The measurement of hard X‐ray polarization in γ‐ray bursts (GRBs) would add yet another piece of information in our effort to resolve the true nature of these enigmatic objects. Here we report on the development of a dedicated polarimeter design with a relatively large FoV that is capable of studying hard X‐ray polarization (50–300 keV) from GRBs. This compact design, based on the use of a large area position‐sensitive PMT (PSPMT), is referred to as GRAPE (Gamma‐RAy Polarimeter Experiment). The feature of GRAPE that is especially attractive for studies of GRBs is the significant off‐axis polarization response (at angles greater than 60°). For an array of GRAPE modules, current sensitivity estimates give minimum detectable polarization (MDP) levels of a few percent for the brightest GRBs

Ledoux, J

Macri, John R

McConnell, Mark L

Ryan, James M.

English

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University of New HampshireUniversity of New Hampshire Scholars' RepositorySpace Science Center Institute for the Study of Earth, Oceans, and Space(EOS)2003The Development of GRAPE, a Gamma RayPolarimeter ExperimentMark L. McConnellUniversity of New Hampshire - Main Campus, mark.mcconnell@unh.eduJ LedouxUniversity of New Hampshire - Main CampusJohn R. MacriUniversity of New Hampshire - Main Campus, John.Macri@unh.eduJames M. RyanUniversity of New Hampshire, James.Ryan@unh.eduFollow 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 CitationThe Development of GRAPE, a Gamma Ray Polarimeter Experiment McConnell, M. L. and Ledoux, J. R. and Macri, J. R. and Ryan, J.M., AIP Conference Proceedings, 662, 503-505 (2003), DOI:http://dx.doi.org/10.1063/1.1579413The Development of GRAPE, a Gamma Ray Polarimeter ExperimentM. L. McConnell, J. R. Ledoux, J. R. Macri, and J. M. Ryan  Citation: AIP Conference Proceedings 662, 503 (2003); doi: 10.1063/1.1579413 View online: http://dx.doi.org/10.1063/1.1579413 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/662?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Development of a graphite polarization analyzer for resonant inelastic x-ray scattering Rev. Sci. Instrum. 82, 113108 (2011); 10.1063/1.3662472  GammaRay Burst Polarimeter aboard IKAROS AIP Conf. Proc. 1279, 227 (2010); 10.1063/1.3509271  Prospects for GRB Polarimetry with GRAPE AIP Conf. Proc. 836, 654 (2006); 10.1063/1.2207970  Development of a hard x-ray polarimeter for gamma-ray bursts AIP Conf. Proc. 428, 889 (1998); 10.1063/1.55415  Polarimeter for soft x rays (abstract) J. Appl. Phys. 81, 5090 (1997); 10.1063/1.364459   This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:132.177.229.79 On: Mon, 24 Nov 2014 13:45:42The Development of GRAPE,a Gamma Ray Polarimeter ExperimentM.L. McConnell , J.R. Ledoux , J.R. Macri  and J.M. Ryan  Space Science Center, University of New Hampshire, Durham, NH 03824Abstract.The measurement of hard X-ray polarization in γ-ray bursts (GRBs) would add yet another piece of informationin our effort to resolve the true nature of these enigmatic objects. Here we report on the development of a dedicatedpolarimeter design with a relatively large FoV that is capable of studying hard X-ray polarization (50-300 keV)from GRBs. This compact design, based on the use of a large area position-sensitive PMT (PSPMT), is referred toas GRAPE (Gamma-RAy Polarimeter Experiment). The feature of GRAPE that is especially attractive for studiesof GRBs is the significant off-axis polarization response (at angles greater than 60Æ). For an array of GRAPEmodules, current sensitivity estimates give minimum detectable polarization (MDP) levels of a few percent forthe brightest GRBs.INTRODUCTIONIn recent years, largely as a result of the observation ofseveral X-ray, optical and radio afterglows, there has de-veloped a growing consensus that classical γ-ray bursts(GRBs) are at cosmological distances. Such great dis-tances imply that a typical GRB releases 1051 1053 ergsor more within a time span of several seconds.The general picture that has emerged is one that de-scribes the GRB phenomenom in terms of a relativisticfireball model [e.g., 1]. One common feature of many ofthese models is that the energy release takes the form ofjets that are directed along the rotation axis of the sys-tem. Several indirect arguments have been used to arguethat such jets are required to explain the observations. Forexample, the shapes of the observed light curves of someof the observed optical afterglows (e.g., GRB 990510)have been used as supporting evidence for jets related tothe external shocks. Since the energy budget of a givenGRB depends  heavily on assumptions about the extentto which the