Development of a Dedicated Hard X-Ray Polarimeter Mark L. McConnell, James R. Ledoux, John R. Macri, and James M. Ryan Space Science Center University of New Hampshire Durham, NH AAS-HEAD Mt. Tremblant, Quebec 23-26 March 23
Hard X-Ray Polarimetry Gamma-Ray Bursts In the context of fireball models of classical GRBs, polarimetry provides useful constraints on the extent to which the flow is beamed. Energy-dependent polarization measurements can also distinguish between synchrotron and inverse-compton emissions. Polarimetry can test the importance of photon splitting in SGRs. Solar Flares Polarimetry provides a means to determine the extent to which the energetic electrons are beamed. 2
Basic Principles of Compton Polarimetry Polarimetry relies on the fact that photons tend to Compton scatter at right angles to the incident polarization vector Z incident photon E o q k scattered photon ds = r 2 o 2 dw Ê Á E Ë E o ˆ 2 Ê Á Ë E o E + E E o ˆ - 2sin 2 q cos 2 h X h k' Y E = 1 + E o mc 2 E o ( 1 - cosq ) q is the Compton Scatter Angle, h is the Azimuthal Scatter Angle 3
The Polarization Signature For a fixed Compton scatter angle (q), the azimuthal distribution of scattered photons contains the polarization signature. 2 Counts 15 1 5 C-max C-min 5 polarization angle 1 15 2 25 Azimuthal Scatter Angle (degs) 3 35 A B Modulation Factor for a 1% polarized beam represents a figure-ofmerit for the polarimeter Q = C max - C min C max + C min = A B C ( h) = A cos 2 (h-j) + B The amplitude of the modulation defines the level of polarization. The scattering angle corresponding to the minimum of the distribution defines the plane of polarization. 4
Minimum Detectable Polarization (MDP) MDP = n s Q 1 S 2( S + B) T S = B = T = Q 1 = source counting rate background counting rate observation time modulation factor for 1% polarization Sensitivity can be improved by : 1) Increasing S (efficiency or geometric area) 2) Decreasing B 3) Increasing T 4) Increasing Q 1 (optimizing geometry) 5
Generating a Polarized Beam Polarized X-rays can be generated in the lab by Compton scattering photons from a g-ray calibration source within a block of plastic scintillator. A signal from the scintillator provides an electronic tag for each scattered photon, which can be used as a coincidence signal with the polarimeter. Polarization Fraction.8.6.4.2. 2 4 6 8 1 12 Scatter Angle (degs) 6 kev 662 kev 12 kev 14 16 18 This graph shows the level of polarization that can be achieved for various input photon energies (corresponding to 241 Am, 137 Cs and 6 Co) and various scatter angles. A 662 kev photon beam, scattered at 9, is ~6% polarized; the scattered energy is 288 kev. 6
The GRAPE Science Model Gamma-RAy Polarimeter Experiment Plastic Scattering Elements : 28 element array of BC-44 Each element optically isolated Wrapped in Tyvek and Kapton tape Each 5 mm square by 5 cm long Readout provided a 5-inch PSPMT Hamamatsu R3292 PSMT Calorimeter Elements : 4 element array of CsI(Tl) Each 1cm square by 5 cm long Readout provided by a MAPMT Hamamatsu R59 MAPMT PSPMT / Plastic Array Housing : 1mm thick aluminum Optically isolated from CsI/MAPMT Plastic Elements BC-44 CsI(Tl) Array Position-Sensitive PMT Hamamatsu R3292 7
GRAPE Polarimetric Response Simulated Results for Monoenergetic On-Axis Beam at 15 kev 12 Polarized Data 12 Unpolarized Data Counts 8 4 Counts 8 4 5 1 15 2 25 3 Azimuthal Scatter Angle (degs) 35 5 1 15 2 25 3 Azimuthal Scatter Angle (degs) 35 The simulated data for an unpolarized beam are used to correct for geometric effects and to extract the true modulation pattern from the data. Counts 12 8 4 Corrected Data 5 1 15 2 25 3 Azimuthal Scatter Angle (degs) 35 8
Simulated Performance: On-Axis Modulation Factor vs. Depth Effective Area vs. Depth 1. 4 Modulation Factor.8.6.4.2. 11.6 mm 88.9 mm 76.2 mm 63.5 mm 5.8 mm 25.4 mm 3 4 5 6 7 8 9 1 2 3 4 5 Effective Area (cm 2 ) 3 2 1 11.6 mm 88.9 mm 76.2 mm 63.5 mm 5.8 mm 25.4 mm 3 4 5 6 7 8 9 1 2 3 4 5 Energy (kev) Energy (kev) Figure of Merit (relative) 7 6 5 4 3 2 1 Figure-of-Merit vs. Depth 76.2 mm 5.8 mm 25.4 mm 63.5 mm 88.9 mm 11.6 mm 3 4 5 6 7 8 9 1 2 3 4 5 Energy (kev) The figure-of-merit incorporates an estimate of the increased background as a function of detector volume. FoM = m 1 ea V det Optimum Depth for this design is ~3 inches 9
Simulated Performance: Off-Axis 7.62 cm depth @ 2 kev 1. 3. Modulation Factor.8.6.4.2. Modulation Factor Effective Area 1 2 3 4 Angle (Degs) 5 6 7 2.5 2. 1.5 1..5. Effective Area (cm 2 ) The GRAPE design offers significant response even at large off-axis angles. This makes GRAPE an attractive design for polarization studies of transient sources (Gamma-Ray Bursts). 1
Science Model Components This photo shows the 5-inch PSPMT, the array of plastic scattering elements and the MAPMT/CsI assembly. 11
GRAPE Science Model Lab Set-Up (drawing not to scale) Photons are partially polarized by scattering off a plastic scintillator detector. This also provides electronic tagging of each scattered photon. 12
