The study of the polarization properties of the gamma‐ray bursts is the one remaining unexplored avenue of research which may help to answer some of the fundamental problems regarding the nature of these mysterious objects. We have designed an instrument to measure linear polarization in cosmic gamma‐ray bursts at energies ≳50 keV. Here we describe the design of this instrument, which we call the Gamma‐ray Burst Polarimeter Experiment (GRAPE)

Forrest, D J

Levenson, K

McConnell, Mark L

Vestrand, W T

English

M. McConnell

D. Forrest

K. Levenson

W. T. Vestrand

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The design of a gamma-ray burst polarimeter

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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)1993The design of a gamma‐ray burst polarimeterMark L. McConnellUniversity of New Hampshire - Main Campus, mark.mcconnell@unh.eduD J. ForrestUniversity of New Hampshire - Main CampusK LevensonUniversity of New Hampshire - Main CampusW T. VestrandUniversity of New Hampshire - Main CampusFollow 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 design of a gamma‐ray burst polarimeter McConnell, M. and Forrest, D. and Levenson, K. and Vestrand, W. T., AIP ConferenceProceedings, 280, 1142-1146 (1993), DOI:http://dx.doi.org/10.1063/1.44145The design of a gammaray burst polarimeterM. McConnell, D. Forrest, K. Levenson, and W. T. Vestrand  Citation: AIP Conference Proceedings 280, 1142 (1993); doi: 10.1063/1.44145 View online: http://dx.doi.org/10.1063/1.44145 View Table of Contents:http://scitation.aip.org/content/aip/proceeding/aipcp/280?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in The spectroscopy of classical gammaray bursts AIP Conf. Proc. 366, 206 (1996); 10.1063/1.50248  Exploration of bimodality in gammaray burst duration and hardness distributions AIP Conf. Proc. 307, 172 (1994); 10.1063/1.45913  DMSP satellites as gammaray burst detectors AIP Conf. Proc. 307, 34 (1994); 10.1063/1.45893  The color of gammaray bursts AIP Conf. Proc. 307, 323 (1994); 10.1063/1.45887  Spectral variations in gammaray bursts AIP Conf. Proc. 265, 195 (1991); 10.1063/1.42823   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 18:09:14THE DESIGN OF A GAMMA-RAY BURST POLARIMETERM. McConnell, D. Forrest, K. Levenson and W.T. VestrandSpace Science Center, University of New Hampshire, Durham, NH 03824ABSTRACTThe study of the polarization properties of the gamma-raybursts is the one remaining unexplored avenue of research which mayhelp to answer some of the fundamental problems regarding the natureof these mysterious objects. We have designed an instrument tomeasure linear polarization in cosmic gamma-ray bursts at energies>50 keV. Here we describe the design of this instrument, which wecall the Gamma-RAy burst Polarimeter Experiment (GRAPE).INTRODUCTIONThe nature of the gamma-ray burst sources has remained amystery ever since their discovery some 25 years ago. Even after thebest efforts of scores of researchers, there is no consensusregarding their origin. The latest results from the Compton Gamma-Ray Observatory (CGRO) have served to heighten the interest in thisproblem without offering any fundamental answers as to their origin.When it comes to the study of electromagnetic radiation, thereare four measurements which an observer can make (all as a functionof time). In the context of gamma-ray burst studies, these are: 1)the source location; 2) the source spectrum (distribution of photonenergies); 3) the source intensity (spectrum normalization); and 4)the source polarization. All studies of gamma-ray bursts to datehave involved the first three types of measurements. Since thesedata have not yet yielded any clear fundamental answers as to thenature of the bursts, we contend that an additional piece ofinformation may be required. The only additional piece ofinformation which is available to the observer is that of thepolarization of the radiation.A few authors have discussed polarization effects in thecontext of burst models. For example, Mitrofanov and Pozanenkol havediscussed the polarization inherent in cyclotron emission in a strongmagnetic field; they predict a polarization fraction of -80% near 100keV for a typical neutron star environment. Bisnovatyi-Kogan 2 hasnoted that the