Gamma-ray observations in the medium energy range (0.50-50.0 MeV) are central to unfolding many outstanding questions in astrophysics. The challenges of medium-energy gamma-ray observations, however, are the low photon statistics and large backgrounds. We review these questions, address the telescope technology requirements, and describe our development of the 3-Dimensional Track Imaging (3-DTI) Compton telescope and its performance for a new mediumenergy gamma-ray mission. The 3-DTI is a large-volume time projection chamber (TPC) with a 2-dimensional gas micro-well detector (MWD) readout

Baker, Robert G

Barbier, Louis M

Bloser, Peter F.

Cooper, La Vida

De Nolfo, Geoergia A

Hunter, Stanley D

Krizmanic, John F

Link, Jason T

McConnell, Mark L

Ryan, James M.

Singh, Satpal

Son, Seunghee

English

University of New Hampshire

University of New Hampshire
University of New Hampshire Scholars' Repository
Space Science Center Institute for the Study of Earth, Oceans, and Space(EOS)
2006
Medium-Energy Gamma-Ray Astrophysics with
the 3-DTI Gamma-Ray Telescope
Stanley D. Hunter
NASA Goddard Space Flight Ctr
Robert G. Baker
NASA Goddard Space Flight Ctr
Louis M. Barbier
NASA/Goddard Space Flight Center
Peter F. Bloser
University of New Hampshire, Peter.Bloser@unh.edu
La Vida Cooper
NASA Goddard Space Flight Ctr
See next page for additional authors
Follow this and additional works at: https://scholars.unh.edu/ssc
Part of the Astrophysics and Astronomy Commons
This Conference Proceeding is brought to you for free and open access by the Institute for the Study of Earth, Oceans, and Space (EOS) at University of
New Hampshire Scholars' Repository. It has been accepted for inclusion in Space Science Center by an authorized administrator of University of New
Hampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.
Recommended Citation
Hunter, S.D.; Baker, R.G.; Barbier, L.M.; Bloser, P.F.; Cooper, L.; Krizmanic, J.F.; Link, J.T.; McConnell, M.L.; de Nolfo, G.A.; Ryan,
J.M.; Singh, S.; Seunghee Son, "Medium-Energy Gamma-Ray Astrophysics with the 3-DTI Gamma-Ray Telescope," Nuclear Science
Symposium Conference Record, 2006. IEEE , vol.2, no., pp.778,782, Oct. 29 2006-Nov. 1 2006
Authors
Stanley D. Hunter, Robert G. Baker, Louis M. Barbier, Peter F. Bloser, La Vida Cooper, John F. Krizmanic,
Jason T. Link, Mark L. McConnell, Geoergia A. De Nolfo, James M. Ryan, Satpal Singh, and Seunghee Son
This conference proceeding is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/ssc/195
 
Medium-Energy Gamma-Ray Astrophysics with  
the 3-DTI Gamma-Ray Telescope  
Stanley D. Hunter, Robert G. Baker, Louis M. Barbier, Peter F. Bloser, LaVida Cooper, John F. Krizmanic,  
Jason T. Link, Mark L. McConnell, Georgia A. de Nolfo, James M. Ryan, Satpal Singh, and Seunghee Son  
 Abstract– Gamma-ray observations in the medium energy 
range (0.50-50.0 MeV) are central to unfolding many outstanding 
questions in astrophysics. The challenges of medium-energy 
gamma-ray observations, however, are the low photon statistics 
and large backgrounds. We review these questions, address the 
telescope technology requirements, and describe our 
development of the 3-Dimensional Track Imaging (3-DTI) 
Compton telescope and its performance for a new medium-
energy gamma-ray mission.  The 3-DTI is a large-volume time 
projection chamber (TPC) with a 2-dimensional gas micro-well 
detector (MWD) readout.  
I. INTRODUCTION 
he first major full-sky survey in medium-energy (0.4 to 30 
MeV) gamma-ray astrophysics was made with the 
COMPTEL instrument on the Gamma Ray Observatory from 
April 5, 1991 to June 4, 2000.  The COMPTEL results 
addressed a broad range of astrophysics including cosmic 
particle accelerators, nucleosynthesis and cosmic radioactivity, 
Galactic diffuse emission, and cosmology.  The COMPTEL 
catalog of objects includes Active Galactic Nuclei & Blazars 
(Mkn 421, 3C273), Super Nova Remnants (Crab Nebula), 
Pulsars & Magnetars (PSR 1509-58), and Black Hole Binaries 
(Cyg X-1).  These observations indicate that we are on the 
threshold of major discoveries addressing fundamental 
questions of thermal and relativistic astrophysics in the 
universe.   
Expanding on the COMPTEL science results requires 
development of a future gamma-ray telescope with greatly 
improved sensitivity by increasing the overall aperture and 
effectively addressing background rejection.  In addition, this 
telescope will need to incorporate several measurement 
techniques since the objects that produce these gamma-rays 
are both extended & point-like, transient & steady, and include 
both continuum & line emissions.  A study of these techniques 
has recently been completed for the Advanced Compton 
Telescope (ACT), one of NASA's future Vision Missions [1].  
The ACT report calls for an increase in sensitivity of about 
100 times that of COMPTEL.  The primary science objective 
assumed for the ACT study report is to detect the 56Co (0.847 
                                                        
