The Gamma RAy Polarimeter Experiment (GRAPE) was first flown on a 26-hour balloon flight in the fall of 2011.
GRAPE consists of an array of Compton polarimeter modules (based on traditional scintillation technologies) designed
to operate in the energy range from 50 keV up to 500 keV. The ultimate goal is to operate GRAPE in a wide FoV
configuration for the study of gamma-ray bursts. For the first (demonstration) balloon flight, GRAPE was configured in
a collimated mode to facilitate observations of known point sources. The Crab nebula/pulsar, the active Sun, and Cygnus
X-1 were the primary targets for the first flight. Although the Crab was detected, the polarization sensitivity was worse
than expected. This paper will review the plans for the next GRAPE balloon flight, which is scheduled to take place in
the fall of 2014 from Ft. Sumner, NM. These plans involve several modifications designed to improve the polarization
sensitivity, including an expansion of the array of polarimeter modules from 16 to 24 and improvements to the
instrument shielding. Sensitivity estimates of the resulting instrument, based on GEANT4 simulations, will be presented.
We have been working on the development of a detector design for a large area coded aperture imaging system operating
in the 10-600 keV energy range. The detector design is based on an array of Lanthanum Bromide (LaBr3) scintillators,
each directly coupled to a Hamamatsu 64-channel multi-anode photomultiplier tube (MAPMT). This paper focuses on
the development of the GEANT4-based simulations as an aid in the optimization of the detector design. The simulations
have been validated by comparisons with various laboratory data sets. We will summarize the current status and latest
findings from this study.
We have developed a design for a hard X-ray polarimeter operating in the energy range from 50 to 500 keV. This
modular design, known as GRAPE (Gamma-Ray Polarimeter Experiment), has been successfully demonstrated in the
lab using partially polarized gamma-ray sources and using fully polarized photon beams at Argonne National Laboratory.
In June of 2007, a GRAPE engineering model, consisting of a single detector module, was flown on a high altitude
balloon flight to further demonstrate the design and to collect background data. We are currently preparing a much larger
balloon payload for a flight in the fall of 2011. Using a large (16-element) array of detector modules, this payload is
being designed to search for polarization from known point sources of radiation, namely the Crab and Cygnus X-1. This
first flight will not only provide a scientific demonstration of the GRAPE design (by measuring polarization from the
Crab nebula), it will also lay the foundation for subsequent long duration balloon flights that will be designed for
studying polarization from gamma-ray bursts and solar flares. Here we shall present data from calibration of the first
flight module detectors, review the latest payload design and update the predicted polarization sensitivity for both the
initial continental US balloon flight and the subsequent long-duration balloon flights.
The Gamma-RAy Polarimeter Experiment (GRAPE) is a concept for an astronomical hard X-ray Compton polarimeter
operating in the 50 - 500 keV energy band. The instrument has been optimized for wide-field polarization measurements
of transient outbursts from energetic astrophysical objects such as gamma-ray bursts and solar flares. The
GRAPE instrument is composed of identical modules, each of which consists of an array of scintillator elements read out
by a multi-anode photomultiplier tube (MAPMT). Incident photons Compton scatter in plastic scintillator elements and
are subsequently absorbed in inorganic scintillator elements; a net polarization signal is revealed by a characteristic
asymmetry in the azimuthal scattering angles. We have constructed a prototype GRAPE module that has been calibrated
at a polarized hard X-ray beam and flown on an engineering balloon test flight. A full-scale scientific balloon payload,
consisting of up to 36 modules, is currently under development. The first flight, a one-day flight scheduled for 2011,
will verify the expected scientific performance with a pointed observation of the Crab Nebula. We will then propose
long-duration balloon flights to observe gamma-ray bursts and solar flares.