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# SPIE Proceedings:
GALI a Gamma-ray Burst Localizing Instrument

Roi Rahin  Department of Physics, Technion, Haifa, Israel  Luca Moleri
Department of Physics, Technion, Haifa, Israel  Alex Vdovin  Department of
Physics, Technion, Haifa, Israel  Amir Feigenboim  Department of Physics,
Technion, Haifa, Israel  Solomon Margolin  Department of Physics, Technion,
Haifa, Israel  Shlomit Tarem  Department of Physics, Technion, Haifa, Israel
Ehud Behar  Department of Physics, Technion, Haifa, Israel  Max Ghelman
Nuclear Research Center Negev, Israel  Alon Osovizky  Nuclear Research Center
Negev, Israel

###### Abstract

The detection of astrophysical Gamma-Ray Bursts (GRBs) has always been
intertwined with the challenge of identifying the direction of the source.
Accurate angular localization of better than a degree has been achieved to
date only with heavy instruments on large satellites, and a limited field of
view. The recent discovery of the association of GRBs with neutron star
mergers gives new motivation for observing the entire $\gamma$-ray sky at once
with high sensitivity and accurate directional capability. We present a novel
$\gamma$-ray detector concept, which utilizes the mutual occultation between
many small scintillators to reconstruct the GRB direction. We built an
instrument with 90 (9 mm)3 CsI(Tl) scintillator cubes attached to silicon
photomultipliers. Our laboratory prototype tested with a 60 keV source
demonstrates an angular accuracy of a few degrees for $\sim$25 ph cm-2 bursts.
Simulations of realistic GRBs and background show that the achievable angular
localization accuracy with a similar instrument occupying $1$l volume is
$<2^{\circ}$. The proposed concept can be easily scaled to fit into small
satellites, as well as large missions.

###### keywords:

Gamma-ray bursts, scintillators, silicon photomultipliers, directional gamma-
ray detector, small satellites, detector simulations

##  1 INTRODUCTION

###  1.1 Astrophysical Context

Astrophysical $\gamma$-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 the collapse
of massive stars. Short GRBs, lasting less than 2 s, are associated with the
coalescence of a neutron stars binary [1, 2], and could also result from a
merger of a NS and a black-hole binary [3].

Following the first discoveries of gravitational waves (GWs) from compact
stellar mergers by Advanced LIGO (Laser Interferometer Gravitational-wave
Observatory), the astrophysics community has concentrated its efforts on
detecting their electromagnetic (EM) counterparts. The first direct detection
of GWs by the two LIGO facilities occurred in September 2015 [4]. In August
2017 the European VIRGO detector joined the LIGO observation run [5]. The
combined detection by three interferometers potentially improves the event
localization from more than a thousand square degrees to tens of square
degrees, depending of course on the strength of the signal.

The first EM counterpart of a GW event was observed on 17 August, 2017. This
LIGO-VIRGO GW event [6]) was followed 1.7 s later by a short GRB (GRB 170817A)
independently detected by the Fermi $\gamma$-ray Burst Monitor (GBM) and by
IBIS on board INTEGRAL [7]. Many other facilities around the world started to
search for the site of the event in all possible wavebands. It took
approximately 11 hours before the source was identified in the lenticular
galaxy NGC 4993 [8].

Increasing the number of simultaneously observed GW and EM counterparts is
paramount to addressing fundamental questions on the nature of coalescing
neutron stars and black-holes. The goal of the method described in the present
paper is to detect GRBs at high sensitivity and to identify their direction
with high accuracy. Such capabilities will not only enable fast follow-up with
telescopes, but will also allow LIGO-VIRGO to search for sub-threshold events
once the time and direction are known.

###  1.2 GRB Localization

A single soft $\gamma$-ray detector unit generally cannot identify the
direction of incident photons. Angular localization of GRBs thus require
multiple detectors. The coded-mask aperture method, most successfully
implemented on Swift-BAT [9], is an array of detectors partially covered by a
mask. The coded mask generates different shading patterns over the detectors
array, varying with the source direction. This technique enables the
reconstruction of the direction of a GRB up to 20’ (minutes of arc). However,
it requires a large detector area and volume due to the required separation
between the mask and the array. Moreover, it has a limited field of view, and
the mask blocks many of the source photons.

