the advanced scintillator compton telescope (ascot

Future Gamma-Ray Observatories - GSFC - March 24, 2016

The Advanced Scintillator Compton Telescope (ASCOT) Balloon Project

P. F. Bloser, M. L. McConnell, J. M. Ryan J. S. Legere, C. M. Bancroft, T. Sharma, A. Wright, T. Gorte

Space Science CenterUniversity of New Hampshire

Philosophy

• The ASCOT project is motivated by the theory that the most cost-effective, low-risk way to implement an advanced, general-purpose Compton telescope is to build directly on the experience of COMPTEL

• A advanced, scintillator-based Compton telescope would use modern detector materials to improve efficiency, energy resolution, and time-of-flight (ToF) resolution for background rejection

• It would also use advanced light readout devices, such as silicon photomultipliers (SiPMs), to reduce passive mass, volume, and power


COMPTEL Background


B

CGRO Platform

COMPTEL EGRET/OSSE

B

DC

AA

C

Kappadath, S. C., 1998, Ph.D. Thesis, University of New Hampshire

B A

C D

C near D1

“Good” ToF Window

ToF was critical to COMPTEL’s sensitivity:

COMPTEL suffered intense background from particle interactions:

Improved ToF resolution will greatly reduce background in straightforward

manner

NaI D2

Organic Liquid D1

Balloon Flight DemonstrationsUNH has conducted two successful balloon flight tests of available technology that would enable an advanced scintillator Compton telescope:

– The FAst Compton TELescope (FACTEL) experiment (September 2011): new scintillators (collaboration with LANL)

– The Solar Compton Telescope (SolCompT) experiment (August 2014): SiPM readouts


FACTEL Experiment• Compton telescope consisting of three 1-inch liquid organic D1

scintillators and three 1-inch LaBr3 D2 scintillators, all read out by fast PMTs

• D1-D2 separation of ~30 cm• D1 surrounded by plastic ACS• Pressure vessel, PC-104 flight computer


Liquid LaBr3 PMT

FACTEL Flight Results (2011)


• ToF spectrum fully described by two Gaussians

• “Down” and “Up” gammas cleanly separated

• FWHM ~ 1.2 ns

• Downward moving gamma spectrum agrees with Geant4 simulations

Julien, M., et al., 2012 IEEE NSS Conference Record, 1893

The Silicon Photomultiplier (SiPM)• Scintillator detectors limited by size, mass, and power of readout• A SiPM (aka SSPM, MPPC) is a summed array of tiny (~50 µm)

silicon APDs reverse-biased slightly above breakdown voltage in limited Geiger mode; recovers in 10s of ns

• SiPMs are compact, light, robust, low power, LV (30 - 70 V), and have gain (105 - 106), timing (~1 ns rise time), and effective quantum efficiency (20% - 30%) equivalent to PMTs

• Replacing PMTs with SiPMs in a Compton telescope would eliminate passive material, reduce mass, and allow closer packing

Future Gamma-Ray Observatories - GSFC - March 24, 2016 (Stapels et al. 2005)


SolCompT Experiment• 2-element Compton telescope: D1 is 1-inch

stilbene, D2 is 26 mm × 26 mm × 26 mm LaBr3

• Both read out using 2 × 2 array of Hamamatsu S11828-3344 MPPCs with transformer FEE for low input impedance

• Payload used hardware (pressure vessel, heaters, readout electronics, and PC-104 computer) flown in 2011 as part of the FACTEL payload

• Tagged 60Co source (~240 nCi) to monitor gain and energy resolution

D1

D2

15 cm

Frame

SolCompT Flight Results (2014)Although only had 3.75 hours at float, tagged events 60Co show good ToF and energy spectra:


-4 -2 0 2 4ToF (ns)

0

20

40

60

80

100

120

Cou

nts

0 500 1000 1500 2000Energy (keV)

0

2

4

6

8

10

Cou

nts

FWHM ~ 760 ps

-4 -2 0 2 4ToF (ns)

0

20

40

60

Cou

nts

Untagged events harder to interpret due to small spacing and surrounding material –but still see up vs. down on top of broad continuum:

Bloser, P. F., et al., 2016, NIM-A, 812, 92

The ASCOT Balloon Project• We are conducting a program to fly a larger scintillator-based

Compton telescope with SiPM readouts on a balloon and observe the Crab in a 1-day flight

• D1 will be p-terphenyl organic scintillator; D2 will be CeBr3 (due to difficulties with Saint-Gobain, also lower internal background)

• Will use the SensL MicroFC-60035-SMT 6 mm × 6 mm SiPM –has “fast” output, good for ToF


