Large-Area Balloon-Borne Polarized Gamma Ray Observer
 - PoGO -
V. Andersson, P. Chen, T. Kamae, G. Madejski, J. Ng, T. Mizuno, H. Tajima, T. Thurston (SLAC, Menlo Park, CA);
L. Barbier, A. Harding, J. Krizmanic, J. Mitchell, R. Streitmatter (GSFC, Greenbelt, MD);
E. Groth, R. Fernholz, D. Marlow (Princeton University, Princeton, NJ);
G. Bogaert (Ecole Polytechnique, Palaiseau, France);
S. Gunji, H. Sakurai, Y. Yamashita (Yamagata University, Yamagata, Japan);
Y. Saito, T. Takahashi (ISAS, Sagamihara, Japan);
M. Arimoto, T. Ikegawa, Y. Kanai, J. Kataoka, N. Kawai, Y. Yatsu (Tokyo Inst. of Technology, Tokyo, Japan);
Y. Fukazawa (Hiroshima Univ., Higashi-Hiroshima, Japan);
P. Carlson, W. Klamra, M. Pearce, M. Suhonen (Royal Inst. of Technology, Stockholm, Sweden);
S. Larsson (Stockholm University, Stockholm, Sweden)
Eincident [kev]
Escatter [keV] Eelectron [keV]
q=120 deg
 200
  100
 
  
     0
 
20     
180     
60     
100     
140     
q=60 deg
q=90 deg
q=60 deg
q=120 deg
q=90 deg
Crude measurement of Escatter and Eelectron by plastic scintillation counters (fwhm~50-100%) can differentiate the scattering site and the photo-absorption site of the scattered photon.
Scattered
photon
(Escatter)
Incident
photon
(Eincident)
Recoil electron (Eelectron)
q
Azimuth
angle
Pol plane
Scattering
Angle q
Simulation of PoGO by EGS4 and revised Geant4 (Y.F, T.M. & H.T.)
23-24%
Mod factor (fraction) = (Max-Min)/(Max+Min)
 