flow is jet-like, determining the reality andnature of jets in GRBs is becoming an important goal offuture observations.Optical polarization measurements, and the theoreticalstudies that have been motivated by such measurements,have provided a better insight into the nature of the GRBphenomena. The optical studies, however, probe only theexternal shock region. In the context of the canonical fire-ball model, measurements of the hard X-ray polarizationduring the prompt phase of the GRB promise to provide asimilar probe of the internal shock region. Since the out-going flow at the internal shock is expected to be moretightly collimated than the flow at the external shock (re-sulting from a continuous spreading of the jet as it pro-gresses outward through the fireball), one can perhapsexpect a somewhat higher level of hard X-ray polariza-tion (assuming that it arises from synchrotron emission)during the prompt phase of the GRB.In contrast to the classical GRBs, the prevailing modelof soft γ-ray repeaters (SGRs) involve emission from thevicinity of magnetars, neutron stars with magnetic fieldsin excess of 1014 G, with the energy release triggered bymassive neutron star crustquakes. Baring [2] suggestedthat the softness of the events can be attributed to photonsplitting in the extremely intense magnetic fields. Thephoton splitting process degrades the high energy γ-rayphotons to hard X-ray energies [3]. One by-product ofphoton splitting is that the reprocessed photons wouldexhibit a polarization level of 25% [2]. Polarizationmeasurements in the 50–300 keV energy range couldtherefore provide a test of the importance of photonsplitting in SGRs.A MODULAR POLARIMETER DESIGNThe basic physical process used to measure linear polar-ization of hard X-rays (50–300 keV) is Compton scat-tering. The measurement is based on the fact that theincident photons tend to be scattered at right angles tothe incident electric field vector. A Compton scatter po- This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:132.177.229.79 On: Mon, 24 Nov 2014 13:45:42Position-SensitivePhotomultiplier TubePlastic Scintillator ArrayCsI Array5 cmFIGURE 1. Schematic diagram of a polarimeter module.larimeter consists of two detectors (or detector arrays)that are used to determine the energies of both the scat-tered photon and the scattered electron. One detector (thescattering detector) provides the medium for the Comp-ton interaction to take place. This detector must be de-signed to maximize the probability of a single Comp-ton interaction with a subsequent escape of the scatteredphoton. The primary purpose of the second detector (thecalorimeter) is to absorb the full energy of the scatteredphoton.We have developed a modular design that places anentire device on the front end of a single 5-inch diameterposition-sensitive PMT (PSPMT) [4, 5, 6]. An array ofplastic scintillator elements (each 5 cm long with a crosssection of 5 mm   5 mm) provides the spatial resolutionin the scattering elements. The spatial resolution is nec-essary for acurately reconstructing the scattering geome-try. The array is arranged in the form of an annulus withan outside diameter of 10 cm. A 2   2 array of 1 cm CsIscintillators is positioned within a well at the center ofthe array. A multi-anode PMT (MAPMT) provides an in-dependent readout of each CsI element (one element peranode). Independent readouts provide the necessary tim-ing information used to define a Compton scatter event.Figure 1 shows a schematic view of a GRAPE module.To be recorded as a polarimeter event, an incident pho-ton Compton scatters from one (and only one) of theplastic scattering detectors into the central calorimeter.The incident photon energy can be determined from thesum of the energy losses in both detectors and the az-imuthal scattering angle (used to define the polarimet-ric response) can be determined by the relative locations120010008006004002000Counts3002001000Azimuthal Scatter Angle (degs)150 keV, 0° incidence FIGURE 2. The simulated polarimetric response of aGRAPE module. Polarized 150 keV photons vertically incidenton the module show an asymmetry in the azimuthal distributionof interactions in the plastic array. The data here have beencorrected for intrinsic asymmetries in the module geometry.1.00.80.60.40.20.0Modulation Factor706050403020100Angle (degs)1.51.20.90.60.30.0Effective Area (cm2) Modulation Factor Effective Area200 keVFIGURE 3. The effective area and modulation factor as afunction of incidence angle for 200 keV photons (from sim-ulations).of the plastic and CsI detectors involved in the event.When the polarimeter is arranged so that the incidentflux is