GALI: a Gamma-ray Burst Localizing Instrument
Roi Rahin, Luca Moleri, Alex Vdovin, Amir Feigenboim, Solomon Margolin, Shlomit Tarem, Ehud Behar, Max Ghelman, Alon Osovizky
SSPIE Proceedings:GALI a Gamma-ray Burst Localizing Instrument
Roi Rahin a , Luca Moleri a , Alex Vdovin a , Amir Feigenboim a , Solomon Margolin a , ShlomitTarem a , Ehud Behar a , Max Ghelman b , and Alon Osovizky ba Department of Physics, Technion, Haifa, Israel b Nuclear Research Center Negev, Israel
ABSTRACT
The detection of astrophysical Gamma-Ray Bursts (GRBs) has always been intertwined with the challenge ofidentifying the direction of the source. Accurate angular localization of better than a degree has been achievedto date only with heavy instruments on large satellites, and a limited field of view. The recent discovery ofthe association of GRBs with neutron star mergers gives new motivation for observing the entire γ -ray sky atonce with high sensitivity and accurate directional capability. We present a novel γ -ray detector concept, whichutilizes the mutual occultation between many small scintillators to reconstruct the GRB direction. We builtan instrument with 90 (9 mm) CsI(Tl) scintillator cubes attached to silicon photomultipliers. Our laboratoryprototype tested with a 60 keV source demonstrates an angular accuracy of a few degrees for ∼
25 ph cm − bursts. Simulations of realistic GRBs and background show that the achievable angular localization accuracywith a similar instrument occupying 1l volume is < ◦ . The proposed concept can be easily scaled to fit intosmall satellites, as well as large missions. Keywords:
Gamma-ray bursts, scintillators, silicon photomultipliers, directional gamma-ray detector, smallsatellites, detector simulations
1. INTRODUCTION1.1 Astrophysical Context
Astrophysical γ -ray Bursts (GRBs) are the most remarkable transient phenomena in high-energy astrophysics.Long GRBs, those lasting more than 2 s, are generally associated with supernovae, the explosion following thecollapse of massive stars. Short GRBs, lasting less than 2 s, are associated with the coalescence of a neutronstars binary,
1, 2 and could also result from a merger of a NS and a black-hole binary. Following the first discoveries of gravitational waves (GWs) from compact stellar mergers by AdvancedLIGO (Laser Interferometer Gravitational-wave Observatory), the astrophysics community has concentrated itsefforts on detecting their electromagnetic (EM) counterparts. The first direct detection of GWs by the twoLIGO facilities occurred in September 2015. In August 2017 the European VIRGO detector joined the LIGOobservation run. The combined detection by three interferometers potentially improves the event localizationfrom more than a thousand square degrees to tens of square degrees, depending of course on the strength of thesignal.The first EM counterpart of a GW event was observed on 17 August, 2017. This LIGO-VIRGO GW event )was followed 1.7 s later by a short GRB (GRB 170817A) independently detected by the Fermi γ -ray Burst Monitor(GBM) and by IBIS on board INTEGRAL. Many other facilities around the world started to search for the siteof the event in all possible wavebands. It took approximately 11 hours before the source was identified in thelenticular galaxy NGC 4993. Increasing the number of simultaneously observed GW and EM counterparts is paramount to addressingfundamental questions on the nature of coalescing neutron stars and black-holes. The goal of the method
Further author information: (Send correspondence to R.R)R.R.: E-mail: [email protected] a r X i v : . [ a s t r o - ph . I M ] F e b escribed in the present paper is to detect GRBs at high sensitivity and to identify their direction with highaccuracy. Such capabilities will not only enable fast follow-up with telescopes, but will also allow LIGO-VIRGOto search for sub-threshold events once the time and direction are known. A single soft γ -ray detector unit generally cannot identify the direction of incident photons. Angular localizationof GRBs thus require multiple detectors. The coded-mask aperture method, most successfully implemented onSwift-BAT, is an array of detectors partially covered by a mask. The coded mask generates different shadingpatterns over the detectors array, varying with the source direction. This technique enables the reconstructionof the direction of a GRB up to 20’ (minutes of arc). However, it requires a large detector area and volume dueto the required separation between the mask and the array. Moreover, it has a limited field of view, and themask blocks many of the source photons.Another method is a sparse array of scintillators, e.g., on Compton-BATSE and Fermi-GBM, whichare distributed over the spacecraft, each facing a different direction. The relative signals in each scintillatorprovide information about the direction of the source. For example, those facing the opposite direction areshielded by the satellite and will have lower count rates. Angular localization of soft γ -ray sources can also beachieved if the detectors are far enough apart, as on different space crafts, and the different times of arrival canbe discriminated. Alternatively, direction reconstruction is possible if the detection is time-coincident withanother observation of a different messenger (e.g. visible light), where better angular accuracy is available.