Manuscript received November 17, 2006.   
S. D. Hunter (telephone: 301-286-7280, e-mail: Stanley.D.Hunter@ 
nasa.gov), L. M. Barbier, R. G. Baker, LaVida Cooper, & S. Singh are with 
NASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA  
G. A. de Nolfo, J. F. Krizmanic, and J. T. Link with CRESST/USRA 
working at NASA/GSFC, Greenbelt, MD 20771 USA 
S. Son is a NASA Post Doctoral fellow working at NASA/GSFC, 
Greenbelt, MD 20771 USA 
P. F. Bloser, M. L. McConnell, and J. M. Ryan are with University of New 
Hampshire, Space Science Center, Durham, NH 03824 USA  
MeV) line produced in thermonuclear supernovae (SNe Ia), 
which will shed light on the dynamics of nuclear burning.  
This narrow science goal was used to define a baseline 
instrument design.  The desired performance of this baseline 
instrument has an effective area of about 103 cm2, broad-line 
(3%) sensitivity of 1.2×10-6 ph cm-2 s-1 at 0.847 MeV, 
narrow-line sensitivity of 5×10-7 ph cm-2 s-1, and continuum 
sensitivity of (1/E) ×10-5 ph cm-2 s-1 MeV-1.  This performance 
represents an increase in narrow- and broad-line sensitivity of 
about 100-fold over COMPTEL.  
The ACT mission is not expected to start before 2012, with 
a launch date well after the end of the INTEGRAL and 
GLAST missions.  However, between the start of the GLAST 
mission and the ACT missions, there is a possibility of an 
intermediate, MIDEX scale, medium-energy gamma-ray 
mission.  
This intermediate mission, with a 10-fold increase in 
narrow-line, broad-line, and continuum sensitivity over 
COMPTEL, would bridge the science from COMPTEL to 
ACT, fill in the multi-wavelength coverage gap between 
INTEGRAL and GLAST, and substantially advance our 
scientific understanding of transition between thermal and 
relativistic processes in the universe.  
The science goals enabled by a 10-fold increase in 
sensitivity translate into three requirements that drive the 
design of a medium-energy telescope.  
 Address the widest range of science goals by optimizing 
the sensitivity to both line and continuum emission, to both 
point-like and extended objects, and to both steady state 
and transient sources.  
 Extend the energy range down to 0.3 MeV and up to 50 
MeV by photon imaging from both Compton scattering 
and pair production interactions.  
 Maximize the effective area by improving the background 
rejection.  
In the following, we describe our efforts towards the design 
and fabrication of a Compton telescope suitable for this 
intermediate mission.  
II. COMPTON SCATTERING TELESCOPE DESIGN  
Compton scattering telescopes, Fig. 1, use two separated, 
position sensitive detectors.  An incident gamma ray scatters 
in the first detector (D1, track imager) producing a recoil 
electron.  The scattered gamma ray is then totally absorbed 
after one or more interactions in the second detector (D2, 
calorimeter).  Measurement of the position of the single 
T 
2006 IEEE Nuclear Science Symposium Conference Record N21-2
1-4244-0561-0/06/$20.00 ©2006 IEEE. 778
 