Another method is a sparse array of scintillators, e.g., on Compton-BATSE [10]
and Fermi-GBM [11], which are distributed over the spacecraft, each facing a
different direction. The relative signals in each scintillator provide
information about the direction of the source. For example, those facing the
opposite direction are shielded by the satellite and will have lower count
rates. Angular localization of soft $\gamma$-ray sources can also be achieved
if the detectors are far enough apart, as on different space crafts, and the
different times of arrival can be discriminated.[12] Alternatively, direction
reconstruction is possible if the detection is time-coincident with another
observation of a different messenger (e.g. visible light), where better
angular accuracy is available.

##  2 The GALI concept

In traditional astrophysical $\gamma$-ray detectors, scintillators are built
with different cross-sections towards different directions to produce a
gradually varying response with angle. These systems rely on the scintillators
facing various orientations to reconstruct the direction of the source. In
contrast, the $\gamma$-ray-burst Localizing Instrument (GALI) concept
presented here exploits mutual occultation of numerous small scintillators,
distributed within a small volume, to provide directional information. The
method relies on the entire array looking significantly different from
different directions. Due to the occultation, the count rates from each
scintillator will vary dramatically as a function of the source direction,
even for small angle differences. In a sense, this is similar to the coded
mask aperture method, but the mask itself is composed of detecting
scintillators, so that no precious photons are lost. The low count rates in
each individual small scintillator are compensated by the large number of
scintillators. As in traditional approaches, the sensitivity to weak sources
depends on the total size of the detecting volume. Fig. 1 shows two prototype
instruments demonstrating the differences between a traditional detector
design and the present concept. The first is the Gamma ray Transient Monitor
(GTM) whose design is similar to the Fermi/GBM, and which has been described
in past SPIE proceedings [13]. The second is the GALI laboratory prototype
described in section 4.2.

(a)

(b)

Figure 1: Two prototype instruments demonstrating the two conceptual detector
designs Left: The GTM - a traditional design based on large, asymmetric box-
shaped scintillators with various orientations. This design is similar to the
Fermi/GBM. Right: The GALI design, which uses (9 mm)3 cubic scintillators in a
3D structure that exploits their different mutual occultation from different
directions.

The GALI laboratory prototype presented here is composed of (9 mm)3 cubic
scintillators. These are read out by 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 the expense of the size of each individual one. An
additional advantage of SiPMs over PMTs is their significantly lower operation
voltage (tens of volt instead of hundreds).

A simulation of the system performance is presented in section 3, a detailed
description of the experimental setup 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[14], a simulation software based on Geant4[15]. In these simulations
all $\gamma$-ray interactions with scintillators are considered as counts in
the relevant scintillator. Each simulation run is composed of two steps: A
bright GRB with no background and the background simulation. We use the bright
GRB simulation to estimate the average counts on each scintillator in the
system with no background: a burst of 1000 ph$\cdot$cm-2 , whose photon
spectrum ($dN/dE$) is a band function[16] (equation 1) with $\alpha=1.1$,
$\beta=2.3$ and $E_{peak}=(2+\alpha)E_{0}=266$ keV.

$\frac{dN(E)}{dE}=A\begin{cases}{(\frac{E}{100keV})^{\alpha}\exp\left({-\frac{E}{E_{0}}}\right)}&E\leq(\alpha-\beta)E_{0}\mbox{
}\\\
{\left[{\left({\alpha-\beta}\right)\frac{E_{0}}{100keV}}\right]^{\left({\alpha-\beta}\right)}(\frac{E}{100keV})^{\beta}\exp\left({\beta-\alpha}\right)}&E>(\alpha-\beta)E_{0}\mbox{
}\end{cases}$ (1)

The simulations are repeated varying the angular coordinates at 5∘ intervals
within the upper hemisphere. The average counts on each scintillator are then
interpolated at either 0.5∘ or 1∘ intervals. We divide the average counts 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 based on various
lower earth orbit (LEO) observations of hard X-rays and $\gamma$-rays [17, 18,
19, 20, 21] and is included in the MEGAlib software package. Background caused
by Leptonic and Hadronic components was not included, being estimated in
simulation to be less than 1% of the photonic background, far less than the
statistical uncertainty. The Hadronic background may need to be simulated
later depending on the spacecraft platform which will host the experiment.