2. THE GALI CONCEPT
In traditional astrophysical γ -ray detectors, scintillators are built with different cross-sections towards differentdirections to produce a gradually varying response with angle. These systems rely on the scintillators facingvarious orientations to reconstruct the direction of the source. In contrast, the γ -ray-burst Localizing Instrument(GALI) concept presented here exploits mutual occultation of numerous small scintillators, distributed withina small volume, to provide directional information. The method relies on the entire array looking significantlydifferent from different directions. Due to the occultation, the count rates from each scintillator will varydramatically as a function of the source direction, even for small angle differences. In a sense, this is similar tothe coded mask aperture method, but the mask itself is composed of detecting scintillators, so that no preciousphotons are lost. The low count rates in each individual small scintillator are compensated by the large numberof scintillators. As in traditional approaches, the sensitivity to weak sources depends on the total size of thedetecting volume. Fig. 1 shows two prototype instruments demonstrating the differences between a traditionaldetector design and the present concept. The first is the Gamma ray Transient Monitor (GTM) whose design issimilar to the Fermi/GBM, and which has been described in past SPIE proceedings. The second is the GALIlaboratory prototype described in section 4.2.The GALI laboratory prototype presented here is composed of (9 mm) cubic scintillators. These are read outby Si Photo-Multipliers (SiPMs), which obviates the traditional, cumbersome, Photo-Multiplier Tubes (PMTs).Adopting such compact light sensors enables filling a volume with a large number of small detectors at theexpense of the size of each individual one. An additional advantage of SiPMs over PMTs is their significantlylower operation voltage (tens of volt instead of hundreds).A simulation of the system performance is presented in section 3, a detailed description of the experimentalsetup is given in section 4, and the results of the experiments are given in section 5.
3. SIMULATIONS
To check the performance of the GALI concept we run simulations using MEGAlib, a simulation softwarebased on Geant4. In these simulations all γ -ray interactions with scintillators are considered as counts in therelevant scintillator. Each simulation run is composed of two steps: A bright GRB with no background and thebackground simulation. We use the bright GRB simulation to estimate the average counts on each scintillator inigure 1: Two prototype instruments demonstrating the two conceptual detector designs Left : The GTM - atraditional design based on large, asymmetric box-shaped scintillators with various orientations. This design issimilar to the Fermi/GBM.
