PowerMac 6100:Desktop Folder:exosat_vol2:exosat_vol2:pg35.html.1

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ME DEAD TIME CONSIDERATIONS

Following a general survey of the EXOSAT instrumentation timing
and dead time characteristics (Express No. 5, p.31) a number of
discussions have taken place at the Data Analysis Workshop and
elsewhere to try to resolve an outstanding discrepancy in the ME
dead time correction factor and its functional form. All observa
tory software and analysis routines use an empirical effective
sample rate to correct spectral counts to fluxes through the
known dead times of the HK QEP counts. Although this method
produces the 'correct' result, several users, notably C. Page at'
Leicester and A. Tennent at Cambridge, have pointed out that for
certain high time-resolution measurements precise knowledge of
the factors contributing to the total loss is important. This
note explores- in more detail the predicted loss of events from
sampling, inherent electronic dead times and subsidiary effects,
which taken together give good agreement with the observational
data.

Figure 1 shows the distribution of time tags (raw channel counts) of a sample of ME events (245760 - mainly background), selected on board according to valid E (-= QEP) with a sample scheme of E, TT at 4K s-1 (sampling scheme eg. E, ID or E, ID, TT has no effect).

Note that the maximum time tag possible is 31, determined by sample interval/clock interval (244.1 rs/7.63 rs) and that the following contributions to overall dead time are evident.

a. loss of events from an expected flat random input
signal because of the restriction of -.<, I event per
sample interval (shaded area).

b. deficit of counts in channel 0 (-25% of expected
counts).

c. c. deficit of counts in channels 26 ( 10% loss), 27-30 (0 counts) appearing as an excess
in channel 31, the maximum time tag possible for a 4K Hz sample rate.

Simple Poisson statistics give the dead time correction factor associated with (a).

fa = MT (I-exp[-MT])-1

where fa = correction factor
M = true total qualified event rate
T = sample period.

M can be obtained with sufficient accuracy from the HK QEP counts, corrected for the electronic chain coincidence logic dead time for events > Emin (factor - 1.005).