measurement of polarization in the observed spectrallines would help to determine the exact nature of the line emission.Baring3 has examined the polarization produced via magnetic photonsplitting; a maximum polarization fraction of about 43% is possiblewith this mechanism.At present, there is no gamma-ray experiment that we know ofwhich is presently in operation or in the planning stages whose majoremphasis is the measurement of polarization. The COMPTEL instrument1142	 © 1993 American Institute of Physics 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 18:09:14M. McConnell et al. 1143on GR0 4 , for example, has only marginal capabilities for detectingpolarized emission due to an inefficient geometry (the major drawbackfor measuring polarization with traditional Compton telescopes).Some of the proposed concepts (such as TTTS 5 ) include the capabilityfor polarization measurements, but once again, they are not optimizedfor that purpose.The prototype GRAPE design has been developed within thecontext of a long-duration balloon platform. However, the modularnature of this experiment will also facilitate its use as an add-onexperiment for satellite platforms. In addition, the simplicity ofthis design, along with its minimal pointing and servicerequirements, would make such an experiment ideally suited fordeployment on Space Station Freedom.POLARIMETER BASICSThe measurement of linear polarization at hard x-ray and gamma-ray energies is based on the fact that the Compton scattering cross-section is a function of angle from the electric field vector. Inparticular, the scattering cross-section is a maximum for photonsscattered perpendicular to the electric field vector and it is aminimum for photons scattered parallel to the electric field vector.The ratio between the minimum and the maximum scattering crosssections gives a good indication of how effective this approach maybe. Although this ratio is quite high (-65) at 100 keV, the ratiodecreases rapidly at energies above 500 keV (-5 at 500 keV). Thismakes this technique increasingly difficult at MeV energies.THE POLARIMETER MODULEGRAPE is designed as a modular experiment, consisting of anumber of independent polarimeters. The basic principle of the GRAPEdesign is to scatter photons from a low-Z plastic scintillator into ahigh-Z NaI(Tl) scintillator which is capable of absorbing theremaining photon energy with a high probability. A singlepolarimeter module consists of a ring of individual plasticscintillators surrounding the NaI(Tl) absorber, as shown in Figure 1.The NaI(T1) detector is cylindrical, 7.6 cm in diameter and 7.6 cmlong. Each plastic scattering element (in the present design) is 7.6cm long and 3 cm thick. A complete module measures 25 cm x 25 cm insize.The goal is to measure the azimuthal distribution of thephotons scattered into the central detector. This distribution isdetermined by recording the coincidence events between each of the 12scattering elements and the central detector. This provides a set of12 energy-loss spectra which can be used to measure the polarizationof the incident flux as a function of energy. 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 18:09:141144 The Design of a Gamma-Ray Burst PolarimeterFigure 1. Schematic diagram of asingle GRAPE module showing thering of plastic scintillatorswhich act to scatter incidentphotons into the central NaI(T1)detector. In the baselinedesign, the NaI(Tl) detector is7.6 cm in diameter by 7.6 cmlong. The plastic scatteringelements are 3 cm thick. Theplastic detectors are centered ona radius of 11.3 cm.We define the measured polarization in terms of the modulationfactor,P(E7,0) = Smax (Er , e) Sn,^„ (E7 9 0)Smax (Er , 0) + Smm (Er , 0)where S refers to the effective area of a single NaI (T1) -plasticscattering pair at incident energy Ey and incident angle 0. In thisexpression, the maximium and minimum values refer to the maximum andminimum of the azimuthal variations. The detection propertiesinherent in this expression have been studied in detail with a seriesof Monte Carlo simulations. These simulations employed a modifiedversion of the CERN GEANT code (modified to properly handle polarizedphotons) .The results of these simulations are shown in Figures 2 and 3.Figure 2 shows the effect of increasing incident energy on