GLAST LAT Project

S. Ritz 1

GLAST Large Area Telescope:GLAST Large Area Telescope:

Adding Converter to the Blank TKR Planes?

Bill Atwood, Tune Kamae, Steve Ritz4 September 2002

Gamma-ray Large Gamma-ray Large Area Space Area Space TelescopeTelescope

GLAST LAT Project

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OutlineOutline

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

GLAST LAT Project

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Statement of the Issue, Based on BFEM Studies Statement of the Issue, Based on BFEM Studies (Kamae, August 2002)(Kamae, August 2002)

“Anomaly found: Lower portion of the tracker Was hit by 2-3 times higher flux from behind.”

“Scaling to LAT: For these events coming from the periphery, multiply by ~5 to scale to LAT.This means ~ 250Hz of albedo gamma triggers.and a good fraction of them to be downlinked”

“Study showed: Low energy e-/e+ produced ininteractions in CAL “evaporate” to Tracker. TheirTypical energy is a few MeV.”

“If the standard 3.6%RL Pb is added to the bottom 3 trays, trigger by gammas decrease to about

40%. T. Kamae’s Recoomendation”

GLAST LAT Project

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Memo from the PIMemo 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

GLAST LAT Project

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AnalysisAnalysis

• 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.

GLAST LAT Project

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(1) L1 Trigger: Albedo Gamma Rate ?(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.

GLAST LAT Project

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Implemented Orbit-max Background FluxesImplemented Orbit-max Background Fluxes

total

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.

orbit-max fluxes used for trigger rate calculations

GLAST LAT Project

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Implemented Orbit-average FluxesImplemented Orbit-average Fluxes

Integrates to ~4.2 kHz/m2

orbit-avg fluxes used for downlink and final background rejection calculations

GLAST LAT Project

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Orbit Max L1 RatesOrbit Max L1 Rates

all chimemax albedo_p_max albedo

gamma

CR e- max albedo e+e-

flux (kHz/m2) 9.9 4.2 2.6 0.92 0.043 2.2

L1T (Hz) 13,134 7,419 3,501 242 79 1,893

L1T frac 1 0.56 0.27 0.02 0.01 0.14

L1V Throttle (Hz)

5,510 2811 1,679 190 37 793

L1V Throttle frac

1 0.51 0.30 0.03 0.01 0.14

Notes:• with the ACD throttle on the TKR trigger, the total max rate is <6 kHz. • albedo gamma rate is for zenith pointed – more on this latermore 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.

GLAST LAT Project

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chime albedo albedo CRe albedo p e+e-

chime albedo albedo CRe albedo p e+e-

5 kHz line

total: 13.1 kHz

L1T unthrottled L1T with Throttle

total: 5.5 kHz

Orbit Max L1 RatesOrbit Max L1 Rates

1 kHz line

100 Hz line

GLAST LAT Project

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(2) “A good fraction to be downlinked” ?(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.

GLAST LAT Project

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On-board FiltersOn-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

Background 100 MeV 100 MeV

outside tile boundary

no tile hit

inside tile

boundary

Rate after ACD selections is 180 Hz orbit-avg(360 Hz orbit-max)

[cm] [cm]

GLAST LAT Project

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On-board Filters (II)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.

X (cm)

Y(c

m)

Rate after CAL selections is ~80 Hz orbit-avg (130 Hz orbit-max)

GLAST LAT Project

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On-board Filters ResultsOn-board Filters Results

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

chime albedo albedo CRe albedo p e+e-

5 Hz line

2 Hz line

1 Hz line

composition:

Additional margin available: much of the residual rate is due to high-energy proton and electron events with CAL E>5GeV -- 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.

16.5 Hz total rate

GLAST LAT Project

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Effects of Rocking: Albedo GammasEffects of Rocking: Albedo Gammas

cos ()

full background flux

L1T with Throttle

frontback

albdeo gamma

As we rock, the spike spreads in :

At zenith, earth horizon is at 113 degrees. Study what happens whenobservatory rocks to 35 and 60 degrees off zenith.

35 degree rock 60 degree rock

Front FrontBack Back

cos( cos(

GLAST LAT Project

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Albedo Gamma RatesAlbedo Gamma Rates

L1T rate [Hz] L1T rate with Throttle [Hz]

After filters [Hz]

After fiducial cut [Hz]

zenith 250 190 2 2

(no cut)

rock 35° 260 200 3 3

(no cut)

rock 60° 300 250 8 1 (<45°)

3 (<53°)

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.

GLAST LAT Project

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Which Filters Remove Albedo Gamma Events?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 > 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.

GLAST LAT Project

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New Studies Using GLEAMNew Studies Using GLEAM

A sequence of runs using the current GLEAM simulations based on Geant 4have been undertaken to directly investigate with high statistics the concern over the albedo flux.

Specifically 3 runs have been completed and are labeled thus:

1) SIGNAL: 50K - 100 MeV ’s, normal incid. over the area of the LAT2) ALBEDO: 100K “Albedo ’s” (E-2 spectrum with -.405 < cos() < -.395)3) SIDE ALBEDO: 100K “Albedo ’s” (E-2 spectrum with -.05 < cos() < .05)

The following plots report the results

GLAST LAT Project

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Where do the 3-in-a-row Triggers Start?Where do the 3-in-a-row Triggers Start?

What is the distribution of the upper most layer in the LAT that beginsa 3-in-a-row trigger?

SIGNAL ALBEDO SIDE ALBEDOTune’s 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 & ACDSIDE ALBEDO - ACD Conversions which move downwards through Tracker

GLAST LAT Project

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Tower RatesTower 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 ALBEDO SIDE 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

GLAST LAT Project

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Calorimeter Energy DistributionsCalorimeter 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 > 20 MeV in CAL for Layer 15 Conversions SIGNAL: ~ 75% remains ALBEDO: < 10% remains SIDE ALBEDO: < 10% remains

GLAST LAT Project

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Negative Impacts? (I)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, Atwood’s 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.

GLAST LAT Project

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Negative Impacts? (II)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.

GLAST LAT Project

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SummarySummary

• 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.

GLAST LAT Project

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Ongoing WorkOngoing 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.