Fig. 1. Schematic of the MEGA medium-energy 
telescope [2]. The Si-strip D1 track imager provides only 
a few (< ~20) position measures of the recoil electron 
tracks from Compton scattering or pair production.  
D1 
D2 
Compton scatter and the total energy of the recoil electron in 
D1, and the position and energy of the scattered gamma ray in 
D2, is sufficient to determine the energy of the incident 
gamma ray and the Compton scattering angle ϕ, Fig. 2. Thus 
the point spread function (PSF) of a single event is a circle on 
the sky called the event circle, Fig. 2a.  The MEGA medium-
energy gamma-ray telescope [2], is one concept for a more 
advanced Compton telescope with tracking of the recoil 
electron, uses multiple layers of planar X- and Y-position 
sensitive silicon-strips as the D1 detector, Fig. 1. The MEGA 
design provides a few (< 20) position measurements of the 
recoil electron track, which constrains the plane of the 
interactions.  Thus the event circle is reduced to an event arc 
with angular length ∆θ, Fig. 2b.  The length of the arc is 
determined by the accuracy with which the initial recoil 
electron direction can be constrained, which is in turn limited 
by multiple scattering in the tracker material. 
The Compton scatter angle, ϕ, and energy of the incident 
gamma ray can be derived from measurements of the energy 
of the recoil electron and the scattered gamma ray using the 
Compton formula.  The direction of the incident gamma ray is 
equally probable from any point on the event circle 
determined by the scatter angle.  Measuring the direction of 
recoil electron reduces the event circle to an event arc with 
azimuthal extent, ∆θ, Fig. 2b. For a Compton telescope, the 
background associated with any observed photon is 
proportional to the area of the event circle, sin ϕ ∆ϕ ∆θ, thus 
reducing the azimuthal extent of the arc reduces the observed 
background and hence increases the sensitivity of the 
telescope. The ability to constrain the incident gamma-ray 
direction with highly precise imaging techniques and thus 
dramatic improvements in sensitivity opens up the field of 
medium-energy gamma-ray observations to many exciting 
new potential discoveries. 
The science goals and design drivers of an intermediate 
Compton telescope mission can be achieved with the MEGA 
two-detector design with an improved D1 track imager. 
Towards this goal, we are developing the Three-Dimensional 
Track Imager (3-DTI) that provides hundreds of track position 
measurements in a low-density gaseous medium as part of a 
complete Compton telescope for an intermediate medium-
energy gamma-ray mission.  
III. THE 3-DTI TRACK IMAGER 
The 3-DTI, Fig. 3, is large volume time projection chamber 
(TPC) with a two-dimensional gas micro-well detector 
(MWD) readout plane. The 3-DTI is designed to measure the 
three-dimension momentum of charged particles by high 
spatial resolution sampling of the charged particle ionization 
tracks in a low density, isotropic, fully active gas volume. The 
MWD provides X- and Y-coordinates and the 3rd dimension 
is obtained by measuring the drift time of the ionization 
electrons.  We describe the 3-DTI TPC and MWD design, 
front-end and transient digitizer electronics, segmented 
calorimeter, and performance in the following sections.  
A. TPC and MWD Design 
The micro-well detector is a type of gas proportional 
counter related to the gas electron multiplier [3].  In contrast to 
multi-wire proportional counter construction, the anodes and 
cathodes of all these detectors are rigidly affixed to an 
Fig. 2. (a) Compton scattering without electron tracking 
constrains the incident gamma ray direction on an event 
circle, (b) Tracking the recoil electron reduces the event 
circle to an arc.  The uncertainty in the electron azimuth 
determines the arc length.  
779
 
insulating substrate, Fig. 4. This greatly simplifies their 
construction and allows the anode and cathode pitches to be 
smaller than is possible with multi-wire construction.  This 
geometry is also inherently two-dimensional; the anodes give 
one coordinate and the cathodes the other.  
An array of wells forms a detector.  The active tracking 
volume is bounded by a drift electrode on the top and an array 
of wells on the bottom.  Ionization electrons produced as a 
charged particle looses energy traversing the gas drift toward 
the anode into the wells of the detector.  An ionization 
avalanche occurs in the well, where there is an intense electric 
field set up by the voltage difference applied between the 
anode and cathode. The micro-wells serve as the proportional 
counter where the electric field strength is large enough to 
produce secondary ionization of gas atoms.  The ensuing 
avalanche of electrons collects on the anode while charge of 
equal magnitude but opposite sign collects on the cathode.  
1) TPC 
The TPC and MWD are housed in a pressure vessel which 
serving as the TPC chamber. Laboratory testing is being done 
with P-10 (90% argon, 10% methane) nominally at 760 Torr.  
The TPC chamber assembly includes a drift electrode held at 1 
kV and employs a field-shaping grid along the chamber.  
2) MWD 
We have previously constructed two-dimensional crossed-
strip MWDs and demonstrated gas gain up to 30,000 and 
spatial resolution of ~200 µm [5]. We are currently exploring 
ways to produce MWDs that are suitable for commercial 
fabrication of detectors up to 50 × 50 cm2 in size. MWDs are 
laminated substrates where the laminate thickness determines 
the well thickness and thus the gain.  Our MWDs consist of 5 
µm copper foil on laminate substrates such as Vacrel and 
Kapton, Fig. 5.  The electrode pattern is a photochemical 
process.  We are considering several different methods for the 
micro-well machining such as a photochemical etch or laser 
ablation which are ideal for large area production with 
relatively little process contamination. Both 5x5 cm2 and 
10x10 cm2 MWDs have been produced.  The wells have a 
diameter and depth of 200µm with a pitch of 400µm and a gap 
between the electrodes of 5µm, resulting in a total of 448 
micro-wells.  
Fig. 4. Schematic view of a micro-well detector.  The anodes, 
blue, are separated from the cathodes, red, by an insulating 
layer, light green.  The MWD is mounted on a rigid substrate, 
dark green. 
2D micro-well 
Fig. 3. The 3-DTI is a large volume time projection chamber 
with 2-D micro-well readout.  The ionization charge in the 
TPC volume drifts towards the MWD and into the individual 
wells.  The strong electric field in the wells produces an 
avalanche of secondary ionization.   The avalanche charge is 
collected on the anode strip under the well and an equal but 
opposite mirror charge appears on the cathode.  
Fig. 5. SEM Micrograph of a micro-well detector.  The wells 
and anode strips can be seen through the holes in the cathode 
strips  
780
 