themeasured azimuthal variations (for an incidence angle of 0 °). Figure3 shows the effect of increasing the incidence angle (for an incidentenergy of 100 key). In all cases, the response both to 100% linearlypolarized radiation and to unpolarized radiation is shown. Solidlines represent aTABLE I. Monte Carlo Results	fit to the data.Some of theseEnergy ST(NaI) ST (plast) ST(coinc)	 p	 results are also(key)	 cm2	 cm2	 cm2	 given in Table I50	 6.5	 153	 4.1	 71%	(where ST refers tothe summed total of100	 6.4	 146	4.2	 71%	all elements in a200	 10.2	 133	 3.6	 73%	single module). We300	 23.8	 125	 3.1	 64%	conclude from these400	 31.6	 117	 2.9	 63%	simulations that: 1)500	 35.3	 111	 2.6	 59%	 there is significant700	 36.9	 101	 2.3	47%	efficiency at900	 35.6	 93	 1.8	 33%	ener ies u to 500g	 P 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 18:09:14NE 0.60.40.2Uw 0.00 100	 200	 300Azimuth Angle (degrees)NE 0.6c^Q 0.4}. 0.2Umw 0.00 100	 200	 300Azimuth Angle (degrees)N 100keV,0=30°Q 0.4 e °0.2Umv-w 0.00 100	 200	 300Azimuth Angle (degrees)NE 0.6m 0.40.2Ue0.00 100	 200	 300Azimuth Angle (degrees)M. McConnell et al. 1145keV; and 2) there is a clear polarization signal even at 30 0 off-axis, thus providing for an effective FOV of -1 steradian.N	 Eer=100 keV, 0= 1000.6\JÎ )/0.4ee0.2W 0.00	 100	 200	 300Azimuth Angle (degrees)NE 0.6 -	 = 300 keV, 0 = 000.4Qm 	eW 0.00	 100	 200	 300Azimuth Angle (degrees)Figure 2. The modulation as afunction of incident energyexpressed as effective area perscattering pair. In each case,the response to both 100 %polarized flux (circles) andunpolarized flux (triangles) isshown. Errors are roughly thesize of the plot symbols.Figure 3. The modulation as afunction of incident angleexpressed as effective area perscattering pair. In each case,the response to both 100 %polarized flux (circles) andunpolarized flux (triangles) isshown. Errors are roughly thesize of the plot symbols. 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 18:09:141146 The Design of a Gamma-Ray Burst PolarimeterBALLOON PAYLOAD CONCEPTIn developing this concept for a balloon payload, it isnecessary for the detector to have an efficiency which would giveuseful scientific results within the time frame of a typical long-duration balloon flight (typically, 2 -3 weeks).	 This requires anarray of polarimeter modules. 	 Our baseline design is an array of 16modules, as shown in Figure 4. 	 As a polarimeter, GRAPE would be ableto detect 50% linearpolarization at >3a in a burstwith a fluence of 5x10 -6 ergscm 2 .	 (More recent simulationsappear to suggest thatrefinements to this design mayimprove the detectionefficiency by as much as 50%.)With a FOV of -1 steradian, wewould expect to see -1 burst atthis fluence level during a 2-3week balloon flight.	 AlthoughGRAPE is optimized for themeasurement of polarization (upto several hundred key), theuse of the individual detectorswill also permit a significantcapability for timing, andspectral measurements of gamma-ray bursts up to several MeV.The use of corner-mounted x-rayFigure 4.	 A schematic of the full detectors (as in the BATSEballoon prototype, consisting of design) will permitan array of 16 individual modules. localization of bursts towithin several degrees; thiswould. be required for a more accurate deconvolution of the GRAPEresponse (due to its dependence on incidence angle).	 Finally,systematic effects within the modules would be alleviated by rotationof the balloon payload.REFERENCES1. I. Mitrofanov and A. Pozanenko, in Gamma-Ray Bursts: Observations,Analysis and Theories, eds. C. Ho, R. Epstein, and E. Fenimore(Cambridge Univ. Press, 1992), p. 430.2. G.S. Bisnovatyi-Kogan, in Gamma-Ray Bursts: Observations, Analysisand Theories, eds. C. Ho, R. Epstein, and E. Fenimore (CambridgeUniv. Press, 1992), p. 89.3. M.G. Baring, in Compton Observatory Science Workshop, eds. C.Shrader, B. Dennis, and N. Gehrels (NASA CP 3137, 1992), p. 293.4. V. Schönfelder et al., submitted to Ap. J. Suppl. (1992)5. K. Hurley, in Gamma-Ray Bursts: Observations, Analysis andTheories, eds. C. Ho, R. Epstein, and E. Fenimore (Cambridge Univ.Press, 1992) , p. 279. 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 18:09:14

The design of a gamma‐ray burst polarimeter

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