JEM-X dead time, grey filter, double triggers, and all that… S. Brandt DNSC, March 2007

1. JEM-X dead time, grey filter, double triggers, and all that…S. Brandt DNSC, March 2007

2. Dead time calculation • HW dead time: HW-trigger is disabled for 5.3 ms • SW dead time: SW-trigger is disabled to read amplifiers, depending on which selection criteria the event will meet/fail. • 17 ms for grey filter and ‘buffer full’ • 79 ms for crude rise time and high anode signal • 179 ms for analysis requiring read-in of all amplifiers • Let L denote the fractional live time, L = (1 – deadtime) • Grey filter introduces an additional dead time by allowing a certain transmission T =(1+GF)/32 • To correct a rate for dead time and grey filter: Rcorrected = Rraw / L / T • Correct if all works as expected SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 2

3. Grey filter mechanism and randomness • Grey filter has 32 settings • Transmission, T = (GF+1)/32 • Grey filter is acting “up-front” on HW triggers, based on a table with 32 entries to ensure randomness of accepted events (so they say) • We have 32 tables, each with 32 on/off bits, maximally spread out • T=100%, Table 31: 1111 1111 1111 1111 1111 1111 1111 1111 • T=50%, Table 15: 1010 1010 1010 1010 1010 1010 1010 1010 • T=3%, Table 0: 1000 0000 0000 0000 0000 0000 0000 0000 • So for T=3%, after proccessing an event, we must ALWAYS wait for 31 HW triggers before processing • This has implications for the distribution of times between events, i.e. grey filter mechanism does NOT appear random after all SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 3

4. Probability of processed event following processed event • Entries in 32 bit grey filter are always maximally spread out • For T<50% a processed (accepted) event is ALWAYS following an rejected due to grey filter Truly random JEM-X random SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 4

5. Distribution of delta-times (log plots) GF=31 GF=2 Dead time effects GF=31 GF=2 SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 5 Ticks 122 micro-sec Ticks 122 micro-sec

6. Double triggers • Number of HW-triggers are greater than what is expected from SW-triggers and dead time calculations • Excess could be caused by double triggers (not statistically independent) • SW trigger is disabled at least 79 ms for normally processed events and 17 ms for grey filter events • Double trigger rate is around 20% (going slightly up) • Possible cause: trails of ionization from particles • Double trigger rate goes dramatically up, when the drift voltage (VC) is lowered (slow drift) • Relative double trigger rate is lower, when only 1 anode segment is active (geometry of tracks) SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 6

7. Relative HW trigger rate (%) Double triggers observed in VC test of orbit 262 Nominal setting VC setting HW trigger rate Dead time corrected SW trigger rate (%) SW trigger rate (%) SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 7

8. Relative double trigger rate as function of anode setting All anodes on D=23% Only 1 anode on D=12% SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 8

9. Dead time correction problems when grey filter active • Fewer accepted events than expected are seen when grey filter is active • SW triggering is enabled 17 ms after a grey filter rejected event • In some cases we may then see the second part of a double trigger • Try to estimate the additional correction factor from available data • Better understanding of the double trigger is needed SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 9

10. Additional correction factor • 10% correction for GF<16, less for GF>16 • Proportional to probability of processed event following GF event Relative Crab rate as function of grey filter setting SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 10

11. Upcoming Crab calibration • Grey filter test (10 ks) • map the additional correction factor in detail as function of GF setting • Anode section and grey filter test (8 ks) • Enable various combinations of anode sections for GF=31 and GF 15 • Measure double trigger rate as function of anode section geometry • Drift voltage test (8 ks) • Determine double trigger rate and performance as function of drift voltage • Check gas contamination • Establish calibration with higher drift voltage (4x25 ks, IBIS pointing) • Determine electronic efficiency relative to nominal setting • Check imaging distortion relative to nominal setting SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 11

12. Conclusions • Grey filter rejection not as random as we thought • Possible impact on power density spectra • New deadtime correction needed – 10% effect! • function of grey filter • Function of background rate? • Is the drift velocity going down? • Crab test will show, hopefully • Can we improve performance by increasing drift voltage? • Crab calibration will show SDAST Meeting,March 2007 JEM-X grey filter etc. Søren Brandt page 12