Converter

1
GLAST Large Area Telescope Adding Converter to
the Blank TKR Planes? Bill Atwood, Tune Kamae,
Steve Ritz 4 September 2002
2
Outline

• Statement of the issue and proposal
• Memo from the PI to the IS
• Investigating the need on-board albedo gamma
  rejection
• Summary of 2001 PDR studies
• New studies using GLEAM
• Potential negative impacts of additional material
• Summary

3
Statement of the Issue, Based on BFEM Studies
(Kamae, August 2002)
Study showed Low energy e-/e produced
in interactions in CAL evaporate to Tracker.
Their Typical energy is a few MeV.
If the standard 3.6RL Pb is added to the bottom
3 trays, trigger by gammas decrease to about
40. T. Kamaes Recoomendation
4
Memo from the PI
Dear Steve, I am writing to you in your
role as Instrument Scientist to request that you
carry out an evaluation of the proposal that Tune
Kamae has made to add radiators to the last two
planes of the LAT tracker. My understanding is
that the intention of this proposed change is to
reduce the trigger rate due to soft events that
boil out of the calorimeter and cause triggers in
the tracker. Given the current state of
the tracker design and the need to redesign
certain aspects of the mechanical structure of
the tracker, now is the time to consider whether
the project should implement the change that Tune
has proposed. However, the timescale is
relatively short for making a decision. After
discussion with Bill Althouse, I agree that we
need to be able to decide within the next 10
days. I realize that this allows very little
time for a thorough analysis and suggest
therefore that you, Tune Kamae and Bill Atwood
consider the possible benefits of adding
radiators and whether, from a science point of
view or with regard to implementation of on-board
triggers, if the proposed change has any likely
negative impacts. I would like to receive
a report from you on this subject by Sept
5. Regards, Peter
5
Analysis

• Review the statements on
• The L1 trigger rate (250 Hz)
• a good fraction to be downlinked
• low energy particles produced in interactions in
  CAL evaporate to Tracker
• Compare with previous studies
• Compare with new studies using GLEAM.
• Consider the potential negative impacts of
  additional material in the TKR.

6
(1) L1 Trigger Albedo Gamma Rate ?

• The following four slides are from the January
  2002 PDR presentation, showing
• the energy spectra of the background fluxes
  (orbit max and orbit average)
• the L1T rates
• Bottom line the 250 Hz estimate by Tune for the
  albedo gamma rate agrees well with the PDR study.

7
Implemented Orbit-max Background Fluxes
total
orbit-max fluxes used for trigger rate
calculations
Integrates to 10 kHz/m2

• LAT-TD-00250-01 Mizuno et al
• Note by Allan Tylka 12 May 2000, and
  presentations by Eric Grove
• AMS Alcaraz et al, Phys Lett B484(2000)p10 and
  Phys Lett B472(2000)p215
• Comparison with EGRET A-Dome rates provides a
  conservative ceiling on the total rate.

8
Implemented Orbit-average Fluxes
Integrates to 4.2 kHz/m2
orbit-avg fluxes used for downlink and final
background rejection calculations
9
Orbit Max L1 Rates

• Notes
• with the ACD throttle on the TKR trigger, the
  total max rate is lt6 kHz.
• albedo gamma rate is for zenith pointed more
  on this later, as a function of rocking angle.
• These rates are high (by 30), due to an
  implementation error in the CAL-LO trigger. They
  will be updated for the collaboration meeting.

10
Orbit Max L1 Rates
L1T unthrottled
L1T with Throttle
total 13.1 kHz
total 5.5 kHz
5 kHz line
1 kHz line
100 Hz line
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(2) A good fraction to be downlinked ?

• The following 5 slides are from the PDR
  presentation outline, showing
• the strawman onboard filter set
• the albedo gamma downlink rate
• the effects on the rates rocking away from
  zenith-pointed
• Note that, although the albedo gamma flux was
  included in all background studies, there was not
  a particular effort required to remove the albedo
  gammas the filters that were used to reject
  other backgrounds were also effective against the
  albedo gammas. Remember, any particle hitting
  the CAL can send energy up into the TKR.
• Bottom line a good fraction need not be
  downlinked. With the strawman filter, the albedo
  gamma rate to the ground is 2 Hz.

