Profiling structured beams produced by X-ray free-electron lasers (FELs) is crucial to both maximizing signal intensity for weakly scattering targets and interpreting their scattering patterns. Earlier ablative imprint studies describe how to infer the X-ray beam profile from the damage that an attenuated beam inflicts on a substrate. However, the beams in-situ profile is not directly accessible with imprint studies because the damage profile could be different from the actual beam profile. On the other hand, although a Shack-Hartmann sensor is capable of in-situ profiling, its lenses may be quickly damaged at the intense focus of hard X-ray FEL beams. We describe a new approach that probes the in-situ morphology of the intense FEL focus. By studying the translations in diffraction patterns from an ensemble of randomly injected sub-micron latex spheres, we were able to determine the non-Gaussian nature of the intense FEL beam at the Linac Coherent Light Source (SLAC National Laboratory) near the FEL focus. We discuss an experimental application of such a beam-profiling technique, and the limitations we need to overcome before it can be widely applied.
The Nuclear Compton Telescope (NCT) is a balloon-borne telescope designed to study astrophysical sources of gammaray
emission with high spectral resolution, moderate angular resolution, and novel sensitivity to gamma-ray polarization.
The heart of NCT is a compact array of cross-strip germanium detectors allowing for wide-field imaging with excellent
efficiency from 0.2-10 MeV. Before 2010, NCT had flown successfully on two conventional balloon flights in Fort
Sumner, New Mexico. The third flight was attempted in Spring 2010 from Alice Springs, Australia, but there was a
launch accident that caused major payload damage and prohibited a balloon flight. The same system configuration
enables us to extend our current results to wider phase space with pre-flight calibrations in 2010 campaign. Here we
summarize the design, the performance of instrument, the pre-flight calibrations, and preliminary results we have
obtained so far.
The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma ray (0.2-10 MeV) telescope designed to study
astrophysical sources of nuclear line emission and polarization. The heart of NCT is an array of 12 cross-strip
germanium detectors, designed to provide 3D positions for each photon interaction with full 3D position resolution to <
2 mm^3. Tracking individual interactions enables Compton imaging, effectively reduces background, and enables the
measurement of polarization. The keys to Compton imaging with NCT's detectors are determining the energy deposited
in the detector at each strip and tracking the gamma-ray photon interaction within the detector. The 3D positions are
provided by the orthogonal X and Y strips, and by determining the interaction depth using the charge collection time
difference (CTD) between the anode and cathode. Calibrations of the energy as well as the 3D position of interactions
have been completed, and extensive calibration campaigns for the whole system were also conducted using radioactive
sources prior to our flights from Ft. Sumner, New Mexico, USA in Spring 2009, and from Alice Springs, Australia in
Spring 2010. Here we will present the techniques and results of our ground calibrations so far, and then compare the
calibration results of the effective area throughout NCT's field of view with Monte Carlo simulations using a detailed
The Medium-Energy Gamma-ray Astronomy library MEGAlib is an open-source object-oriented software library
designed to simulate and analyze data of low-to-medium-energy gamma-ray telescopes, especially Compton telescopes.
The library comprises all necessary simulation and data analysis tools including geometry construction,
Monte-Carlo simulation, response creation, event reconstruction, image reconstruction, and other high-level
The Gamma-Ray Imager (GRI) is a novel mission concept that will provide an unprecedented sensitivity leap in the soft gamma-ray domain by using for the first time a focusing lens built of Laue diffracting crystals. The lens will cover an energy band from 200 - 1300 keV with an effective
area reaching 600 cm<sup>2</sup>. It will be complemented by a single reflection multilayer coated mirror, extending the GRI energy band into the hard X-ray regime, down to ~10 keV. The concentrated photons will be collected by a position sensitive
pixelised CZT stack detector. We estimate continuum sensitivities of better than 10<sup>-7</sup> ph cm<sup>-2</sup>s<sup>-1</sup>keV<sup>-1</sup> for a 100 ks exposure; the narrow line sensitivity will be better than 3 x 10<sup>-6</sup> ph cm<sup>-2</sup>s<sup>-1</sup> for the same integration time. As focusing instrument, GRI will have an angular resolution of better than 30 arcsec within a field of view of roughly 5 arcmin - an unprecedented achievement in the gamma-ray domain. Owing to the large focal length of 100 m of the lens and the mirror, the optics and detector will be placed on two separate spacecrafts flying in formation in a high elliptical orbit. R&D work to enable the lens focusing technology and to develop the required focal plane detector is currently underway, financed by ASI, CNES, ESA, and the Spanish Ministery of Education and Science. The GRI mission is proposed as class M mission for ESA's Cosmic Vision 2015-2025 program. GRI will allow studies of particle acceleration processes and explosion physics in unprecedented detail, providing essential clues on the innermost nature of the most violent and most energetic processes in the Universe.