After running the simulations, we estimate the directional capabilities of the
detector system. We consider 1 second bursts of 10 ph cm-2 s-1 from a given
direction in the presence of the background. The burst is generated using the
aforementioned relative average counts by applying poisson statistics to the
expected average of each scintillator. We then add the background with poisson
uncertainty. We reconstruct the burst direction using a cstat estimator [22]
between the generated counts and the simulated ones at each angle. The
reconstructed direction is that which gives minimal cstat value. The burst
generation and direction reconstruction process is repeated 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)3 scintillator cubes spread in a 6$\times$6$\times$7 cm3 volume
and one of 350 cubes in a 10$\times$10$\times$10 cm3 volume. To compare the
potential improvement of the GALI accuracy with respect to previous existing
concepts, we also simulated the a GTM detector with four 3” diameter 1” thick
cylindrical scintillators. The GTM has similar effective area compared to the
350-scintillator GALI, but has approximately twice that of the 90-scintillator
GALI. The simulated systems are shown in figure 2. Notice that the
90-scintillator GALI contains the PCB boards upon which the detectors are
mounted as in the the lab prototype described in section 4.2, whereas no
350-scintillator GALI prototype exists, 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 each
system is 13.1∘ for the GTM, 6.5∘ for a 90-scintillator GALI and 1.7∘ for the
350-scintillator GALI. From these simulations we conclude that the
localization accuracy of a system increases significantly with the number of
detectors, 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)3 cubic scintillators.
Between the detectors are PCB boards. This system is simulated as close as
possible to a lab model. Right: A GALI configuration of 350 (9 mm)3 cubic
scintillators.

Figure 3:  A comparison between the simulation-generated GRBs direction
reconstructed by different detector configurations. Each dot represents a 1 s
burst of 10 ph cm-2 s-1 at lower earth orbit. Dots are grouped by color
according to the actual burst direction, which is represented by a $\times$
mark. Top: GTM. Middle: 90-scintillator GALI. bottom: 350-scintillator GALI.
Notice the clear improvement in localization accuracy when the number of
scintillators is increased.

##  4 Experimental setup

###  4.1 Individual detector unit characterization

We conducted a preliminary study to characterize single detector units in
order to choose among different scintillating crystals, reflective wrappings
and SiPM sensors. Each detector unit consists of a (9 mm)3 cubic scintillator
wrapped or coated by a reflective material and coupled to a SiPM using
Cargille Meltmount111https://www.cargille.com/mounting-media optical glue,
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 negligible intrinsic radioactivity: CsI(Tl) (see for example Balamurugan
et al.[23]) and Ce:GAGG[24]. Both scintillators were polished by and purchased
from Advatek222https://www.advatech-uk.co.uk. We coated the CsI(Tl) crystals
by sputtering a 50 nm thick layer of SiO2, which protects the delicate and
slightly hygroscopic crystal from mechanical degradation and water absorption.
Ce:GAGG suffers from afterglow for hours after exposure to light due to
lattice defects[25]; therefore, it should be always kept in the dark so as not
to alter the measurements. For space applications, the afterglow caused by
electron and $\gamma$-ray dose needs to be considered.