12
On-board Filters

• select quantities that are simple to calculate
  and that do not require individual sensor
  calibration constants. Filter scheme is flexible
  current set is basis for flight implementation.
• order of selections to be optimized. Grouped by
  category for presentation purposes
• ACD info match track to hit tile, count hit
  tiles at low energy

outside tile boundary
inside tile boundary
Background
100 MeV g
no tile hit
Rate after ACD selections is 180 Hz
orbit-avg (360 Hz orbit-max)
cm
cm
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On-board Filters (II)

• CAL info most of the residual rate at this point
  is due to albedo events and other upward-going
  energy events. Require track-CAL energy centroid
  loose match, fractional energy deposit in front
  layer reasonably consistent with downward EM
  energy flow. If no CAL energy, require track
  pattern inconsistent with single-prong.
• TKR info low-energy particles up the ACD-TKR gap
  easily dealt with
• project track to CAL face and
• require XY position outside this
• band for low CAL energy,
• require TKR hit pattern
• inconsistent with single prong.

Rate after CAL selections is 80 Hz orbit-avg
(130 Hz orbit-max)
Y(cm)
X (cm)
14
On-board Filters Results

• After all selections, orbit-average background
  rate is 17 Hz.

16.5 Hz total rate
composition
5 Hz line
2 Hz line
1 Hz line
Additional margin available much of the residual
rate is due to high-energy proton and electron
events with CAL Egt5GeV -- if apply ACD selections
onboard to higher energy, rate can be cut in half
(to 8 Hz), with 5 reduction in Aeff at 10 GeV.
15
Effects of Rocking Albedo Gammas
albdeo gamma
L1T with Throttle
full background flux
front
back
cos (q)
As we rock, the spike spreads in q, f
At zenith, earth horizon is at 113 degrees.
Study what happens when observatory rocks to 35
and 60 degrees off zenith.
f
f
Front
Front
Back
Back
cos(q)
cos(q)
35 degree rock
60 degree rock
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Albedo Gamma Rates

• Notes
• rates for other backgrounds will be reduced
  somewhat by the same angle cut, not taken into
  account here.
• small incremental L1T rate not a problem
• calculating the gamma direction only happens at
  a relatively low rate, if needed (after other
  filters), so incremental CPU load not a problem.
• can reduce the downlink contribution to whatever
  we need with a tighter fiducial cut.

17
Which Filters Remove Albedo Gamma Events?

• The TKR info provides the greatest reduction of
  albedo gamma events (note, these selections are
  used to reject other backgrounds, too). Require
• at least one track to be found and, if no CAL
  energy, require the bottom of the best track NOT
  point to the ACD skirt gap 60 Hz
• track to be inconsistent with a single prong (no
  extra hits anywhere near the best track) OR CAL
  energy gt 350 MeV 28 Hz
• if there is any CAL energy, the bottom of track
  must reach at least down to the 3rd TKR layer
  above the CAL 10 Hz
• NOTE both gammas and e/e- will evaporate up
  into the TKR from the CAL. The last cut in the
  above list removes the gammas. Placing more
  material closer to the CAL will kill this
  distinguishing characteristic for the subset of
  upward gammas that convert in the extra material.
• The remaining reduction to 2 Hz comes from simple
  CAL shape cuts and the ACD info (tracks exiting
  through the ACD). Again, these are used to
  remove other backgrounds, too.