With focusing of gamma rays in the nuclear-line energy regime establishing itself as a feasible and very promising approach for high-sensitivity gamma-ray studies of individual sources, optimizing the focal plane instrumentation for gamma-lens telescopes is a prime objective. The detector of choice for a focusing nuclear-line spectroscopy mission would be the one with the best energy resolution available over the energy range of interest: Germanium. Using a Compton detector focal plane has three advantages over monolithic detectors: additional knowledge about (Compton) events enhances background rejection capabilities, the inherently finely pixellated detector naturally allows the selection of events according to the focal spot size and position and could enable source imaging, and Compton detectors are inherently sensitive to gamma-ray polarization. Suitable Ge-strip detectors that could be assembled into a sensitive high-resolution focal plane for a gamma-ray lens are available today. They have been extensively tested in the laboratory and flown on the Nuclear Compton Telescope balloon from Ft. Sumner in 2005. In the course of the ACT vision mission study, an extensive simulation and analysis package for Compton telescopes has been assembled. We leverage off this work to explore achievable sensitivities for different Ge Compton focal plane configurations - and compare them to sensitivities achievable with less complex detectors - as a step towards determining an optimum configuration.
The Advanced Compton Telescope (ACT), the next major step in gamma-ray astronomy, will probe the fires where
chemical elements are formed by enabling high-resolution spectroscopy of nuclear emission from supernova explosions.
During the past two years, our collaboration has been undertaking a NASA mission concept study for ACT. This study
was designed to (1) transform the key scientific objectives into specific instrument requirements, (2) to identify the most
promising technologies to meet those requirements, and (3) to design a viable mission concept for this instrument. We
present the results of this study, including scientific goals and expected performance, mission design, and technology
The Nuclear Compton Telescope (NCT) is a balloon-borne soft
gamma-ray (0.2MeV-10MeV) telescope designed to study astrophysical
sources of nuclear line emission and polarization. A prototype
instrument was successfully launched from Ft. Sumner, NM on June 1,
2005. The NCT prototype consists of two 3D position sensitive
High-Purity-Germanium (HPGe) strip detectors fabricated with
amorphous Ge contacts. The novel ultra-compact design and new
technologies allow NCT to achieve high efficiencies with excellent
spectral resolution and background reduction. Energy and positioning calibration data was acquired pre-flight in Fort Sumner, NM after the full instrument integration. Here we discuss our calibration techniques and results, and detector efficiencies. Comparisons with simulations are presented as well.
ESA's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) will be launched in October 2002. Its two main instruments are the imager IBIS and the spectrometer SPI. Both emply coded apertures to obtain directional information on the incoming radiation. SPI's detection plane consists of 19 hexagonal Ge detectors, its coded aperture has 63 tungsten-alloy elements of 30 mm thickness.
SPI, the Spectrometer on board the ESA INTEGRAL satellite, to be launched in October 2002, will study the gamma-ray sky in the 20 keV to 8 MeV energy band with a spectral resolution of 2 keV for photons of 1 MeV, thanks to its 19 germanium detectors spanning an active area of 500 cm<sup>2</sup>. A coded mask imaging technique provides a 2° angular resolution. The 16° field of view is defined by an active BGO veto shield, furthermore used for background rejection. In April 2001 the flight model of SPI underwent a one-month calibration campaign at CEA in Bruyères le Châtel using low intensity radioactive sources and the CEA accelerator for homogeneity measurements and high intensity radioactive sources for imaging performance measurements. After integration of all scientific payloads (the spectrometer SPI, the imager IBIS and the monitors JEM-X and OMC) on the INTEGRAL satellite, a cross-calibration campaign has been performed at the ESA center in Noordwijk. A set of sources has been placed in the field of view of the different instruments in order to compare their performances and determine their mutual influence. We report on the scientific goals of this calibration activity, and present the measurements performed as well as some preliminary results.
We report the scientific motivation for and performance measurements of a prototype detector system for SONTRAC, a solar neutron tracking experiment designed to study high- energy solar flare processes. The full SONTRAC instrument will measure the energy and direction of 20 to 200 MeV neutrons by imaging the ionization tracks of the recoil protons in a densely packed bundle of scintillating plastic fibers. The prototype detector consists of a 12.7 mm square bundle of 250 micrometer scintillating plastic fibers, 10 cm long. A photomultiplier detects scintillation light from one end of the fiber bundle and provides a detection trigger to an image intensifier/CCD camera system at the opposite end. The image of the scintillation light is recorded. By tracking the recoil protons from individual neutrons the kinematics of the scattering are determined, providing a high signal to noise measurement. The predicted energy resolution is 10% at 20 MeV, improving with energy. This energy resolution translates into an uncertainty in the production time of the neutron at the Sun of 30 s for a 20 MeV neutron, also improving with energy. A SONTRAC instrument will also be capable of detecting and measuring high-energy gamma rays greater than 20 MeV as a 'solid-state spark chamber.' The self-triggering and track imaging features of the prototype are demonstrated with cosmic ray muons and 14 MeV neutrons. Design considerations for a space flight instrument are presented.