parallel to the symmetry axis, unpolarized radia-tion will produce an axially symmetric coincidence rate.If the incident radiation is linearly polarized, then the co-incidence rate will show an azimuthal asymmetry whosephase depends on the position angle of the incident ra-diation’s electric vector and whose magnitude dependson the degree of polarization. Figure 2 shows the simu-lated polarimetric response of a GRAPE module to ver-tically incident polarized radiation at an energy of 150keV. The amplitude of the modulation pattern serves todefine the level of polarization. The angle at which themodulation pattern is a minimum corresponds to the po-larization angle. Figure 3 shows that the module main-tains a very good polarimetric response out to very largeincidence angles (beyond 60Æ). This makes the GRAPEdesign especially appealing for GRB studies.We anticipate that this design would be used in thecontext of an array of polarimeter modules (a bunch ofGRAPEs). Table 1 shows the sensitivity of an array of36 modules, based on Monte Carlo simulations of thisdesign and reasonable estimates of the background atballoon altitudes. The sensitivity is presented in terms of This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:132.177.229.79 On: Mon, 24 Nov 2014 13:45:42TABLE 1. GRB Polarization Sensitivity (50–300 keV)Fluence(ergs cm 2)MDPT = 10 sMDPT = 100 sObserved Rate(N   Fluence)1 10 4 2.8% 2.9% 1 per 320 days5 10 5 3.9% 4.4% 1 per 80 days1 10 5 9.3% 13.7% 1 per 10 days5 10 6 14.0% 24.4% 1 per 6 days3 10 6 19.4% 38.6% 1 per 4 days1 10 6 43.2% — 1 per 2 daysFIGURE 4. The components of the laboratory sciencemodel, showing (left to right) the PSPMT, the array of plasticscintillators, and the MAPMT/CsI assembly that fits into thecentral well of the plastic array.both the minimum detectable polarization (MDP) and thefrequency of observed events above a given fluence level.Note that, for a given fluence level, the MDP is lowerfor shorter events due to the larger signal-to-backgroundintegrated over the event duration. These results suggestthat a balloon borne GRAPE would achieve a sensitivitylevel of a few percent for the strongest GRBs.LABORATORY SCIENCE MODELWe have assembled a laboratory science model (SM)for the purpose of validating the GRAPE design. Thecomponents of the SM are shown in Figure 4. The cur-rent configuration uses a charge division readout for thePSPMT that provides an event location using only fourchannels (two readouts for X and two readouts for Y ).During the summer of 2001, we carried out an LED map-ping of the PSPMT spatial response using the four chan-nel readout. The LED was stepped through a sequenceof locations with a fixed grid spacing of 2.5 mm. The ar-ray of measured grid locations is shown in Figure 3. Notethe distortions of the grid pattern that are evident near theedge of the PSPMT.300250200150100Y-Location (arbitrary units)300250200150100 X-Location (arbitrary units)Measured LED LocationsFIGURE 5. Plot of the measured (relative) positions of theLED calibration points.We have recently started collecting data with the fullyassembled science model using a partially polarizedbeam that is generated by scattering 662 keV photonsfrom a 137Cs source off a block of plastic scintillator.This particular arrangement also provides for the elec-tronic tagging of polarized photons. A preliminary set ofuncorrected data shows evidence for a polarization signa-ture. Further processing and analysis of these data are inprogress. In the coming months we plan to complete theanalysis of laboratory SM data and make detailed com-parisons with Monte Carlo simulations. This effort willprovide the basis for continued development of a futurespacecraft or balloon payload.ACKNOWLEDGMENTSThis work is currently supported by NASA grant NAG5-5324.REFERENCES1. Piran, T., Phys. Rept. 333, 529-533 (2000).2. Baring, M., ApJ Letters 440, L69-L72 (1995).3. Baring, M., MNRAS 262, 20-26 (1993).4. McConnell, M. L., Forrest, D. J., Macri, J., McClish, M.,Osgood, M., Ryan, J. M., Vestrand, W. T., and Zanes, C.,IEE Trans. Nucl. Sci. 45, 910-914 (1998).5. McConnell, M. L., Macri, J. R., McClish, M., Ryan, J.,Forrest, D. J., and Vestrand, W. T., IEE Trans. Nucl. Sci.46, 890-896 (1999).6. McConnell, M. L., Macri, J. R., McClish, M., and Ryan, J.,SPIE Conf. Proc. 3764, 70-78 (1999). This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:132.177.229.79 On: Mon, 24 Nov 2014 13:45:42

The Development of GRAPE, a Gamma Ray Polarimeter Experiment

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The Development of GRAPE, a Gamma Ray Polarimeter Experiment

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