Date: Tue, 15 Oct 1996 12:18:44 -0500
To: ccr-team@lanl.gov
From: spence@bu.edu (Harlan E. Spence)
Subject: IPS reconfiguration
Precedence: bulk
Sender: ccr-team-request@nis.lanl.gov
Dear All,
On Monday October 7, 1996 (13-14UT), Dan Mabry uploaded commands to
reconfigure the energy binning of the IPS. We have now received the data
from that day and the results of the remapping are in. The intent of the
exercise was to see if we could lower the lowest energy threshold without
encountering bad noise problems thereby gaining sensitivity for PEPs and
ENAs.
The IPS energy bins are set by toggling bits on high and low reference
points on our custom analog-to-digital chips. Once these are set at a
particular voltage, sixteen logarithmically spaced voltage steps are
established and pulses are binned then through comparison to the voltage
thresholds. On the 7th, we lowered the lowest voltage setting on every
channel by three bits which corresponds to an electronic energy of ~3keV
(~1 kev/bit). This lowering reduced our true energy threshold from the
current settings of 16.8 to 13.9 keV (these values include both the
electronic and window energy). At the same time, we changed the bottom
energy for the integral channel (which is computed on-board at the highest
possible time resolution of 185 msec) to be bin 1 (rather than the present
bin 0) to avoid any possible noise contamination.
The results are somewhat mixed but overall very good. The new settings
now have the following energies compared to the old:
Channel Threshold (keV) Midpoint Energy (keV)
Old New Old New
E0 16.8 <- 13.9 18.9 <- 15.6
E1 21.2 17.5 <- 24.4 19.9 <-
E2 27.9 22.6 32.4 26.2
E3 37.5 30.3 43.1 35.4
E4 49.6 41.4 57.2 48.1
E5 65.9 55.9 76.0 65.2
E6 87.7 75.9 101.7 88.4
E7 118 103 138 121
E8 161 142 188 168
E9 221 198 259 234
E10 303 277 355 327
E11 417 387 489 459
E12 574 543 674 643
E13 791 762 929 903
E14 1091 1071 1281 1269
E15 1505 1505 N/A N/A
As you can see, owing to the logarithmic nature of the energy bins and the
linear offset of the change, the largest effects are seen at the lower end
of the spectrum. This is to our advantage as this is where the counts are!
I have marked with arrows the channel at which the energy integral channel
begins. As you can see, the combination of the lowering of energy
threshold and raising of the integral channel has nearly maintained the
integrity of the integral channel. IN addition, in the new scheme, we now
have a channel which is sensitive to 13.9-17.5 keV ions with a midpoint
of
15.6 keV!
As you will see when you inspect the latest plot on the summary page,
this change has mixed results. Several of the IPS heads now have
low-count-rate but constant noise in channel E0. On the other hand, many
others don't, so in these, we now have an extremely low energy detector
(perhaps better than one ever flown?). Even in the worst case, the noise
is at the rate of about 20 counts/second (10 degree head) so in regions
of
high foreground, the background can become inconsequential. In a steeply
falling spectra, lowering the low end can produce a substantial increase
in
counting statistics. Time will now tell whether the benefits outweigh the
deficits. My initial feeling is that we should stay in this configuration
for at least a week and reassess at that time. One option might be to
raise the levels a bit at a time to see if we can get them all above
background. If so then we can lower the integral back to channel 0 and
then be in Fat City!
Regards, --- Harlan
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