3) Front-end Electronics and Transient Digitizer 
Each anode and cathode electrode of the MWD is connected 
to a front-end electronics (FEE) channel consisting of a charge 
sensitive preamplifier, a CR-RC pulse shaper with gain, and a 
differential driver. The FEE is a bipolar design to 
accommodate the negative anode signal and positive, cathode 
mirror signal. To maximize design flexibility, we have chosen 
to implement the FEE design using discrete components. This 
initial approach greatly reduced the time required for the 
design and testing, and permits in-house fabrication. The 
components used are the AD8610, AD8672 and AD8137 for 
the charge sensitive preamplifier, the shaping and gain amps, 
and the driver, respectively. Sixteen FEE channels were 
fabricated on each front-end card (FEC) which also has the 
high voltage distribution buss. Our data acquisition is done in 
LabView. The response of each stage of the FEE to a test 
pulse input is shown in Fig. 6. 
The arrival time of charge pulses from each FEE channel is 
recorded with a transient digitizer. Since the ionization charge 
drifts at a constant velocity, the relative pulse arrival time 
provides the third coordinate of the charge position. In order to 
achieve equal resolution in all three spatial coordinates, the 
timing resolution must be equal to the pixel pitch divided by 
the electron drift velocity. To satisfy the above condition, a 2.5 
MHz, 12 bit resolution custom Transient Digitizer (TD) board 
was designed and has been fabricated. The digitized positive 
and negative output from an anode and cathode is shown in 
Fig. 7.  
B. Calorimeter (D2 Detector) 
Surrounding the 3-DTI on all four sides and along the 
bottom will be a segmented calorimeter with an inner layer of 
LaBr3 crystals and an outer layer of NaI crystals.  Surrounding 
the calorimeter will be anti-coincidence scintillator.  We are 
currently testing, simulating, and optimizing the configuration 
of scintillation crystals to provide the needed energy and 
position resolution, and to enable a viable, affordable detector.   
C. Instrument Performance 
We tested the 3-DTI prototype in the laboratory using an 
55Fe source.  Figure 4 shows the energy spectrum for 5.9 keV 
X-rays with a secondary peak from the Ar escape peak at 2.7 
keV.  Our energy resolution is ~ 20% based on the 5.9 keV x-
ray peak in 90% argon with 10% methane at a gain of 103.   
Gas-gain measurements have been made with a similar 
micro-well configuration [4] and exhibit stable operation at 
gas-gains in excess of 30,000.  Fig. 8 shows the avalanche or 
gas gain versus high voltage curves for a 200 µm thick MWD.  
The maximum gain observed to date is ~103. The energy 
resolution of this MWD is shown in Fig. 9 for a voltage 
difference between the anode and cathode of 1000 V and.  The 
charge collected on the MWDs is sent to front-end electronics 
boards (FEE) for charge-integrating amplification and shaping 
and to transient digitizer boards (TD) for time projection 
determination. 
Fig. 7. Digitized positive and negative outputs from an anode 
and cathode in response to two separate 55Fe events.  
Fig. 8. Micro-well detector (MWD) gas gain (avalanche 
charge multiplication) measured in P-10 (90% Ar, 10% CH4) 
as a function of anode-cathode voltage. 
Test pulse input 
Preamp output 
Shaper, gain amp output 
Fig. 6. Response of front-end electronics (FEE) to a test 
input pulse across a 2 pF capacitor.  The overall gain is 
about 8 mV/fC.  
781
 