Right : The GALI design, which uses (9 mm) cubic scintillators in a 3D structurethat exploits their different mutual occultation from different directions.the system with no background: a burst of 1000 ph · cm − , whose photon spectrum ( dN/dE ) is a band function (equation 1) with α = 1 . β = 2 . E peak = (2 + α ) E = 266 keV. dN ( E ) dE = A ( E keV ) α exp (cid:16) − EE (cid:17) E ≤ ( α − β ) E (cid:2) ( α − β ) E keV (cid:3) ( α − β ) ( E keV ) β exp ( β − α ) E > ( α − β ) E (1)The simulations are repeated varying the angular coordinates at 5 ◦ intervals within the upper hemisphere. Theaverage counts on each scintillator are then interpolated at either 0.5 ◦ or 1 ◦ intervals. We divide the averagecounts by the total number of counts for each burst to obtain the relative average counts on each scintillator.The result is an array of relative counts for each angle in the hemisphere. The background simulation is basedon various lower earth orbit (LEO) observations of hard X-rays and γ -rays and is included in the MEGAlibsoftware package. Background caused by Leptonic and Hadronic components was not included, being estimatedin simulation to be less than 1% of the photonic background, far less than the statistical uncertainty. TheHadronic background may need to be simulated later depending on the spacecraft platform which will host theexperiment.After running the simulations, we estimate the directional capabilities of the detector system. We consider 1second bursts of 10 ph cm − s − from a given direction in the presence of the background. The burst is generatedusing the aforementioned relative average counts by applying poisson statistics to the expected average of eachscintillator. We then add the background with poisson uncertainty. We reconstruct the burst direction usinga cstat estimator between the generated counts and the simulated ones at each angle. The reconstructeddirection is that which gives minimal cstat value. The burst generation and direction reconstruction process isrepeated 100 times for each angle at 5 ◦ intervals so as to generate an accuracy map of the entire hemisphere.The simulated detectors are two different GALI systems: a random configuration of 90 (9 mm) scintillatorcubes spread in a 6 × × volume and one of 350 cubes in a 10 × ×
10 cm volume. To compare the potentialimprovement of the GALI accuracy with respect to previous existing concepts, we also simulated the a GTMdetector with four 3” diameter 1” thick cylindrical scintillators. The GTM has similar effective area compared tothe 350-scintillator GALI, but has approximately twice that of the 90-scintillator GALI. The simulated systemsare shown in figure 2. Notice that the 90-scintillator GALI contains the PCB boards upon which the detectorsre mounted as in the the lab prototype described in section 4.2, whereas no 350-scintillator GALI prototypeexists, so no PCB boards have been included in this case.A small sample of the generated bursts’ direction reconstructed by the three systems is shown in figure 3.The all-sky average deviation of the reconstructed source direction from the original source direction in eachsystem is 13.1 ◦ for the GTM, 6.5 ◦ for a 90-scintillator GALI and 1.7 ◦ for the 350-scintillator GALI. From thesesimulations we conclude that the localization accuracy of a system increases significantly with the number ofdetectors, even when the effective area of the entire system is reduced.Figure 2: Three detector configurations compared using simulations. Left : A GTM configuration of four 3”diameter 1” thick cylindrical detectors - the GTM.
Middle : A GALI configuration of ninety (9 mm) cubicscintillators. Between the detectors are PCB boards. This system is simulated as close as possible to a labmodel. Right : A GALI configuration of 350 (9 mm) cubic scintillators.
4. EXPERIMENTAL SETUP4.1 Individual detector unit characterization
We conducted a preliminary study to characterize single detector units in order to choose among differentscintillating crystals, reflective wrappings and SiPM sensors. Each detector unit consists of a (9 mm) cubicscintillator wrapped or coated by a reflective material and coupled to a SiPM using Cargille Meltmount ∗ opticalglue, which has a refractive index of 1.7. Scintillating crystals of two kinds were selected for their high density and light yield, as well as their negligibleintrinsic radioactivity: CsI(Tl) (see for example Balamurugan et al. ) and Ce:GAGG. Both scintillators werepolished by and purchased from Advatek † . We coated the CsI(Tl) crystals by sputtering a 50 nm thick layerof SiO , which protects the delicate and slightly hygroscopic crystal from mechanical degradation and waterabsorption. Ce:GAGG suffers from afterglow for hours after exposure to light due to lattice defects; therefore,it should be always kept in the dark so as not to alter the measurements. For space applications, the afterglowcaused by electron and γ -ray dose needs to be considered. γ -ray signals from our CsI(Tl) and Ce:GAGG crystals are read by a Sensl J60035 SiPM ‡ through a 10 kΩresistor and recorded with an oscilloscope with 1 MΩ termination, and are shown in figure 4. In these measure-ments the SiPM was operated at a 27.2 V bias. The recorded voltage pulse is a convolution of the SiPM responsefunction and the scintillation time evolution. For both crystals in the present configuration (RC ∼ µ s, givenC ∼ ∗ † ‡ igure 3: A comparison between the simulation-generated GRBs direction reconstructed by different detectorconfigurations. Each dot represents a 1 s burst of 10 ph cm − s − at lower earth orbit. Dots are grouped by coloraccording to the actual burst direction, which is represented by a × mark. Top : GTM.
