Transcript ppt

GLAST LAT Project
Gamma-ray Large
Area Space
Telescope
Instrument Analysis meeting, June, 24, 2005
CAL retriggering study
with SLAC data.
Alexandre Chekhtman
NRL/GMU
A.Chekhtman
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Data used for analysis
• Previous presentation was based on NRL data
– Event time tag was assigned in the SBC – could contain significant
error because of buffering delays
• Now I analysed 2 nuon runs collected at SLAC:
– Run 135001500: low FLE/FHE thresholds (3 steps above the noise
level), 616011 events, 2500 seconds
– Run 135002134: flight FLE/FHE thresholds (FLE=100 MeV, FHE=1
GeV), 462675 events, 5500 seconds
• I used following time parameters:
– GemTriggerTime: 20 MHz counter which rolls over at 225 (every 1.6
seconds)
• I add 225 every time the parameter goes back to zero, so I get the
trigger time in 20 MHz ticks, increasing monotonically from the beginning
to the end of run
– GemDeltaEventTime: 20 MHz counter giving the time between the
current trigger and the previous one.
A.Chekhtman
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Time from previous event
• Run 135001500 – low thresholds
– Huge peak at
GemDeltaEventTime < 200 µs
A.Chekhtman
• Run 135002134 – flight
thresholds
–
pure exponential
distribution
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Time from previous event - zoomed
•
Run 135001500 (low thresholds)
– Peak of retriggered events is
between dead time (26 μs) and
200 μs
– This is the period of event data
transmission from TEM to GASU
A.Chekhtman
•
Run 135002134 (flight
thresholds)
– No retriggering
– Exponential distribution goes
down to dead time (26 μs)
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Trigger bits
•
Run 135001500 (low thresholds)
– >300K events have
GemConditionsWord=12:
•
Cal_LO and Cal_HI, no Tkr
A.Chekhtman
•
Run 135002134 (flight thresholds)
– almost all events have
GemConditionsWord=2:
•
Tkr, no Cal_Hi, no Cal_LO
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Trigger bits for muons and retriggered events
•
Run 135001500, retriggered events
(GemDeltaEventTime<4000 ticks)
–
Almost all retriggered events have
GemConditionsWord=12:
•
Cal_LO and Cal_HI, no Tkr
A.Chekhtman
•
Run 135001500, muon events
(GemDeltaEventTime>4000 ticks)
– Most part of events have
GemConditionsWord=2:
•
Tkr, no Cal_Hi, no Cal_LO
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Time from previous event – “fine structure”
•
Run 135001500 (low thresholds)
–
–
–
The distribution of GemDeltaEventTime for retriggered events contains narrow and high
peaks with distance between peaks = 132 ticks
This time period corresponds to the transfer of one “data cell” when sending event data
from TEM to GASU (see LAT Inter-module communication document, page 15-17)
This fine structure confirms that the retriggering is caused by crosstalk noise produced by
TEM->GASU data transfer
A.Chekhtman
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
No fine structure, if retriggered event follows
muon event
Both plots for run 135001500 with low thresholds
–
Retriggered event after another
retriggered event
•
–
Fine structure
Explaination: retriggered events are
synchronized with the transfer of data
cells
•
Time between two retriggered events with
big probability is equal to multiple of 132
ticks
A.Chekhtman
–
Retriggered event after muon event
•
–
No fine structure (almost)
Data transfer is not synchronized
with muon event, except the case
when TEM FIFO is empty
•
Time between muon event and
retriggered event could have any
value
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Periodic event rate variations
•
Run 135001500 (low thresholds)
–
GemTriggerTime histogram (with bin=10
seconds) shows significant variations with
regular pattern and the period ~500
seconds
A.Chekhtman
•
Run 135002134 (flight thresholds)
–
GemTriggerTime histogram (with
bin=20 seconds) is flat
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Isn’t it a software bug ?
