Introduction
The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma-ray (0.2-15 MeV)
telescope designed to study astrophysical sources of nuclear line emission and
gamma-ray polarization.
It employs a novel Compton telescope design, utilizing
twelve 3D imaging, high spectral resolution germanium detectors (GeDs), enclosed
on the sides and bottom by an active BGO well, and with an overall field-of-view
(FOV) of 25% of the sky.
The Compton imaging serves three purposes: imaging
the sky, measuring polarization, and very effectively reducing background.
NCT's guiding principle is that high efficiency and
excellent background reduction are
critical for advances in soft gamma-ray sensitivity.
The compact geometry achieves high photopeak efficiencies -- NCT
increases the effective area per unit detector volume by a factor
of 100 over COMPTEL. The combination
of Compton imaging, active shielding, and analysis techniques
made possible with our 3D GeDs serve to dramatically decrease the
background - a factor of 30-100 per unit volume over INTEGRAL/SPI.
Scientific Objectives
Nuclear astrophysics studies the lifecycle and evolution of matter in our Universe: stellar evolution
ending in supernovae, with the ejection of heavy nuclei back into the galaxy to be reborn in new stars.
Radioactive nuclei produced through this cycle of creation emit characteristic photons that fingerprint
the isotopes themselves, and quantify their abundance, speed, temperature, and attenuation. High spectral
resolution measurements of these line emissions probe deep into the center of a supernova explosion,
revealing the nuclear burning and dynamics in the core. Nuclear excitations and positron annihilations
also reflect the extreme environment on the surface of neutron stars and white dwarfs, and near the event
horizon of black holes. NCT is an important advance over current and historical gamma-ray instruments in
two regards: its development and flight will provide a testing ground for novel event analysis, background
reduction, and imaging techniques for gamma-ray astronomy, and break new ground in the measurement of
polarized gamma-ray emission from astrophysical sources.
Instrument Design
The Nuclear Compton Telescope will employ a novel Compton telescope design, utilizing twelve 3-D
imaging, high spectral resolution germanium detectors (GeDs). The Compton imaging serves three purposes:
imaging the sky, measuring polarization, and very effectively reducing background. NCT is designed to optimize sensitivity to nuclear
line emission over the crucial 0.5-2 MeV range, and sensitivity to polarization in the 0.2-0.5 MeV range.
NCT's guiding principle is that high efficiency and excellent background reduction are critical for advances
in soft gamma-ray sensitivity.
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Back-projection of Compton circles from simulated 0.662 MeV point source.
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10 iterations of LMEM algorithm on simulated 0.662 MeV point source.
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The heart of NCT is an array of 12 crossed-strip cryogenic germanium detectors (GeDs)
with 3-D position resolution,
excellent spectroscopy, and high efficiency.
Compton telescopes image gamma-rays by inversion of the Compton scatter formula.
An incoming photon at MeV energies will undergo a Compton
scatter at an initial position in the instrument,
losing an energy related to the scattered photon direction by the Compton
scatter formula. The scattered photon then looses the
rest of its energy in the instrument in a series of one or more interactions ending
in a photoelectric absorption.
By measuring the position and energy of
the interactions with high precision, the photon event can be reconstructed
through the Compton formula to determine the initial photon
direction to within an annulus on the sky.
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NCT flight detector, wire-bonded in its cryostat mounting. The heart of NCT is an array of 12 of these crossed-strip GeDs with 3D tracking resolution. By measuring the position and energy of each photon interaction, the initial photon direction can be determined through the Compton scatter formula to within an annulus (event circle) on the sky.
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The NCT payload with its full long duration balloon flight systems (except solar power) on the flight line in Fort Sumner, NM. Long duration balloon flights allow payloads to get above 99% of the atmosphere to perform satellite-class observations for 1-2 weeks at a time.
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NCT "all-sky" image of a lab Co-60 (1.173-MeV) source. The source is ~60 degrees off-axis, demonstrating NCT's wide-field imaging capabilities. This data was processed though a list-mode maximum-likelihood reconstruction algorithm.
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Co-60 (1.173, 1.333-MeV) spectrum of the Compton-scatter events, 4.0 keV FWHM resolutions are currently achieved near optimal for the current generation of electronics.
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