Conceptual design of the instrument  (number of units will be greater than shown here): a) Isometric view; (b) View from the front of the instrument; (c) Vertical cross-section of the instrument.   The proposed instrument will have 200-400 units and L1 + L2 in (c) will be ~60cm.
(a)
(b)
(c)
Design of PoGO: Concept  – Well-type Phoswich Counters
Trigger and Pulse-Shape-Discrimination: L0, L1, L2
Unit
Detector Assembly
Pulse-Shape Discrimination
polarization
plane
30degree
y
x
Beam Direction
1
2
3
4
5
6
7
§   Beam goes into the slide at the center of Unit#4. 
§   Polarization plane is along x-axis.
§   Set-up rotated about the beam at 30 deg steps.
•  Coincidence between #4 and a peripheral counter
•  Eincident=60.2, 73.2, 83.5 keV
Consistent with single
Compton scatt. in #4
Gap btwn
valid Compton events and background events
ADC reading of Unit#4
Sum of the 2 ADC readings
Likely to have scattered
more than once in #4
EX=73.2keV
Key Features of PoGO
ü  Energy 25-80 keV: Compton scatt. around Black Holes, Neutron Stars, AGNs.
ü  FOV 5 sq-deg: Well-type Phoswich Design for low background (<10mCrab).
ü  Inexpensive and easy-to-maintain: plastic scinti., BGO, and PMTs.
ü  Detect to 6-10% at 3s in 100 mCrab sources in a 6-hr balloon flight. 
Timeline of PoGO
Ø Summer, 2002: Group formation and prototyping began
Ø Feb. 20, 2003:   Kick-off meeting
Ø Apr. 18, 2003:   Application to a Research Opportunities in Space Science program (NASA)
Ø July 7-8, 03:      First short beam test at Spring 8
Ø  Fall, 2003:        Selected by NASA as a Research Opportunities in Space Science program
Ø  Nov. 12-18, 03: Beam test of a prototype (a system of 7 unit) at Advanced Photon Source (ANL)
Ø  Apr. 2004:        Selected by Monkasho (Japan) as a Grant-in-Aid project.
Ø  Sept. 2004:        Application for funds submitted in Sweden.
Ø  Dec. 2004:         Beam test of a proto-flight unit at Photon Factory (KEK)
Ø  Spring 2005:     Beam test of a prototype (a system of 7 unit) at Advanced Photon Source (ANL)
Ø  Summer 2007:  The flight instrument (the 217 unit configuration) complete
Principle: Yes,  good old Compton scattering in Plastic Scintillators
Background rejection: An array of Well-type Phoswich Counter made of Fast and Slow Plastic Scintillators, and BGO
Flight PMT Assembly
(Incl. HV DC-DC Converter)
Proven to work very well: Nov. 2003 beam test at Advanced Photon Source (ANL)
Single photo-
electron peak
55Fe peak
Flight PMT with small fast scinti,
241Am source. FWHM = 26.5%
Flight PMT with small fast scinti.
55Fe source. 8 p.e. / 5.9 keV
241Am (60 keV)
55Fe (6 keV)
FWHM = 26.5%
5.8 p.e.
31.8%
FM 3
6.0 p.e.
31.0%
FM 2
5.5 p.e.
32.5%
FM 1
6.2 p.e.
30.0%
Prototype
Light Yield
(5.9 keV)
FWHM
(60 keV)
Fast Scinti.
PoGO fast scinti. + BGO attached to R580 PMT radiated by 241Am
BGO branch
Fast scintillator
branch
0
30
60
90
120
150
0
30
60
90
120
150
(keV)
(keV)
Fast Scintillator (Eljen Technology) and PMT (Hamamatsu 1” f, H3178X)
Slow Scintillator: (5 prototypes received)
- Good light transmission achieved -
Light transmission along 60cm lenght of
2mm thick hexagonal slow scintillator tube.
Positions of 90Sr-source
Ratio peak6 / peak1
#1: 3.4, #2: 3.0, #3: 2.8, #4: 3.0, #5: 2.5
Pulse-Shape Discrimination works
Comparison between Pulse Heights
from Fast and Slow Shaping Amplifiers
PMT
“Cosmic-ray background”
Sr90 (b-:Emax=2.28MeV)
at 3 locations
60cm
20cm
“Signal”
Am241
(g:60keV)
Fast Shaper
(t~20ns)
Slow Shaper
(t-300ns)
Improvement by fitting the fast pulse shape
Spctr after “Signal Region” cut
Spctr without Sr90 bkgnd
60keV
Spctr of the Signal
Spectrum of Am241 (60keV) after cut
Scatter plot: Fast vs Slow
Signal Region
Bkgnd Region
Figure: Simulated data that typifies the statistical accuracy expected from a 6-hour observation by PoGO.
Can determine the Pulsar Model at ~20s in one 6hr balloon flight !!
Design-397
Design-397
Can measure Pol of hard X-rays from Cygnus X-1 in ANY know states to s ~ 2-5% 
Design-397/217
Design-397/217
Characterization of crucial flight model components done: PMTs, fast scintillators, and slow scintillators meet requirement
Slow Scinti(active collimator)
Fast Scinti
60cm
20cm
One Well-Unit
PMT
BGO
Polarized hard X-ray beam: Pol>98%, E=60.2, 73.2, 83.5 keV
Fast Scinti. (Eljen Technology) and PMT (1” f, H3178X)
Ratio: 3.4
Pulse Height
Compton scatt. site and photo-abs. site separated
ch1
ch3
ch5
ch2
ch6
ch7
Mod. factor measure with the set-up (different from the FM)
73.2keV
• MF~(2400-1000)/(2400+1000)~41%
• Small differences btwn ch1/ch7, ch2/ch6, and ch3/ch5.
   are a little smaller than those at 60.2keV
41%
Background Rejection Scheme Demonstrated: Test in laboratory
With fitting
Fit for fast shaper output based on three sample points.
w/o fitting
Signal unaffected even in
>100 times background
Great science capability: Pol of phase-resolved Crab pulsar, Cygnus X-1 in any phase
We can measure Pol
for P1 and P2 to s~6-10%
in one 6-hour balloon flight