Transcript Slide 1

Can FERMI-LAT Measure
Gamma Ray Polarization?
Bill Atwood, SCIPP/UCSC
8-Oct-2008
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Polarization Angle
If the incident Gamma Ray is
linearly polarized, the plane of the
e+,e- pair shows a modulation in
azimuth, f, about the direction of
the Gamma Ray..
High Energy
Gamma Ray
High Electric
Field
ee+
Pair
Z=74
Tungsten
e+
f
e-
qOP
g
Details of the LAT Conversion Telescope
Recoil
Nucleus
Polarization
Tungsten Conversion
Silicon Conversions
TUNGSTEN RADIATOR
TOP Conversions
SILICON STRIP DETECTORS
First 12 LAT Decks
Radiator = 2.8%
Silicon = 2 x .4%
Trays
= .5%
BOTTOM Conversions
(68% Convert Here)
(20% Convert Here)
SUPPORT TRAY
(12% Convert Here)
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Other Angles in the Problem
4me
 20m rad (Eg = 100 MeV)
Eg
13.6
q MS 
 (1  .038ln( ))
Eg 2
qMS in mrad, Eg in GeV,  in rad. Len.
q op 
Eg = 100 MeV
qop = .017
qMS = 21.1 mrad for Silicon Conversions!
(34.5 mrad for Tungsten Conversions)
All Conversions
Top Silicon Conversions
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Polarization Jargon
Asymetries are often used
indicating the difference in eg. a
rate for two orthogonal incoming
polarizations.
T
=
T
=
 
A
 
Polarization Report
Spring-8, 2001
The “analyzing power” of a
polarimeter is given by the
theoretical analyzing power times
dilution factors.
The dilution factors include purity
of sample and multiple scattering
due to radiator material:
LAT: Xo = .4%
AMAX = 9%
<A> = 5%
Polarization Report
Spring-8, 2001
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Separation of Silicon Conversions
Tray Level
Overall
THIN
THICK
TOP
BOTTOM
Monte Carlo location of conversions
The reconstruction places the
start of the track in the middle of
the lower SSD measuring plane
or in the middle of the tungsten
radiator above the upper SSD
measuring plane.
Reconstruction
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Separation Analysis
Bottom/Top Split: Tkr1TrayZ0 < 0
Bottom CT
(easy)
Top CT
(HARD)
These include
the Tungsten
Conversions
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The Selection of Silicon Conversions
The multiplicative factor
APurity 
Right Wrong
Right Wrong
in combination with the statistics (higher purity – fewer events)
lead to an optimization
ARelative 
APurity
Silicon Probability = .7
N Events
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Putting It All Together …
AObs  AQED  APurity  AGammaRays
APurity 
Right Wrong 664 91

 .76
Right Wrong 664 91
AObs  .05 .76 AGammaRays  .038 AGammaRays
This is probably a bit high as the average Xo is > .4% …
As a Polarimeter, LAT has an “analyzing power” of ~ 3%
Fraction of events surviving: 755 out of 11661 = 6.5% (Thin radiator & E < 2000, AGs)
Efficiency: 664 events correctly tagged / 1545 Silicon Conversions
Analysis Efficiency = 42%
AObs 
1
N Events
And so it will take
N Events  1
2
Analyzer
A
to measure AGammaRays (assumed = 1.0) to
 1 2  1111
.03
1
8
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Prognosis for Vela Polarization Measurement
In All Sky Scanning Mode the LAT detects ~ 1000 events/day from Vela
Of these >18 are tagged to be Silicon Conversion.
In 5 years: NEvents = 5 x 365 x 20. = 33 k events
AObs  .0055
AVela  .0055/ .03  .2
Next Steps
Make a real MC simulation INCLUDING polarization
effects. It seems that this is not out of the question
particularly if one is mainly concerned with the
energies above 10 MeV (B. Giebels, private comm.)
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