 
D. Development Plans 
Arrangements have been made to test a 10 cm2 3-DTI at the 
3 MV Tandem Ion Accelerator located at the Naval Surface 
Warfare Center – Carderock Division in Bethesda Maryland.  
This single-user facility accelerator allows for the production 
of mono-energetic gamma-rays between 0.2-15 MeV by 
making an appropriate selection of ion, target and ion energy.  
The ability to generate a mono-energetic gamma-ray beam 
will allow us to calibrate and characterize the 3-DTI detector 
tracking and verify simulation results.  The accelerator testing 
will also enable the determination of the optimal calorimeter 
configuration.  
IV. CONCLUSIONS 
The MWD fabrication process and the 3-DTI design are 
being developed with the goal of producing 50 × 50 cm2 or 
larger MWDs without extensive retooling.  Upon the 
conclusion of the 10 × 10 cm2 accelerator tests we will begin 
the design and fabricate a complete 50× 50 cm2 3-DTI 
prototype Compton telescope including a complete 
calorimeter and anti-coincidence.  We are planning to fly this 
prototype onboard a NASA high altitude balloon in 2008. 
REFERENCES 
[1] S. Boggs, et al. ACT, Advanced Compton Telescope, NASA Vision 
Mission Concept Study Report,” December 2005  
[2] G. Kanbach, et al. “The MEGA project,” New Astron. Rev. 48, 275 
2004  
[3] F. Sauli “ ,” NIM-A, 386, 531, 1997  
[4] P. Deines-Jones et al. “ ,” NIM-A, 477, 55, 2002 
[5] P. Deines-Jones et al. “ ,” NIM-A, 478, 130, 2002 
 
 
Fig. 9. Measured spectral response of a MWD to 55Fe.  The 6 
keV absorption peak and the 3 keV Argon escape peak are 
clearly visible.  The energy resolution is about 20%.  
782


