correlated shifts in the pedestals in the BTOW have been seen. It is unclear where this comes from, but shifts of the order of 4-5 ADC counts have been seen (link to study on this effect?).
There are a number of ways to treat this effect, but the way that has been used in the past is to essentially worsen the energy resolution from the ADC (originally an idea from Steve Vigdor).

The current prescription to deal with these pedestal shifts is as follows:

  1. Shift the pedestal peak in each channel to be at the ADC value 24
  2. Bit shift the ADC value to the right (give it less bits) by 4 bits
  3. Bit shift the ADC value to the left (give it more bits) by 4 bits

This is consistent with setting the last 4 bits of the 12 bit ADC value from the BTOW to 1000 (which is just 8) which is in the ~middle of the 0-15 of the information that is being lost. Really you have the freedom to set the last 4 bits to whatever you want since this is going to be implicit in your energy resolution now so it was chosen to put it in the middle of the range.

  • No labels