$\gamma$-ray signals from our CsI(Tl) and Ce:GAGG crystals are read by a Sensl
J60035 SiPM333https://www.onsemi.com/pub/Collateral/MICROJ-SERIES-D.PDF
through a 10 k$\Omega$ resistor and recorded with an oscilloscope with 1
M$\Omega$ termination, and are shown in figure 4. In these measurements the
SiPM was operated at a 27.2 V bias. The recorded voltage pulse is a
convolution of the SiPM response function and the scintillation time
evolution[26]. For both crystals in the present configuration (RC$\sim$40
$\upmu$s, given C$\sim$4 nF) the scintillation time is shorter than the
circuit typical decay time. Therefore, the measured voltage is proportional to
the integrated charge produced by the SiPM, and the peaks in figure 4
corresponds to the end of the scintillation light production. The voltage
recovery follows the circuit RC value. Figure 4b shows the full Ce:GAGG light
emission signal, which presents a fast component ($\sim$100 ns) and a slow
tail (ending at $\sim$500 ns) that can be distinguished by the signal slope
change. In the same figure it can be seen that the CsI(Tl) light emission is
much slower than the Ce:GAGG one. During the 500 ns in which the Ce:GAGG
emission is completed, CsI(Tl) produces only $\sim$40$\%$ of the total charge.
In figure 4a the full CsI(Tl) signal is shown, reaching its maximum value
within $\sim$6 $\upmu$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 $\sim$1 $\upmu$s
integration time (like the TOFPET2 ASICs described in section 4.2) there is a
clear advantage in using Ce:GAGG for better signal-to-noise.

(a)

(b)

Figure 4: $\gamma$-ray signals from CsI(Tl) and Ce:GAGG crystals read by a
SiPM coupled to ground through a 10 k$\Omega$ resistor. The right hand side
shows the first 500 ns of the plot on the left. The measured voltage is
proportional to the integrated charge produced by the SiPM. The peak
corresponds to the end of the light production, which is faster for Ce:GAGG.
The voltage recovery follows the circuit RC value.

We measured charge spectra produced by the two crystals using an Ortec 671
spectroscopy amplifier with an Ortec Aspec-927 MCA. We set the amplifier
shaping time to 0.5 $\upmu$s for Ce:GAGG and to 3 $\upmu$s for CsI(Tl) to
include the full signal rise. The latter value is due to the signal shortening
because of the low input impedance (465 $\Omega$) of the amplifier. Spectra
from a 137Cs source from both scintillators are presented in figure 5. The
ratio of the number of counts in the 662 keV peak between the two
scintillators is $\sim$2 in favor of Ce:GAGG, whereas for the 32 keV peak the
ratio is comparable (taking into account the higher Compton baseline of
Ce:GAGG). This indicates a higher detection efficiency of Ce:GAGG due to its
higher density. On the other hand, it has been reported[27] that at low
energies CsI(Tl) provides $\sim$120% response with respect to the response at
high energy while Ce:GAGG provides only $\sim$85%. In conditions in which the
signal to noise ratio is low, such as high temperature or high SiPM dark count
rates (for example due to radiation damage), the noise threshold rises, so the
advantage due to higher Ce:GAGG efficiency can be jeopardized. In general, the
spectra peak positions result from a few factors: the crystal light yield, the
SiPM gain and the effect of the amplifier. Investigating these factors and
their $\gamma$-ray energy dependence requires a dedicated study with multiple
radioactive sources, which is beyond the scope of the present study.

Figure 5: Spectra from a 137Cs source detected by (9 mm)3 CsI(Tl) and Ce:GAGG
cubic scintillators. The 662 keV peaks clearly show the higher detection
efficiency of Ce:GAGG at high energies.

Based on the presented results we conclude that there might be advantages in
choosing Ce:GAGG for its faster light emission and higher detection
efficiency. Despite that, for our first system prototype we chose CsI(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[28]444https://www.hamamatsu.com/resources/pdf/ssd/s14160_s14161_series_kapd1064e.pdf.
The most relevant features for the two kinds are summarized in table 4.1. The
Sensl J60035 SiPM has two clear advantages: a 13.5 V lower breakdown voltage
and a smaller cell size, which implies a linear response until higher gamma
energies. On the other hand, Hamamatsu 14160-6050HS has lower dark current,
which is important for space applications. Additionally, according to vendor
data the Hamamatsu 14160-6050HS has a $\sim$10$\%$ enhanced PDE in the region
above 450 nm, where the emission of both scintillators peaks.