•
Run 135001500 (low thresholds)
–
Histogram plotted by ROOT
using SVAC ntuple variable
GemTriggerTime
•
Run 135001500 (low thresholds)
–
–
Histogram plotted by IDL using ldf file, read in by
Byron’s IDL reader
Only first 150K events are on the plot
•
Picture is identical to the first period of the left plot
• Results obtained from 2 different software tools are identical –
it cannot be a software bug
A.Chekhtman
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Correlation with event number
•
To verify, if the rate variations are correlated with event number, the following
procedure was used
–
Events with event numbers from 0 to (219 -1) = 524287 were split into 1024 groups (bins)
containing 512 events each:
•
–
For each bin I calculated the time dt required to generate its 512 events by subtracting
GemTriggerTime of the first event of this bin from GemTriggerTime of the first event of
next bin:
•
•
•
•
–
Rate[i]=512/Dt[i]
For each bin I calculate:
•
•
–
Dt[0] = GemTriggerTime(event=512)-GemTriggerTime(event=0)
Dt[1] = GemTriggerTime(event=1024)-GemTriggerTime(event=512)
. . . . . . . . . . . . .
Dt(1023) = GemTriggerTime(event=524288)-GemTriggerTime(event=523776)
total event rate is calculated for each bin as:
•
–
0-511, 512-1024, 1024-1536, … , 523776-524287
number of muon events Nmu[i], satisfying the condition GemDeltaEventTime>4000ticks (>200 μs)
number of retriggered events Nrtg[i], satisfying the condition GemDeltaEventTime<4000 ticks
(<200 μs)
The rates of muon events and retriggered events are calculated for each bin:
•
•
A.Chekhtman
muRate[i]=Nmu[i]/Dt[i]
rtgRate[i]=Nrtg[i]/Dt[i]
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Time per bin and the number of retriggered
events per bin vs event number
•
Run 135001500 (low thresholds)
–
–
–
•
Time per 512 triggers as a function of
event number divided by 217 = 131072
There are points with very small time
~20-50 ms per 512 events,
corresponding to multiple of 0.5
There are visible “bands” of points
A.Chekhtman
Run 135001500 (low thresholds)
–
–
Points at multiple of 0.5 corresponds to (almost) all
512 in the bin being retriggered events
Similar to left plot, there is “band” structure
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
What is magic number 131072 ?
•
•
The event number in the TEM contribution to the event data contains 17
bits: 15 bits in Event ID plus 2 bits in the Event sequence number
I suspect that the “band structure” on the previous slide is related the
number of bits set in the higher byte of the Event ID
– To verify this I calculate the number of bits set to 1 in higher byte of event
number for each group of 512 events – shown on the plot below
A.Chekhtman
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Rate of retriggered events for different number of
bits set to 1
•
•
Rate of retriggered
events strongly
correlated with number
of bits set to 1 in the
higher byte of event ID
In addition to that
there is slow decreasing
of rate at constant
number of bits within a
period
–
A.Chekhtman
another factor ?
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Rate of muon events is constant !
• The rate of muon events from the same run is shown
below – it has no variation with event number
•
A.Chekhtman
The points with big
error bars
correspond to the
bins with very few (
or no) muons due to
very high
retriggering rate.
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Discussion
• Event ID is a part of TEM contribution to event data
which is transferred to from TEM to GASU
• The noise being produced during this data transfer
process causes the calorimeter retriggering when
trigger thresholds are sufficiently low
• Probability of retriggering increases when there are
more data bits set to 1 in the event number
• Could other data bits in the TEM contribution (adc’s
etc) also produce retriggering ?
• What will happen when number of towers will be
bigger and the data transfer rate will be higher ?
Could this lead to the retriggering at higher
thresholds ?
A.Chekhtman
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Possible tests
• As proposed at the trigger meeting, we could try to
repeat the data collection with low thresholds, but
disable the data transfer of the TEM contributions to
the GASU
– In this case we could expect the the retriggering will
disappear
• Lets try to the test with different FLE/FHE
thresholds (5,10,20,50 MeV) to find where the
retriggering stops
• We could try to the charge injection with self
triggering and with auto ranging OFF, to readout
saturated LEX8 range for all crystals( containing many
bits set to 1
– will it produce high retriggering rate ?
A.Chekhtman
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GLAST LAT Project
Instrument Analysis meeting, June, 24, 2005
Conclusion
• Calorimeter retriggering at low FLE/FHE threshold is
caused by crosstalk noise produced by the data
transfer from TEM to GASU
• There is unexpected correlation of retriggering rate
with event ID
• For flight setting this effect hasn’t been found so far
• Additional tests are needed to estimate the potential
danger of this effect for complete LAT working at
high event rate.
A.Chekhtman
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