Middle : 90-scintillatorGALI. bottom : 350-scintillator GALI. Notice the clear improvement in localization accuracy when the numberof scintillators is increased.proportional to the integrated charge produced by the SiPM, and the peaks in figure 4 corresponds to the endof the scintillation light production. The voltage recovery follows the circuit RC value. Figure 4b shows the fullCe:GAGG light emission signal, which presents a fast component ( ∼
100 ns) and a slow tail (ending at ∼
500 ns)that can be distinguished by the signal slope change. In the same figure it can be seen that the CsI(Tl) lightemission is much slower than the Ce:GAGG one. During the 500 ns in which the Ce:GAGG emission is com-pleted, CsI(Tl) produces only ∼
40% of the total charge. In figure 4a the full CsI(Tl) signal is shown, reaching itsmaximum value within ∼ µ s. Most existing integrated readout electronics have much shorter integration times,which may hinder the sensitivity of CsI(Tl) scintillators. Considering a readout electronics having ∼ µ s integra-tion time (like the TOFPET2 ASICs described in section 4.2) there is a clear advantage in using Ce:GAGG forbetter signal-to-noise.We measured charge spectra produced by the two crystals using an Ortec 671 spectroscopy amplifier withan Ortec Aspec-927 MCA. We set the amplifier shaping time to 0.5 µ s for Ce:GAGG and to 3 µ s for CsI(Tl) toinclude the full signal rise. The latter value is due to the signal shortening because of the low input impedance(465 Ω) of the amplifier. Spectra from a Cs source from both scintillators are presented in figure 5. The ratioof the number of counts in the 662 keV peak between the two scintillators is ∼ - - - · time [s] i n t eg r a t ed c ha r ge [ a . u .] CsI(Tl)Ce:GAGG - - · time [s] i n t eg r a t ed c ha r ge [ a . u .] CsI(Tl)Ce:GAGG
Figure 4: γ -ray signals from CsI(Tl) and Ce:GAGG crystals read by a SiPM coupled to ground through a 10 kΩresistor. The right hand side shows the first 500 ns of the plot on the left. The measured voltage is proportionalto the integrated charge produced by the SiPM. The peak corresponds to the end of the light production, whichis faster for Ce:GAGG. The voltage recovery follows the circuit RC value.been reported that at low energies CsI(Tl) provides ∼ ∼ γ -ray energy dependence requires a dedicated study with multiple radioactive sources,which is beyond the scope of the present study.Based on the presented results we conclude that there might be advantages in choosing Ce:GAGG for itsfaster light emission and higher detection efficiency. Despite that, for our first system prototype we choseCsI(Tl) because of its lower cost. Next we plan to build a full Ce:GAGG prototype to compare their performance. The SiPM models that we considered are: Sensl J60035 and Hamamatsu 14160-6050HS § . The most relevantfeatures for the two kinds are summarized in table 4.1. The Sensl J60035 SiPM has two clear advantages: a13.5 V lower breakdown voltage and a smaller cell size, which implies a linear response until higher gammaenergies. On the other hand, Hamamatsu 14160-6050HS has lower dark current, which is important for spaceapplications. Additionally, according to vendor data the Hamamatsu 14160-6050HS has a ∼
10% enhanced PDEin the region above 450 nm, where the emission of both scintillators peaks.Type size V bd [V] max I dark [ µ A cell size [ µ m] capacitance [nF]Sensl J60035 6 mm 24.5 12 35 4Hamamatsu 14160-6050HS 6 mm 38 7.5 50 2Another important parameter for space applications is radiation hardness, meaning capability to withstanddoses of highly ionizing particles with an acceptable performance degradation. We could not find any study § Entries 1024Mean 339.6Std Dev 287.4
ADC c oun t s h1 Entries 1024Mean 339.6Std Dev 287.4
Ce:GAGGCsI(Tl)
Figure 5: Spectra from a
Cs source detected by (9 mm) CsI(Tl) and Ce:GAGG cubic scintillators. The662 keV peaks clearly show the higher detection efficiency of Ce:GAGG at high energies.of performance degradation due to intense irradiation for Sensl J60035, whereas recent studies were publishedfor Hamamatsu 14160-6050HS within the context of the CAMELOT mission. One study shows that forHamamatsu 14160-6050HS the performance degradation - in terms of dark current and noise threshold - dueto 200 MeV protons recovers over time, as it undergoes an annealing process at room temperature. Anotherstudy reports that heavy-ion irradiation causes an increased dark current and a worse energy resolution, but theeffect is overall mild. Clearly, more testing of radiation damage on SiPM is required. Nonetheless, taking all this into account wechose Hamamatsu 14160-6050HS for our prototype.