Type | size |  $V_{bd}$[V] | max Idark[$\upmu$A | cell size [$\upmu$m] | capacitance [nF]
---|---|---|---|---|---
Sensl J60035 | 6 mm | 24.5 | 12 | 35 | 4
Hamamatsu 14160-6050HS | 6 mm | 38 | 7.5 | 50 | 2

Another important parameter for space applications is radiation hardness,
meaning capability to withstand doses of highly ionizing particles with an
acceptable performance degradation. We could not find any study of performance
degradation due to intense irradiation for Sensl J60035, whereas recent
studies were published for Hamamatsu 14160-6050HS within the context of the
CAMELOT mission [29]. One study shows that for Hamamatsu 14160-6050HS the
performance degradation - in terms of dark current and noise threshold - due
to 200 MeV protons recovers over time, as it undergoes an annealing process at
room temperature [30]. Another study reports that heavy-ion irradiation causes
an increased dark current and a worse energy resolution, but the effect is
overall mild [31].

Clearly, more testing of radiation damage on SiPM is required. Nonetheless,
taking all this into account we chose Hamamatsu 14160-6050HS for our
prototype.

#### Reflective materials

of various kinds can be applied to the crystal faces in order to maximize the
light collection from the crystal to the SiPM. The reflectors considered here
are: Teflon tape (diffusive), Ag-Al coating (specular), and 3M-VikuitiTM
enhanced specular reflector (ESR).

The Teflon tape is 80 $\upmu$m thick and needs to be winded a few times around
the crystal, which is glued onto the SiPM, as shown in figure 6a. We discarded
Teflon wrapping because of its large thickness and the difficulty of applying
it uniformly and repeatably in the same fashion for each unit.

We sputtered SiO2-coated CsI(Tl) crystals with a 200 nm Ag layer and then a
more robust 200 nm Al layer on top of it. A 6 mm square area at the center of
one cube face was left open to couple the SiPM. An example of such a unit is
shown in figure 6b.

(a)

(b)

Figure 6: Left: A CsI(Tl) crystal coupled to a SiPM and then wrapped in Teflon
tape. Right: A CsI(Tl) crystal coated with Al and Ag and coupled to a SiPM.

The Vikuiti ESR is a 65 $\upmu$m thick film. It was laser-cut and
glue5553M-9471 was applied to specific areas to keep it closed after folding.
The wrapping process is illustrated in figure 7.

(a)

(b)

(c)

(d)

Figure 7: From left to right, the CsI(Tl) crystal wrapping process with
Vikuiti ESR.

A comparison of a 137Cs spectrum measured by a CsI(Tl) crystal coated with Ag
and Al with one measured by a Vikuiti ESR wrapped crystal is presented in
figure 8. From the ratio of the 662 keV peaks position, one can see that the
light transport in the coated crystal results in a $\sim$2.2 times lower
signal than the Vikuiti ESR wrapped one. This is surprising, given the fact
that both Ag and Al have a reflectivity higher than 90$\%$ in the CsI(Tl)
spectral range. This effect deserves further investigation. Since we repeated
a similar experiment with a Ce:GAGG crystal obtaining similar results, we
conclude that the cause is probably not a chemical reaction that alters the
properties of the surface. For Vikuiti ESR wrapping there is a layer of air
between the crystal and the reflector which causes internal reflection to take
place. This may affect the results, but it is not clear if this fact alone can
explain the significant difference from the coated crystal.

Figure 8: Comparison of a 137Cs spectrum measured by a CsI(Tl) crystal coated
with Ag and then Al, and one measured by a Vikuiti ESR wrapped crystal. The
light transport in the coated crystal results in noticeably less charge.

###  4.2 GALI detector prototype

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 SiO2, 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,
each hosting 13 or 12 units. First, the SiPMs are soldered onto the PCBs, then
the wrapped crystals are glued to them by means of a positioning jig. During
the gluing stage the PCB and the glue dispenser are kept in an oven so that
the glue melts. An assembled layer is shown in figure 9a. The layers are then
stacked by means of poly-carbonate rods and spacers and aluminium holders that
allow mounting the entire stack on motorized axes for automatic angle
scanning. On the back of the vertical axis a PETSys DAQ system is
mounted666https://www.petsyselectronics.com: a scalable readout based on the
TOFPET2 ASICs. Each detector layer is connected to one of the two ASICs front-
end boards through a PCB adapter. The front-end boards are connected to the
DAQ through flat cables. Each ASIC has 64 channels independently connected to
a single SiPM. On top of providing bias voltage to the SiPM, each channel
integrates its output current and digitizes the integral whenever a threshold
value is passed. The raw data are stored conveniently in a
ROOT777https://root.cern.ch tree for offline analysis. The data acquisition
software can be controlled by python888https://www.python.org 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 box where back-fed peltier plates keep the inner
temperature constant at 24$\pm 0.5^{\circ}$C. This is crucial for a stable
gain of the SiPMs and of the DAQ.