PSPMT Response to Plastic Array The spatial resolution in the PSPMT is sufficient to resolve the individual 5 mm plastic scattering elements. This figure shows the spatial distribution of events in the plastic scintillator array. These are coincident events, representing photons that have scattered from one of the plastic elements into the central CsI array. The individual plastic elements are clearly resolved, as is the square well for the central CsI array. Distribution of 137 Cs Events Distortions near the edge of the PSPMT are also evident in these raw data. 13
Results from Science Model 25 2 Polarized Source Spectrum (Nominal Energy = 288 kev) The incident energy spectrum is reconstructed by combining the energy loss in both the CsI and in the plastic array. Counts 15 1 E o = 284.2 (±.9) kev E/E = 39% This energy spectrum is that of a 662 kev beam that has been scattered once. 5 1 2 3 4 5 Energy (kev) 6 7 8 The variable scattering geometry results in an incident beam that covers a broad range of energies, thus accounting for the broad nature of this spectrum. 14
Lab Results from Science Model Runs taken with the incident beam having a polarization angle that differs by ~9. 15 1 5-5 -1 1 8 GRAPE Science Model Incidence f = 1 5-5 -1 6 5 GRAPE Science Model Incidence f = 9 Corrected Counts 6 4 Corrected Counts 4 3 2 2 1 6 12 18 24 3 36 6 12 18 24 3 36 Azimuthal Scatter Angle (degs) Azimuthal Scatter Angle (degs) measured polarization = 61(±3)% polarization angle = 49(±1) measured polarization = 73(±7)% polarization angle = 135(±3) 15
GRB Sensitivity We have estimated the polarization sensitivity of a long-duration balloon payload consisting of a planar array of 36 GRAPE modules. Such an array would have a FoV of ~2ϖ steradian and cover less than 1 m 2 of area. The table below gives the polarization sensitivity over the full energy range (5-3 kev) and the expected frequency of observed events (based on the fluence distribution of the 4B catalog). GRB Polarization Sensitivity 5-3 kev energy band Fluence (ergs cm -2 ) MDP T = 1 sec MDP T= 1 sec Observed Rate (N > Fluence) 1 1-4 2.8% 2.9% 1 every 32 days 5 1-5 3.9% 4.4% 1 every 8 days 1 1-5 9.3% 13.7% 1 every 1 days 5 1-6 14.% 24.4% 1 every 6 days 3 1-6 19.4% 38.6% 1 every 4 days 1 1-6 43.2% 1 every 2 days 16
Solar Flare Sensitivity The solar flare polarization sensitivity for various energy bands. These data assume a balloon-borne array of 16 GRAPE modules. 3 Minimum Detectable Polarization (%) 25 2 15 1 5 X1-Class Flare X1-Class Flare 1 2 3 4 Energy (kev) 5 6 17
A New Design We are currently preparing to conduct lab tests with a more compact design based on Hamamatsu s new flat-panel MAPMT. This new PMT design (Hamamatsu H85) provides an 8 x 8 array of anodes. The anode size of 5.6 mm (on a 6 mm pitch) is an excellent match to the 5 mm plastic elements that we are currently using. The MAPMT can be used to read out both the plastic scattering elements and the CsI calorimeter elements. This also eliminates a significant amount of mass in front of the detector array. 18
Polarimeter Design Based on MAPMT CsI plastic elements MAPMT FEE Single module with MAPMT and FEE electronics. Array of modules suitable for use as a coded aperture imaging plane. 19
Development Status We have developed a design for a compact polarimeter module that could be adapted to a variety of configurations. Testing of a science model based on the design has so far yielded encouraging results. Further testing of the science model will be completed in the coming months. We will soon be testing a new design based on the use a flat panel multi-anode PMT. Long-term goal will be to develop a dedicated balloon payload. An array of GRAPE modules (a bunch of GRAPEs ) will provide sufficient sensitivity to constrain models GRBs and solar flares. 2
Results from Science Model Data from the science model are shown below. These are energy selected data from both a polarized beam (left) and an unpolarized beam (middle). The corrected data is on the right. Measured Polarization = 53(±3)% Polarization Angle = 48 (±1) 1 4 1 8 3 8 Counts 6 4 Counts 2 Counts 6 4 2 1 2 Polarized Data Unpolarized Data Corrected Data 6 12 18 24 3 36 6 12 18 24 3 36 6 12 18 24 3 36 Scatter Angle (degs) Scatter Angle (degs) Scatter Angle (degs) 288 kev 356 kev 288 kev 21
PSPMT Calibration The PSPMT response was calibrated during the summer of 21. These data show both the calibration grid (2.5 mm spacing) and the reproduced locations. Distortions near the edge of the PSPMT are evident. LED Calibration Grid Measured LED Locations 4 3 Y-Location (mm) 2-2 Y-Location (arbitrary units) 25 2 15-4 1-4 -2 2 4 1 15 2 25 3 X-Location (mm) X-Location (arbitrary units) 22
Compton Scatter Polarimetry A Compton scatter polarimeter measures the angular distribution of the scattered photons in a plane which is perpendicular to the incident photon direction. The asymmetry of this azimuthal scatter angle distribution can be exploited to measure the linear polarization of the incident flux. Asymmetry Ratio 1 1 1 5 kev 1 kev 25 kev 5 kev 1 MeV 6 8 1 12 Scattering Angle (degrees) A Compton scatter polarimeter consists of two basic components: 1) low-z scattering detector(s) to provide Compton scattering medium 2) high-z calorimeter detector(s) to absorb the scattered photon 23