18
New Studies Using GLEAM
A sequence of runs using the current GLEAM
simulations based on Geant 4 have been undertaken
to directly investigate with high statistics the
concern over the g albedo flux. Specifically 3
runs have been completed and are labeled
thus 1) SIGNAL 50K - 100 MeV gs,
normal incid. over the area of the LAT 2) ALBEDO
100K Albedo gs (E-2 spectrum with
-.405 lt cos(q) lt -.395) 3) SIDE ALBEDO 100K
Albedo gs (E-2 spectrum with -.05 lt cos(q) lt
.05) The following plots report the results
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Where do the 3-in-a-row Triggers Start?
What is the distribution of the upper most layer
in the LAT that begins a 3-in-a-row trigger?
SIDE ALBEDO
ALBEDO
SIGNAL
Tunes Spike
Layer 0 is at the TOP of the LAT - Layer 15 is
the last GLAST Super Layer Note that Layer 0
always has more - why? SIGNAL - ACD conversions
(ACD 4 rad. lens.) ALBEDO - upward moving
events leaving through the Layer 0 ACD SIDE
ALBEDO - ACD Conversions which move downwards
through Tracker
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Tower Rates
Which Towers are hit most - Corners, Sides, or
Core?
The following plots require at least a valid
3-in-a-row that the Start Layer 15 (Last
GLAST Super Layer)
SIGNAL
SIDE ALBEDO
ALBEDO
Expectation Signal would be strongest in Corners
and Sides Conclusion SIGNAL is FLAT
ALBEDO is peaked in Corners /
lesser in Sides SIDE ALBEDO
is FLAT
21
Calorimeter Energy Distributions
Expectation Events will be typified by low
energy deposited in the CAL Data cut on a valid
3-in-a-row which starts in Layer 15
SIGNAL
ALBEDO
SIDE ALBEDO
Straw-man Cut Require gt 20 MeV in CAL for Layer
15 Conversions SIGNAL
75 remains ALBEDO lt
10 remains SIDE ALBEDO lt 10
remains
22
Negative Impacts? (I)

• Many years have been spent on detailed and
  systematic background rejection studies.
  Detector modifications that would make the
  analysis easier have always been welcome provided
  they do not add significant complexity or cause
  other problems. Indeed, Atwoods original design
  was shaped by these studies.
• The main concern with adding material in the TKR
  is the impact on the background rejection
  analysis.
• Rerunning the 107 background events required to
  do a quantitative study in the proposed
  configuration would derail the ongoing software
  effort on GLEAM, so instead we rely on our
  experience.

23
Negative Impacts? (II)

• There are two categories of background that will
  likely be worsened with additional tracker
  material
• horizontal primary particles (not tracked) that
  interact in the additional material creating
  secondaries that either look like, or are,
  gammas. The lowest row of ACD tiles were added
  to help reject these in the last layers of the
  TKR, however the efficiency requirement on these
  tiles is less strict since no candidate gammas
  come from this region. Additional converter
  (which does not add effective area for science)
  will be a target for background generation.
• a major advance of GLAST over EGRET is the lack
  of a TOF system, enabling a much larger FOV. It
  is necessary for the instrument to distinguish
  upward from downward-going energy by other means.
  One method of removing upward gammas from
  primary interactions in the CAL is requiring a
  found track to be somewhere close to the CAL.
  The additional material will convert 6 more
  upward-going photons closer to the CAL, removing
  this useful distinction. The additional
  converter in the TKR will make the problem of
  upward-going event rejection worse.
• There are also concerns about good gamma
  reconstruction. Both our beam tests (1997 and
  1999-2000), which provided our detailed
  experimental check of the simulation, had no
  converter in the last tracking layers. We have
  no operational experience without the blank
  layers.
• Assessing these concerns quantitatively would
  require a detailed study.

24
Summary

• The requirement for the additional material has
  not been demonstrated. There does not appear to
  be an anomaly events of this type have been
  included in the simulations. Means for removing
  the events of concern on orbit have been
  identified. These will be reviewed at the
  collaboration meeting in October.
• Concerns with adding additional material in the
  blank tracker layers have been identified. A
  detailed study would be required to assess the
  actual impacts quantitatively.
• An easy additional, optional filter
• A CAL Energy requirement for each Start Layer
• - Would give us a natural and flexible
  throttle for downlink
• - If backgrounds are even close to those
  anticipated, only last Super layers (if that)
  would have non-zero cuts.

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Ongoing Work

• Improve angular distributions of the background
  flux implementations.
• Finish flight software filter implementation (the
  results presented were based on the strawman
  algorithms that are the starting point for the
  flight software filter design). Include the
  flight algorithms in reconstruction/analysis
  packages to study the effects in detail.