(a)

(b)

Figure 9: Left: A GALI detector layer composed of 13 units assembled on a PCB
board. Right: The present prototype, equipped with PETSys DAQ system. The
assembly is installed on motorized axes, which allows automatic angle
scanning.

##  5 Localization measurements and results

In order to test the direction reconstruction capabilities of GALI we expose
it to a 10 mCi 241Am source placed approximately 3.5 meters away to simulate a
distant source. The effective flux of the 59.6 keV line at this distance is
approximately 50 ph cm-2 s-1. We scan the entire hemisphere by varying
$\theta$ between 0 and 90∘ and $\phi$ between 0 and 360∘ with 5∘ intervals.
For each angle we acquire two kinds of measurements. First a 60 s long
exposure (corresponding to up to $\sim 3000$ counts on each scintillator), and
then a series of 100 bursts 0.5 s long. Measurements are analyzed offline. We
use the long measurements to define the spectrum region of interest for each
detector unit, namely, 2$\sigma$ region around the 59.6 keV Gaussian peak. The
long measurements also provide the average counts in each detector at each
angle. These are then interpolated at 0.5∘ intervals. We then test the 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
6.35$\times$6.35$\times$2.54 cm3 box NaI scintillators. For practical reasons
the GTM experiment is limited to a quarter of the hemisphere. The GTM system
is exposed to the source for 30 s. Due to the larger effective area of each
scintillator, a 30 s exposure (corresponding to $\gtrsim 50000$ counts on each
scintillator), is enough to establish the reference average counts. We Test
the GTM localization capabilities using 50 bursts of $\sim 0.5\pm 0.05$ s.
This uncertainty in burst length is caused by limitations of the DAQ system
(CAEN DT5724999https://www.caen.it). The GTM data are analyzed 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 figure 10 for 12 different burst directions. As can be seen,
the 90-scintillator system demonstrates better accuracy overall. The average
error for the shown reconstructed bursts ranges between 1.3∘ and 2.8∘ for the
90-scintillator GALI and between 3∘ and 21∘ for the GTM. GALI performs
strictly better in all measured directions. The improvement in accuracy is
often greater than 50%, despite the significantly smaller total detecting
volume of GALI with respect to the GTM.

Figure 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 are grouped by
color according to the actual source direction, which is represented by a
$\times$ mark. Confirming the simulations, the superior angular localization
accuracy of GALI is clear.

##  6 Conclusions

The present experiments can be summarized as follows

  * •

We present a novel directional GRB detector concept based on mutual
occultation of numerous, small scintillator elements.

  * •

For this purpose, we explored both CsI(Tl) and Ce:GAGG scintillators, as well
as their coating and wrapping procedures. Two SiPM readout elements were also
compared. We chose for the current experiment to use CsI(Tl) coated with SiO2
and wrapped by a Vikuiti ESR reflector.

  * •

We built a laboratory prototype consisting of 90 CsI(Tl) (9 mm)3 cubes stacked
in 7 layers. Relative count rates 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 241Am show that short bursts
($\sim$25 ph cm-2) can be localized to within 1∘–3∘.

  * •

Simulations of a 350-scintillator instrument, including true LEO background
and GRB spectra, show an accuracy of $\sim 1.7^{\circ}$, which outperforms any
existing scintillator-based GRB instrument that we are aware of, including
much bigger ones.

###### Acknowledgements.

We acknowledge the work of project students Ori Zaberchik and Joseph Mualem in
the laboratory. We thank Guy Ankonina for proficiently coating the crystals.
We acknowledge support by a grant from ISA, a Center of Excellence of the ISF
(grant No. 2752/19), and a grant from the Pazy Foundation. R.R. is supported
by a Ramon scholarship from the Israeli Ministry of Science and Technology.

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