1.8 m

CSBF Rotator

Elevation Drive

Batteries

Power Relay Board

Mini-SIP

GPS Antenna

1.4 m

ASCOT Balloon Instrument• Instrument concept: basic “module” with 8 × 8 scintillator array

optically coupled to a 8 × 8 SiPM array• Each scintillator 15 × 15 × 25 mm3

• Each scintillator read out by 2 × 2 SiPM array (same as lab tests)• Detector layers each 2 × 2 array of modules• Two D1 layers, one D2 layer (cost)• Estimate ~4σ Crab detection


Cover

Scintillator

SiPM (2x2)SiPM Array Board

15 cm

VETO Panel

D1 Layers

(p-terphenyl)

15 cm

D2 Layer

(CeBr3)

FRP Frame

Instrument

Electronics

Flight

Computer

35 cm

ASCOT D1: P-terphenyl• 142 p-terphenyl crystals have been delivered • Show good uniformity, light output• Will be assembled into 8 × 8 array with Delrin housing


0 200 400 600 800 1000 1200Energy (keV)

0

500

1000

1500

Cou

nts

DataSimulation

0 1000 2000 3000 4000Pulse Height

1100

1200

1300

1400

1500

1600

PSD

252Cf

Neutrons

Gammas

Test data with test SiPM array

Project SMART Balloon Flight• A prototype D1 detector was flown as a weather balloon payload

by high school students participating in UNH’s Project SMART• Reached altitude of ~31 km, counting rate shows expected

behavior (Pfotzer Maximum)• “Poor man’s thermal/vac test”


0 5.0×103 1.0×104 1.5×104 2.0×104 2.5×104 3.0×104

Altitude (m)

0

5

10

15

20

Scin

tilla

tor R

ate

(cts

s-1)

ASCOT D2: CeBr3 Array• 8 × 8 vacuum-rated array made by Scionix• Initial testing shows slightly reduced energy resolution

compared to 13 × 13 × 13 mm3 LaBr3 crystal• Due to light loss via entrance window (i.e., cross talk):


0 500 1000 1500 2000 2500Energy (keV)

0

5

10

15

20

25

Ener

gy R

esol

utio

n (%

FWH

M)

CeBr3LaBr3

0 200 400 600 800Energy (keV)

0

50

100

150

Cou

nts/

keV

∆E/E = 6.2% @ 662 keV

Time of Flight

• Measured ToF resolution between one p-terphenyl and one CeBr3 pixel using custom CFD/TAC board

• We see very little PH-dependent walk in timing• For coincident 60Co events with large PH, get:


527 ± 34 ps (FWHM)

ASCOT Instrument• Mechanical design is underway• Electronics design as well• SiPMs will be mounted on 8-pixel “strip” boards,

plugged into motherboard


Initial Simulations• Preliminary simulations done with MGGPOD indicate

~4σ Crab detection• Have begun using MEGAlib for more detailed work


1 10Energy (MeV)

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

Effe

ctive

Are

a (c

m2 )

5

6

7

8

9

10

ARM

(deg

FW

HM)

Effective Area

Angular Resolution

ASCOT Balloon Response

1Energy (MeV)

1

10

100

1000

Cou

nts

MeV

-1

Simulation of Potential Explorer Mission

• Explorer-sized instrument concept: 7 × 7 array of modules forms a “layer”

• Three D1 layers and three D2 layers, 50 cm separation

• Assume FRP frame (as in FACTEL), plastic ACS

• Estimate 120 × 120 × 100 cm3 instrument, ~1000 kg payload

• Simulate response and background with MGGPOD – assume radiation inputs from Advanced Compton Telescope Concept Study for 5º inclination, 550 km LEO

• Use measured detector response


MGGPOD Payload Model

Hmm – why not on sides??

Simulated Compton Telescope Performance

Simulations indicate that an Explorer-sized Compton telescope using this technology would greatly improve on the performance of COMPTEL:


1 10Energy (MeV)

0

10

20

30

40

50

60

70

Effe

ctive

Are

a (c

m2 )

COMPTEL 0o

ASCOT 0o

ASCOT 30o

ASCOT 60o

1 10Energy (MeV)

10-6

10-5

10-4

10-3

phot

ons

cm-2 s

-1 M

eV-1

COMPTELASCOT

Cyg X-1 Hard State

Cyg X-1 Soft State

3m Sensitivity4 x 105 s

1Energy (MeV)

10-6

10-5

10-4

10-3

10-2

10-1

phot

ons

cm-2 s

-1 M

eV-1

3m Sensitivity

Tobs

= 106 sT

obs = 107 s

Much greater effective area than COMPTEL, especially below 1 MeV

~8 times better on-axis continuum sensitivity around 1 MeV for 2-week observation (compare to COMPTEL Cyg X-1 spectrum)

Instrument could study MeV spectra of multiple Galactic black holes (spectra from OSSE)

the advanced scintillator compton telescope (ascot

Documents