Reflective materials of various kinds can be applied to the crystal faces in order to maximize the lightcollection from the crystal to the SiPM. The reflectors considered here are: Teflon tape (diffusive), Ag-Al coating(specular), and 3M-Vikuiti
T M enhanced specular reflector (ESR).The Teflon tape is 80 µ m thick and needs to be winded a few times around the crystal, which is glued ontothe SiPM, as shown in figure 6a. We discarded Teflon wrapping because of its large thickness and the difficultyof applying it uniformly and repeatably in the same fashion for each unit.We sputtered SiO -coated CsI(Tl) crystals with a 200 nm Ag layer and then a more robust 200 nm Al layeron top of it. A 6 mm square area at the center of one cube face was left open to couple the SiPM. An exampleof such a unit is shown in figure 6b.The Vikuiti ESR is a 65 µ m thick film. It was laser-cut and glue ¶ was applied to specific areas to keep itclosed after folding. The wrapping process is illustrated in figure 7. ¶ igure 6: Left:
A CsI(Tl) crystal coupled to a SiPM and then wrapped in Teflon tape.
Right:
A CsI(Tl) crystalcoated with Al and Ag and coupled to a SiPM.Figure 7: From left to right, the CsI(Tl) crystal wrapping process with Vikuiti ESR.A comparison of a
Cs spectrum measured by a CsI(Tl) crystal coated with Ag and Al with one measuredby a Vikuiti ESR wrapped crystal is presented in figure 8. From the ratio of the 662 keV peaks position, onecan see that the light transport in the coated crystal results in a ∼ Our first GALI prototype is a system of 90 detector units assembled in the way described in section 4.1.Each CsI(Tl) crystal is coated with SiO , wrapped in Vikuiti ESR film and coupled to a Hamamatsu 14160-6050HS SiPM with optical glue. The detector units are arranged on 7 layers of standard FR4 PCB boards, eachhosting 13 or 12 units. First, the SiPMs are soldered onto the PCBs, then the wrapped crystals are glued tothem by means of a positioning jig. During the gluing stage the PCB and the glue dispenser are kept in anoven so that the glue melts. An assembled layer is shown in figure 9a. The layers are then stacked by means ofpoly-carbonate rods and spacers and aluminium holders that allow mounting the entire stack on motorized axesfor automatic angle scanning. On the back of the vertical axis a PETSys DAQ system is mounted ‖ : a scalablereadout based on the TOFPET2 ASICs. Each detector layer is connected to one of the two ASICs front-endboards through a PCB adapter. The front-end boards are connected to the DAQ through flat cables. EachASIC has 64 channels independently connected to a single SiPM. On top of providing bias voltage to the SiPM, ‖ Entries 1024Mean 101.7Std Dev 121.3 charge [a.u.] e v en t s h1 Entries 1024Mean 101.7Std Dev 121.3
Ag-AlVikuiti
Figure 8: Comparison of a
Cs spectrum measured by a CsI(Tl) crystal coated with Ag and then Al, and onemeasured by a Vikuiti ESR wrapped crystal. The light transport in the coated crystal results in noticeably lesscharge.each channel integrates its output current and digitizes the integral whenever a threshold value is passed. Theraw data are stored conveniently in a ROOT ∗∗ tree for offline analysis. The data acquisition software can becontrolled by python †† scripts, as well as the rotating axes, so that the measurements are fully automated.The entire assembly is shown in figure 9b. We installed the assembly in a thermally insulating dark boxwhere back-fed peltier plates keep the inner temperature constant at 24 ± . ◦ C. This is crucial for a stable gainof the SiPMs and of the DAQ.