Medium-Energy Gamma-Ray Astrophysics with the 3-DTI Gamma-Ray Telescope

UNH Scholars' Repository

University of New HampshireUniversity of New Hampshire Scholars' RepositorySpace Science Center Institute for the Study of Earth, Oceans, and Space(EOS)2006Medium-Energy Gamma-Ray Astrophysics withthe 3-DTI Gamma-Ray TelescopeStanley D. HunterNASA Goddard Space Flight CtrRobert G. BakerNASA Goddard Space Flight CtrLouis M. BarbierNASA/Goddard Space Flight CenterPeter F. BloserUniversity of New Hampshire, Peter.Bloser@unh.eduLa Vida CooperNASA Goddard Space Flight CtrSee next page for additional authorsFollow this and additional works at: https://scholars.unh.edu/sscPart of the Astrophysics and Astronomy CommonsThis Conference Proceeding is brought to you for free and open access by the Institute for the Study of Earth, Oceans, and Space (EOS) at University ofNew Hampshire Scholars' Repository. It has been accepted for inclusion in Space Science Center by an authorized administrator of University of NewHampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.Recommended CitationHunter, S.D.; Baker, R.G.; Barbier, L.M.; Bloser, P.F.; Cooper, L.; Krizmanic, J.F.; Link, J.T.; McConnell, M.L.; de Nolfo, G.A.; Ryan,J.M.; Singh, S.; Seunghee Son, "Medium-Energy Gamma-Ray Astrophysics with the 3-DTI Gamma-Ray Telescope," Nuclear ScienceSymposium Conference Record, 2006. IEEE , vol.2, no., pp.778,782, Oct. 29 2006-Nov. 1 2006AuthorsStanley D. Hunter, Robert G. Baker, Louis M. Barbier, Peter F. Bloser, La Vida Cooper, John F. Krizmanic,Jason T. Link, Mark L. McConnell, Geoergia A. De Nolfo, James M. Ryan, Satpal Singh, and Seunghee SonThis conference proceeding is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/ssc/195 Medium-Energy Gamma-Ray Astrophysics with  the 3-DTI Gamma-Ray Telescope  Stanley D. Hunter, Robert G. Baker, Louis M. Barbier, Peter F. Bloser, LaVida Cooper, John F. Krizmanic,  Jason T. Link, Mark L. McConnell, Georgia A. de Nolfo, James M. Ryan, Satpal Singh, and Seunghee Son   Abstract– Gamma-ray observations in the medium energy range (0.50-50.0 MeV) are central to unfolding many outstanding questions in astrophysics. The challenges of medium-energy gamma-ray observations, however, are the low photon statistics and large backgrounds. We review these questions, address the telescope technology requirements, and describe our development of the 3-Dimensional Track Imaging (3-DTI) Compton telescope and its performance for a new medium-energy gamma-ray mission.  The 3-DTI is a large-volume time projection chamber (TPC) with a 2-dimensional gas micro-well detector (MWD) readout.  I. INTRODUCTION he first major full-sky survey in medium-energy (0.4 to 30 MeV) gamma-ray astrophysics was made with the COMPTEL instrument on the Gamma Ray Observatory from April 5, 1991 to June 4, 2000.  The COMPTEL results addressed a broad range of astrophysics including cosmic particle accelerators, nucleosynthesis and cosmic radioactivity, Galactic diffuse emission, and cosmology.  The COMPTEL catalog of objects includes Active Galactic Nuclei & Blazars (Mkn 421, 3C273), Super Nova Remnants (Crab Nebula), Pulsars & Magnetars (PSR 1509-58), and Black Hole Binaries (Cyg X-1).  These observations indicate that we are on the threshold of major discoveries addressing fundamental questions of thermal and relativistic astrophysics in the universe.   Expanding on the COMPTEL science results requires development of a future gamma-ray telescope with greatly improved sensitivity by increasing the overall aperture and effectively addressing background rejection.  In addition, this telescope will need to incorporate several measurement techniques since the objects that produce these gamma-rays are both extended & point-like, transient & steady, and include both continuum & line emissions.  A study of these techniques has recently been completed for the Advanced Compton Telescope (ACT), one of NASA's future Vision Missions [1].  The ACT report calls for an increase in sensitivity of about 100 times that of COMPTEL.  The primary science objective assumed for the ACT study report is to detect the 56Co (0.847                                                         Manuscript received November 17, 2006.   S. D. Hunter (telephone: 301-286-7280, e-mail: Stanley.D.Hunter@ nasa.gov), L. M. Barbier, R. G. Baker, LaVida Cooper, & S. Singh are with NASA/Goddard Space Flight Center, Greenbelt, MD 20771 USA  G. A. de Nolfo, J. F. Krizmanic, and J. T. Link with CRESST/USRA working at NASA/GSFC, Greenbelt, MD 20771 USA S. Son is a NASA Post Doctoral fellow working at NASA/GSFC, Greenbelt, MD 20771 USA P. F. Bloser, M. L. McConnell, and J. M. Ryan are with University of New Hampshire, Space Science Center, Durham, NH 03824 USA  MeV) line produced in thermonuclear supernovae (SNe Ia), which will shed light on the dynamics of nuclear burning.  This narrow science goal was used to define a baseline instrument design.  The desired performance of this baseline instrument has an effective area of about 103 cm2, broad-line (3%) sensitivity of 1.2×10-6 ph cm-2 s-1 at 0.847 MeV, narrow-line sensitivity of 5×10-7 ph cm-2 s-1, and continuum sensitivity of (1/E) ×10-5 ph cm-2 s-1 MeV-1.  This performance represents an increase in narrow- and broad-line sensitivity of about 100-fold over COMPTEL.  The ACT mission is not expected to start before 2012, with a launch date well after the end of the INTEGRAL and GLAST missions.  However, between the start of the GLAST mission and the ACT missions, there is a possibility of an intermediate, MIDEX scale, medium-energy gamma-ray mission.  This intermediate mission, with a 10-fold increase in narrow-line, broad-line, and continuum sensitivity over COMPTEL, would bridge the science from COMPTEL to ACT, fill in the multi-wavelength coverage gap between INTEGRAL and GLAST, and substantially advance our scientific understanding of transition between thermal and relativistic processes in the universe.  The science goals enabled by a 10-fold increase in sensitivity translate into three requirements that drive the design of a medium-energy telescope.  ? Address the widest range of science goals by optimizing the sensitivity to both line and continuum emission, to both point-like and extended objects, and to both steady state and transient sources.  ? Extend the energy range down to 0.3 MeV and up to 50 MeV by photon imaging from both Compton scattering and pair production interactions.  ? Maximize the effective area by improving the background rejection.  In the following, we describe our efforts towards the design and fabrication of a Compton telescope suitable for this intermediate mission.  II. COMPTON SCATTERING TELESCOPE DESIGN  Compton scattering telescopes, Fig. 1, use two separated, position sensitive detectors.  An incident gamma ray scatters in the first detector (D1, track imager) producing a recoil electron.  The scattered gamma ray is then totally absorbed after one or more interactions in the second detector (D2, calorimeter).  Measurement of the position of the single T 2006 IEEE Nuclear Science Symposium Conference Record N21-21-4244-0561-0/06/$20.00 ©2006 IEEE. 778 Fig. 1. Schematic of the MEGA medium-energy telescope [2]. The Si-strip D1 track imager provides only a few (< ~20) position measures of the recoil electron tracks from Compton scattering or pair production.  D1 D2 Compton scatter and the total energy of the recoil electron in D1, and the position and energy of the scattered gamma ray in D2, is sufficient to determine the energy of the incident gamma ray and the Compton scattering angle ϕ, Fig. 2. Thus the point spread function (PSF) of a single event is a circle on the sky called the event circle, Fig. 2a.  The MEGA medium-energy gamma-ray telescope [2], is one concept for a more advanced Compton telescope with tracking of the recoil electron, uses multiple layers of planar X- and Y-position sensitive silicon-strips as the D1 detector, Fig. 1. The MEGA design provides a few (< 20) position measurements of the recoil electron track, which constrains the plane of the interactions.  Thus the event circle is reduced to an event arc with angular length ∆θ, Fig. 2b.  The length of the arc is determined by the accuracy with which the initial recoil electron direction can be constrained, which is in turn limited by multiple scattering in the tracker material. The Compton scatter angle, ϕ, and energy of the incident gamma ray can be derived from measurements of the energy of the recoil electron and the scattered gamma ray using the Compton formula.  The direction of the incident gamma ray is equally probable from any point on the event circle determined by the scatter angle.  Measuring the direction of recoil electron reduces the event circle to an event arc with azimuthal extent, ∆θ, Fig. 2b. For a Compton telescope, the background associated with any observed photon is proportional to the area of the event circle, sin ϕ ∆ϕ ∆θ, thus reducing the azimuthal extent of the arc reduces the observed background and hence increases the sensitivity of the telescope. The ability to constrain the incident gamma-ray direction with highly precise imaging techniques and thus dramatic improvements in sensitivity opens up the field of medium-energy gamma-ray observations to many exciting new potential discoveries. The science goals and design drivers of an intermediate Compton telescope mission can be achieved with the MEGA two-detector design with an improved D1 track imager. Towards this goal, we are developing the Three-Dimensional Track Imager (3-DTI) that provides hundreds of track position measurements in a low-density gaseous medium as part of a complete Compton telescope for an intermediate medium-energy gamma-ray mission.  III. THE 3-DTI TRACK IMAGER The 3-DTI, Fig. 3, is large volume time projection chamber (TPC) with a two-dimensional gas micro-well detector (MWD) readout plane. The 3-DTI is designed to measure the three-dimension momentum of charged particles by high spatial resolution sampling of the charged particle ionization tracks in a low density, isotropic, fully active gas volume. The MWD provides X- and Y-coordinates and the 3rd dimension is obtained by measuring the drift time of the ionization electrons.  We describe the 3-DTI TPC and MWD design, front-end and transient digitizer electronics, segmented calorimeter, and performance in the following sections.  A. TPC and MWD Design The micro-well detector is a type of gas proportional counter related to the gas electron multiplier [3].  In contrast to multi-wire proportional counter construction, the anodes and cathodes of all these detectors are rigidly affixed to an Fig. 2. (a) Compton scattering without electron tracking constrains the incident gamma ray direction on an event circle, (b) Tracking the recoil electron reduces the event circle to an arc.  The uncertainty in the electron azimuth determines the arc length.  779 insulating substrate, Fig. 4. This greatly simplifies their construction and allows the anode and cathode pitches to be smaller than is possible with multi-wire construction.  This geometry is also inherently two-dimensional; the anodes give one coordinate and the cathodes the other.  An array of wells forms a detector.  The active tracking volume is bounded by a drift electrode on the top and an array of wells on the bottom.  Ionization electrons produced as a charged particle looses energy traversing the gas drift toward the anode into the wells of the detector.  An ionization avalanche occurs in the well, where there is an intense electric field set up by the voltage difference applied between the anode and cathode. The micro-wells serve as the proportional counter where the electric field strength is large enough to produce secondary ionization of gas atoms.  The ensuing avalanche of electrons collects on the anode while charge of equal magnitude but opposite sign collects on the cathode.  1) TPC The TPC and MWD are housed in a pressure vessel which serving as the TPC chamber. Laboratory testing is being done with P-10 (90% argon, 10% methane) nominally at 760 Torr.  The TPC chamber assembly includes a drift electrode held at 1 kV and employs a field-shaping grid along the chamber.  2) MWD We have previously constructed two-dimensional crossed-strip MWDs and demonstrated gas gain up to 30,000 and spatial resolution of ~200 µm [5]. We are currently exploring ways to produce MWDs that are suitable for commercial fabrication of detectors up to 50 × 50 cm2 in size. MWDs are laminated substrates where the laminate thickness determines the well thickness and thus the gain.  Our MWDs consist of 5 µm copper foil on laminate substrates such as Vacrel and Kapton, Fig. 5.  The electrode pattern is a photochemical process.  