220 200 180 160 140 2 120
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so ë «0 40 20 220 200 160 ISO 120 140 0 20 40 60 SO 100 In p u t C ount R ate (Hz)Fig 4.9a. Simulated affect o f MCP intensifier dead tim e on the point source dynamic range response o f the MIC detector. Intensifier dead tim es 0.1ms (Curve 1 (o)), 1ms
(Curve 2 (x ) ) and 10ms (Curve 3 (•)).
The curves represent the dynamic range response of the MIC detector when:
• The radius of the intensifier dead area is 45/zm i.e. three pore spacings. Because the intensifier dead radius is larger th an the FW HM of a point source illum ination every event is assumed to be spatially coincident w ith each other.
• The CCD frame tim e is 1.5ms (using the spectroscopic form at).
Fig 4.9a
shows th e affect of a changing intensifier dead tim e on the dynam ic range response of the MIC detector. Because curves 1 and 2 are associated w ith a detector whose intensifier dead tim e is smaller than the CCD fram e tim e, these two curves are very similar. For an intensifier dead time of 0.1ms the inp ut count rate a t which 10% coincidence occurs is a t 180Hz, and for a dead time of 1ms it occurs a t lOOHz. Most of the coincidence losses will take place on the CCD because the probability of two events arrivingw ithin th e same CCD frame is higher than an event arriving w ithin th e (intensifier) dead tim e of a previous event.
Curve 3 is associated w ith an intensifier dead tim e of 10ms which is some seven times greater th an th e CCD frame time. In this case most of the coincidence losses take place w ithin the intensifier and 10% coincidence occurs a t a count ra te of llH z .
This set of simulations indicate th a t the dynamic range is highly dependent upon the intensifier dead tim e especially if the dead tim e is large compared to the CCD frame tim e. Because the MIC intensifier has an estim ated dead tim e of 0.1ms these simulations predict th a t MIC has a point source dynamic range whose bright lim it is 180Hz assuming the spectroscopic form at is employed.
F ig 4 .9 b shows the affect of a change in CCD frame tim e on the dynamic range response of the MIC detector. The curves represent frame times of 1.5ms (o) and 10ms ( x ) , with the following characteristics applied:
220 200 160 160 140 3 (§ 120 C 3 CJ 100
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80 9 O 40 20 180 20 0 220 0 20 40 60 80 100 120 140 160 In p u t C ount R ate (Hz)F ig 4 .9 b . S im u la te d affect o f C C D fram e tim e o n th e p o in t so u r c e d y n a m ic ra n g e r e sp o n se o f th e M IC d e te c to r
• The radius of the intensifier dead area is 45//m i.e. three pore spacings. • The intensifier dead tim e is 0.1ms.
Curve 1 in F ig 4 .9 b is the same curve as curve 1 in F ig 4 .9 a and represents th e case where the MIC detector has a CCD frame tim e of 1.5ms. In this case 10% coincidence is
estim ated to occur at an input count rate of 180Hz.
Curve 2 represents the case where MIC has a CCD frame tim e of 10ms which corre sponds to im aging w ith the full CCD form at (256x256 CCD pixels). In this case 10% coincidence is estim ated to occur at a count rate of only 25Hz, some seven times smaller th a n when employing a spectroscopic format.
There are three points th a t can be made about the results of these point source dynamic range simulations:
• For small intensifier dead times the 10% coincidence point is nearly inversely pro portional to the CCD frame time.
• For intensifier dead times which are large compared to the CCD fram e tim e a small change in the CCD frame rate will hardly affect the 10% coincident point. This is due to most of the events being lost within the intensifier.
• If coincidences within the intensifier and the CCD cam era are treated separately then for an intensifier dead tim e which is equal to the CCD frame tim e, the 10% coincident point will be more sensitive to changes in the intensifier dead time. Events paralyse neighbouring pores of the intensifier for an entire dead tim e after they arrive. Events captured by the CCD are only coincident w ith other events captured w ithin the same frame independent of its arrival time with respect to the s ta rt of a frame period.
4 .5 .6