Produit, N

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29th International Cosmic Ray Conference Pune (2005) 3, 385388POLAR a compact detector for measuring GRB polarization.N. Produit , W. Hajdas, F. Barao, S. Deluit , C. Leluc, A. Mschedlishvili, M. Pohl,D. Rapin, E. Suarez-Garcia, J.-P. Vialle, R. Walter and C. Wigger(a) ISDC, Universit·e de Geneve, Switzerland(b) PSI, Villigen, Switzerland(c) LIP, Lisboa, Portugal.(d) DPNC, Universite de Geneve, Switzerland(e) LAPP/IN2P3/CNRS, Annecy, FrancePresenter: N. Produit (Nicolas.Produit@obs.unige.ch), swi-produit-N-abs2-og15-posterThe POLAR detector is a simple and compact detector capable of measuring the polarization level of a typicalgamma-ray burst (GRB). POLAR don’t try to measure itself the sky coordinate of the incoming GRB but willrely on the availability of such information from other instruments. Simple plastic scintillator is used. MonteCarlo simulation show that a reliable and non biased polarization signal can be achieved. Fast coincidencetechniques are used to reject the noise.1. GRB polarizationGRB phenomenon still remains a mystery. Competing theoretical models like fireball [1], cannonball [2]or electromagnetic [3] make very different prediction about the polarization of the prompt  -rays photons.Systematic bursts observations with a dedicated polarization detector would enable to discriminate betweenthe different models.To date, despite of many attempts, the polarization data remain very scarce and its measurements proved quitedifficult. Well designed GRB polarimeter might be also very beneficial in studies of other hard X-ray andgamma ray sources like X-ray pulsars, neutron stars or AGNs. Currently, there are several laboratory projectsaimed to build a hard X-ray polarimeter based on Compton scattering technique. They utilize various gammadetector arrays like e.g. low-high Z scintillator hybrids [4], homogeneous low Z fast plastics scintillators [5] orCdTl semiconductor pixel detectors [6].2. Detector DesignThe fast and precise localisation of GRB is nowadays provided rapidly and freely by many observatory (IN-TEGRAL, SWIFT, GCN). We foresee that this will be true also in the future. It is therefore not important thata detector is able, by itself, to localise the direction of a GRB.Releasing the constrain of localisation is simplifying the design. We take here for granted that the positionof the GRB are known and that the detector platform can tell where the detector is pointing. We can rely onMonte Carlo simulation to correct for the difference in response due to the position of each GRB. We will show(figure 3) that those corrections are small for a reasonable solid angle.Following requirements and considerations guided us to the proposed detector design. The polarimeter shouldcover energy range characteristic for GRBs i.e. from few up to several hundred keV. It will have large effectivearea of at least few hundred cm  and high efficiency for the Compton scattering. Both can be assured by usinglow Z detector materials like e.g. light weight plastic scintillators. The modulation factor for polarization mea-surement will be kept high by selecting the single cell (pixel) size similar to the photon mean free path and bysetting the energy threshold as low as possible. Above features should also be kept within possibly wide rangeof incoming gamma angles. This requirement would effectively cause an increase of the overall sensitivity386 N. Produit et al.of the polarimeter to a large fraction of the sky. Moreover, the design should tolerate small angle scatteringinevitable in the low-Z scintillators while the initial direction of the polarization vector should be roughly thesame until a large angle scattering occurs.With a good angular resolution defined by a suitable pixel cross section one will use the large angle scatteringto determine polarization direction of the event. This will be done by selecting two coincidence events withlargest energy depositions. Schematic view of the polarimeter design is shown in figure 1. The instrument iscomposed of 2304 plastic scintillator bars with dimension 4x4x200 mm  . Scintillation light is converted bya mosaic of flat panel multianode photomultipliers (PM). Electronics, power supply, readout and communi-cation units are behind PMs and serve also as a passive shield. Background events from cosmic rays can berejected by analysing total energy deposition in the detector while the rate of accidental coincidences from thegamma background will be reduced both applying thin absorber material on the detector walls and by usingfast scintillators and short coincidence widths.FlatpannelPM200 mmCC H8  8198mmFigure 1. Schematic view of the detector scintillator array and photomultiplier on the right side.3. Response ModellingDetector response was simulated using GEANT4 code from CERN. The computer model consists of the scin-tillator array shielded by thin carbon sheets and coupled to the PM and read-out electronic blocks approximatedby a 10 cm thick Aluminium box. Incoming photons have been generated uniformly over the area covering thewhole detector and their entrance angles varied from 0 to 70  in the polar ( 	 ) and from 0 to 45  in azimuth (  )directions respectively. All generated photons were fully polarized while their energy was sampled from theBand [7] model distribution (  =1,  =2.5 and E  =200 keV) between 10 keV and 300 keV. On average, 14%of the events fulfil the double energy deposition coincidence required by the trigger and about 50% of suchcoincidences occur within 10 