We are considering several different methods for the micro-well machining such as a photochemical etch or laser ablation which are ideal for large area production with relatively little process contamination. Both 5x5 cm2 and 10x10 cm2 MWDs have been produced.  The wells have a diameter and depth of 200µm with a pitch of 400µm and a gap between the electrodes of 5µm, resulting in a total of 448 micro-wells.  Fig. 4. Schematic view of a micro-well detector.  The anodes, blue, are separated from the cathodes, red, by an insulating layer, light green.  The MWD is mounted on a rigid substrate, dark green. 2D micro-well Fig. 3. The 3-DTI is a large volume time projection chamber with 2-D micro-well readout.  The ionization charge in the TPC volume drifts towards the MWD and into the individual wells.  The strong electric field in the wells produces an avalanche of secondary ionization.   The avalanche charge is collected on the anode strip under the well and an equal but opposite mirror charge appears on the cathode.  Fig. 5. SEM Micrograph of a micro-well detector.  The wells and anode strips can be seen through the holes in the cathode strips  780 3) Front-end Electronics and Transient Digitizer Each anode and cathode electrode of the MWD is connected to a front-end electronics (FEE) channel consisting of a charge sensitive preamplifier, a CR-RC pulse shaper with gain, and a differential driver. The FEE is a bipolar design to accommodate the negative anode signal and positive, cathode mirror signal. To maximize design flexibility, we have chosen to implement the FEE design using discrete components. This initial approach greatly reduced the time required for the design and testing, and permits in-house fabrication. The components used are the AD8610, AD8672 and AD8137 for the charge sensitive preamplifier, the shaping and gain amps, and the driver, respectively. Sixteen FEE channels were fabricated on each front-end card (FEC) which also has the high voltage distribution buss. Our data acquisition is done in LabView. The response of each stage of the FEE to a test pulse input is shown in Fig. 6. The arrival time of charge pulses from each FEE channel is recorded with a transient digitizer. Since the ionization charge drifts at a constant velocity, the relative pulse arrival time provides the third coordinate of the charge position. In order to achieve equal resolution in all three spatial coordinates, the timing resolution must be equal to the pixel pitch divided by the electron drift velocity. To satisfy the above condition, a 2.5 MHz, 12 bit resolution custom Transient Digitizer (TD) board was designed and has been fabricated. The digitized positive and negative output from an anode and cathode is shown in Fig. 7.  B. Calorimeter (D2 Detector) Surrounding the 3-DTI on all four sides and along the bottom will be a segmented calorimeter with an inner layer of LaBr3 crystals and an outer layer of NaI crystals.  Surrounding the calorimeter will be anti-coincidence scintillator.  We are currently testing, simulating, and optimizing the configuration of scintillation crystals to provide the needed energy and position resolution, and to enable a viable, affordable detector.   C. Instrument Performance We tested the 3-DTI prototype in the laboratory using an 55Fe source.  Figure 4 shows the energy spectrum for 5.9 keV X-rays with a secondary peak from the Ar escape peak at 2.7 keV.  Our energy resolution is ~ 20% based on the 5.9 keV x-ray peak in 90% argon with 10% methane at a gain of 103.   Gas-gain measurements have been made with a similar micro-well configuration [4] and exhibit stable operation at gas-gains in excess of 30,000.  Fig. 8 shows the avalanche or gas gain versus high voltage curves for a 200 µm thick MWD.  The maximum gain observed to date is ~103. The energy resolution of this MWD is shown in Fig. 9 for a voltage difference between the anode and cathode of 1000 V and.  The charge collected on the MWDs is sent to front-end electronics boards (FEE) for charge-integrating amplification and shaping and to transient digitizer boards (TD) for time projection determination. Fig. 7. Digitized positive and negative outputs from an anode and cathode in response to two separate 55Fe events.  Fig. 8. Micro-well detector (MWD) gas gain (avalanche charge multiplication) measured in P-10 (90% Ar, 10% CH4) as a function of anode-cathode voltage. Test pulse input Preamp output Shaper, gain amp output Fig. 6. Response of front-end electronics (FEE) to a test input pulse across a 2 pF capacitor.  The overall gain is about 8 mV/fC.  781  D. Development Plans Arrangements have been made to test a 10 cm2 3-DTI at the 3 MV Tandem Ion Accelerator located at the Naval Surface Warfare Center – Carderock Division in Bethesda Maryland.  This single-user facility accelerator allows for the production of mono-energetic gamma-rays between 0.2-15 MeV by making an appropriate selection of ion, target and ion energy.  The ability to generate a mono-energetic gamma-ray beam will allow us to calibrate and characterize the 3-DTI detector tracking and verify simulation results.  The accelerator testing will also enable the determination of the optimal calorimeter configuration.  IV. CONCLUSIONS The MWD fabrication process and the 3-DTI design are being developed with the goal of producing 50 × 50 cm2 or larger MWDs without extensive retooling.  Upon the conclusion of the 10 × 10 cm2 accelerator tests we will begin the design and fabricate a complete 50× 50 cm2 3-DTI prototype Compton telescope including a complete calorimeter and anti-coincidence.  We are planning to fly this prototype onboard a NASA high altitude balloon in 2008. REFERENCES [1] S. Boggs, et al. ACT, Advanced Compton Telescope, NASA Vision Mission Concept Study Report,” December 2005  [2] G. Kanbach, et al. “The MEGA project,” New Astron. Rev. 48, 275 2004  [3] F. Sauli “ ,” NIM-A, 386, 531, 1997  [4] P. Deines-Jones et al. “ ,” NIM-A, 477, 55, 2002 [5] P. Deines-Jones et al. “ ,” NIM-A, 478, 130, 2002   Fig. 9. Measured spectral response of a MWD to 55Fe.  The 6 keV absorption peak and the 3 keV Argon escape peak are clearly visible.  The energy resolution is about 20%.  782

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Medium-Energy Gamma-Ray Astrophysics with the 3-DTI Gamma-Ray Telescope

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