5. LOCALIZATION MEASUREMENTS AND RESULTS
In order to test the direction reconstruction capabilities of GALI we expose it to a 10 mCi
Am source placedapproximately 3.5 meters away to simulate a distant source. The effective flux of the 59.6 keV line at thisdistance is approximately 50 ph cm − s − . We scan the entire hemisphere by varying θ between 0 and 90 ◦ and φ between 0 and 360 ◦ with 5 ◦ intervals. For each angle we acquire two kinds of measurements. First a 60 s longexposure (corresponding to up to ∼ σ region around the 59.6 keV Gaussian peak. The long measurements also providethe average counts in each detector at each angle. These are then interpolated at 0.5 ◦ intervals. We then testthe GALI direction reconstruction accuracy of the bursts, similar to the method described for the simulations(section 3).To quantify the advantages of the new concept, we compare GALI to the traditional design used in the GTM.The results of the 90-scintillator GALI are compared to those obtained by a GTM prototype composed of four ∗∗ https://root.cern.ch †† igure 9: Left:
A GALI detector layer composed of 13 units assembled on a PCB board.
Right:
The presentprototype, equipped with PETSys DAQ system. The assembly is installed on motorized axes, which allowsautomatic angle scanning.6.35 × × box NaI scintillators. For practical reasons the GTM experiment is limited to a quarterof the hemisphere. The GTM system is exposed to the source for 30 s. Due to the larger effective area of eachscintillator, a 30 s exposure (corresponding to (cid:38) ∼ . ± .
05 s. Thisuncertainty in burst length is caused by limitations of the DAQ system (CAEN DT5724 ‡‡ ). The GTM data areanalyzed in a similar manner to the GALI data as described above.A comparison between the performance of the GTM system and the 90-scintillator GALI is shown in figure10 for 12 different burst directions. As can be seen, the 90-scintillator system demonstrates better accuracyoverall. The average error for the shown reconstructed bursts ranges between 1.3 ◦ and 2.8 ◦ for the 90-scintillatorGALI and between 3 ◦ and 21 ◦ for the GTM. GALI performs strictly better in all measured directions. Theimprovement in accuracy is often greater than 50%, despite the significantly smaller total detecting volume ofGALI with respect to the GTM.
6. CONCLUSIONS
The present experiments can be summarized as follows • We present a novel directional GRB detector concept based on mutual occultation of numerous, smallscintillator elements. • For this purpose, we explored both CsI(Tl) and Ce:GAGG scintillators, as well as their coating and wrap-ping procedures. Two SiPM readout elements were also compared. We chose for the current experimentto use CsI(Tl) coated with SiO and wrapped by a Vikuiti ESR reflector. • We built a laboratory prototype consisting of 90 CsI(Tl) (9 mm) cubes stacked in 7 layers. Relative countrates were measured at angles over the entire hemisphere with long (60 s) exposures, and used as reference.Subsequently, the direction reconstruction capability was tested on short bursts (0.5 s). • Experimental results with the 59.6 keV peak of
Am show that short bursts ( ∼
25 ph cm − ) can belocalized to within 1 ◦ –3 ◦ . • Simulations of a 350-scintillator instrument, including true LEO background and GRB spectra, show anaccuracy of ∼ . ◦ , which outperforms any existing scintillator-based GRB instrument that we are awareof, including much bigger ones. ‡‡ igure 10: A comparison between laboratory tests of the GTM ( Top ) and a 90-scintillator GALI (
Bottom ).The reconstructed direction of repeated 0.5 s bursts are plotted where each dot represents a burst. Dots aregrouped by color according to the actual source direction, which is represented by a × mark. Confirming thesimulations, the superior angular localization accuracy of GALI is clear. CKNOWLEDGMENTS
We acknowledge the work of project students Ori Zaberchik and Joseph Mualem in the laboratory. We thankGuy Ankonina for proficiently coating the crystals. We acknowledge support by a grant from ISA, a Centerof Excellence of the ISF (grant No. 2752/19), and a grant from the Pazy Foundation. R.R. is supported by aRamon scholarship from the Israeli Ministry of Science and Technology.
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