closest detection cells. Simulations also confirmed that photon interactions aredominated by the low energy Compton scattering with a small energy transfer to the electron and only a tinychange in the initial polarization direction for the scattered photon. However in every case where a large anglescattering occurs, a clear modulation of the azimuth scatter angle is observed as the direction of the Comptonscattered photon is more likely to be perpendicular to the polarization direction. Simulations clearly indicatethat it is sufficient to select only two most energetic processes and their corresponding cells. In addition, theline connecting both cells is well correlated with the outgoing azimuth photon direction for wide range of in-coming photon polar angles. An example in figure 2 shows the reconstructed azimuth angle distribution foran intense GRB coming perpendicularly to the detector and with the polarization orientation along   =0  .The maximum signal rate for coincidences should be observed at scattering angles   =90  as it is showedPOLAR a compact detector for measuring GRB polarization. 3870 20 40 60 80 100 120 140 160 180050100150200250300ξCoincidence EventsAzimuthal angle (deg)Figure 2. Azimuth angle distribution for the Compton scattered events from photons coming at ﬀ =0 ﬁ and with the polar-ization direction along ﬂ =0ﬁ.in the figure 2. Depending on both photon energy and impinging angle, the modulation function can vary ina wide range though its sinus-like shape is always well preserved. For the Band function spectrum of GRBwith the parameters as above, the modulation factor varies in the range from 0.32 to 0.22 for the photon in-coming angles from 	 =0 to 70  (see Fig. 3 Left). Similar results were obtained for polarization vectors rotatedalong the  angle. It confirms the requirement put on the polarization detector design to keep large values ofmodulation factors over a wide range of the gamma directions from the sky. Although the geometrical area ofthe detector reaches about 400 cm  , its effective size for Compton scattered events with the predefined Bandtype of the spectrum (as given by the geometrical surface multiplied by the photon detection probability) variesfrom 100 cm  down to 40 cm  , depending on the photon entrance angle. As shown in figure 3, the acceptancearea changes relatively slowly over about 1.3 ﬃ sr. Within this fiducial solid angle the average area is equal toabout 80 cm  . Simulations show that these results are virtually independent on the photon azimuth incomingangle  .4. Background ReductionThe background photons are coming from the diffused X-ray sky, non-GRB gamma ray sources as well asfrom the induced radioactivity in the spacecraft and the detector itself. The X-ray diffused background cancos(theta) 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1modulation factor0.20.220.240.260.280.30.320.34cos(theta)0.3 0.4 0.5 0.6 0.7 0.8 0.9 12cm405060708090100effective areaFigure 3. Left: GEANT4 simulations of the modulation factor as a function of the photon polar angle with polarizationdirection set in the plane ﬂ =0 ﬁ . Right: Effective area for Compton scattered photons with energies from 10 and 300 keVas a function of the incoming angle ﬀ ( ﬂ was set to 0ﬁ).388 N. Produit et al.dominate at lower energies and one will apply 1 mm thick carbon absorbers in front of the scintillator array.Other background contributions from the gamma ray sources like e.g. Crab nebula will have only a smallfraction of the diffused X-ray background rate. The induced gamma background is expected to be negligibleusing estimates from the INTEGRAL and RHESSI satellites. The Cosmic Ray charged particle backgroundwill penetrate the whole detector but due to a very high energy deposition (at least 800 keV) one can get rid ofit by setting the upper energy threshold.5. Polarization Detection from TransientsThe minimum detectable polarization for GRBs and other transient sources was estimated with the help ofMonte Carlo calculations using typical parameters for both the burst and the background. Statistical fluctu-ations of the background and of the signal rate have been taken into account properly. Assuming 80 cm effective detector area, POLAR instrument can determine just within 1 second the 50% polarization value (3 level) for events with fluxes equal to about 15 photons s  "! cm  # . Fig. 4 shows the minimum detectable po-larization values for GRBs as a function of the mean waiting time given in days. The predictions are based onthe GUSBAD [8] catalog with a supposition that only about a third of all events will be in the detector field ofview. According to figure 4, the proposed polarimeter would be able to measure the GRB polarization valuesfrom 20% up (3  levels) for about 10 to 15 GRB per year.waiting time [days]1 10210minimum detectable polarisation level−1101( )minimum detectable polarisation levelFigure 4. Minimum detectable polarization value of the GRB plotted as a function of the waiting time. Predictions arebased on the previous GRB records.References[1] Piran et al. Phys.Rept. 314 (1999) 575-667[2] S. Dado et al., astro-ph/0403015[3] M Lyutikov et al., Astrophys.J. 597 (2003) 998-1009[4] McConnel et al. Vestrand, IEEE Trans. Nucl. Sci. 46(4)[5] N. Produit astro-ph/0504605, accepted for publication in NIM[6] Caroli, E., et al. (2000), SPIE Conf. Proc., 4140, 573.[7] D. Band et al. ApJ 413:281-292, 1993[8] M. Schmidt, Astrophys.J. 616 (2004) 1072-1077

POLAR a compact detector for measuring GRB polarization

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